Environmental science is the interdisciplinary study of the interactions among living and non-living parts of the environment, with a special focus on the impact of humans on the environment. Our future depends on our ability to understand and evaluate evidence-based arguments about the environmental consequences of human actions and technologies and to make informed decisions based on those arguments – environmental science provides much of the evidence needed.

From global climate change to habitat loss driven by human population growth and development, Earth is becoming a different planet—right before our eyes. The global scale and rate of environmental change are beyond anything in recorded human history. Our challenge is to understand better Earth’s complex environmental systems, systems characterized by interactions within and among their natural and human components that link local to global and short-term to long-term phenomena, and individual behavior to collective action.

In this section, we’ll lay a foundation for discussing environmental sustainability science – a general understanding of how energy and temperature move around the planet and a bit of chemistry, so we can talk about physical stuff, too.

Energy, temperature, and a spinning planet – the climate system

Our environmental problems take place on a planet with lots of moving parts, particularly in the air and oceans. Some of our environmental problems (air pollution, for example) hitch rides on the moving parts! As part of our foundation, we need to know the basic moving parts and the forces that drive them and that move with them.

First, three videos from the UK meteorological service (Met Office) that provide helpful visual support for understanding how air and temperature move on a spinning planet.

How does the climate system work?

What is global circulation – the three cells

What is global circulation – the Coriolis effect & winds 

The combination of waters of different temperature and salinity, plus the Coriolis effect give rise to the global conveyor belt of ocean currents also known as the Meridional Overturning Circulation (MOC) This conveyor belt distributes warm water away from the equator and helps to moderate ocean temperatures in temperate and polar latitudes, just as the atmospheric cells discussed in the videos move warm air away from the equator into cooler latitudes. The Atlantic portion of this pattern is important enough to the Northern Hemisphere nations around the Atlantic that is often discussed separately, as the Atlantic meridional overturning circulation, or AMOC. This video from the PBS NOVA collection explains the basics of global ocean circulation.

Click here to watch: Global Ocean Circulation

Chemistry for understanding environmental sustainability

Atoms and molecules

Atoms are the basic unit of chemistry – the smallest bits of single chemical entities. Atoms combine to form molecules. Some molecules are composed of a single chemical element (for example, O₂ or N₂ – oxygen and nitrogen as they exist in their usual form as gases in the atmosphere). Water is composed of three atoms of two elements – 2 H atoms and 1 O atom, and we write its formula as H₂O. Larger molecules have increasingly complex formulas – glucose is a simple example: C₆H₁₂O₆.

Atomic abbreviations come from the periodic table (Fig 1) – a formal presentation of all the elements known to science. As of early 2025, there are 118 chemical elements formally described to science, but several of these only exist when researchers undertake rather violent experiments in carefully controlled laboratory conditions, and many more are uncommon.

Many of the abbreviations seem very sensible: C for carbon, O for oxygen, H for hydrogen, He for helium, Al for aluminum, and so forth. Others seem very odd: Fe for iron, Na for sodium, K for potassium, for example. Most of the oddballs were discovered rather early, when Latin was still an important language for science, and reflect Latin names: ferrum, natrium, and kalium for these examples. Note that whereas the abbreviations for the names of elements are capitalized (being abbreviations), the names of the elements themselves are not capitalized.

The periodic table groups similar kinds of elements – several different kinds of metals, for example, or the unreactive chemicals called the noble gases (column VIIIA in Figure 1) that don’t participate in chemical reactions. These groups have somewhat similar chemistries, and their behaviors can be predicted, to some extent, by their membership in these groups.

Column VIIA of the periodic table contains the elements called halogens. Chlorine through iodine are the most familiar. Chlorine (Cl), fluorine (F), and bromine (Br), in their elemental forms, are all toxic. But chemists find them useful and use them to create compounds of many kinds, many of which have become environmental problems due to toxicity. The words halogenated, chlorinated, fluorinated, or brominated signal a potentially harmful compound, particularly if it’s a synthetic compound. But note that plenty of fairly harmless, naturally occurring compounds, including table salt (sodium chloride – NaCl) also contain halogens.

Atoms combine to form molecules because atoms have electrical charges that attract them to each other (forming ionic bonds) or because they become more stable in the combined state (forming covalent bonds). Some can attract themselves and occur in their pure forms in nature – O₂ and N₂ are gaseous examples and soot, graphite and diamond are all solid examples of pure C – but other elements rarely in their pure form in nature. For example, iron occurs primarily with oxygen in ores such as hematite, Fe₂O₃, and magnetite, Fe₃O₄.

Some of the bonds between atoms are easily undone in certain circumstances – table salt, NaCl, and many other ionically-bonded compounds dissolve (dissociate, technically) easily in water. Table salt dissociates to leave charged ions of sodium (Na⁺) and chlorine (Cl^–) separately floating around.

Adding energy through sunlight or heat can break covalent bonds. For example, sunlight is an important part of the atmospheric chemistry that creates smog. Heat from fire can break the bonds of lignin, the very large molecule that is the basis for wood, turning it into carbon dioxide and water.

Figure 1. The periodic table of the elements. Each cell provides information on a single chemical element. Elements are grouped into subsets based on similarities in chemical behavior. Offnfopt. Public domain.

Living things are composed mostly of molecules called carbohydrates, fats, and proteins. Carbohydrates (including sugars) and fats are made up of carbon, hydrogen, and oxygen. Lignin, the main component of wood, is a carbohydrate and thus contains only C, H, and O. Many biologically important molecules are polymers – large molecules made up of repeating subunits of one or more smaller molecules called monomers. Lignin is a biopolymer made up of monomers called phenols that have a ring structure (see examples of ring structures in Figure 2).

Proteins all add nitrogen; they are made up of amino acids. Fur, made of keratin is made of the same amino acids that make up protein, and requires only C, H, O, and N.  Some amino acids have sulfur (S) atoms included in them, which help keep proteins folded into their proper shapes. Some proteins add phosphorus (P) and a variety of trace amounts of other elements. Bones and teeth need calcium (Ca) and phosphorus. Enzymes, which are proteins that serve as catalysts to help chemistry happen quickly in living things, use a variety of trace elements, including several metals: zinc (Zi), selenium (Se), copper (Cu – cuprum), manganese (Mn), iron, cobalt (Co), molybdenum (Mo), and nickel (Ni).

Although C, H, O, and N make up more than 90% of the weight of all living things, very small quantities of elements can be hugely important to the function of a molecule: a single atom of magnesium (Mg) is at the heart of chlorophyll – the green stuff that makes photosynthesis (and most of life) possible – and four atoms of iron (Fe) occur in hemoglobin – the stuff that makes blood red and that captures oxygen in our lungs and allows it to be delivered throughout our bodies, far from the atmosphere.

Organic compounds

Most of the compounds in living things are organic compounds – compounds built on a backbone of carbon atoms in chains and rings (Fig 2). The simplest organic compounds, simple hydrocarbons composed only of C and H, are not found in living things but derive from components of living things. They are  found in fossil fuels components: methane (CH₄, also called natural gas or swamp gas) and ethane (C₂H₆) (Fig 2). Remember that fossil fuels resulted from buried organic material – mostly buried vegetation – subjected to heat and pressure over geological time. No surprise, then, that fossil fuels should be a complicated mix of organic compounds. Plastics are an example of anthropogenic (human-made – synthetic) organic, polymeric materials, generally created from fossil fuels. Many other synthetic chemical compounds, including important hazardous substances are also organic compounds.

Figure 2. Simple organic molecules. Top row from left: methane, ethane, methanol. Bottom row from left: cyclohexane, benzene. Images built with MolView by Meretsky. CC0.

The 3-D molecular models built here show carbon atoms in gray, hydrogen in white, and oxygen in red. Each carbon atom can form four bonds. Methane and ethane are the two simplest hydrocarbons (molecules that contain only C and H), with one and two carbons, respectively. Methanol is the simplest alcohol, with a single carbon. The OH group (the red atom connected to the white atom) makes it an alcohol. The lower row shows two hydrocarbon ring structures, each with 6 carbons but different numbers of hydrogen atoms. Cyclohexane has single bonds between the carbon atoms, leaving each C with 2 bonds to share with hydrogen atoms. Benzene has double bonds between every other pair of carbon atoms, so each C atom has only a single bond left to share with hydrogen. Benzene is a common unit found in lignin, soot, and some toxic organic pollutants including polychlorinated biphenyls (PCBs) and dioxins.

The phrase persistent organic pollutants refers to synthetic organic compounds that may take decades or centuries to be broken down, and that can continue to harm living things during that time. “Forever chemicals” is a newer, more emotional phrase that describes the same phenomenon. Keep in mind that naturally occurring but toxic elements such as lead, mercury, and other heavy metals cannot ever break down because they are already simply elements.

Some persistent pollutants, including heavy metals, avoid being broken down or eliminated by digestion when they enter the bodies of organisms (from plankton to polar bears) through contaminated food and water. Instead, the body may treat them as fats and store them in fat deposits, in the case of organic pollutants or use them in place of useful elements such as calcium, in the case of heavy metals.

Figure 3. Bioaccumulation is the process by which individuals accumulate more and more toxin over time because it is not processed out of their bodies when they eat or absorb it. Biomagnification is the process in which toxins increase with each higher level on the food web because individuals at higher levels eat many individuals at lower levels and retain much of the toxin in their food. Meretsky using ChatGPT 5.2. CC0.

Plankton in contaminated water are tiny and likely only hold a tiny amount of the contaminant. But over the short lifetime of the plankton, it will continue to take in more of the contaminant and will not lose most of it. This gradual build-up of a toxin over the lifetime of an organism is termed bioaccumulation. Fish that eat the plankton will also retain the contaminant, and will bioaccumulate more over their lives, eating many plankton each day. Because they eat lots of plankton, the level of toxin in the fish is much higher than the level in the plankton – termed biomagnification or bioamplification (Fig 3). As organisms at each successive level of a food chain encounter the toxin, the levels of toxin rise sharply, and the likelihood of harm increases. Different toxins cause different kinds of harm – neurological or developmental damage, for example. Birds of prey and other predators, along with humans, who also eat at the tops of some food chains, are at most risk or harm. Persistent organic pollutants but also some inorganic toxins such as heavy metals may bioaccumulate and biomagnify.

One class of forever chemicals has become a particular focus in the 2010s and 2020s: PFAS – perfluorolalkyl and polyfluorolalkyl substances. Thousands of these compounds exist; they have been in use since the 1930s and 1940s and are now widely dispersed throughout the world in all ecosystems and all human habitations.  They occur in a wide variety of compounds including waterproofing and stainproofing materials,  and firefighting foams. PFAS are responsible for a wide range of health impacts including cancer, endocrine disruption, developmental issues, and immune deficiencies. Regulation of PFAS in drinking water only began in 2024.

Inorganic compounds

Inorganic compounds – compounds that are not based on carbon chains and rings – are also important. Salts, metals, ores, crystals, and all the other forms of minerals are considered to be inorganic. While inorganic compounds are often used for construction and industry, they are important to living things, as well. Any athlete knows the importance of replacing salts lost to sweat. The iron in hemoglobin anchors the chemical component that captures oxygen in the lungs and delivers it throughout our bodies.

When we discuss soil, we divide it into its mineral components, which derive from the breakdown of rock – sand, silt, and clay – and its organic components that derive from the breakdown of living things. Good topsoil, for agricultural purposes, has 3-6% organic material by weight, along with about 45% mineral (inorganic) material by weight; 50% is pore space containing water and air.

Mixes of organic and inorganic compounds occur in living things, for example, in bones and teeth that incorporate calcium and phosphorus. A number of grasses incorporate silica (silicon dioxide: SiO₂ – in larger amounts it is called sand) into their leaves to make their tissues more resistant to grazing and digestion, to deter herbivores.

Sometimes inorganic compounds and organic compounds can mix to create highly toxic pollutants.  For example, ethyl lead – Pb(C₂H₅)₄ – was an additive in gasoline in the 20th century that ended up in car exhaust, causing widespread lead contamination of the environment. In addition, when the heavy metal mercury, Hg, is processed by some bacteria at the bottom of rivers and the ocean, a compound called methyl mercury (CH₃Hg) is formed, which acts like an organic pollutant in that it bioaccumulates, but it behaves like a highly toxic heavy metal. Both Pb and Hg are highly potent neurotoxins.

Elements of air

Earth’s atmosphere has changed immensely over the history of the planet. The early atmosphere would have been poisonous to most present life forms, and lacked oxygen. Today, Earth’s atmosphere is dominated by nitrogen (78%) and oxygen (21%). The remaining gases, including carbon dioxide (CO₂), methane (CH₄), water vapor, and other trace gases, compose only 1% of the atmosphere but include the gases that regulate planetary temperatures and affect human and environmental health. Volcanos, which contributed much of the gas volume for the early Earth atmosphere, can change the balance of trace gases in the present, during eruptions.

The oxygen in our present atmosphere is created by plants, which produce oxygen as a byproduct of photosynthesis – the process of using light energy to combine water and CO₂ to produce sugar  – that is the basis for all food webs (almost all – a few microorganisms do really interesting things with chemistry, instead). Plants and animals take in oxygen to fuel metabolism – the process that maintains, builds, and repairs organisms – and all these organisms produce CO₂ as a result of metabolic processes called respiration. Plant biomass is approximately 180 times the animal biomass on Earth, and plants undertake photosynthesis, so they are the major players in this O₂ – CO₂ swapping process.

Greenhouse gases (GHG) are atmospheric gases that absorb heat radiated by Earth and re-radiate some of it back to the planet. Greenhouse gases are the reason that Earth has temperatures that are much milder than temperature on the Moon, which is the same average distance from the sun as Earth. Without greenhouse gases, life on Earth would not be possible. Water vapor is the primary greenhouse gas in our atmosphere, but because humans do not directly affect the concentration of water vapor in the atmosphere, it is not a target for control. Carbon dioxide, methane (swamp gas, natural gas), and nitrous oxide (N₂O) are the main anthropogenic greenhouse gases. We will see more of them in the next section.

In the stratosphere, one of the higher atmospheric layers well above the Earth’s surface, the sun’s energy is strong enough to split oxygen molecules, which then interact with unsplit molecules to form a layer of ozone (O₃), a highly reactive, short-lived compound that absorbs ultraviolet (UV) radiation. This stratospheric ozone layer is important for protecting life on Earth from heavy doses of ultraviolet radiation – UV – that could damage DNA, harm exposed body surfaces and vegetation, and cause cancers. Air pollution can weaken the ozone layer by thinning it, creating health risks and other environmental harm.

Acids, bases, and pH

Acids and bases are a large class of chemicals that play an important role as biomolecules (remember amino acids) but can also be players in environmental pollution. In the world of hazardous materials, some acids and bases (many of them are inorganic, but some are organic) are in the group of chemicals classed as corrosive – that can chemically attack other compounds, chemically burning them and breaking them down. In nature, acidic compounds are more common than basic compounds and can be more corrosive.

Acids and bases can exist in solid, liquid, or gaseous form, but in the environment, they are most commonly dissolved in water. Familiar acids include sulfuric acid (H₂SO₄ – a component of acid rain), hydrochloric acid (HCl – a component of stomach acid) and acetic acid (CH₃COOH; vinegar – an organic acid).  Familiar bases include lye (NaOH), ammonia (NH₃), and calcium carbonate (CaCO₃ – often used to treat an acid stomach). Acids and bases neutralize each other, with acids donating hydrogen ions (H⁺) and bases accepting them. If you spill acid, you can limit the damage by adding a base, and vice versa.

Measuring the strength of acids is not completely straightforward, but it’s not rocket science, either. Bear with me. Because hydrogen ions are a major part of the chemistry, their concentration is the quantity that is measured. pH stands for the potential of hydrogen; it is always written with a lower-case ‘p’ and an upper-case ‘H’, even at the start of a sentence.

Some molecules in water tend to fall apart (dissociate) a bit or a lot. Water also falls apart a bit, so there’s always some hydrogen and hydroxide ions (H⁺ and OH^–) in the solution. If we analyzed a sample of pure water, we would find a concentration of hydrogen ions of 10^-7 moles/liter of solution. [Detour: in chemistry, moles are not small mammals, but rather an amount equal to the molecular weight of the substance (for practical purposes). Hydrogen has a molecular weight of 1 gram/mole, so pure water has 10^-7 grams (0.0000001 grams) of hydrogen ions per liter of water – a tiny bit.]

Acids dissolve in water, releasing more hydrogen ions, so acid solutions have more hydrogen ions than pure water. The pH scale, which ranges from 0 to 14, measures hydrogen ions as the negative logarithm (in base 10) of the concentration of hydrogen ions. Don’t gibber. It’s not that bad.

The negative logarithm of 10^-7 is 7. So, the pH of pure water – a neutral substance – is 7 (Fig 4). More hydrogen ions means a higher concentration – perhaps 10^-5. The pH of that solution would be 5. Each unit of pH is an order of magnitude – a multiplication by 10. So, a pH of 5 is 100 times more concentrated with hydrogen ions than a pH of 7. Higher acidity means lower pH, which can get confusing – use your words carefully when discussing pH.

The pH scale is neutral at 7, acid at less than 7, and basic at greater than 7. Rainwater, which contains some carbonic acid (H₂CO₃) due to dissolved CO₂, has a pH around 5.5, which is 10^1.5 or 31.6 times more concentrated than pure (neutral) water. Stomach acid in humans has a pH of about 2 – very corrosive – but the stomach acid of vultures is near 0 which is 100 times stronger than human stomach acid, and suitable for destroying bacteria and disease organisms in their food.

Basic substances have pH values between 7 and 14. The pH of blood is between 7.35 and 7.45 – just slightly basic. Soils that form over limestone have pH values of 7.5-8, still only one pH unit from neutral. Seawater has a pH of 8.1. A baking soda solution can reach pH 9. The most basic, naturally occurring liquid I can find in the literature would be produced when a volcano superheats limestone (calcium carbonate – CaCO₃), creating calcium oxide (CaO), which dissolves in water to create Ca(OH)₂ – calcium hydroxide. Note the similarity to lye, which is sodium hydroxide – NaOH. The pH of the strongest possible concentration of calcium hydroxide would be about 12.4. But you need a volcano to do it. Natural, but not common! And not associated with living things. As I mentioned earlier, in nature, acidic compounds are more common than basic compounds and can be more corrosive.

Figure 4. The pH scale indicates the concentration of hydrogen ions on a scale ranging from very acidic (pH 0) to very basic (pH 14). Pure water, which is neutral, has a pH value of 7. Meretsky CC0.

Limiting substances

Limiting substances are a concept at the interface between chemistry and biology. Living things take in nutrients – by eating food if they are animals or by taking up dissolved materials through their roots if they are plants. If animals don’t get enough calories or plants don’t get enough light (which they use for photosynthesis, which allows them to create carbohydrates), they eventually die. If the diet provides enough energy but does not contain nutrients in the proportions that are needed, then deficiencies can slow growth, harm health, and, in severe cases, kill.

In the past, when nutrition was not well understood and when many humans ate diets low in important substances (Vitamin C for early sailors, protein and other vitamins for the poor and for people living in harsh areas with limited kinds and amounts of food), limiting substances were a problem for humans, and still are, in some parts of the world.

In science, limiting substances are most often studied in plants. Because plants can photosynthesize and create carbohydrates, they can generally have enough C, H, and O. Nitrogen and phosphorus are important nutrients that they cannot make for themselves; most plants must absorb N and P from the soil. In situations where a unit of added nitrogen produces more plant growth than a unit of added phosphorus, then nitrogen is the limiting substance. If a unit of added phosphorus produces more growth, then it is the limiting substance. These two nutrients are the most common limiting substances for plants, but on some kinds of soil, other nutrients can be limiting. Occasionally, too much of a nutrient (particularly trace elements such as selenium – Se) can be a greater problem, acting as a poison.

In most terrestrial ecosystems, and in freshwater, N is the limiting substance for plant and algal growth, although P will likely also increase growth, to a lesser extent. In saltwater, P is the limiting substance, although N will generally also increase growth to a lesser extent. The issue of N and P limitations will come up when we look at water pollution, because nutrient pollution from fertilizer runoff from agricultural lands, from manure runoff from grazing lands and confined-animal feeding operations, and sewage from municipal water treatment can all change the nutrient balance in receiving waters, leading to nutrient pollution. Knowing which nutrient is limiting can suggest efficient ways of dealing with the problem.

Cycles of elements, pyramids of energy 

The accompanying video clarifies why elements in the world are always shown as being involved in cycles (for example, the carbon cycle), but energy is always shown in a pyramid. This difference has implications for how we manage people and resources. [The speaking pace is slow, and you may be able to watch it comfortably at a faster speed.]

“Natural” does not mean “beneficial to humans;” “synthetic” does not mean “harmful to humans.”

One final note here that applies particularly to chemical compounds but also to other environmental factors. The natural world, as you likely have noticed, is not composed solely of puppies, butterflies, and rainbows. Its substances and phenomena can be powerfully harmful. The stomach acid of vultures is approximately the pH of battery acid. We use the venoms of animals and poisons of plants to create many products – some help individuals (pain relievers, for example) while others are weapons (mustard gas and ricin, e.g.). Vultures benefit from their strong stomachs, and plants and animals benefit from their offensive and defensive chemicals, but that doesn’t mean humans can encounter these substances without harm.

In a similar contrast, disease organisms that have killed large portions of human populations arose naturally, although they can also be weaponized. Vaccines created to combat disease are often merely weakened forms of those organisms and are very helpful.

On the “unnatural” side, synthetic substances created in laboratories also cannot easily be categorized. Plastics are becoming a major environmental problem not merely from the mass of waste that has accumulated, but also because they break down into microplastics that are increasingly seen as a health hazard to organisms of all kinds. However, plastics have also improved quality of life for many people through inexpensive clothing, lower costs for shipping due to light-weight containers, safer, more nutritious food due to impermeable wrappings, medical appliances that depend on the properties of particular plastics, etc. As in most undertakings, we can make the best progress towards sustainability by basing decisions on careful analysis and thorough information.

Knowledge Check

Take a moment to complete the short quiz below to assess your understanding of this section. Read each question carefully and refer back to the content as needed. This quiz is not graded – it’s simply an opportunity for you to reflect on what you’ve learned and reinforce key concepts.

A history of sustainability

The concept of sustainability can be found in the oral histories of indigenous cultures. For example, the Native American Haudenosaunee (the Iroquois Confederacy) included a version of sustainability, the ‘Law of the Seventh Generation’ in their Great Law. According to this, before any major action was undertaken, its potential consequences on the seventh generation had to be considered. For a species that at present is only 6,000 generations old and whose current political decision-makers operate on time scales of months or a few years at most, the thought that other human cultures have based their decision-making systems on time scales of many decades seems wise but perhaps inconceivable in the current political climate.

Our Common Future (1987), the World Commission on Environment and Development report, is widely credited with popularizing the concept of sustainable development. It defines sustainable development in the following ways…

Useful concepts related to sustainability

The ecological footprint (EF) was developed in 1990 by Canadian ecologist and planner William Rees and Swiss sustainability advocate Mathis Wackernagel. It is an accounting tool that uses the land as the unit of measurement to assess per-capita consumption, production, and discharge needs. It starts from the assumption that every category of energy, material consumption, and waste discharge requires the productive or absorptive capacity of a finite area of land or water. If we determine all the land requirements for all categories of consumption and waste discharge by a defined population, the total area represents the ecological footprint of that population on Earth, whether or not this area coincides with the population’s home region. Land area is a useful measure because land is literally the foundation and source for the goods and services humans receive from the environment and without which life would not be possible.

Ecological footprint analysis can tell us in an easily grasped manner how much of the Earth’s environmental functions are needed to support human activities. It also makes visible the extent to which consumer lifestyles and behaviors are ecologically sustainable. According to the Global Footprint Network, the ecological footprint of the average person living in the US in 2019 was – conservatively – 5.1 hectares of productive land per capita (12.6 acres – an acre is about the size of a US football field or soccer pitch). By their calculations, humanity exceeded earth’s ecological carrying capacity in 1970, and in 2019 needed about 1.7 Earths in order to support current consumption, sustainably.

The precautionary principle is another important concept in environmental sustainability. The Wingspread Statement, a 1998 consensus statement by an international group of researchers, policy makers, and advocates, characterized the precautionary principle: “When an activity raises threats of harm to human health or the environment, precautionary measures should be taken even if some cause-and-effect relationships are not fully established scientifically.” For example, if a new pesticide chemical is created, the precautionary principle would dictate that we presume, for safety, that the chemical may have potentially negative consequences for the environment and/or human health, even if such consequences have not been proven yet. In other words, it is best to proceed cautiously in the face of incomplete knowledge about something’s potential harm.

Planetary boundaries are limits to particular aspects of the planetary conditions (including temperature, and nutrient levels, for example) required for the functioning of life on Earth as we presently know it (Fig 1). The original 9 limits were proposed in 2009 by a group of environmental scientists; they include both familiar items such as ozone depletion that causes the so-called “ozone hole” and much less familiar items such as ocean acidification and radiative forcing. All are considered crucial to the planet’s ability to support life. By 2023, 6 of the 9 boundaries were considered to have been crossed. We will address individual planetary boundaries in the chapters to which they relate. More information on the planetary boundaries model and information on the status of each is [click] here.

Figure 1. The nine original proposed planetary boundaries and their status in 2023. For example, stratospheric ozone – the high-atmosphere ozone that protects life from high UV radiation levels – is within the safe operating space for life as we know it – below the green line – but radiative forcing – the balance between warming and cooling processes on the planet – is dangerously out of balance. By Potsdam Institute for Climate Impact Research (PIK)  CC BY 4.0 .

A number of additional variables have been suggested as planetary boundaries, including soil health. Numeric limits for the various zones of safety have not yet been set for novel entities (synthetic chemicals, genetically modified or synthetic organisms, and any other human-created additions to the planet), but scientists are certain that, whatever the safe limit may be, it has been passed. In June, 2025, researchers determined that most of the oceans of the world had passed the safe-operating-space boundary for ocean acidification.

The UN Sustainable Development Goals (SDGs ) is a set of 17 goals designed to ensure that environmental, social, and economic sustainability is considered, and regularly assessed, as nations undertake development and ongoing governance. They cover all aspects of sustainability. The goals are part of the 2030 Agenda for Sustainable Development, that was created in 2015 and adopted by all nations.

Video 1. Watch the short video  that quickly introduces the UN Sustainability Development Goals.

Each development goal has multiple targets that provide ways to measure progress toward the goal. For example, the first target for the Clean Water and Sanitation goal is By 2030, achieve universal and equitable access to safe and affordable drinking water for all. Overall, 169 targets are identified. Reports are produced annually to review progress. You can read the 2023 report here.

The human population and environmental impacts

Figure 2. World population size and growth rate, 1700-2100. 

The fundamental cause of the acceleration of the human growth rate in the past 200 years has been the reduced death rate due to changes in public health and sanitation. Clean drinking water and proper disposal of sewage have drastically improved health in developed nations. Also, medical innovations such as the use of antibiotics and vaccines have decreased the ability of infectious diseases to limit human population growth. In the past, diseases such as the bubonic plague of the fourteenth century killed between 30 and 60 percent of Europe’s population, reducing the world population by as many as one hundred million. Infectious disease continues to impact human population growth, especially in poorer nations. For example, life expectancy in sub-Saharan Africa, which increased from 1950 to 1990, began declining after 1985, largely due to HIV/AIDS mortality. Life expectancy in sub-Saharan Africa was reduced by 25%, according to a study in 1999. Since then, education and access to medicines have recovered most of the loss. A comprehensive report shows SARS/COVID caused a global decrease in life expectancy of 1.6 years, during 2019-2020, with enormous variation in mortality among countries, but overall rapid recovery due to medical research to develop vaccines.

Discussions of sustainability of humanity’s environmental impacts hinge not only on population size, but also on the impacts of each individual. Resource use per capita varies widely around the world, as we will see in coming chapters.

The Anthropocene

Many environmental scientists consider the current age of the world to be sufficiently different from the pre-industrial world that it needs a different name. The name of choice, now used in scientific and popular literature, is the Anthropocene – the era marked by human impacts. Although the geologists are not yet completely on board, those who deal with sustainability and aspects of the living world use the term freely, if not with great joy.

The following is a brief list of some of the most commonly noted aspects of the current world condition, as affected by human actions.

IN 2005, researchers calculated showed that humans now move almost 10 times more earth and rock each year – for example, through construction and mining activities – than all the natural forces – rivers, glaciers, rain, wind, etc. – on Earth.

Deforestation remains an issue of concern, with the greatest forest loss occurring in tropical forests. Between 2001 and 2015, an estimated 63% of tree-cover loss occurred in order to create open land for cattle grazing. In addition to representing a loss of timber and wildlife resources and water filtration, deforestation contributes to global warming.

As much as 33% of the Earth’s vegetated surface is now at least moderately degraded, according to a 2015 report coauthored by the FAO (Food and Agriculture Organization of the United Nations). Trends in soil quality and management of irrigated land raise serious questions about longer-term sustainability.

Some 26% of the world’s population lacks access to safe water according to the 2023 UN World Water Development Report, and 46% lack access to safe sanitation.

According to a 2024 FAO report based on 2021 data, although 77% of the the weight of harvested fish came from sustainably-managed stocks, only 62.3% of marine fish stocks were being harvested within sustainable levels.

World Wildlife Fund’s Living Planet Report 2024 stated that the size of the global wildlife populations declined an average of 73% between 1970 and 2020.Extinction rates, already significantly higher than historical trends, are predicted to rise sharply under climate change. Under current climate trends, more than 95% of coral reefs are predicted to be lost by the end of the century. 

In 2024, planetary temperatures were more than 1.5ºC higher than historical averages, the first time the world crossed a boundary that nations agreed to try to avoid crossing, in order to protect human and environmental health from rising temperatures. Average planetary temperatures are still below the 1.5ºC boundary.

Over 350,000 chemicals may now be in commercial use. The most protective legal statutes in the world do not require any testing for cumulative effects of chemicals that commonly co-occur in the environment. Older, so-called “persistent” organic pollutants, microplastics, and newer “forever chemicals” are now so widely distributed by air and ocean currents that they are found in the tissues of people and wildlife everywhere.

Pollution from heavy metals, especially from their use in industry and mining, also creates serious health consequences in many parts of the world. In addition, incidents and accidents involving uncontrolled radioactive sources continue to increase, and particular risks are posed by the legacy of contaminated areas left by military activities involving nuclear materials.

The UN and the International Solid Waste Association estimate the global cost of solid waste management at $252 billion, with an additional $109 billion in hidden costs. Annual municipal solid waste, alone, was predicted to grow to 2.1 billion tons by 2050.

Knowledge Check:

Take a moment to complete the short quiz below to assess your understanding of this section. Read each question carefully and refer back to the content as needed. This quiz is not graded—it’s simply an opportunity for you to reflect on what you’ve learned and reinforce key concepts.

This is a text about science, but applied science is often used to create environmental regulations, and environmental regulations are often needed to protect environmental health. Nevertheless, environmental regulations are often subject to attack by people who believe they are an economic mistake that burdens businesses and raises prices for consumers.

Many thanks to Professor Ken Richards of the O’Neill School of Public and Environmental Affairs at Indiana University-Bloomington for his care and thoroughness in the review of section 1.3.

A free market exists when people can freely choose what they want to buy or sell and can pay or receive a fair price for it. In a free market, all goods and services are privately owned; the benefits and costs of the goods and provision of service are received (benefits) and borne (costs) only by the owners of the goods or the providers of the services, who can protect their ownership (theft can be prevented). Consumers have enough information to know whether they want to pay the offered price, and there is competition among providers.

Adam Smith, a Scottish economist and philosopher, wrote An Inquiry into the Nature and Causes of the Wealth of Nations (often shown just as The Wealth of Nations) in 1776, just as the US was becoming a nation. In it, he suggested that individuals acting only for their own self-interest in buying and selling could still benefit society at large. He suggested that free markets were one way this could happen (economists these days also call them perfectly competitive markets). Economists maintain that true free markets – free markets that meet all the requirements to be called free markets – result in a situation in which everyone in the market is as well off as they can be, without hurting others, without outside regulation.

Note that the prices that result from a free market are not the same as low prices, or convenient prices. Rather, they are prices that accurately reflect the cost of the good or services that consumers are willing to pay and that owners are willing to offer. If the owner sets the price too high, consumers will have enough information to understand that the prices is higher than they need to pay, and there will be competitors to offer lower prices, but only to the point that owners can still make the profits they need in order to continue to produce the items involved. Prices, in the long run, will support the continued existence of owners, who invest money to provide the goods and services, but will not rise far above that point for long, because competition will bring them back down. Owners are driven to be efficient in their production of goods and services, so that they may become cheaper over time and consumers that make themselves aware of alternative prices can benefit from that increased efficiency. If a good or service is rare or difficult to provide, then owners will only be able to ask higher prices if the item is attractive enough that consumers will pay high prices, and consumers will understand the situation well enough to choose whether to pay high prices.

This theoretical ability of free markets to self-regulate is very attractive to those who dislike the idea of government regulation, and when it works, it is a very efficient means of setting prices that benefit all parties in the market to the extent possible.

Requirements for free-market conditions

As in all good stories, the devil is in the details. Let’s look first at the requirement for private ownership of goods and services. Economists term this universality because, in a free market, all goods are privately owned.

Not all goods and services are privately owned: private, public, club, and common-pool resources

Economists divide goods (things, mostly) into categories depending on whether there is competition for their use (usually, no one competes for air, but lots of people compete for trees that produce expensive timber), and whether or not it’s possible to prevent people from accessing the goods (not really, for air; possibly, but not always, for the trees). Figure 1 shows the formal names given to the resulting categories. If access to the resource can be controlled, it is considered excludable; otherwise, as for many natural resources, it is non-excludable. If competition occurs, the resource is considered rivalrous. In the absence of some system of governance, most natural resources are non-excludable and rivalrous and are therefore considered to be common-pool resources. Because air is non-excludable but not a resource for which there is competition, it is classified as a public good.

Some of the boundaries between categories can become blurry under some circumstances. Public, non-toll roads are typically considered to be public goods for which people do not compete. Taxes pay for the roads and everyone can use them. But any city at rush hour demonstrates that, if there isn’t competition, precisely, easy access is certainly not available! Economists use the term congestible for this halfway situation. Managers of popular public parks may monitor user numbers and close a park (if access can be controlled) if user numbers become too high for safety, resource protection, or enjoyment.

Many natural resources are common-pool resources. There is competition for access, use, and consumption and, depending on the relevant legal system, they are not owned or controlled, and as a result do not qualify as ideal free markets. Wildlife, timber, ocean resources, and fresh water are often examples. Because access is not controlled, the distribution of benefits and harms that accompany access is not controlled, and overuse or overharvest are likely to occur – deforestation, extinction, water pollution, etc. Both the resource and the public may suffer harm as a result. The phrase Tragedy of the Commons is used to describe the results of easy access to a resource no one protects.

Regulations can confer legal ownership for some common-pool resources (mining laws, for example, allow miners to own the minerals they recover), and can regulate to prevent harm, without mimicking private ownership, in others (air pollution laws, for example, in which a government takes stewardship of the air resource). Only the first case – mimicking private ownership – seeks to create something like a free market that can provide the benefits of protection of the resource that free markets can offer. So-called extractive resources – resources that are removed in order to use or consume them – are often good candidates for legal ownership. Laws often convey legal ownership of timber, plants, minerals, and water but may vary in how well they achieve the conditions of free markets and sustainability of the resource (for renewable resources).

Not all natural resources are easily protected by these mechanisms. Ocean fisheries provide an example of complexity. By international law, nations control the seas within 200 km of their borders (their exclusive economic zones – EEZs), and can regulate fishing in EEZs, conveying legal ownership of fish to allow commerce and a market. Nations each have their own systems, and some create more sustainable conditions than others. But no international law conveys ownership-like status in the open ocean. Fish that only inhabit the open ocean and fish that migrate across EEZs and the open ocean can be protected by international treaties among nations but not all nations sign such treaties, monitoring can be difficult or impossible, and enforcement requires willing and cooperative partners. As a result, sustainable management of ocean fisheries is still very much a work in progress. Minerals from the deep ocean floor are a common-pool resource for which treaty discussions are currently very dynamic.

Some aspects of freshwater make it difficult to create something approaching free-market conditions. The ability to withdraw water for industrial, municipal, agricultural, and household use is often legally controlled, but these laws may run into problems dealing with surface water and ground water, because these are often more closely connected than the laws recognize. Access to swim or boat on water may be controlled by a property owner for a pond or lake entirely within a single property, but may be more complicated if multiple property owners, towns, states, or nations are involved. Access to discharge pollution into water is similarly complicated by jurisdictional issues. Pollution that enters the water at one point can spread to other points that may not have jurisdiction to prevent the pollution at the source. National regulations and international treaties can used to control the problem, but, as with oceans, bad actors – from individuals to nations – can degrade and diminish these shared-access resources. Water-pollution laws are most often permit/penalty systems, rather than systems that involve markets.

Because air is not usually subject to competition, and is not excludable, it is considered a public good. Air pollution can be limited through regulations of the sources of air pollution, but air pollution, like water pollution, can cross jurisdictional lines. National-level regulation can maintain air quality within a nation, but international treaties are needed to maintain air quality when air pollution cross international lines. Market-based mechanisms called cap-and-trade agreements exist for some air pollutants, notably the main precursors for acid rain – sulfur dioxide and NOx – and carbon in the form of CO₂ equivalents. These create a market for air pollution, and regulations seek to approximate a free market. In other cases, laws are directly regulatory – like water-pollution laws – setting limits and imposing penalties, so no market conditions exist.

As you can see, the free-market requirement for universality – private ownership of all goods and services – is not so universal. Now let’s look at exclusivity.

Externalities are failures in the exclusivity of costs and benefits to owners

Under theoretical free-market conditions, all the benefits of having a good or providing a service accrue exclusively to the owner, and all the costs of creating the good or providing the service likewise are borne by the owner. This characteristic is termed exclusivity. In the real world, things are often not so neat.

Even with something as simple as a shirt or a can of peas, pollution from fabric dyes or erosion from agriculture may enter air or water, creating a burden borne by members of the public, whose taxes often pay for pollution monitoring and enforcement. If regulations recoup the cost through permitting fees and penalties, then the public is made whole and does not pay to clean up after the private owners. But often, this is not the case. Monitoring may not occur at all, or may have gaps in coverage. Enforcement may be understaffed. Regulations may not exist in the first place, so that neither monitoring nor enforcement exists. Then the public bears the costs of these unintended but unavoidable results of industry and agriculture in harm to themselves and their property, and in harm to the common-pool and public resources on which they rely.

An externality is a cost or benefit that arises as a result of provision of a good or service but is not received by or borne by the owner. The problems with shirt and pea production in the previous paragraph are negative externalities.

Not all externalities are negative. If your neighbors keep beautiful homes and lovely gardens, your property values may rise, as a result, providing you with a positive externality. If one property owner along a lake restores shoreline wetlands that improve water quality, all users of the lake benefit.

Externalities result from the production and consumption of many private goods and services, but are particularly likely to exist when natural resources are harvested or extracted, because of the complicated connections among the components of the natural world. The hunter who kills a wolf in a legal hunt affects the deer populations in the area, which in turn affects the vegetation, which may affect timber availability and soil health. The logger who extracts timber from a forest may create erosion that affects water quality; reduce water filtration at the harvest site; and alter the composition of wildlife, fish, and plants in the vicinity. In theory, externalities can be prevented (or at least mitigated) by laws that protect environmental health, but legal protection and enforcement are often incomplete, and externalities are common, as a result.

The real world is not doing well so far with universality or exclusivity. On to perfect information.

Perfect information is rarely available

Because information about externalities is often unavailable, consumers cannot always make informed decisions about what to purchase from whom, which also defeats the requirement for so-called perfect information in a free market. Non-economists can be forgiven for assuming that perfect information is the same as complete information, but consumers don’t need to know absolutely everything about goods in order to make informed decisions. We probably don’t need to know the color of the walls in the office of the plant in which the peas are canned, for example. But consumers may want a wide range of information about environmental, economic, and societal impacts before they decide what to purchase, and from whom. Information can be required by law, but often is not, and enforcement of requirements to provide accurate information may also be insufficient.

The real world often fails to meet major requirements for free markets, as we have seen. Environmental regulations can help.

Environmental regulations are often required t o meet free-market conditions

A common accusation levelled against environmental regulations and regulators is that they are interfering with free markets. People often know that free markets can operate “without regulation,” but often do not know about the many requirements that must be met in order for a free market to exist in the first place.

Common-pool resources, theft and poaching, externalities, lack of information, and monopolies all defeat free markets, and all can be addressed, if imperfectly, through regulation. Thus, environmental regulations can be necessary to mimic free-market conditions for goods and services that are not private. Without such protections, the public is left with the cost of cleaning up environmental harm or is deprived of opportunities and services that cannot be replaced.

Costs for clean-up and loss of ecosystem service are separate from the costs of shirts and peas and timber and fish. As a result, people may not see the link between the low cost of timber or a shirt and the high price for drinking water or the loss of wildlife. One phrase used in environmental activism that makes this link more visible is “polluter pays.” But when harm is not as obvious as dirty air or water, it may still be hard for people to see connections between environmental regulation, environmental health, and efficient prices for goods and services.

Not all regulations support free-market conditions. Some regulations are created for other purposes, such as tariffs that are used for political reasons. But even regulations designed to support free-market conditions may be imperfect. For example, laws may impose weak penalties that fail to protect resources, public health, etc. The process of creating environmental regulations thus requires firm foundations in both environmental science and environmental economics.

Free markets do not ensure sustainable use of traded resources

Obviously, consumable, non-renewable resources such as oil cannot be used sustainably because the processes that create oil in nature are much slower than the processes that use oil and convert it to CO₂ and H₂O. So, a market in oil cannot achieve sustainability. But markets are designed for buying, selling, and trading, without any dilution by other aims such as sustainability. For example, laws usually allow buyers to legally own wood. If wood from a particular tree becomes very attractive to buyers, and sellers believe the attraction will be short-lived, then the market may exhaust the available supply of timber, or even, in a worst case, cause the extinction of the tree species, if buyers and sellers are willing to do so. Often, sellers are invested in continuing to provide goods and services and buyers are opposed to extinction, which can support conservation measures if everyone is aware of a risk of extinction due to trade. Where such interests are not strong enough, if society has an interest in avoiding unsustainable use or extinction, then other kinds of regulations are needed to identify at-risk species and halt trade before extinction occurs.  One such regulation is the international treaty known as the Convention on International Trade in Endangered Species, or CITES.

Knowledge Check:

This section introduced several new vocabulary terms. Use the interactive activity below to test how many you can recognize after just one read-through. If you miss a question, don’t worry – return to the section to review the word and better understand its meaning.

Air pollution occurs in many forms but can generally be thought of as gaseous and particulate contaminants that are present in the earth’s atmosphere. Several UN Sustainability Goals related to air pollution, notably good health and well-being, clean water, and sustainable cities and communities.

Chemicals discharged into the air that have a direct impact on the environment are called primary pollutants. These primary pollutants sometimes react with other chemicals in the air to produce secondary pollutants.

Air pollution takes the form of aerosols and gases. Aerosols are made up of liquid and solid particles, sometimes called particulates, suspended in the atmosphere such as soot, dust, and volcanic ash.  Air-polluting gases include carbon dioxide (CO₂), sulfur dioxide (SO₂), a variety of nitrogen oxides (mostly NO and NO₂, cumulatively written as NOx) and ozone (O₃). The components of air pollution undergo chemical reactions in the atmosphere that can generate additional pollutants. For example, sulfur dioxide gas can dissolve in water droplets to create an aerosol of sulfuric acid droplets that contributes to acid rain. Reactions of VOCs and NOx can lead to the formation of O₃ and aerosols, the primary components of photochemical smog. Chemical changes are driven by energy from sunlight that helps to produce highly reactive chemical components such as ozone.

Aerosol loading is one of the 9 planetary boundaries. Air pollution is the primary way that humans change aerosol levels in the atmosphere.

Outdoor and indoor air pollution are usually described separately. Outdoor air pollution involves exposures that take place outside of the built environment. Examples include fine particles produced by coal burning, noxious gases such as sulfur dioxide, nitrogen oxides, carbon monoxide, and ground-level ozone. Indoor air pollution involves exposure to particulates, carbon oxides, and other indoor air or dust pollutants. Examples include household products and chemicals, out-gassing of building materials, allergens (cockroach and mouse dropping, mold, pollen), and tobacco smoke.

Sources of air pollution

A stationary source of air pollution refers to an emission source that does not move, also known as a point source. Stationary sources include factories and power plants. The term area source describes many small, dispersed sources of air pollution located together whose individual emissions may be below thresholds of concern but whose collective emissions can be significant. Residential wood burners, gas stations, and dry cleaners are good examples of small sources; when they occur at high densities, they can contribute to local and regional air pollution levels. Area sources of air pollution are considered to be nonpoint sources.

A mobile source of air pollution refers to a source capable of moving under its own power. Mobile sources generally imply “on-road” transportation, including cars, sport utility vehicles, and buses. In addition, there is also a “non-road” or “off-road” category that includes gas-powered lawn tools and mowers, farm and construction equipment, recreational vehicles, boats, planes, and trains.

Agricultural sources arise from operations that raise animals and grow crops, which can generate emissions of gases and particulate matter. For example, animals confined to a barn or restricted area produce large amounts of manure. Manure emits various gases, particularly ammonia and methane, into the air. These gases can be emitted from animal houses, manure storage areas, or land after the manure is applied. In crop production, the misapplication of fertilizers, herbicides, and pesticides can potentially result in aerial drift of these materials, and harm may be caused. We will see manure as a source of water pollution, as well, in a later chapter.

Unlike the anthropogenic sources of air pollution described above, air pollution caused by natural sources is not caused by people or their activities. An erupting volcano emits particulate matter and gases, forest, and prairie fires can emit large quantities of “pollutants,” dust storms can create large amounts of particulate matter, and plants and trees naturally emit volatile organic compounds which can form aerosols that can cause a natural blue haze. Wild animals in their natural habitats are also considered natural sources of “pollution” in the form of methane, urine and manure.

Air pollutants regulated in the US 

In the US, the six most common outdoor air pollutants during the 1970s were targeted for clean-up in the Clean Air Act: particulate matter, ground-level ozone, carbon monoxide, sulfur oxides, nitrogen oxides, and lead. These are the pollutants classed here as “traditional” air pollutants because they have been the subject of the longest-running efforts at control and regulation in the US and internationally. In the US they are referred to as criteria pollutants because the EPA sets criteria to control their use. These pollutants can harm health and the environment and cause property damage. At present, of the six criteria pollutants, particulates and ground-level ozone are the most widespread health threats. The U.S. Environmental Protection Agency (EPA) regulates them by developing criteria based on human and environmental health considerations. Sources for major air pollutants in the US are shown in Figure 2, below. Greenhouse gases (GHG) which cause global warming and climate change are regulated similarly to more traditional air pollutants but are controlled and regulated differently. They are addressed in a later section of this chapter.

Ground-level ozone is not emitted directly into the air but is created by chemical reactions in smog between nitrogen oxides (NOx) and volatile organic compounds (VOCs), in the presence of sunlight. Emissions from industrial facilities and electric utilities, motor vehicle exhaust, gasoline vapors, and chemical solvents are some of the major sources of NOx and VOCs. Breathing ozone can trigger various health problems, particularly for children, the elderly, and people of all ages with lung diseases such as asthma. Ground-level ozone can also have harmful effects on vegetation and ecosystems and can reduce crop production. (Ground-level ozone should not be confused with the ozone layer, which is high in the atmosphere and protects Earth from ultraviolet light; ground-level ozone provides no such protection).

Figure 1. Particle size comparison for PM2.5 and PM10. US EPA. Public domain.

Particulate matter, also known as particle pollution, comprises extremely small particles and liquid droplets. Particle pollution comprises several components, including acids (such as nitrates and sulfates), organic chemicals, metals, and soil or dust particles. The size of particles is directly linked to their potential to cause health problems. EPA is concerned about particles that are 10 micrometers in diameter or smaller because those are the particles that generally pass through the throat and nose and enter the lungs. Once inhaled, these particles can affect the heart and lungs and cause serious health effects. The most harmful are less than 2.5 micrometers in diameter (human hairs range from 50-180 micrometers in diameter – Fig 1); some particles that enter the lungs can enter the bloodstream.

Carbon monoxide (CO) is a colorless, odorless gas emitted from combustion processes.  Nationally, and, particularly in urban areas, the majority of outdoor CO emissions come from mobile sources.  CO can cause harmful health effects by reducing oxygen delivery to the body’s organs (like the heart and brain) and tissues. At extremely high levels, CO can cause death.

Nitrogen dioxide (NO₂) is one of a group of highly reactive gases known as “oxides of nitrogen,” or nitrogen oxides (NOx). Other nitrogen oxides include nitrous acid and nitric acid. EPA’s National Ambient Air Quality Standard uses NO₂ as the indicator for the larger group of nitrogen oxides. NO₂ forms quickly from emissions from cars, trucks, buses, power plants, and off-road equipment. In addition to contributing to the formation of ground-level ozone, and fine particle pollution, NO₂ is linked with a number of adverse effects on the respiratory system.

Sulfur dioxide (SO₂) is one of a group of highly reactive gases known as “oxides of sulfur.”  In 2020, the largest sources of SO₂ emissions were from coal combustion at power plants (48%) and emissions from industrial facilities (19%).  Smaller sources of SO₂ emissions include industrial processes such as extracting metal from ore and burning high sulfur-containing fuels by locomotives, large ships, and non-road equipment. SO₂ is linked with adverse effects on the respiratory system.

Lead is a metal found naturally in the environment and manufactured products. The major sources of lead emissions, historically, were fuels for on-road motor vehicles (such as cars and trucks) and industrial sources.  As a result of regulatory efforts in the U.S. to remove lead from on-road motor vehicle gasoline (it had been used as an additive to improve engine performance), emissions of lead from the transportation sector declined by 95% between 1980 and 1999, and levels of lead in the air decreased by 94% between 1980 and 1999. Internationally, all countries have banned lead as an additive to gasoline, since 2021. Today, the highest lead levels in the air are usually found near lead smelters; today’s major sources of lead emissions to the air are ore and metals processing and old-fashioned aircraft that still use leaded aviation gasoline. Incinerators used to process solid waste and lead-acid battery manufacturers are other significant sources.

In addition to these criteria pollutants, the US regulates radioactive and toxic (or hazardous) air pollutants, pollutants that affect the protective ozone layer in the stratosphere that protects living things from harmful UV radiation from the sun, and CO₂ emissions that contribute to global warming. Toxic air pollutants including many chemicals used in industry, asbestos, and heavy metals such as lead, mercury, cadmium, and chromium; they cause a variety of harms to health including neurological reproductive, developmental, respiratory and cardiac problems.

Many toxic air pollutants can settle out of the atmosphere or rain out onto the soil or into the water, where they can be absorbed to soil, or taken in by drinking (animals) or root uptake (plant). Pollutants, particularly persistent organic pollutants that have long lifetimes, can travel long distances in the air before finally settling and raining out – termed long-range transport. Because planetary winds tend to spiral towards the poles, pollutant levels on the ground and in the water near the poles can be unexpectedly high, despite being far from industry or human settlement.

Pollutants that affect the ozone layer are often chemicals in refrigerants, foams, aerosols, solvents, and fire suppressants. By reducing ozone in the stratosphere, they increase UV radiation that can cause skin cancer, harm vegetation, and decrease crop production.

Figure 2. Sources for major air pollutants in the US, from Air Pollutant Emissions Trends Data, US EPA, as of 2024.

Generally, emissions of air pollution come from

• stationary fuel combustion sources (such as electric utilities and industrial boilers)
• industrial and other processes (such as metal smelters, petroleum refineries, cement kilns and dry cleaners)
• highway vehicles
• non-road mobile sources (such as recreational and construction equipment, marine vessels, aircraft and locomotives)

Acid rain

Figure 3. Spruce trees killed by acid rain in the Jizera Mountains of the Czech Republic. Acidification from industrial air pollution began in the early 1950s and peaked in the mid-1980s, killing 40 to 80% of trees in spruce stands. Wikimedia Commons, public domain.

The primary contributors to acid rain are SO₂ and NOx. Acid rain harms building materials, especially marble and limestone that dissolve in acid water, metals, and paints. In ecosystems, acid rain harms vegetation and can kill it if exposure is severe and prolonged. Acid rain acidifies soil and water, releasing aluminum from soils and substrates; at pH levels below approximately 5.0, fish and most invertebrates in soil and water cannot persist due to aluminum toxicity. In areas where soils contain neutralizing, buffering materials such as calcium carbonate from limestone, soil and water acidity are lessened, but the process depletes the soils of the buffering materials.

At the height of acid-rain deposition, in the 1980s, entire forests in Europe were killed (Fig 3). In the northeastern US and Canada, where acid rain was severe due to both local and upwind burning of fossil fuels, and where granite soils have only low concentrations of buffering materials, most lakes became “dead,” devoid of fish, invertebrates, and even algae. With the lower levels of the food web eliminated, higher-level predators that relied on aquatic foods, such as osprey, a fish-eating bird of prey, also disappeared. In these lakes, with little buffering material to neutralize acid waters, pH levels are recovering very slowly, and buffering materials remain reduced. However, due to chemistry involving plants, toxic aluminum concentrations are declining more quickly. Fish are returning to some lakes.

Acid rain can also harm human health if people are breathing air with acid aerosols – usually in polluted urban environments, during rain events. Eye and throat irritation and damage to lungs can occur. Asthma and other respiratory conditions and some heart conditions may be exacerbated. This is just an extension of air pollution health effects – SO₂ and NOx pollution can cause these problems in the absence of rain, too.

Acid rain is a worst-case example of air pollution affecting the planetary boundary for  modification of biogeochemical flows of N and P. By spreading N all over the planet through NOx pollution, humans alter the N cycle in all ecosystems.

International air pollutants of note 

The World Health Organization (WHO) provides guidelines for all of the 6 US criteria pollutants, plus three major indoor air pollutants: polycyclic aromatic hydrocarbons (PAHs – like carbon monoxide, these are mostly formed by incomplete combustion of fossil fuels and natural fuels), formaldehyde (a common indoor air pollutant and a VOC), radon (a radioactive gas and indoor air pollutant), In addition, the WHO provides good practice statements, but not guidelines, for three additional air pollutants. Black carbon, also known as soot, is an important indoor and outdoor air pollutant in countries where household fires are still a common means of heating and cooking and where diesel fuels are burned without accompanying pollution controls. Ultrafine particles (PM0.1 – particles less than 0.1 micrometers in diameter) mostly result from incomplete burning of fuels in transportation, power generation, and heating; they increase the risk for heart and lung diseases. Molds are fungi that can be allergens and irritants in indoor air, usually resulting from poor ventilation and moisture build-up; they can cause respiratory damage including asthma attacks.

Indoor air pollution 

Most people spend approximately 90% of their time indoors. However, the indoor air we breathe in homes and other buildings can be more polluted than outdoor air and can increase the risk of illness. There are many sources of indoor air pollution in homes. They include biological contaminants such as bacteria, molds, and pollen, burning fuels and environmental tobacco smoke, chemicals, including volatile organic compounds, in building materials and furnishings, household products, central heating and cooling systems, and outdoor sources. Outdoor air pollution can enter buildings and become a source of indoor air pollution. Where cooking and heating use diesel and other dirtier fossil fuels, manure, wood, or other biomass, indoor air pollution can also include soot and higher levels of carbon monoxide.

Sick building syndrome is a term used to describe situations where building occupants have health symptoms associated only with spending time in that building. Causes of sick building syndrome include inadequate ventilation, indoor air pollution, and biological contaminants. Usually, indoor air quality problems only cause discomfort. Most people feel better as soon as they remove the source of the pollution.

Natural sources of air pollution

Many air pollutants occur naturally in measurable quantities. Wildfires, in particular, create many problematical substances, including CO₂, CO, CH₄, NOx, VOC (recall that NOx and VOC can be precursors to ground-level ozone) and particulates including black carbon. Wind-borne dust from deserts, rangelands and degraded agricultural lands can contribute to very high levels of particulates. Trees emit VOCs that can contribute to ground-level ozone. Volcanoes produce a wide array of substances including all of the traditional air pollutants, plus heavy metals. The term “vog” is used to describe the combination of gases and particulates produced by volcanoes, which can be hazardous to human and environmental health and harmful to property. SO₂ is usually a substantial component of volcanic gas and can contribute to very acidic aerosols; volcanic particulates include fine, very sharp particles of volcanic glass that can damage lungs, skin, plants, and equipment.

Knowledge Check:

Take a moment to complete the short quiz below to assess your understanding of this section. Read each question carefully and refer back to the content as needed. This quiz is not graded – it’s simply an opportunity for you to reflect on what you’ve learned and reinforce key concepts.

Climate change is a topic that comes into all aspects of environmental sustainability. Here, we will address the atmospheric and climatic aspects of climate change, but we will see it many other chapters as well. UN Sustainability Goals for clean energy and climate action are most relevant.

Earth’s temperature is a balancing act

As we saw in Chapter 1, Earth’s temperature depends on the balance between energy entering and leaving the planet. When incoming energy from the sun is absorbed, Earth warms. When the sun’s energy is reflected back into space, Earth avoids warming. When energy is released from Earth into space, the planet cools. Many factors, both natural and human, can cause changes in Earth’s energy balance, including

• changes in the greenhouse effect, which affects the amount of heat retained by Earth’s atmosphere;
• variations in the sun’s energy reaching Earth; and
• changes in the reflectivity of Earth’s atmosphere and surface.

Scientists have pieced together a picture of Earth’s climate, dating back hundreds of thousands of years, by analyzing a variety of indirect measures of climate, such as ice cores, tree rings, glacier size, pollen counts, and ocean sediments. Scientists have also studied changes in Earth’s orbit around the sun and the activity of the sun itself. 

The historical record shows that the climate varies naturally over various time scales. In general, climate changes before the Industrial Revolution in the 1700s can be explained by natural causes, such as changes in solar energy, volcanic eruptions, and natural changes in greenhouse gas (GHG) concentrations. However, recent warming cannot be explained by natural causes alone, especially since the mid-20th century. Rather, human activities, especially our combustion of fossil fuels, explain most of that warming. The scientific consensus is clear: through alterations in the carbon cycle, humans are changing the global climate by increasing the impacts of the greenhouse effect.

The greenhouse effect causes the atmosphere to retain heat, allowing life to exist

Much like the glass in a greenhouse, GHGs allow incoming visible light energy from the sun to pass, but they block infrared radiation radiating from Earth toward space (Fig 1). In this way, they help trap heat energy that subsequently raises air temperature. Being a greenhouse gas is a physical property of certain types of gases; because of their molecular structure, they absorb wavelengths of infrared radiation but are transparent to visible light. Scientists estimate that the average temperature on Earth would be -18º C without naturally occurring GHGs.

FIgure 1. Enhanced greenhouse effect. Essentials of Environmental Science by Kamala Doršner is licensed under CC BY 4.0. Modified from the original by Matthew R. Fisher.

The most important GHGs directly emitted by humans are CO₂, methane (CH₄), and nitrous oxide (N₂O). Carbon dioxide is the primary greenhouse gas that is contributing to recent global climate change. CO₂ is a natural component of the carbon cycle, involved in such activities as photosynthesis, respiration, volcanic eruptions, and ocean-atmosphere exchange. Human activities, primarily the burning of fossil fuels and changes in land use, release very large amounts of CO₂ into the atmosphere, causing its concentration in the atmosphere to rise. Water vapor is also an important greenhouse gas, but humans do not significantly, directly affect its presence in the atmosphere.

Carbon dioxide (CO₂)

Atmospheric CO₂ concentrations have increased by 45% since pre-industrial times, from approximately 280 parts per million (ppm) in the 18th century to 425 ppm in 2025 (Fig 2). The last time that Earth’s CO₂ concentration was so high was 3-5 million years ago. Human activities currently release over 40 billion tons of CO₂ into the atmosphere every year, about 88% of which was from burning of fossil fuels (Fig 3, 4).  While some volcanic eruptions released large quantities of CO₂ in the distant past, in the present, volcanos emit, on average, less than half a billion tons of CO₂, annually.

Figure 2. Atmospheric carbon dioxide (CO2) in parts per million (ppm) for the past 800,000 years based on ice-core data (light purple line) compared to 2022 concentration (bright purple dot).  US NOAA. Public Domain.

Deep ice cores from Antarctica preserve a record of atmospheric gases going back 800,00 years, in small bubbles trapped in the ice as it formed. [The time span shown here is mostly from the Pleistocene – the age of ice ages – which ended approximately 11,700 years ago.] The peaks and valleys in the line show ice ages (low CO₂) and warmer interglacials (higher CO₂). Throughout that time, CO₂ was never higher than 300 ppm (light purple dot, between 300,000 and 400,000 years ago). The increase over the last 60 years is 100 times faster than previous natural increases. In fact, on the geologic time scale, the increase from the end of the last ice age to the present (dashed purple line) looks virtually instantaneous. Graph by National Oceanic and Atmospheric Association (NOAA) Climate.gov based on data from Lüthi, et al., 2008, via NOAA National Centers for Environmental Information (NCEI) Paleoclimatology Program. Text by Rebecca Lindsey, NOAA, bracketed addition by Meretsky.

Methane (CH₄)

Methane is produced through both natural and human activities. For example, wetlands, agricultural activities, fossil fuel extraction, and transportation emit CH₄. Methane is more abundant in Earth’s atmosphere now than in at least the past 650,000 years.  Due to human activities, CH₄ concentrations increased sharply during most of the 20th century and are now more than two-and-a-half times pre-industrial levels. In recent decades, the rate of increase has slowed considerably.  

Nitrous oxide (N₂O)

Nitrous oxide is also produced both naturally and anthropogenically. Globally, 40% of N₂O comes from anthropogenic sources. In the US, 75% of anthropogenic N₂O comes directly from agriculture, primarily as a result of use of nitrogen fertilizers.

A number of other greenhouse gases are produced in smaller quantities than these leading three. Some, including fluorinated gases, also harm the stratospheric ozone layer.

Figure 3. Recent increases in concentrations of major greenhouse gases. The measurement of Gt CO₂-eq (gigatonnes of CO₂ equivalents) provides a means of comparing GHG emissions across different gases by converting the GHG impact of all gases into multiples of the GHG impact of CO₂. From https://www.epa.gov/ghgemissions/global-greenhouse-gas-overview. Data from the Working Group III report of the IPCC (2022).

EPA describes their fluorinated gas category this way: “Industrial processes, refrigeration, and the use of a variety of consumer products contribute to emissions of F-gases, which include hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF₆).”  These compounds are also chemicals that harm the ozone layer. https://www.epa.gov/ghgemissions/global-greenhouse-gas-overview.

Figure 4. Recent increases in greenhouse gas emission by commercial sector. AFOLU represents agriculture, forestry, and other land uses including clearing of forests for livestock grazing. From https://www.epa.gov/ghgemissions/global-greenhouse-gas-overview. Data from the Working Group III report of the IPCC (2022).

Natural sources of greenhouse gases

Just as traditional air pollutants can occur naturally, so can greenhouse gases. CO₂ is, of course, a normal component of the atmosphere, produced by respiration, decomposition of dead plant and animal material, forest fires, volcanoes and weathering of rocks. Methane and N₂O are produced by respiration and decomposition in wet environments. Methane is also produced by seepage from fossil-fuel deposits, by digestion, especially by the group of herbivores called ruminants (including cows, sheep, and goats but also deer, antelope, and giraffe), and by melting of an ice-like form of methane, gas hydrate, found on the ocean floor and in permafrost. Currently, approximately 60% of both methane emissions and nitrous oxide emissions are from natural sources. The proportion is larger for carbon dioxide. But keep in mind that Nature has always produced these gases, and we need GHGs in the atmosphere to create a livable Earth – we don’t want to be like the moon! Global warming is the result of humans increasing GHGs in the atmosphere.

Global warming potentials

Greenhouse gases differ in their lifetime in the atmosphere and in their global-warming potentials (GWP). CO₂ takes part in many atmospheric interactions and can disappear from the atmosphere in a few decades or persist for thousands of years; its lifetime in the atmosphere is not reported. It is the standard against which the global-warming potential of other gases are measured.

Methane is a relatively short-lived GHG, lasting approximately 12 years in the atmosphere. As a result of this shorter life, its global warming potential, relative to CO₂, depends on the timeframe. Over a 20-year period, methane is approximately 80 times as effective as CO₂ at warming the atmosphere. Most data presentations use a 100-year timeframe, and over that longer period, methane is approximately 28 times as effective as CO₂.

Nitrous oxide is 273 times more effective than CO₂ over a 100-year period, and has an approximately 100-year lifespan.

Because of their higher warming potentials, methane and nitrous oxide are well worth regulating and reducing, despite their lower concentrations in the atmosphere. Some much less common gases are far more potent: sulfur hexafluoride (SF₆), used in electrical and electronic applications, is 25,200 more potent than CO₂ and lasts some 3,200 years in the atmosphere.

Changes in reflectivity (albedo) affect how much energy enters Earth’s system

Humans cannot affect how much sunlight reaches the outer atmosphere, but we can affect how much stays to warm the Earth. When sunlight energy reaches Earth, it can be reflected, thereby not warming the planet, or absorbed. The amount that is reflected or absorbed depends on Earth’s surface and atmosphere. Light-colored objects and surfaces, like snow and clouds, tend to reflect most sunlight, while darker objects and surfaces, like the ocean and forests, tend to absorb more sunlight. Albedo refers to the amount of solar radiation reflected from an object or surface, often expressed as a percentage. Earth as a whole has an albedo of about 30%, meaning that 70% of the sunlight that reaches the planet is absorbed.  Sunlight that is absorbed warms Earth’s land, water, and atmosphere.

Albedo is also affected by aerosols. Aerosols are small particles or liquid droplets in the atmosphere that can absorb or reflect sunlight. Unlike greenhouse gases (GHGs), the climate effects of aerosols vary depending on what they are made of and where they are emitted. Those aerosols that reflect sunlight, such as particles from volcanic eruptions, have a cooling effect. Those that absorb sunlight, such as black carbon (a part of soot), have a warming effect.

In an ironic twist, efforts to reduce air pollution have reduced Earth’s albedo somewhat, contributing to warming. Sulfur dioxide, in its aerosol form as tiny droplets of sulfuric acid, is highly reflective. Thus, one form of industrial air pollution – a form that causes acid rain – reflects the sun’s rays and reduces radiation entering Earth’s atmosphere. The global clean-up of sulfur dioxide air pollution caused a slight but measurable increase in planetary temperatures.

Natural changes in albedo, like the melting of sea ice or increases in cloud cover, have contributed to climate change in the past, often acting as feedback to other processes. Volcanoes have also  played a noticeable role in climate. Volcanic particles that reach the upper atmosphere can reflect enough sunlight back to space to cool the planet’s surface by a few tenths of a degree for several years.  Volcanic particles from a single eruption do not produce long-term change because they remain in the atmosphere for much shorter times than GHGs.

Human changes in land use and land cover also have changed Earth’s albedo. Processes such as deforestation, reforestation, desertification, and urbanization often contribute to changes in climate in the places they occur. These effects may be significant regionally but have a lesser impact at the global scale.

Current levels of warming

Climate change is one of the planetary boundaries. Presently the Earth is considered to be experiencing a heightened risk from increased CO₂ and to be at high risk from the overall levels of GHG in the atmosphere.

The text that follows, under this heading, is taken verbatim from the Global Temperature Report for 2024 by Berkeley Earth with their permission (CC BY-NC). Berkeley Earth is a nonprofit that provides open-source environmental data and analysis on global temperature and air pollution.

The year 2024 began with six months of continuous record high monthly-average temperatures (Fig 5). This is partly due to the El Niño event that peaked in late 2023. However high temperatures persisted well beyond the end of El Niño in June 2024. Similar warmth occurred in the second half on 2024, though only August set a new record, as the other five months each ranked behind the records set in 2023. In 2024, our analysis placed every month at least 1.5 °C (2.7 °F) above the 1850-1900 average for that month.

Figure 5. Global mean temperatures, by month, from 1850-2024. From Berkeley Earth Global Temperature Report for 2024. CC BY NC

“The warming spike in 2023/2024 suggests that the past warming rate is no longer a reliable predictor of the future, and additional factors have created conditions for faster warming, at least in the short-term.” Berkeley Earth Global Temperature Report for 2024 CC BY NC

The warming pattern in 2023/2024 has been extraordinary. It appears to have been caused by a combination of natural and man-made factors.

While global warming controls the long-term trend, it changes only gradually. Short-term fluctuations in global mean temperature are primarily driven by internal variations in the climate system, such as the state of the El Niño / La Niña oscillation. To a lesser extent, they can also be affected by external processes such as the solar cycle and volcanic eruptions.

Though it is interesting to understand the characteristics of individual years, global warming is ultimately about the long-term evolution of Earth’s climate. The exceptional nature of the warming in 2023/2024 makes future forecasting more difficult, since it likely points to a deviation from the historical trend.

Since 1980, the overall trend has been about +0.20°C/decade (+0.36°F/decade). The extreme warmth in 2023/2024 likely points to a period of greater warming. However, whether that greater warming rate persists over the long-term or is only present briefly is hard to predict. To the extent that excess recent warming is likely driven by reductions in man-made aerosol pollution (mostly sulfate aerosols, as mentioned above), future warming from this source will also depend directly on human choices regarding the regulation of such aerosols.

That said, our long-term trend estimate (a 30-year window) has already crossed 1.4°C (2.5°F) above the average temperature from 1850-1900. Given recent rates of warming it may take only ~5 years for our long-term trend to reach 1.5°C (2.7°F).

The Paris Agreement on Climate Change aims to keep the long-term average global temperature rise to well below 2°C (3.6°F) and encourages parties to strive for warming of no more than 1.5°C (2.7°F). It has been clear for some time that the 1.5°C (2.7°F) goal will not be achieved. Too little time remains and efforts at mitigation fall far short of what would have been needed to meet that target.

Nonetheless, effective mitigation can still limit global warming and reduce the severity of negative outcomes. The increasing abundance of greenhouse gases in the atmosphere due to human activities is the primary cause of recent global warming. If the Paris Agreement’s goal of no more than 2°C (3.6°F) warming is to be reached, significant progress towards reducing greenhouse gas emissions needs to be made soon.

Feedbacks, tipping points, thresholds, and climate change

On a warm day, water evaporates readily, clouds form, and the atmosphere cools because the high albedo of the sun reflects some of the sun’s heat away from the planet. Thunderstorms may occur and rainfall will further cool the area. These are examples of negative feedback. You may be more familiar with some examples from human physiology – sweating and shivering. When you are hot, you sweat, and evaporation cools you. When you are cold, you shiver, and the muscular contractions of shivering raise your body temperature. Negative feedback loops help to maintain the stability of a system, avoiding extreme conditions.

Positive or forward feedback loops are destabilizing forces. A familiar example from human physiology is a fever. When you have an infection, the body responds by raising your body temperature, seeking to kill the disease organisms. If the disease persists, the body raises its temperature further, moving farther away from its stable condition of normal body temperature. The body will continue to raise its temperature until the disease organisms are killed or the body is too weak to continue the warming process.

Physiological feedback loops are the result of evolution. You could think of them as having a job to do – if they didn’t benefit organisms in some way, they likely would not have developed. But climate is not subject to evolution, and climate feedback forces occur without any limitations regarding harm to organisms or ecosystems.

Figure 6. A conceptual diagram of a tipping point between two states or regimes. Vicky Meretsky. CC0.

Tipping points are situations in which a small amount of change of one kind can lead to large changes of another kind – potentially, large, irreversible changes. Consider the diagram in Figure 6. The ball started in the valley on the left of the diagram. Some force or change gave it enough energy to go over the hump between the valleys, and it has fallen into the valley on the right. The valley on the right is lower than the valley on the left, and it will take more energy to go to the left and return to the original valley than it took to go to the right.

An example of a climate tipping point occurs with the Meridional Overturning Circulation of the world’s oceans, which we met in Chapter 1. The Atlantic portion of the circulation (the AMOC) contains one of the areas (between Greenland and Canada) in which cold, salty, dense water sinks from the surface to the deep ocean – one of the engines of the circulation. The current AMOC circulation is Regime 1 in the diagram. Under climate change, melt water from glaciers is pouring into the region of ocean where cold, salty dense water sinks to the ocean floor. Glacial meltwater is freshwater, which is not salty and is less dense than seawater. With the area full of much less salty, less dense water, the sinking process is slowing down. If the climate warms enough (the small change) to send enough meltwater into the area, it’s possible the AMOC might stop altogether, which would change the overall ocean circulation considerably (the large change). That would be Regime 2 in the diagram.

Without climate change, it would be incredibly hard to stop the AMOC. In term of the diagram, one can think of climate change as lowering the hump – the threshold – between Regime 1 and Regime 2, until it’s very easy for the ball to drop down into Regime 2. But returning the ball to Regime 1 – restoring the AMOC – would require reversing recent climate change to restore conditions that allow cold, salty, dense water to sink – a difficult change to arrange, and one that will get more difficult, and will likely take longer, the longer that anthropogenic global warming continues. It will not be enough to halt climate change at its current level – it will be necessary to reverse it to the point that the melting of the Greenland ice sheet slows or stops.

Visit the European Space Agency’s website Understanding climate tipping points . Read down to the world map graphic and click on some of the nonpolar climate tipping-point dots to understand some of the other tipping point possibilities. We will see tipping points again when we explore how climate change and ecosystems interact.

Jonathan Röckstrom, an environmental scientist who has been deeply involved with planetary boundary and climate change research presented a TED talk in 2024 on The Tipping Points of Climate Change – and Where We Stand, which you can view – click here.

Past and present-day GHG emissions will affect climate far into the future

Many greenhouse gases stay in the atmosphere for long periods of time. Researchers presently believe that if we were to cease all anthropogenic GHG emissions, temperatures would level off in a few decades. But a return to pre-industrial temperatures – a decrease in planetary temperature – will require many centuries. Partly this is due to the lifetime of greenhouse gases in the atmosphere, but CO₂ and heat have also been absorbed by the ocean, including the deep ocean, and all of that must be purged to return the planet to pre-industrial temperatures.

Future temperature changes 

Climate change touches directly on Sustainable Development Goal 13 Climate Action. However, its impacts on the planet are sufficiently severe that all other goals are also affected by climate change. In their progress report on the SDGs for 2024, the UN referred to a “global failure to meet climate goals.”

The primary scientific authority on climate change is the Intergovernmental Panel on Climate Change (IPCC), which issues large, complex assessments of climate change every 5-7 years. [Click here for the] most recent report , the 6th, often referred to as AR6, was issued in 2023. In order to estimate future climate change, the IPCC uses a range of scenarios  from the lowest, SSP1.9, in which emissions are reduced very quickly to the highest, SSP 8.5, in which emissions continue to rise throughout the 21st century. SSP stands for shared socio-economic pathway and the numbers represent the force of warming, or radiative forcing, in watts per square meter.

The Summary for Policy Makers for the synthesis volume of the 2023 report provides the following summary of estimated future warming. The figure of 1.5°C (2.7°F) represents a level of warming above pre-industrial temperatures that nations agreed to work to avoid – an increase that represents clear risks, but less severe risks than under 2°C or higher warming. In 2024, the average planetary temperature broke the 1.5°C mark, but longer-term planetary averages are still below that level. Note that the IPCC report is based on data through approximately 2022. It did not include the 2023 and 2024 data that are discussed in the section above, on current levels of warming.

Global warming will continue to increase in the near term (2021–2040) mainly due to increased cumulative CO₂ emissions in nearly all considered scenarios and modelled pathways. In the near term, global warming is more likely than not to reach 1.5°C even under the very low GHG-emission scenario (SSP1.9) and likely or very likely to exceed 1.5°C under higher emissions scenarios. In the considered scenarios and modelled pathways, the best estimates of the time when the level of global warming of 1.5°C is reached lie in the near term. Global warming declines back to below 1.5°C by the end of the 21st century in some scenarios and modelled pathways. The assessed climate response to GHG-emissions scenarios results in a best estimate of warming for 2081–2100 that spans a range from 1.4°C for a very low GHG-emissions scenario (SSP1.9) to 2.7°C for an intermediate GHG-emissions scenario (SSP4.5) and 4.4°C for a very high GHG-emissions scenario (SSP8.5).

Aspects of climate change beyond global warming – future precipitation and storm events

Patterns of precipitation and storm events, including rain, snowfall, and frequency and intensity of tornadoes and cyclones are already changing due to global warming (in the meteorological world, thunderstorms and tornados are smaller, land-based storms, whereas cyclones are larger storms that arise over oceans and include hurricanes and typhoons). The nature of these changes in the future is generally less certain than the changes associated with temperature because they involve more complicated climate mechanisms.

For example, increasing temperatures warm the oceans and land, increasing evaporation and creating conditions that give rise to thunderstorms, tornadoes, and cyclones. However, increasing temperatures also change winds in ways that can make it harder for the vortexes that drive such storms to build to the heights that create severe storms. Current predictions suggest cyclones will increase in intensity, but perhaps not in frequency, and may travel further towards the poles than in the past (because warmer waters will support their growth and travel). In January 2025, a storm with hurricane-force winds struck Anchorage, Alaska, in the US. January 2025 was the warmest January on record, globally, up to that point in time.

Due to increased evaporation, average precipitation is expected to increase, but a number of presently dry areas are predicted to become drier, including the American Southwest, southern Africa, the Mediterranean, and the Caribbean; in these areas, although precipitation may increase, evaporation will increase more. In addition to change in precipitation, timing of precipitation is expected to change, with more precipitation falling outside of the growing season in temperate latitudes, and more intense precipitation occurring, generally, which increases risk of crop damage, catastrophic erosion, and flooding. Formerly predictable precipitation patterns such as the African and Asian monsoons have already become much less predictable.

Due to warming, more precipitation will fall as rain in areas that once received snow. Snowpack holds water for longer periods, releasing it more slowly, and watering mountains and their streams over an extended period of time. A change from snow to rain means a longer dry period both for mountains and for extensive downstream areas that rely on montane water delivery.

The general pattern of warming is quite consistent around the planet and frequency and severity of heat waves are also increasing. Night-time temperatures are higher because the increased heat-absorbing properties of the atmosphere mean that at night, when the sun is not heating the dark side of the planet, heat is now retained that was once radiated to space under lower levels of GHGs. In some areas, cold extremes may also occur. Global warming has weakened the winds that once confined the Arctic cold-air mass more tightly to the Arctic. As the winds weaken, their course becomes more erratic, allowing Arctic air to come further south more often than it once did, causing severe winter cold that can last for several days or more – polar vortexes.

Climate-change impacts will be discussed in most of the remaining chapters, as the impacts related to the specific chapter topics.

Knowledge Check

Take a moment to complete the short quiz below to assess your understanding of this section. Read each question carefully and refer back to the content as needed. This quiz is not graded – it’s simply an opportunity for you to reflect on what you’ve learned and reinforce key concepts.

Trends in levels of traditional air pollutants

US trends

The graphs below are from the EPA 2024 air quality report and show trends in the average US criteria air pollutant levels relative to the national standards and in terms of mass; of these pollutants, only PM2.5, fine particulates, shows signs of increasing, and all are below the current national standard (Figs 1, 2). These are national averages; several areas of the US violate standards for one or more criteria pollutants.

Figure 1. Trends in US criteria air pollutants, relative to the national standards.  US EPA. Public domain.

Nationally, concentrations of air pollutants have dropped significantly since 1990:

• Carbon monoxide (CO) 8-hour,  down 79%
• Lead (Pb) 3-month average,  down 87% (from 2010)
• Nitrogen dioxide (NO₂) annual, down 62%
• Nitrogen dioxide (NO₂) 1-hour,  down 55%
• Ozone (O₃) 8-hour,  down 18%
• Particulate matter 10 microns (PM₁₀) 24-hour,  down 29%
• Particulate matter 2.5 microns (PM_2.5) annual,  down 37% (from 2000)
• Particulate matter 2.5 microns (PM_2.5) 24-hour,  down 29% (from 2000)
• Sulfur dioxide (SO₂) 1-Hour,  down 92%
• Numerous hazardous air pollutants, or air toxics, have declined with percentages varying by pollutant

Despite increases in air concentrations of pollutants associated with fires (carbon monoxide, particle pollution, and ozone), national average air quality concentrations remain below the current, national standards.

Figure 2. Trends in US air pollutants by weight. US EPA. Public domain.

Emissions of most key air pollutants continue to decline from 1990 levels:

• Carbon monoxide (CO),  down 71%
• Ammonia (NH₃),  up 15%
• Nitrogen oxides (NO_x),  down 73%
• Direct particulate matter 2.5 microns (PM_2.5),  down 28%
• Direct particulate matter 10 microns (PM₁₀),  down 27%
• Sulfur dioxide (SO₂),  down 93%
• Volatile organic compounds (VOC),  down 46%

In addition, from 1990 to 2017 emissions of air toxics declined by 74 percent, largely driven by federal and state implementation of stationary and mobile source regulations, and technological advancements from American innovators.

Global trends

Worldwide, mortality associated with air pollution is declining (Fig 3).  But international progress in reducing air pollution varies considerably by region.

Figure 3. Global trends in deaths caused by air pollution, by type of air pollution. OurWorldinData.org. CC BY.

 

At present, the most polluted cities (Fig 4) and countries are concentrated in Southeast Asia and Africa, but instances of poor air quality can be found much more widely. Within regions, air temperature, weather, agricultural activity, home heating, and local phenomena such as wildfires and dust storms can cause seasonal changes in air pollution, as well. Temperature inversions, which occur when warm air at low elevation becomes trapped by a higher cold-air layer that prevents the warm air from rising, can trap air in valleys, or in cold, surface air layers in winter, concentrating local pollutants for several days at a time and significantly increasing pollution concentrations and resulting health impacts.

International air-quality rankings such as those shown in Figure 4 may rely solely on measurements of PM2.5, which are easy and inexpensive to obtain. Nations have not agreed upon an international air-quality index, but international reporting of useful component measurements is becoming more common. Although measurements are not universally available for all the major air pollutants, observations such as the improvements in the ozone hole, and reduction in acid rain provide evidence of the global trends in some air pollutants.

Figure 4.  Measurements of PM2.5, relative to the WHO standard, in cities around the world, in 2024.  IQAir. Used with permission.

Control techniques for traditional air pollution

Air-pollution control is achieved by modifying processes that produce air pollutants and by cleaning dirty air. Reductions in use of fossil fuels and emission controls have reduced CO₂, CO, NO_x, SO₂, and VOCs, which has led to reductions in ground-level ozone, and particulates. Changes to refrigerants, construction materials and home furnishings have reduced levels of chemicals that deplete stratospheric ozone. Restoration to repair degraded land can reduce dust (particulates). At mining sites, roads may be wetted daily or more often to reduce dust, where control is required. Cover crops – plants raised during the non-growing season for crops, to protect soil – can reduce dust from fallow agricultural lands.

Air-pollution control technologies to reduce particulates in industry and transportation use static electricity, strong air currents, protective bags, and filters to capture the particles where they are produced. Scrubbers use liquid or treated surfaces (activated charcoal is an example) to wash out or trap polluting chemicals from flue-gas stacks (“smoke stacks”). Air-polluting substances can be oxidized – burned – using catalysts (substances that help to speed up chemical reactions), converting them to CO₂.

Effectiveness of air pollution control has varied among air pollutants, even in developed countries with strong laws to protect air quality. The differences between reductions in SO₂ and reductions in NOx demonstrate some of the issues involved. Both pollutants result primarily from burning of fossil fuels, but SO₂ originates more from stationary industrial sources and less from mobile vehicular sources than NOx (3.1, Fig 2). Scrubbers and similar technology used to remove pollutants from flue-gas stacks work more effectively on SO₂ than on NOx. Increases in numbers of vehicles on the road and in number of miles driven further increase NOx emissions in many countries. Air pollution control of both these emissions has resulted in substantial declines in acid rain – one of the primary effects of both pollutants – but whereas the EPA reported a 92% drop in SO₂ between 1990 and 2023, NOx levels in the same period declined by 55% (Fig 1, 1-hour standards).

Air pollution policies and standards

US air pollution regulation

In the US, criteria air pollutants are regulated by the Clean Air Act (CAA), as are toxic air pollutants and pollutants that deplete stratospheric ozone. The Environmental Protection Agency is tasked with implementing the Clean Air Act, which includes permitting of major sources and some area sources of air pollution, requirements for air-pollution control technology used to reduce air pollution at major sources, and limits on the levels of certain chemicals in products such as construction materials, solvents, paints, etc. that can contribute to air pollution. Many of these chemicals are “novel entities” – synthetic materials – whose production has put the planetary boundary in novel entities into the high-risk zone.

The EPA sets National Ambient Air Quality Standards (NAAQS) for each criteria air pollutant. State agencies responsible for air quality develop State Implementation Plans to indicate how they will stay within the standards and monitor pollutants to ensure standards are met. Areas that exceed the NAAQS are labeled non-attainment areas; state and local agencies that manage air quality are required to develop plans to bring non-attainment areas into compliance with NAAQs.

Major new stationary sources of air pollution must meet high standards of pollution control technology in order to obtain permits to emit air pollution, particularly if they will be built in non-attainment areas. Manufacturers of new chemicals that may be air pollutants are required, under the Toxic Substance Control Act (TOSCA) to notify EPA about new chemicals or new uses of existing chemicals that may contribute to air pollution. If the EPA finds that new chemicals or new uses pose unreasonable threats to human or environmental health, the agency can require additional testing to provide further information, and can impose restrictions on use.

International air pollution regulation

Figure 5. The ozone layer on 28 September, 2024, showing strong thinning over the South Pole. Scientists predict the hole could close by 2066. US National Aeronautic and Space Administration. Public domain.

International treaties cover some air pollutants, particularly pollutants that deplete stratospheric ozone and cause the “ozone hole” over the South Pole (Fig 5). The Montreal Protocol on on Substances That Deplete the Ozone Layer  (1987; usually simply the Montreal Protocol) was a binding implementation agreement that grew out of the Vienna Convention on the Protection of the Ozone Layer (1985). It was the first of only a handful of UN treaties to achieve ratification by all nations, and has been called the most successful environmental agreement. The Protocol sets limits on production and use of ozone-depleting substances. Most of these chemicals are early refrigerants, aerosols, and related chemicals – chlorofluorocarbons (CFCs) and hydrofluorocarbons (HFCs) – but the list of controlled chemicals is updated regularly to ensure ongoing protection of the ozone layer. The Protocol includes a so-called “ratcheting” mechanism that decreases allowable emissions over time – an important reason for its success.

Ozone depletion over the South Pole continued to worsen during the early years of implementation of the Montreal Protocol, with the largest thinned area occurring in the 2000s with a total area of the ozone hole of 28.4 million square kilometers or more than 4 times the size of the US. In 2024, the maximum extent of the ozone hole was 21.9 million square kilometers – still a significant proportion of the all-time maximum. Nevertheless, the Protocol has led to reductions of emissions of ozone-depleting substances by more than 99% and production of many of them has ceased. Scientists now predict that the ozone hole could “heal” completely well before the end of the century.

An earlier, and less heralded air pollution treaty, the 1979 Convention on Long-Range Transboundary Air Pollution paved the way for the Montreal Protocol. It was the first multilateral agreement to address the problem of air pollution crossing international boundaries, and allowed Europe, Russia, and North America to work together to understand and reduce long-range air pollution. It not only reduced air pollution emissions as intended but also began the process of uncoupling air pollution from economic growth, and important step for better air quality in the long term.

Not all air pollution treaties involve many nations. The US-Canada Air Quality Agreement (1991) is a binational treaty, similar to the Convention on Long-Range Transboundary Air Pollution, to address air pollutants that contribute to acid rain and smog. By 2007, both nations had reduced ozone levels sufficiently to meet the main requirement of the treaty, and a 2023 review and assessment recommended updating the agreement to develop and implement strategies for additional air pollutants such as PM2.5.

National air pollution standards very widely among the nations of the world, due, in part, to technological capacity. Whereas Figure 5 demonstrates observed levels of PM2.5 around the world, Figure 6 shows the standards set by nations for PM2.5. Because pollutants are not measured uniformly, comparing national standards and national pollution levels ranges can be difficult. For example, Figure 6 shows the PM2.5 standard for a 1-year averaging period; a map of the PM2.5 standards for a 24-hour averaging period would rank nations differently. WHO guidelines are available for both measurement frameworks, which allows comparison to a standard, but comparisons between countries can vary depending on the specific measurement. Similarly, observed pollution levels may be measured with different instrumentation, at different elevations off the ground, with different densities of measurements, at different time intervals, etc.

Figure 6. PM2.5 standards for nations around the world, 2025, for a one-year averaging period. Note that the WHO guideline for PM2.5 is 5 μg/m3 in that timeframe. World Health Organization. Permitted use – see terms of use at https://worldhealthorg.shinyapps.io/Air_Quality_Standards_V2_1/.

Regulation and control of greenhouse gases

The UN Framework Convention on Climate Change (UNFCCC; 1992) was the first international treaty to address greenhouse gas (GHG) emissions that cause global warming. The Paris Agreement, adopted in 2015, built on the UNFCCC to create a binding agreement requiring nations to set firm goals (nationally determined contributions: NDCs) to reduce GHG emissions. Like the Montreal Protocol, it has a ratcheting mechanism that requires nations to be more and more ambitious in their reductions over time. However, the Paris Agreement has not enjoyed the success of the Montreal Protocol; emissions of GHGs continue to increase.

Developed countries bear most of the blame for the lack of progress: the US and the European Union are the 2nd and 4th greatest emitters and have the technology and the greater economic stability needed to undertake deep cuts and have not done so. Since 2025, the US is not even participating in the treaty – the only major emitter to withdraw from the Paris Agreement (twice – the US withdrew from 2017-2021, as well). The Paris Agreement explicitly encourages developed countries to take the lead in emissions reductions. It acknowledges that transitional and developing countries need time to balance decreased emissions with economic growth.

Emissions from China (the largest emitter by a considerable margin) and India (3rd greatest emitter) demonstrate the air-quality outcomes of a slower transition to clean energy. China now produces more GHGs than the developed countries, combined. However, in recent years, China has led the world in installing renewable energy; in 2023, China’s additions were over 60% of world additions in renewable capacity. Globally, however, national goals of developed, transitional, and developing countries are insufficient to reduce warming, policies and actions typically lag behind national goals, worsening the outlook for success.

The “promises” that nations make in their NDCs become actual reductions in GHG emissions and GHG levels through a variety of mechanisms. Cap-and-trade agreements, also called emission trading systems, create an economic market by allowing potential GHG emitters to emit a certain amount of GHG, and allowing fast-reducing emitters to sell credits to emitters that are slower to reduce their emissions. Like the ratcheting mechanism used to reduce sulfate air pollution in the US, cap-and-trade agreements generally tighten emissions limits over time, creating pressure on all emitter to find efficiencies and move away from GHG-emitting energy sources. The EU uses cap-and-trade systems, as does China. In the US, a regional cap-and-trade system called RGGI – the Regional Greenhouse Gas Initiative – binds several eastern states, and California and Washington state have state-based systems.

Governments use subsidies, tax credits and other financial incentives to lighten the financial burden associated with transitions to clean energy. These may be aimed at individuals, corporations, local and state governments, and cooperatives. At the level of individuals, subsidies may support addition of solar panels, heat pumps, and electric vehicles. At higher levels, subsidies can support infrastructural development such as energy grid expansions and updates. Subsidies can also support research and investment in evolving technologies such as those involved in carbon capture and storage, which removes CO₂ already in the atmosphere, to hasten reductions in GHG levels over what can be achieved through reduced emissions.

Monitoring and reporting are important elements of achieving reductions in GHG emissions. An accurate understanding of changes in emissions is needed to ensure that government and corporate targets are being met and to allow appropriate enforcement of regulations. Monitoring is carried out at emissions sites, by ground-based stations across the land, and by remote sensing using satellites, aircraft, and drones. In some areas, so-called third parties – neither the emitters nor the enforcers – may undertake monitoring, often under stringent oversight to insure impartiality and accuracy.

Section 6.5 in the Energy chapter addresses the changes in energy production and use and in management of atmospheric CO₂ required to control GHG levels.

Knowledge Check

Take a moment to complete the short quiz below to assess your understanding of this section. Read each question carefully and refer back to the content as needed. This quiz is not graded – it’s simply an opportunity for you to reflect on what you’ve learned and reinforce key concepts.

The video linked click here to watch, created by Meretsky, provides a short case study on a natural source of PM2.5 air pollution – the Sahara Desert. Saharan dust storms affect weather and aspects of human and environmental health over a wide area.

Properties of water

Although water is common in many parts of the world, it is unusual among liquids. Unlike many substances, water doesn’t become denser and denser as it gets colder. Solid metal sinks in liquid metal. Solid paraffin sinks in liquid paraffin. But solid water floats on liquid water, and for this reason, when ponds and lakes freeze over in winter, most of the contents remain liquid, continuing to support aquatic ecosystems and their fish, invertebrates, and aquatic plants.

Water is called the universal solvent because so many substances dissolve in water. As a result of this property, blood and plant sap can carry circulate through plant and animal bodies, delivering gases and nutrients and carrying away wastes. Water can support aquatic ecosystems in the same way.

Lastly among water’s interesting and useful properties is its slow response to changing air temperature. It takes considerable energy to heat water, and once heated, it cools slowly. This property of thermal inertia, a result of water’s high specific heat capacity , is the reason that coastal areas near oceans and large lakes often have rather mild climates relative to areas at the same latitude that are farther from water. Because water cools slower than the surrounding air in the winter and warms slower in the summer, nearby land experiences less extreme temperature swings. Moist soil also warms and cools more slowly than dry soil, due to its water content.

Kinds of water

Of the various kinds of water, humans rely most heavily on surface freshwater; it is, arguably, the most heavily managed natural resource on the planet. Yet, this comprises only a tiny fraction of all the water on Earth (Fig 1).

Figure 1. Global water availability. US Geological Survey. Public domain.

Liquid freshwater in groundwater is about 30% of 2.5% = 0.7% of total global water. Liquid, surface freshwater in rivers, lakes, and wetlands is about 24% of 1.2% of 2.5% = 0.007%. Less than 1% of the world’s water is easily used for drinking water. Further, a 2021  United Nations report estimated that about 40% of monitored freshwater bodies did not meet the standard for good ambient water quality due to water pollution. Many freshwater bodies are not monitored.

Figure 1 divides water resources between saltwater and freshwater, and then further divides freshwater into a variety of liquid and frozen forms. Liquid freshwater resources can be surface water or groundwater (Fig 2). Surface water occurs above the ground, in lakes, rivers, and wetlands. Groundwater, as the name suggests, is water in the ground. But it may be water immediately under the land’s surface or it may be water under rock layers far below the Earth’s surface.

Figure 2. Groundwater, aquifers, and groundwater withdrawal. T.C. Winter, J.W. Harvey, O.L. Franke, and W.M. Alley, US Geological Survey. Public domain.

An unconfined aquifer is a water layer just below the land’s surface, in the soil or sand or gravel (Fig 2). The top of the unconfined aquifer is called the water table. If you ever dug a hole near a lake shore or ocean shore, to reach water, you reached the water table. Where the land dips below the water table, surface water occurs in the forms of lakes, rivers, and wetlands. Surface water, rainfall or melting snowfall can add water to the unconfined aquifer (called recharge) quickly because the water merely needs to percolate through the soil to reach the unconfined aquifer. If water is plentiful, the unconfined aquifer can contribute water to surface-water bodies, as in the figure above, where blue lines point to the stream; if water is less plentiful, then the water table will be lower and flow direction will likely be from surface water into the unconfined aquifer.

Confined aquifers are separated from the land’s surface by one or more confining layers that significantly slow percolation of water (an aquitard – for example, a clay layer) or prevents it entirely (an aquiclude – for example, a rock layer) (Fig 2). Confined aquifers can occur in several layers, separated by confining layers. Just as the unconfined aquifer isn’t pure water but is soil or sand or gravel saturated with water, confined aquifers are layers of porous rock or sand or gravel saturated with water. Precipitation recharges confined aquifers over long periods, ranging from years to millennia.

A well that taps into an unconfined aquifer and removes water creates a local depression in the water table (Fig 2). The area and depth of the depression is a balance between the amount of water withdrawn and the amount of recharge from local precipitation. Withdrawal may thus be sustainable or unsustainable. If the depression is extensive or prolonged, the ground in the area may become compressed without water to support it, and the land may subside, creating a surface depression called a sinkhole.

A well that taps into a confined aquifer is tapping an essentially nonrenewable resource – human use occurs faster than recharge, so the confined aquifer is constantly depleted. As the confined aquifer is drained, the overlying confining layers lose the support of the water and may crack or subside, leading to subsidence at the surface, as well. In addition, collapse of deeper layers may lead to drainage of any unconfined aquifer as well as surface water in rivers or lakes, into the deeper layers.

Figure 3. Saltwater intrusion. New York Water Science Center. Public domain.

In the upper diagram, the water table holds enough water to push against the ocean and maintain freshwater under the land surface. In the lower diagram, water withdrawal from the water table reduces the amount of pressure from the water table outward against the ocean; salt water is drawn towards the well.

Over pumping of aquifers in coastal regions can lead to saltwater intrusion (Fig 3), a process in which saline ocean water moves into the aquifer when freshwater is withdrawn, and potentially renders it unfit for use as a water supply. Saltwater intrusion has occurred in many coastal areas in the US and is particularly problematic in Florida. The problem of saltwater intrusion is exacerbated by rising sea levels due to global warming.

Knowledge Check

Take a moment to complete the short quiz below to assess your understanding of this section. Read each question carefully and refer to the section content as needed. This quiz is not graded – it’s simply an opportunity for you to reflect on what you’ve learned and reinforce key concepts.

Water pollution is a major part of the UN Sustainable Development Goal for clean water and sanitation, and is also involved in the goals for good health and well being, life in water and sustainable cities and communities.

Sources of water pollution

As with air pollution, water pollution sources can be categorized as point sources or nonpoint sources. Point sources of water pollution are usually pipes or other sources that drain mining, shipping, industrial, agricultural, and municipal waste into bodies of water. Nonpoint water pollution generally enters bodies of water through runoff over the ground or travel through the ground. Nonpoint water pollution can come from pollution on roadways, from yard and agricultural chemicals, mine drainage, faulty septic systems, and pet waste. Air pollution that settles out of the air or is washed out of the air into water or onto land and then into water is also a nonpoint source of water pollution.

Types of water pollution

Water pollution is a subset of water contamination. Contaminants are simply foreign substances, or substances present at higher-than-normal levels, whereas pollutants are contaminants that are harmful to the environment and/or organisms. Thus, all pollutants are contaminants, but not all contaminants are pollutants.

The US EPA classifies drinking-water pollutants broadly into physical, chemical, biological, and radiological categories, and also addresses thermal pollution.

Physical water pollution

Physical contaminants are often referred to as sediments and suspended solids. These are solid materials that do not dissolve in water. Although they may carry adsorbed chemicals (chemicals adhering to the surface of the solids), this category of water pollution is concerned only with their impacts as solid materials. Sediment in water is sand, silt, and other kinds of soil that enter waterways as a result of runoff, often from storm water. Suspended solids is a broader category that can also include any debris carried into bodies of water, from microplastics to automobiles – trash, plant and animal debris, microorganisms, soil, etc. Physical contamination reduces water clarity, blocking light to aquatic plants and animals. It may clog fish gills, reduce stream flow, fill in shallow bodies of water, block drainage pipes and restrict water passage for dams and culverts. It degrades habitat for fish and aquatic invertebrates and can also impede the exchange of water between surface water and groundwater.

Chemical water pollution

Chemical contamination may enter water bodies dissolved in inflowing water (including from point-source discharges such as industrial waste discharge), runoff, or groundwater, or may enter as adsorbed chemicals on sediment and other suspended solids that leaches into receiving waters – the lakes and rivers that receive the contaminated water.

Inorganic chemical pollutants

Inorganic chemical pollutants include strong acids and bases, nutrients from yard and agricultural fertilizers and sewage, salts, heavy metals and radioactive materials (these last are treated by EPA as a separate category).

Acid rain

Acid rain or snow that reaches streams and lakes can lower their pH. Aquatic life forms typically cannot tolerate pH values below 5. In severe cases, “dead” lakes can occur, in which most plankton, fish, amphibians, and aquatic invertebrates are absent. As we learned earlier, acid rain was a greater problem in the past but can still occur in the present where high-sulfur coal is burned, particularly in Asia and parts of Russia.

Nutrient pollution

Nutrient pollution is mostly composed of biologically available forms of N (nitrates, ammonia) and P (phosphates) that enter waterways as runoff or drainage from sewage systems, agricultural areas, livestock operations, and municipal and residential properties where fertilizers are produced or applied. N and P may enter as fairly simple compounds, often from fertilizer or as the urine component (urine is a nitrogenous waste) of livestock and human waste. But organic materials in general are high in bioavailable nutrients, and if they are readily decomposed, they will contribute to nutrient pollution.

The process of human contamination of waters with nutrient pollution is termed anthropogenic eutrophication (sometimes called cultural eutrophication). Where soils are naturally higher in nutrients, nearby waters can be naturally eutrophic or highly productive (the opposite – for low-nutrient waters – is oligotrophic). Anthropogenic eutrophication creates even higher levels of nutrients in the water than occurs naturally on productive soils. The high levels of nutrients support heavy growth of algae; algae become numerous and reduce water clarity. In fact, a standard measure of eutrophication is the level of chlorophyll-a in the water, which is due to the algae. Algae are short-lived – usually only a few days – and when they die, they sink to the bottom, where bacteria decompose them, using oxygen. But  oxygen only diffuses from the atmosphere to the bottom of lakes slowly. As a result, oxygen levels near the bottom of the lake can become too low to support most aquatic life, resulting in the death of less mobile bottom life – mollusks and other invertebrates – and any more mobile life that cannot find more oxygenated waters – often including many fish.

Severely nutrient-polluted water bodies are termed hypertrophic. Often dense algal mats cover the surface, blocking light to aquatic plants, and fish kills are common, as fish run out of oxygen. In estuaries where large rivers carrying heavy nutrient loads meet the ocean, the introduction of such large amounts of nutrients can result in dead zones – areas of ocean that are almost devoid of life.

When large amounts organic waste may enter water, usually from industry or from wastewater treatment plants, biological oxygen demand (BOD) is used to monitor the problem. BOD measures the amount of oxygen needed by oxygen-using decomposers to decompose the organic matter in the body of water. BOD rises with nutrient levels and also with water temperature, because decomposers can decompose more quickly in warmer waters (up to a point).

Both acid rain and nutrient pollution push the planetary boundary in modification of biogeochemical flows by changing the N and P cycles. Both through atmospheric deposition of NOx air pollution (as we saw in Chapter 2) and through additions of N and P to surface waters, humans increase availability of these important nutrients, leading to the present “increasing risk” status for this boundary.

Heavy metals

Familiar, toxic heavy metals include arsenic (As), mercury and lead. Many common metals used in everyday life are also toxic – copper, iron, silver, zinc, nickel. Others include chromium (Cr) and platinum (Pt). Heavy metals vary in the nature of their toxicity, but can inactivate enzymes, cause neurological problems, and trigger cancers, among other forms of toxicity.

Lead is of particular concern in drinking water because many early water-distribution systems in the US and elsewhere were built using lead pipes. Lead is relatively inexpensive and easy to work, and the lead industry pushed hard to continue its use. The risk from lead was recognized by the late 1800s, and cities began moving away from lead pipes by the 1920s. Nevertheless, a nationwide ban on new lead pipes in drinking-water systems was not passed until 1986, when it became part of the Safe Drinking Water Act. That ban did not require replacing existing lead pipes, and older cities and homes still contain some lead pipes.

In some parts of the world, arsenic occurs naturally and contaminates ground water. In Bangladesh and India, many deep wells were built by international efforts, partly to provide reliable water and also to avoid disease organisms in fouled surface water. Many of these wells were later found to contain high levels of arsenic.

Salts

Salt pollution occurs most often from road salt (mostly NaCl – the same chemical as table salt) but can also result from mining and industrial wastes. Most aquatic organisms have somewhat narrow tolerances for salinity, and organisms that live in freshwater cannot usually tolerate much salinity. Infrastructure – water treatment plants and dams, for example – can be damaged by corrosive salts. If metal drinking-water pipes that are not designed to carry salty water nevertheless receive salty water, metals from the pipes can leach into the water they carry, affecting human health.

Figure 2. Salinization in a field in the San Joaquin Valley, California, USA. Scott Bauer, USDA ARS. Public domain.

Deep, ancient aquifers that hold water from ancient seas are naturally salty. Not all aquifers have been discovered or tested, so we do not know the total proportion of so-called “fossil water” that is salty. An early (1962) state report indicated as much as 75% of aquifer water in New Mexico, in the US, was too salty to use without treatment. Some areas in the world, including some arid areas of the western US and central Australia have salt deposits or saline groundwater close to the surface. In the most saline areas, rainwater is enough to dissolve salt and bring it to the surface (Fig 2); in such areas, only plants that tolerate high salinity can grow. In other areas, under natural conditions, sparse rainwater would be taken up by plant roots before it reaches these saline water bodies. But if the land is cleared, then, in the absence of plant roots, particularly if irrigation water is added, rainwater or irrigation water can penetrate deeply enough to reach the saline water, resulting in salt being wicked up to the surface of the land, sharply reducing the land’s ability to support agriculture or even native vegetation that is not adapted to saline conditions.

Sea-level rise is an important source of salt pollution for coastal areas and islands. Sea-level rise in the present is a result of global warming and has two main components. Melting ice of terrestrial glaciers and ice sheets on Greenland and in the northern extremes of North American and Eurasia and on Antarctica flows into the oceans and, in 2025, contributed about one-third of sea-level rise. Thermal expansion – the increase in the volume of water as it warms – contributes the other two-thirds. As climate change continues, and melting of terrestrial ice accelerates, meltwater will contribute more to sea-level rise, and thermal expansion will contribute less. But water that is warmed will always increase in volume, and thermal expansion will always be an important part of sea-level rise, so long as warming continues.

As sea level rises, ocean waters push inwards against the edges of continents and islands, pushing salt water into aquifers and up rivers that flow into the oceans. The problem is particularly severe for unconfined aquifers – aquifers in sand and gravel beds and in soil – and aquifers in porous or cracked rock layers such as limestones. Coastal swamps fed by freshwater from local water tables and rivers are already experiencing tree death as a result of saltwater coming up the rivers during high tide and contaminating groundwater. Coastal agriculture is also at risk from salinization of coastal surface aquifers.

Organic chemical pollutants

Organic chemical pollutants of water include PFAS, many pesticides, herbicides, artificial hormones (mainly birth-control chemicals that are excreted in urine and enter waterways from water treatment plants that often do not remove such chemicals), biodegradable organic matter (including raw sewage), fire retardants, industrial chemicals, pharmaceuticals, and related chemicals such as caffeine. Many organic pollutants are persistent and remain in the environment for long periods. Some bioaccumulate and biomagnify, causing increasing health impacts as they move higher up food chains. Environmental chemists group a number of problematic organic pollutants into a category called Persistent, Bioaccumulative, and Toxic (PBT) compounds; these include the persistent organic pollutants (POPs) mentioned in Chapter 1.

Biological water pollution

Figure 3. Zebra mussels clog a water filter in a hydroelectric power plant in Gavins Point, South Dakota, USA. In 2020, the plant reported that water strainers and cooling systems clogged up with zebra mussels required regular shutdowns due to over heating and the prevention of water flowing through the powerhouse. Michael Schnetzer, US Army Corps of Engineers. Public domain.

Biological contaminants include disease organisms (also called pathogens), parasites, and invasive plant and animal species. Water polluted with sewage and animal wastes is more likely to support disease organisms that live in such wastes, including Escherichia coli (E. coli), Salmonella, Cryptosporidium, Giardia, and the organisms responsible for cholera, hepatitis, and typhoid. Some parasites and invasive species are more common in eutrophic water. Invasive plant and animal species can outcompete, harm, or eat native species; clog waterways; and damage infrastructure (Fig 3). Some contribute to eutrophication and some are harmful to humans. Once they are widely established, invasive species are often expensive to control and impossible to eradicate completely. Modified genetic material from genetically modified organisms and from organisms that have developed antibiotic resistance is also classified as a biological pollutant. Warming conditions make once-cold waters more hospitable to invasive species, and nutrient pollution also can facilitate their spread.

Several unrelated varieties of algae produce toxins that contribute to water pollution. As a group, their occurrences are termed harmful algal blooms (HABs). The most common HABs are associated with freshwater algal blooms in eutrophic waters fed by excess fertilizer and sewage. In 2014, a particularly large HAB occurred on Lake Erie, in North America, near Toledo, Ohio, in the US, caused the shutdown of the drinking-water treatment plant there for several days, due to the presence of the toxin microcystin.

So-called “red tides” are examples of coastal marine HABs that produce brevetoxins that build up in shellfish (which are not affected) and sicken or kill vertebrate species that eat shellfish, including humans. Ciguatera poisoning is a result of another algal-produced toxin that builds up in fish and invertebrates and becomes harmful at higher levels, up the food chain. Saxitoxins are similarly harmful to high levels of food chain and were found to be associated with northern fur seal deaths in the southeast Bering Sea in 2025.

Algal populations reproduce faster in warmer waters, particularly when nutrient pollution is present to fuel their growth. The fur-seal incident in the southeast Bering Sea is attributed to global warming impacts on ocean waters in the polar region.

Radiological water pollution

Radioactive contamination of water can occur naturally when radioactive elements such as radon, radium or uranium occur near wells or groundwater. Mines that target radioactive minerals, or that have radioactive minerals co-occurring with non-radioactive minerals, may release mining wastes into water. Nuclear powerplants may similarly release radioactive materials into waterways, and nuclear accidents can release large volumes of radioactive material. Radioactivity can cause cancers and radioactive elements may accumulate in organs and bones, as heavy metals can, leading to longer exposures and more severe harm.

Thermal pollution of water

Water changes its temperature slowly, providing a stable thermal environment for aquatic life. As a result, many aquatic invertebrates, fish, and aquatic mammals have rather narrow temperature tolerance. In addition, warm water carries less oxygen than cold water; for fish that cannot gulp air at the surface, lack of oxygen in unusually warm water can be lethal.

In industry and in power production, water pumped from rivers or lakes is often used to cool machinery. If it is discharged back to receiving waters while it is still warm or hot, it can cause thermal pollution, killing organisms that cannot tolerate either the temperature or the low oxygen levels. Global warming can exacerbate this problem.

Deforestation and timber harvesting near stream sides can expose small, previously shaded streams to direct sunlight, causing increased water temperatures that inhibit fish movement and reduce reproduction. In contrast, construction of dams on naturally warm rivers can lead to harm from cold water that comes from deep in the resulting reservoirs, at the depth that hydropower dams draw water to produce electricity. Rivers with such dams may lose many of their native fish and invertebrates for considerable distance downstream of the dams.

Off the coast of Florida, a population of West Indian manatees that exists at the northernmost limit of their temperature tolerance, benefits from warm-water discharges from power plants along the coast. In winter, when water temperatures drop below 68°F (20°C), warm water discharges can be vital for their survival, along with naturally warm springs and thermal basins. But most impacts from thermal pollution are less benign.

Knowledge Check

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Climate change affects water quality indirectly, by affecting precipitation frequency and intensity, by increasing the variety and growth of invasive species, and by causing sea-level rise.

Under climate change, the variability, timing and intensity of precipitation are changing. Wetter periods are becoming wetter, and dry spells are becoming drier. When storms occur, they are more severe, with higher winds, and higher rainfall in shorter periods of time. Precipitation in temperate and boreal regions is becoming more concentrated in the non-growing season, when plants are not active in water uptake. Throughout the world, droughts may occur at unexpected times of the year, and, in general, may occur earlier in the growing season.

Both drought and flooding can affect water quality. During droughts, water levels drop, concentrating pollutants. Increased levels of toxins and, often, decreased oxygen levels lead to deaths of aquatic organisms, which further increase pollution levels.

Floods increase erosion and sedimentation, overcome water-pollution control structures, and result in dead organisms in the water, which can increase pathogen levels. Settling ponds used to trap water to reduce erosion may overflow, releasing sediment into flood waters. Ponds used to trap animal wastes at large-scale livestock operations known as confined-animal feeding operations (CAFOs) similarly overflow, releasing animal wastes and associated pollutants, including pathogens. Many older cities combine their industrial waste, sewage, and stormwater systems, with the same pipes carrying all three kinds of water to treatment plants. Combined sewer overflows (CSOs) occur when stormwater volumes overwhelm the capacity of water treatment plants to accept inflows. The treatment plants are forced to dump the overflow, including industrial waste and raw sewage, into the receiving waters that usually receive treated water. As flooding increases under climate change, CSOs are also increasing. Projects that separate stormwater pipes from waste/sewage pipes can eliminate CSOs but are expensive and time-consuming.

During severe floods, contaminated water from the all the kinds of overflows described above combine with flood waters to cover the entire landscape, flooding and contaminating fields used to grow food crops, contaminating and damaging homes and businesses, and contaminating wells and bodies of water used for recreation and drinking water. Water treatment plants may be inactive for some period of time due to damage, so that residents must boil water or use bottled water. This further increases the risk of human health impacts.

We have seen in the previous section that warming water can also increase numbers and population sizes of invasive species and can trigger harmful algal blooms. More tropical species continue to move into more polar regions as a result of warming, giving management agencies, industries, and private citizens an increasing number of problem species to address. In addition, global warming can exacerbate existing thermal pollution.

Knowledge Check

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An introduction to water treatment

Water treatment has two main forms. Water pumped from surface water and groundwater is treated before it is piped out to the public to use. Typically, such water is brought up to drinking-water standards, since public water systems serve many uses. Water that has been used but not used up – wastewater from residential, municipal, and industrial uses – is treated to remove some pollutants before it can be returned to  receiving surface waters or groundwater. Treatment of water that is not to be used for drinking typically achieves a lesser level of water quality than drinking-water treatment, both in terms of number and level of pollutants.

Watch the video click here to watch to learn about standard methods of drinking water treatment and wastewater treatment.

Limitations of water treatment

Water treatment can eliminate many forms of contamination. However, some contaminants are not removed by standard treatment methods. Some of these difficult contaminants have only been developed or detected recently. Others have been known for a long time but are difficult or expensive to address.

Nitrate ions, one of the common forms of fertilizer, are not removed from standard water treatment because the only effective methods are quite expensive. Nitrates are not usually a health problem, but in agricultural areas, they can rise to high levels and then they pose a risk to pregnant women and infants. “Blue baby syndrome” is the result of high nitrate levels that impede oxygen delivery through the blood of very young children.

Heavy metals are well-known category of pollutants that cannot be degraded by bacteria during water treatment because they are elements and cannot be broken down. In addition, they undergo complicated chemistry with organic molecules that defeats most of the standard treatment approaches. The different heavy metals behave differently in terms of solubility and chemical reactivity, making it difficult to develop a single solution for their removal.

Microplastics – tiny fragments of plastic (< 5 mm in length and often much smaller) that result from physical fragmentation of larger pieces – can be too small to be removed during most treatment methods. The smallest fragments don’t settle well, and none can be biodegraded by bacteria (studies have found some bacteria that can “digest” plastics, but they are not available for general use).  The smallest microplastics are small enough to travel from drinking water through the walls of the gut and into the blood stream and then into organs, including the brain. Microplastics can clog the digestive processes of aquatic organisms, particularly planktonic animals, and can then bioaccumulate and biomagnify in aquatic food chains. Because they do not biodegrade but simply get smaller and smaller, they are classified as “forever chemicals.” Research on impacts to humans is ongoing, but so far has suggested that cancer, heart problems and reproductive problems may be involved, as well as neurological damage and inflammatory diseases.

Many pharmaceutical compounds are not removed during water treatment for the simple reason that the treatment processes are not designed to remove them. Some also resist being broken down. As a result, birth-control drugs and caffeine eliminated in urine produce daily spikes in municipal waterways, and fish behaviors including migration and reproduction are altered by measurables levels of antidepressants, hormones, opiates, and other drugs. Resistance to antibiotics increases due to continual environmental exposure. The impacts of these inadvertent drug exposures on humans include antibiotic-resistant diseases, endocrine disruption (disruption to hormone pathways including reproductive hormones), and possibly immune-response and neurological effects.

Water treatment, itself, can introduce harmful compounds into drinking water. Chemical disinfection to address pathogens often uses halogens, particularly chlorinated compounds that produce chlorinated byproducts that are toxic and are not removed by other aspects of water treatment. Disinfection byproducts from chlorination can cause mutations and cancers. Ozone disinfection produces fewer and different harmful byproducts, but ozonation is more expensive than traditional chlorination methods and is less often used, particularly in smaller treatment plants.

Knowledge Check

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Trends in the US

In the 1960s, some rivers in the US carried so much pollution that it was possible for them to be on fire. Many of the industrial wastes in water of that period have been significantly reduced, but a constant stream of new chemicals, many of them persistent in the environment and difficult to treat, is being produced, and some kinds of wastes, such as heavy metals, continue to be a problem. Clean-up is further complicated because, for many pollutants, no safe levels exist – to render drinking water safe and to protect water quality in rivers and lakes to protect aquatic ecosystems, these materials should be entirely removed, which is likely impossible.

Under the Clean Water Act of 1972, pollutant means “dredged spoil, solid waste, incinerator residue, sewage, garbage, sewage sludge, munitions, chemical wastes, biological materials, radioactive materials, heat, wrecked or discarded equipment, rock, sand, cellar dirt and industrial, municipal, and agricultural waste discharged into water.” Such a comprehensive definition makes for challenging enforcement.

Drinking water

The USEPA is charged with enforcing most US water-quality laws. The agency issues an annual Report on the Environment that provides information on aspects of air quality, water quality,  land condition, human health, and ecological condition. The following information is from the 2022 report section on drinking water.

Community water systems (CWS) are public water systems that supply water to the same population year-round. In fiscal year (FY) 2021, more than 315 million Americans (U.S. EPA, 2021)—roughly 95 percent of the U.S. population (U.S. Census Bureau, 2021)—got at least some of their drinking water from a CWS. This indicator [the EPA indicator for drinking water] presents the percentage of Americans served by CWS for which states reported no violations of EPA health-based standards for more than 90 contaminants (U.S. EPA, 2022a).

Health-based standards include Maximum Contaminant Levels (MCLs), Maximum Residual Disinfection Levels (MRDLs), and Treatment Techniques (TTs). An MCL is the highest level of a contaminant that is allowed in drinking water. An MRDL is the highest level of a disinfectant allowed in drinking water (U.S. EPA, 2022c). A TT is an enforceable procedure or level of technological performance which public water systems must follow to ensure control of a contaminant (U.S. EPA, 2022b). TTs are adopted where it is not economically or technologically feasible to ascertain the level of a contaminant, such as microbes, where even single organisms that occur unpredictably or episodically can cause adverse health effects. Compliance with TTs may require a variety of actions to protect public health, such as assessment of the system, filtration and disinfection, and optimized corrosion control (U.S. EPA, 2022b).

This indicator tracks the population served by CWS for which no violations of health-based standards were reported to EPA annually for the period from FY 1993 to FY 2021, the latest year for which data are available. Results are reported as a percentage of the overall population served by CWS, both nationally and by EPA Region. This indicator also reports the number of persons served by systems with reported violations of standards covering surface water treatment, microbial contaminants (microorganisms that can cause disease), disinfection byproducts (chemicals that may form when disinfectants, such as chlorine, react with naturally occurring materials in water and may pose health risks), and other contaminants. The indicator is based on violations reported quarterly by EPA, states, territories, and the Navajo Nation, who each review monitoring results for the CWS that they oversee.

Of the population served by CWS nationally, the percentage served by systems for which no health-based violations were reported for the entire year increased overall from 79 percent in 1993 to 92 percent in FY 2021 (Fig 1). Drinking water regulations have changed in recent years. This indicator is based on reported violations of the standards in effect in any given year.

When results are broken down by EPA Region, some variability over time is evident (Fig 2). Between FY 1998 and FY 2021, most Regions were consistently above the national percentage. Only Region 2 remained consistently below the national percentage over the entire period of record, largely because of a small number of public water systems serving large populations.

In FY 2021, reported violations involving surface water treatment rules were responsible for exceeding health-based standards for 15.6 million people (4.9 percent of the population served by CWS nationally). Reported violations of the health-based, disinfection-byproducts rules affected 4.3 million people (1.4 percent of the CWS-served population).

Figure 1. Proportion of the US population that is served by community water systems that reported no violations of EPA health-based standards. US Environmental Protection Agency. Public domain.              

Figure 2. Proportion of the US population that is served by community water systems that reported no violations of EPA health-based standards, broken down by EPA region. US Environmental Protection Agency. Public domain.

EPA Region 1, shown by the turquoise line in Figure 2, comprises the New England states: Maine, Vermont, New Hampshire, Massachusetts, Connecticut, and Rhode Island. Region 2, the rust-colored line in Figure 2, comprises New York and New Jersey.

Households that do not get water from community water systems rely largely on private wells (groundwater) or take water directly from rivers or lakes (surface water). Well owners and direct pumpers are responsible for monitoring their own water quality.

Surface water

National and global information on specific water pollutants in surface waters is less easy to obtain than information on air pollutants. Whereas air pollution is somewhat well mixed, at least at the level of cities, water pollution varies considerably among bodies of water and with distance from sources of pollution. In addition, whereas treatment for air pollution tends to be controlled before the pollutant is discharged into the air, water pollution is controlled at two levels; it is regulated by permit before it is discharged into bodies of water, but it is also treated when water is withdrawn from surface or groundwater, before it is used for municipal or industrial use. Air pollution regulations seek to make air safe to breathe because there is no treatment after smokestacks, before the air is breathed. But water pollution regulations need not make surface or groundwater safe to drink because surface and groundwater used for drinking will be treated before it reaches households.

Under EPA regulations, states set water quality standards, depending on the major uses of water bodies (e.g., drinking water, fishing, industrial, recreational). Water bodies with measurements exceeding the standards are considered to be impaired. Figure 3 shows a map of impaired waters – bodies of water found to have exceeded at least water quality standard, of 2015.

Figure 3. Waters of the US found to be impaired as of 2015. US Government Accounting Office. Public domain.

The sampling methods, sampling intensity, and definition of impairment differ among states because the Clean Water Act gives considerable authority to the states. The concentration of red in Ohio and Michigan (or lack of red in Pennsylvania) does not mean waterways in these states are more (or less) polluted than others. Rather, it reflects differences in sampling intensity and state-level definitions of impairment.

Nutrient pollution is a good example of the complexities involved in water pollution trends.  Nutrient pollution from point sources such as wastewater treatment plants is regulated by permit programs developed in the Clean Water Act. However, nonpoint sources of nutrient pollution, particularly from agriculture, are presently unregulated by states or by the EPA. The EPA estimates that 44% of rivers and streams, 50% of lakes and 17% of coastal waters have nutrient levels above recommended values, with only about half of the relevant water bodies having been assessed as of 2017.

One indirect measure of nutrient pollution from the Midwest, where the majority of high-intensity agriculture occurs, is the size of the dead zone in the Gulf of Mexico. Because the Mississippi River drains the Missouri and the Ohio Rivers, in addition to its own flow, it encompasses 41% of the contiguous US (the US excluding Alaska and Hawai’i) and most of the high-intensity agricultural area. As a result, it collects the nutrient pollution of that region, which is then discharged into the northern coastal region of the Gulf of Mexico. The nitrogen and phosphorus trigger algal blooms that then rapidly decompose.

The bacterial decomposers exhaust the oxygen supply in the region, creating a large hypoxic zone (a zone with too little oxygen to support most life) that is deadly to much marine life – hence “dead zone” (Fig 4). The size of the dead zone varies depending on the winter and spring precipitation in the basin, including the storm history and resulting runoff and associated sediment and other pollutants.  Once crops begin to grow and bacteria become more active, in late spring, nutrient run-off lessens. The size of the hypoxic zone is mapped each year, providing evidence of the ongoing problem with nutrient pollution. The dead zone affects marine ecosystem health in the region as well as human health and economic health of the Gulf states, due to loss of fisheries and recreation.

Figure 4. The area of the summer 2024 Gulf hypoxia zone (the area with dissolved oxygen < 2 mg/L) and the history of the size of the zone from 1985. The flat dotted line is the target goal of the Mississippi River/Gulf of Mexico Watershed Nutrient Task Force. The irregular dotted line is the 5-year running-average size of the bottom-water hypoxic zone. Image: LUMCON/LSU/NOAA. Public domain.

Normal oxygen levels in the Gulf of Mexico are >8 mg/L. Benthic organisms – those that live in the bottom water – tend to be less mobile and are at greater risk of dying as a result of hypoxia. The summer 2024 hypoxic zone was approximately 17,366 sq km (6,705 sq mi; equal to about 18% of the size of Indiana and more than half the size of Belgium).

US water pollution control

Federal water pollution law applies to those bodies of waters considered to be waters of the US, a designation that is subject to legal interpretation, and has varied over time. So-called “navigable waters,” interstate waters, coastal waters up to 12 miles from shore, and certain tributaries and wetlands associated with these are presently considered to be waters of the US. States may choose to further regulate water quality – for example, to protect a greater proportion of wetlands from being filled in – so long as those waters are within state boundaries.

The primary laws regulating water pollution in the US are the Clean Water Act of 1972 and the Safe Drinking Water Act of 1974. Both are the responsibility of the US EPA, which works with states on many aspects of water quality. Entities of all kinds that seek to discharge pollutants into US waters from a point source (usually a pipe of some kind) must apply for permits called National Pollutant Discharge Elimination System permits (NPDES permits). The definition of “pollutant” is very broad and includes municipal, industrial, and agricultural wastes. Point sources, by law, include concentrated animal feeding operations such as stockyards and poultry barns and exclude agricultural stormwater discharges and return flows from irrigated agriculture.

NPDES permits set a maximum threshold for pollutant amounts or levels that can be discharged; the goal of the permits is to ensure that waters meet state water quality standards and minimum Federal standards. The EPA sets specific numeric standards for a subset of pollutants that can harm human health, fisheries, and wildlife. States may set more stringent standards and may regulate substances for which federal regulations are not yet available. State standards are set through a process that involves public input and EPA review. Depending on the nature of the pollutant and the polluter, public input may strengthen or weaken state standards.

The regulatory process for water pollutants inevitably goes more slowly than the creation of chemical compounds. In the US, for example, EPA’s PFAS regulations only require monitoring of 6 classes of PFAS by 2027, and public water systems must begin to implement measures to remove those classes of PFAS by 2029. Pharmaceutical compounds are also largely unregulated as water pollutants.

Drinking water standards around the world

Individual nations each establish their own water pollution standards for surface water, groundwater, and drinking water. Many countries rely on guidelines from the World Health Organization (WHO) as the basis for their drinking water standards; these have been published since 1958. Because the WHO is not a regulatory body, it does not enforce standards but only recommends them. In addition to providing guidelines, the WHO also reports on the drinking water standards adopted by many nations, comparing national standards to the WHO guidelines for a wide range of pollutants. The latest report, from 2021, shows that many countries have not yet adopted standards for many kinds of pollutants. However, most nations do have standards for heavy metals, for nutrients (nitrate and nitrite), and for E. coli. Except for some heavy metals (e.g., cadmium), most national standards are at least as protective as the WHO guidelines.

Knowledge Check

Take a moment to complete the short quiz below to assess your understanding of this section. Read each question carefully and refer to the section content as needed. This quiz is not graded – it’s simply an opportunity for you to reflect on what you’ve learned and reinforce key concepts.

View the videos here to watch the first and here to watch the second for a history of water pollution in the Seine River in France and efforts to clean up the Paris segment of the river for the 2024 Summer Olympics and for the public, in general.

In chapter 3, we saw that accessible freshwater resource is a tiny fraction of the water on Earth. Fortunately, water is a renewable resource and is difficult to destroy. Evaporation and precipitation combine to replenish our fresh water supply constantly; however, water availability is complicated by its uneven distribution over the Earth. Arid climates and densely populated areas have combined in many parts of the world to create water shortages, which are projected to worsen in the coming years due to population growth and climate change. Human activities such as water overuse and pollution have significantly compounded the problem of providing clean freshwater to the world. Freshwater availability is central to UN Sustainable Development Goals for clean water, life below water, and sustainable cities and communities.

The water cycle

The water (or hydrologic) cycle shows the movement of water through different reservoirs or pools, including oceans, the atmosphere, glaciers, groundwater, lakes, rivers, and the biosphere. Solar energy and gravity drive the motion of water in the water cycle. Simply put, the water cycle involves water moving from oceans, rivers, and lakes to the atmosphere by evaporation, forming clouds. From clouds, it falls as precipitation (rain and snow) on both water and land. The water on land can either return to the ocean by surface runoff, rivers, glaciers, and subsurface groundwater flow or return to the atmosphere by evaporation and evapotranspiration (sometimes just transpiration; loss of water by plants to the atmosphere directly from their leaves and during respiration) (Fig 1).

Figure 1. The water cycle or hydrologic cycle. US Geological Survey. Public domain.

Sublimation occurs when water in solid form as ice or snow goes directly into water vapor, by evaporating. Evapotranspiration is evaporation that takes place when plants breathe and lose water vapor as they exchange oxygen and carbon dioxide.

Living things are affected both by (1) surface water and soil water available for drinking and root uptake and (2) water in the atmosphere. The need for drinking water is familiar to all of us, but the atmospheric aspect is less obvious. You may have experienced your nose and mouth and eyes and even skin feeling dry when the air is dry – when relative humidity is low. Because atmospheric humidity is low, moisture is moving from you into the atmosphere, by diffusion. You represent a body of highly concentrated moisture, and the atmosphere represents a body of low-concentration moisture. Diffusion moves water along the gradient from you, on the high end of the gradient, to the atmosphere, on the low end of the gradient. You don’t breathe in order to lose moisture, but rather to exchange gases. But you breathe through a system of moist membranes in your lungs and nasal passages, and you can lose water from these surfaces. Similarly, when plants exchange gases, they breathe through small pores in their leaves (called stomata) and can lose water during this process – part of evapotranspiration. And just as your skin can lose moisture directly, many plant leaves can lose moisture directly – the other part of evapotranspiration. Plants in arid areas often have leaf surfaces modified to minimize such losses. But they still need to exchange gases and still lose water to dry air.

For living things, relative humidity is more important than absolute humidity. “Relative” means “as a percent,” whereas “absolute” refers to actual quantity. Cold air can hold very little absolute moisture. A cubic meter of air at freezing temperature (32°F, 0°C) can hold a maximum of about 3.5 grams of water in vapor form (less than 1 teaspoon) Air at room temperature can hold more absolute moisture – a maximum of about 17 grams of water, or 3.4 teaspoons. Saturated air – air that is holding all the moisture it can hold – has 100% relative humidity. Air during a rainstorm is saturated and has 100% relative humidity. If the air is holding half of what it can hold at maximum, then it has 50% relative humidity (considered a comfortable indoor humidity).

Living things deal with relatively humidity, because that is the measurement that indicates how much more moisture the air could hold than it presently does. The lower the relative humidity, the greater the difference between living things (as somewhat permeable bags containers of moisture with some evaporative surfaces) and the atmosphere. If the atmosphere could hold much more water than it has – if the relative humidity is low – then moisture will move out of living things and into the atmosphere by diffusion, through evaporation, as described above.

An important part of the water cycle is how water varies in salinity, which is the abundance of dissolved ions in water. The saltwater in the oceans is highly saline, with about 35,000 mg of dissolved ions per liter of seawater. Evaporation (water changing from liquid to gas at ambient temperatures) is a distillation process that produces nearly pure water with almost no dissolved ions. Water vaporizes and leaves the dissolved ions in the original liquid. Eventually, condensation  (water changing from gas to liquid) forms fog and clouds and sometimes precipitation (rain and hail and snow). After rainwater falls onto land, it dissolves minerals in rock and soil, which increases its salinity. Most lakes, rivers, and near-surface groundwater are called freshwater and have a relatively low salinity. The next sections discuss important parts of the water cycle relative to freshwater resources.

Figure 2. Variation in rainfall around the world, by month. PZmaps in Wikimedia Commons. CC BY-SA.

Take a moment and watch the pattern of precipitation through the months over the Sahara in northern Africa, the west coast of central South America, and over the Himalayas (north of India). Watch the differences in different places (eastern compared to western North America, for example). Which places are always wet? Which are always or very often dry? Compare this to information about precipitation related to atmospheric cells from Chapter 1.

 

Figure 3. Surface runoff is part of overland flow in the water cycle.  James M. Pease at Wikimedia Commons. Public domain.

Flowing water from rain and melted snow on land enters river channels by surface runoff (Fig 3) and groundwater seepage. River discharge describes the volume of water moving through a river channel over time (Figure 4). The relative contributions of surface runoff vs. groundwater seepage to river discharge depend on precipitation patterns, vegetation, topography, land use, and soil characteristics. Soon after a heavy rainstorm, river discharge increases due to surface runoff. The steady normal flow of river water, when storms aren’t involved, is mainly from groundwater that discharges into the river. Gravity pulls river water downhill toward the ocean. Along the way, the moving water of a river can erode soil particles and dissolve minerals. Groundwater also contributes a large amount of the dissolved minerals in river water.

The geographic area drained by a river and its tributaries is called a drainage basin or watershed. The Mississippi River drainage basin includes approximately 40% of the U.S., an area that includes the smaller drainage basins, such as the Ohio River and Missouri River, that help to comprise it. Rivers are an important water resource for cropland irrigation and drinking water for many cities worldwide. Rivers that have had international disputes over water supply include Colorado (Mexico, southwest U.S.), Nile (Egypt, Ethiopia, Sudan), Euphrates (Iraq, Syria, Turkey), Ganges (Bangladesh, India), and Jordan (Israel, Jordan, Syria).

Although glaciersrepresent the largest reservoir of fresh water, they generally are not used as a direct water source because they are located too far from most people (Figure 4). However, meltwater from glaciers provides a natural source of river

Figure 4. Briksdalsbreen glacier in Norway. Vicrogo, Wikimedia Commons. Public domain.

water and groundwater, due to the cycle of melting during summer and building during winter. Under climate change, the melt-build cycles are increasingly pushed to more melt and less build, as less and less precipitation falls as snow and more falls as rain. Obviously, this can only last until glaciers melt completely.

Water use in the U.S. and world – withdrawals from surface and groundwater

Humans use water to produce the food, energy, and mineral resources they use.  Consider, for example, these approximate water requirements for some things people in the developed world use every day: one tomato = 3 gallons (11.4 L); one kilowatt-hour of electricity from a thermoelectric power plant = 21 gallons (79 L); one loaf of bread = 150 gallons (568 L); two pounds of beef (0.9 kg) = 3,200 gallons (12,100 L); and one ton (or tonne) of steel = 63,000 gallons (238,500 L). Human beings require only about 1 gallon (3.8 L) per day to survive. Still, a typical person in a U.S. household uses approximately 100 gallons (380 L) per day, which includes cooking, washing dishes and clothes, flushing the toilet, and bathing – indoor uses.  The water demand of an area is a function of the population and other uses of water.

In the US, In the years immediately after World War II, agriculture was still the primary undertaking using water (Fig 5). As municipal and industrial demand for energy increased, use of water for cooling mostly coal-fired power plants increased, passing agricultural water use in the mid-1960s. In the 21st century, as the nation’s energy portfolio shifted from coal-fired power plants to natural-gas power plants that need less cooling, overall water use began to decline, helped along by the economic downturn in 2008. In 2015, use of water for power plants was again only slightly higher than use for agriculture. The decline in water use in the US has come largely in surface water use, with groundwater use holding steady in the 21st century (Fig 6).

Figure 5. US freshwater withdrawals by water-use category, 1950-2015. US Geological Survey. Public domain.

Water use declined after 2005 for several reasons, including reduced irrigation demand for fruits and vegetables during the 2008 economic downturn, reduction in coal-fired power plants and associated water use for cooling, and increased efficiency of cooling technology.

 

Figure 6. Freshwater withdrawals from groundwater and surface water, shown with US human population, 1950-2015. US Geological Survey. Public domain.

Although use of water in the energy sector has declined in the past, the anticipated expansion of electrical energy use, in response to increases in electrical vehicles, expansion of US manufacturing, the development of hydrogen fuels, growth of AI, and general increases in electrification will likely increase water demand. Water use to cool AI data centers is anticipated to increase 2-3 times between 2024 and 2028, for example, causing concern for local water availability.

The sectoral use of water in the US varies geographically (Fig 7), with some regions having higher agricultural withdrawal, and others higher industrial withdrawal. In the wetter eastern part of the country, rain-fed agriculture is more common, and irrigation is less necessary than in the drier, western part of the country. Power plants tend to cluster near population centers, and industrial use has both historical and recent components to its distribution. Note that water used for agricultural purposes mostly returns to the atmosphere through evaporation and evapotranspiration, whereas a significant proportion of water used for many other uses, including cooling of power plants, industrial use, and municipal use, is often returned to surface water bodies.

Figure 7. Variation in kind and amount of water withdrawals across the US in 2015. US Geological Survey. Public domain.

Global total water use is steadily increasing at a rate greater than world population growth (Fig 8). During the 20th century, global population tripled, and water demand grew by a factor of six. The increase in global water demand beyond the population growth rate was due to an improved standard of living without an offset from water conservation. Increased production of goods and energy entails a large increase in water demand.

 

Figure 8. Global water withdrawal by sector, 1900-1910, with global human population. FAO, Aquastat. Public domain.

Rates of increase in global water use are slowing in the 21st century, as water use is decoupled from economic growth by improvements in technology in all sectors, from irrigation, to industry, to municipal use. Globally, the US, India, and China have the highest level of water withdrawal (Fig 9).

Figure 9. Annual water withdrawals by country in 2021. FAO data presented by Hannah Ritchie and Max Roser of OurWorldinData. CC BY.

The largest water users shown are the US, India, and China. Despite the aridity of many countries, water withdrawal in Africa is comparatively low.

Continents with more agriculture on drier land rely heavily on irrigation, and agricultural water use continues to be the largest use of water, worldwide (Figure 10). Although domestic use in countries like the US may be much higher than the global average, owing to watered lawns, water-intensive appliances, etc., water for agriculture is a much larger share of the country’s water portfolio.

Figure 10. Proportions of water withdrawal by sector across continents. FAO Aquastat. Public domain.

Knowledge Check

Take a moment to complete the short quiz below to assess your understanding of this section. Read each question carefully and refer to the section content as needed. This quiz is not graded – it’s simply an opportunity for you to reflect on what you’ve learned and reinforce key concepts.

As we saw in the previous section, water withdrawals have increased over time. Water is a renewable resource, and the amount of water on the planet is essentially constant. However, we have seen that most of the water on the planet is not freshwater. In addition, many freshwater resources have been polluted to the extent that they cannot safely be used for human purposes without treatment.

In addition, water, even uncontaminated freshwater, can become less available, depending on where it occurs and where it is needed. Confined aquifers can be emptied and do not recharge in a useful timeframe. Much of the water used for irrigation does not return to surface water or groundwater but evaporates; it remains in the hydrologic cycle, but is lost from the water source from which it was withdrawn.

Fresh groundwater and surface freshwater both exist as patchy resources. In addition to being used for direct human purposes, surface water is also the environment for freshwater ecosystems. Soil moisture, the water in soil above the water table is the water source for rainfed agriculture and for naturally occurring vegetation. Atmospheric moisture – humidity – is also important for agriculture and vegetation in general. This section primarily examines issues of water supply for direct human uses. Ecosystem issues are addressed in a later chapter.

Unsustainable withdrawals – the global picture

So far, we have studied the availability of water around the world, and the levels of withdrawal. How do those quantities compare? We know that withdrawals from confined aquifers must be unsustainable, because these do not recharge on a useful timescale. How fast are we drawing down our unconfined groundwater resources? Are withdrawals from surface waters within the limits of these renewable water bodies?

Blue-water scarcity is a measure of water deficit developed by Mekonnen and Hoekstra. Blue water is a term that combines surface water and groundwater, ignoring precipitation (green water, in this jargon). Blue-water scarcity is calculated as unreturned blue-water withdrawal divided by the net additions to blue-water availability (area runoff input minus upstream withdrawals minus downstream river outflows). A blue-water scarcity value of 1.0 indicates that withdrawals just balance inputs. Lower values (shown in green in Figure 1) indicate that blue-water withdrawals are less than additions. Blue-water scarcity values >1 indicate shortfalls. Not surprisingly, Figure 1 looks very much like a map of aridity – less sustainable water use tends to occur in drier areas.

Figure 1. Average monthly blue water scarcity, 1996-2005.  Figure 2 of Mekonnen & Hoekstra, 2016. CC BY-NC.

The groundwater resource is spatially patchier than surface water, because aquifers are limited in their extent, whereas precipitation falls everywhere. A 2015 [click here to read the] study of the 37 largest aquifers on Earth used remote sensing to detect water use and modeling to estimate recharge and discharge of rechargeable aquifers (Fig 2). The aquifers with greatest depletion for the 2003-2013 study period were the Ganges-Brahmaputra system that drains the Himalayas, the North Caucasus (mountains between the Black and Caspian Seas), the Canning Basin in Australia; the Arabian aquifer system, and the aquifer system under the Central Valley of California. Of these, only the Arabian aquifer system has negative recharge due to evaporative losses that exceed precipitation; the other 4 are declining rapidly despite some amount of recharge. Stressed aquifers may occur in arid areas, near high population centers, in areas with high irrigated agriculture, high livestock use, high industrial use, or a combination of these stresses.

Depletion of rechargeable groundwater lowers water tables and reduces spring flow to rivers. Lowered water tables can dewater wetlands and make arable land less suitable for rainfed farming. If irrigation is introduced to allow continued farming, it will further deplete surface and groundwater resources. Reduced spring flow to rivers reduces flow of the rivers themselves and can cause a perennially flowing river to become a seasonal or ephemeral stream. The Phoenix-Tucson area of the southwestern US is notable for large, dry river beds where the Santa Cruz, Gila, Salt, and Agua Fria rivers, among others, once flowed. The rivers were eliminated as surface features by overpumping of groundwater and diversion of upstream flows.

Figure 2. Groundwater storage trends for Earth’s 37 largest aquifers using remotely-sensed data, showing depletion (red/orange) and replenishment (blues) in millimeters of water per year from 2003-2013. Twenty-one aquifers were being depleted, and 13 of these were considered significantly distressed, threatening regional water security and resilience. NASA/JPL-Caltech. Public domain.

Unsustainable withdrawals – the local picture

As groundwater is pumped from water wells, there usually is a localized drop in the water table around the well called a cone of depression (Fig 3). When a large number of wells have been pumping water for a long time, the regional water table can drop significantly. This is called

Figure 3. Cone of depression around a groundwater well, with shallower dry well and salt-water intrusion. Water Utility Management. Used with permission.

groundwater mining, which can force the drilling of deeper, more expensive wells. In addition, on coastlines, the reduced outward pressure of the local water table can allow salt water to intrude into the surface aquifer, leading to salination which may render the water undrinkable and unsuitable for agriculture or industry.

Water in aquifers supports the land above the aquifer. As pumping withdraws that water, the land itself may settle, in localized sinkholes. If larger areas of an aquifer are substantially drained, subsidence of the land surface over a larger area may occur, for example when a sand-and-gravel aquifer compacts as the water is withdrawn. Figure 4 shows the results of years of groundwater pumping of sand and silt aquifers in the San Joaquin Valley of California. Over the period of 1925-1977, the land surface subsided by more than 33 ft (10 m).

Figure 4. Results of groundwater withdrawal in the San Joaquin Valley of California, showing land subsidence from 1925-1977. US Geological Survey. Public domain.

Sources of water loss

As we saw earlier, water pollution can render freshwater sources unusable or can increase the cost of making it useable. In addition, water may be lost between the point of extraction from the source and the point of use. Around the world, water is moved in open canals from its source to distant location, even locations in other countries. Canals and raised flumes constructed of metal or wood require maintenance that is not always forthcoming. In Central Asia, much of the water infrastructure was constructed when those nations were part of the Soviet Union. Even in well-maintained systems, leakage occurs, and illegal “poaching” of water from canals also occurs. Remote sensing can often detect leakage; careful monitoring of flow volumes can detect unsanctioned withdrawals but is expensive. Leakage is not only a problem in delivery infrastructure, but also occurs in storage containers, and even to groundwater, from natural streams and from reservoirs.

A Food and Agriculture Organization manual estimates that earthen canals in sand lose 20-40% of their water, depending on length; canals in clay lose 10-20%. Lined canals are estimated to retain 95% of their water, but poor maintenance can reduce all these figures by as much as 50%!

In addition to leakage, water delivery systems that are open to the atmosphere are subject to losses due to evaporation. Canals through arid landscapes are particularly at risk for such loss. Researchers estimate that covering California’s almost 4000-mi (6350-km) irrigation canal system could save 63 billion gallons of water annually (>238 million cubic meters). They suggest covering the canals with solar panels.

Flood irrigation systems, which water entire fields or furrows throughout fields (for example, for rice paddies, but also alfalfa, tomatoes, and other crops) and sprinkler irrigation systems are another significant source of water loss. Flood irrigation loses water to evaporation, run-off, and percolation too deep for crops to reach. Sprinklers can lose water in all these ways, plus drift of water spray off the intended plants. One set of estimates for well-maintained flood and spray irrigation systems estimated that the proportion of water reaching the appropriate root zone varied between 40% and 90%, depending on the method. Drip irrigation methods put >90% of water into the intended root zone.

Climate change and water availability 

The Intergovernmental Panel on Climate Change (IPCC)

The IPCC is the primary international science body responsible for reporting on climate-change issues. Three working groups underpin the reporting efforts. Working Group I focuses on bringing together the physical science related to climate change, studying aspects of climate, atmosphere, hydrology, oceanography, etc. Working Group II studies impacts of, vulnerability to, and adaptation to climate change, for humans and for the natural world. All aspects of ecology, human health, social sciences, and economics are addressed here, to understand the best changes to minimize harm to humans and the natural world and to increase adaptation to climate-change impacts. Working Group III studies ways to reduce levels of greenhouse gases (GHG) in order to slow and eventually reduce climate change – referred to as mitigating climate change. In the specialized focus of climate change, the term “mitigation” refers only to reductions in greenhouse gases. Any other kinds of mitigation, such as mitigation of harms from various aspects of climate change, require a longer phrase, for clarity – “mitigation of heat impacts,” for example. Because GHGs are produced and absorbed naturally and are also produced by a wide range of human undertakings, WGIII, like WGII, comprises both natural and social scientists. However, the WGIII focus is much tighter, on reducing production and concentration of GHGs.

Every 5-7 years, the working groups produce an assessment report (AR) summarizing new science since the last report. AR6 was produced during 2021-2023. Each working group produces a report, and after those reports are available, a synthesis report is produced that integrates the findings of the three working group reports as well as any special reports that have been produced since the last assessment report. IPCC reports are used throughout the world to understand how climate change will affect the world and what opportunities are available to adapt to it and to reduce it.

Climate-change impacts on water availability

Under climate change, although precipitation will increase as temperatures warm, due to increased evaporation from the oceans and other bodies of water, less precipitation will fall as snow, and evaporation from the land will increase, leading to a loss of groundwater, soil moisture, and streamflow, and decreased water levels in lakes and reservoirs. Carbon dioxide will permit some increase in plant growth and water efficiency, but reductions in soil moisture are expected to offset these potential gains in many places.

Precipitation forecasts are much less certain than temperature forecasts due to the greater complexity of the hydrological cycle – precipitation, evaporation, evapotranspiration, oceans, clouds, rain-vs-snow issues, etc. The IPCC long-term predictions (for the end of the 21st century) suggest reductions in precipitation in winter (December, January, February – DJF – in the Northern Hemisphere; June, July, August – JJA – in the Southern Hemisphere) will be strongest in the American Southwest, around the Mediterranean and in northern Africa, in parts of the Amazon Basin, in southern Africa, and in parts of Oceania (Fig 5). Reductions in summer precipitation will hit hardest in western Europe and the Mediterranean, the Caribbean, along the southeast coasts of Africa and South America. The Mediterranean thus has reductions in both seasons.

Figure 5. IPCC projections of precipitation changes for two seasons under four different scenarios ranging from modest increases in GHG (SSP1-2.6) or more severe increases (SSP3-7.0) for the end of the 21st century. Most of the figures are covered in hatching, indicating that the findings are not statistically significant, but darker colors, indicating greater proportional change, are not hatched, indicating they rise to the level of statistical significance. Only slight areas of crosshatching occur in this figure, in the right-side graphics.

The IPCC figure caption is rather technical, but provides numerical information about the level of uncertainty of the results. Here is the full caption from the WGI report of AR 6, 2021.

Figure 4.24 | Long-term change of seasonal mean precipitation. Displayed are projected spatial patterns of multi-model mean change (%) in (top) December –January–February (DJF) and (bottom) June–July–August (JJA) mean precipitation in 2081–2100 relative to 1995–2014, for (left) SSP1‑2.6 and (right) SSP3‑7.0. The number of models used is indicated in the top right of the maps. No map overlay indicates regions where the change is robust and likely emerges from internal variability, that is, where at least 66% of the models show a change greater than the internal-variability threshold (Section 4.2.6) and at least 80% of the models agree on the sign of change. Diagonal lines indicate regions with no change or no robust significant change, where fewer than 66% of the models show change greater than the internal-variability threshold. Crossed lines indicate areas of conflicting signals where at least 66% of the models show change greater than the internal-variability threshold but fewer than 80% of all models agree on the sign of change. Further details on data sources and processing are available in the chapter data table (Table 4.SM.1).

To see the impacts of changing hydrologic regimes on agriculture, we can look at the level of water in the atmosphere and in the soil – the places that control how much dryness plant stems and leaves experience and how much water is available to roots. Water-pressure deficit is a variation of the relative humidity measurements seen in local weather forecasts and is the preferred measure of atmospheric wetness and dryness. Atmospheric wetness affects how much water plants lose through evapotranspiration (think of what your nose feels like when the humidity is low) and how much water is lost from the soil surface due to evaporation.

Soil moisture is more straightforward, but can be measured in the upper levels of the soil, where most evaporation occurs, and where most agricultural crops have their roots, or throughout the entire water column. On average across a variety of temperate-zone crops, 50% of crop roots are in the first 15 cm of soil. In Figure 6, below, the measure of water in the top 10 cm (4 in) of soil best reflects vulnerability to evaporation of recent precipitation due to dryness, whereas total-column moisture reflects moisture over a longer term, including from previous seasons; it includes the full range of depths from which plant roots might draw moisture but, in deeper soils, also includes depths that most plant roots don’t reach.

Under more severe climate change estimates (SSP5-8.5, the lower line of graphics), all of earth’s land surfaces are projected to experience increasing vapor-pressure deficits – more atmospheric drying (Figure 6). Upper-soil moisture losses will be significant in many places, with significant increases in some presently dry portions of Africa and parts of central China. Soil moisture throughout the soil column shows similar patterns to upper soil moisture, but with reduced strength.

Figure 6. IPCC projections of change (as a %) in vapor-pressure deficit and soil moisture for mild, medium, and severe climate-change scenarios (SSP1-2.6, SSP2-4.5, and SSP5-8.5) for the end of the 21st century. As in Figure 5, hatching indicates findings that are not statistically significant. No areas of crosshatching occur in this figure.

The full IPCC caption for this figure follows.

Figure 8.19 | Projected long-term relative changes in annual mean soil moisture and vapour pressure deficit. Global maps of projected relative changes (%) in annual mean vapor pressure deficit (left), surface soil moisture (top 10cm, middle) and total column soil moisture (right) from available CMIP6 models (number provided at the top right of each panel) for the SSP1.2-6 (a, b, c), SSP2-4.5 (d, e, f) and SSP5-8.5 (g, h, i) scenarios respectively. All changes are estimated for 2081–2100 relative to a 1995–2014 base period. Uncertainty is represented using the simple approach. No overlay indicates regions with high model agreement (‘Robust change’), where ≥80% of models agree on sign of change, diagonal lines indicate regions with low model agreement, where <80% of models agree on sign of change. For more information on the simple approach, please refer to the Cross-Chapter Box Atlas.1. Further details on data sources and processing are available in the chapter data table (Table 8.SM.1).

Climatologists measure drought of three kinds: meteorological, hydrological and agricultural/ecological. Meteorological drought is measured against regional expectations for precipitation at daily, weekly, monthly, or annual levels. Hydrological drought is measured by the condition of primary water sources – surface and ground water. Because primary water sources get their water in part from runoff that may come from snowmelt or percolation through the soil, hydrological drought may lag behind meteorological drought. Agricultural and ecological drought is measured through conditions for plants, especially soil moisture and evapotranspiration (water loss through plant respiration). Environmental scientists also define environmental drought, which combines not only the water situation, but also the ripple effects from declining water availability – tree mortality, fire, habitat loss, erosion, water quality, etc. In a similar fashion, social scientists look at the socioeconomic impacts of drought including results of crop failure, livestock mortality, and reduced water supply to human undertakings including hydropower.   

The IPCC AR6 projections show at least medium confidence in an increase of agricultural/ecological drought at or above 2°C of warming in the US West and Midwest, across most of Mexico and Central America, through most of northern South America, Chile, and southern Argentina, through Europe, the Mediterranean, and southern Africa, and in southern and southeastern Australia. At that level of warming, drought intensity is predicted to increase by more than 50% (with 90% of estimates ranging from approximately 20-230%) and drought frequency is predicted to increase between 2-fold and 3-fold (with 90% of estimates ranging from approximately 1.5-6-fold) (From Figure 11.18 of the WGI1 AR6 report). The IPCC scenario that best matches the current trend in GHG – SSP5-8.5 – predicts warming of 2°C by around 2040, so these increases in drought may not be far in the future.

Approaches to improving sustainability in water availability

The current and future water shortages described above require multiple approaches to extending the fresh water supply to improve sustainability. A variety of solutions are in use.

Reservoirs that form behind dams in rivers can collect water during wet times and store it for use during dry spells. They also can be used for urban water supplies and to support irrigation. Other benefits of dams and reservoirs are hydroelectricity, flood control, and recreation. However, reservoirs permanently flood land, displacing the original inhabitants, whether these are humans and related agriculture and industry or natural systems. Reservoirs lose water to evaporation, particularly in arid climates; downstream river channels experience increased erosion, and the original river ecosystems are transformed into lake habitats, while the dams interfere with migration and spawning of fish. Fisheries impacts from dams reduce food availability and can also significant economic losses. Cambodia’s Tonle Sap fishery has been sharply reduced by dams on the Mekong by China and downstream countries. Salmon fisheries in the US and Canada have been strongly reduced by dam construction, impacting both commercial and indigenous fisheries.

Dams on rivers that cross international boundaries can create conflict among nations, as upstream nations with dams control the amount and timing of water releases to downstream nations. For example, in 2019, operation of 11 dams on the Mekong within Chinese borders caused the river to run dry for downstream nations during a severe drought.

Aqueducts can move water from where it is plentiful to where it is needed. Like dams, aqueducts can be controversial and politically difficult, especially if the water transfer distances are large. One drawback is that water diversion can cause drought in the area from where the water is drawn. For example, Owens Lake and Mono Lake in central California began disappearing after their river flow was diverted to the Los Angeles aqueduct. Owens Lake remains almost completely dry, but Mono Lake has recovered more significantly due to legal intervention. Water diversion can also move contaminants and invasive species to new areas. Evaporation during diversion in open canals, which occurs in many parts of the world, increases salinity and other contamination of diverted water.

Desalination, which involves removing salts from seawater or saline groundwater, can create additional fresh water. It involves considerable energy and moderate to high expense. Solar desalination has been demonstrated to be feasible, but is used for less than 1% of desalination. Desalination is most common in the Middle East, where aridity, oil, and salt water all occur in the same region. Desalination also creates concentrated salt water as a waste product that is potentially hazardous to local water resources.

Rainwater harvesting involves catching and storing rainwater for reuse before it reaches the ground. It is mostly used for home water use. In the US, rainwater harvesting is illegal in some states, particularly drier western states, because it diminishes the water that reaches streams, and legal water rights are tied to surface water. Fog harvesting is used in a few parts of the world in which fog is somewhat predictable, but like rainwater harvesting, it does not scale up to levels suitable for entire cities or larger regions.

Conservation encompasses using less water and using it more efficiently. Drip irrigation is one of the most widely practiced forms of water conservation. As we saw in water use statistics, use of water by industry and energy production has become more conservative over time, reducing overall water use. Xeriscaping – the use of dry-adapted plants in arid areas – can significantly reduce water used in landscaping. In the home, high-efficiency appliances and an informed approach to water use can also reduce water use significantly.

The majority of the world’s water use is for agriculture, and precision irrigation, drip irrigation, and similar methods are available to reduce water use, but are not widely adopted. Variability in precipitation due to climate change may lead to more irrigation due to potential failure of rainfed agriculture. Increasing irrigation then further stresses area water resources. Such an increase in irrigation is occurring in corn-and-soybean-growing regions of the US.

In developing countries, water infrastructure is often old and inefficient. Much of the water infrastructure of the arid Central Asian nations still dates to Soviet times. Conservation efforts cannot proceed under such conditions.

Given the loss of water from aquifers and the growing uncertainty of water levels in reservoirs and streams, most experts agree that fully sustainable water use will require recycling, or reclaiming, water.

Visit this site from the Colorado School of Mines to understand the process of recycling municipal water. The mobile system shown here is best for emergencies. Permanent versions of this recycling process are in place in a growing number of cities, worldwide, including cities in the US, Namibia, Australia, India, Spain, and Singapore.

Municipal water reclamation is sometimes referred to as toilet-to-tap water use, which does not reduce the “ick” factor but does at least provide transparency. Some industrial water users already recycle at least a part of their water in order to reduce costs and uncertainty.

Not all reused water needs to be treated to the level of drinking water standards. Water professionals distinguish two levels of recyclable water: black water – water from toilets or septic systems, and gray water – domestic water from other sources such as showers, laundry, or sinks. If the water is to be returned to municipal use, it must be cleaned to drinking water standards. But under carefully controlled conditions, gray water can be used for irrigation, watering gardens, parks, and golf courses, or even for “rewatering” short stretches of streams that have been dewatered by overpumping of groundwater and/or diversion of upstream water. In Arizona, over 20 miles (32 km) of the Santa Cruz river, dry since 1940, now flow, through use of treated gray water.

Reclaimed water can also be used for artificial recharge of aquifers, allowing water managers to use the depleted aquifer as a convenient storage area – evaporation is essentially eliminated. Both treated wastewater and transported water (e.g., from an aqueduct) can be used. However, an aquifer is not an inert container. Aquifers have their own chemical properties, depending on the materials (rock, sediment, residual water) in the aquifer. In addition, water-delivery materials have their own chemical properties, and some still include lead pipes or lead-soldered pipes. Interactions between water chemistry, aquifer properties, and water infrastructure can result in changes in water quality and so much be carefully considered and tested. A change in the river used to supply water to Flint, Michigan in the US mobilized lead from the water infrastructure into the drinking water system, raising blood-lead levels in the city’s children, and creating a major scandal.

Progress towards sustainability in water availability

The issue of water availability appears among the UN Sustainable Development Goals as Goal 6: Clean Water and Sanitation. In their description of progress on the goal, the UN reports these numbers:

From 2015 to 2024, the population using safely managed drinking water, safely managed sanitation and basic hygiene services increased from 68 to 74 per cent, from 48 to 58 per cent and from 66 to 80 per cent, respectively. However, in 2024, 2.1 billion people were without safely managed drinking water, 3.4 billion without safely managed sanitation and 1.7 billion without basic hygiene services. In schools around the world in 2023, 447 million children lacked a basic drinking water service, 427 million lacked a basic sanitation service, and 646 million lacked a basic hygiene service.

Estimates based on data for 129 countries covering 89 per cent of the world’s population suggest that the proportion of domestic wastewater that is safely treated was 56 per cent in 2022 (no change since 2020).

From 2015 to 2022, global water use efficiency improved from $17.5/m3 to$21.5/m3, a 23 per cent increase. However, 57 per cent of countries still face challenges, with low efficiency of below $20/m3. Globally, water stress showed little change from 2015 to 2022. Water stress varies significantly across regions, with Northern Africa and Western Asia as well as Southern and Central Asia facing extreme scarcity.

We have many tools to use to improve sustainability of water availability, but the capacity to employ those tools, both in terms of funding and in terms of knowledge, is not sufficient to achieve sustainability. Progress is being made, but not at the pace anticipated when the Sustainable Development Goals were drafted.

Knowledge Check

Take a moment to complete the short quiz below to assess your understanding of this section. Read each question carefully and refer to the section content as needed. This quiz is not graded – it’s simply an opportunity for you to reflect on what you’ve learned and reinforce key concepts.

Read the following article about how climate change and natural disasters affect drinking water around the world: click here to read.

Kinds of waste

Waste is often categorized by its source, its makeup,  or its destination, and then may be broken down further. The US Resource Conservation and Recovery Act (RCRA defines solid waste as “any waste or  sludge from a wastewater treatment plant, water supply treatment plant, or air pollution control facility and other discarded material, including solid, liquid, semi-solid, or contained gaseous material resulting from industrial, commercial, mining, and agricultural operations, and from community activities. In general, solid waste can be categorized as either non-hazardous waste or hazardous waste.” This is a trifle confusing, as it includes things that are clearly not solid, and separates out some sources very specifically while grouping other sources very broadly. In the US, waste production is tracked by the EPA, but irregularly; In 2025, no major reports were available for conditions after 2018.

Non-hazardous wastes

Municipal waste includes non-hazardous waste generated by households and offices – what we often call “trash” or “garbage.” In the US, food and paper waste comprise almost half of all wastes   (Fig 1). In developing countries, waste related to packaging comprises a lower proportion of waste, resulting in a larger proportion of food-related waste (Fig 2). Medical wastes that are not hazardous are typically treated as part of municipal wastes. Hazardous waste from the municipal waste stream includes many categories of material from batteries of all kinds to radioactive medical waste.

Figure 1. Breakdown of municipal waste in the US in 2018. Total volume was 292.4 million tons (265 metric tons). US EPA. Public domain.

Figure 2. Global average and regional breakdown of municipal solid waste composition. United Nations Environment Programme and International Solid Waste Association. Blanket permission for educational purposes.

Agricultural wastes include residue from crop processing and food processing such as residues from brewing and distilling of alcoholic beverages, juice production, grain processing, animal wastes and carcasses, packaging from feed and fertilizers, etc. Crop residues left in the fields are not categorized as agricultural waste. Pesticide containers and left-over pesticides are classified as hazardous wastes.

Industrial wastes vary considerably by industry and within industries. Construction wastes may include a large component of organic material from wood products or no wood waste at all, for example. Wastes may be largely unprocessed such as rock and mined water from mining operations, or may be heavily processed, such as paints, solvents, and other industrial chemicals. Many industrial wastes are classified as hazardous wastes.

Because they are classified as non-hazardous materials, many municipal, agricultural, and industrial waste components have potential for recycling and reuse.

Hazardous wastes

In the US, hazardous wastes are defined by the US EPA as having (at least) one of four characteristics.

Ignitable: a material that can ignite on its own, without any separate ignition sources such as a spark or flame. The material may combust as a result of heat, chemical changes, changes in moisture, friction, etc. (Combustible materials require an outside ignition source in order to burn).

Corrosive: corrosive materials can destroy other materials on contact. In particular, they can corrode containers such as metal containers, which makes them difficult to contain. Corrosives are commonly strong acids with pH <2 or strong bases with pH > 12.5 .

Reactive: materials that combust, explode, or emit harmful vapors or fumes under specific conditions. They may react to heat, compression, or the addition of water.

Toxic: materials that can cause harm or kill when ingested, absorbed through the skin, or inhaled.

In international law, hazardous wastes are largely defined in documents related to the Basel Convention on the Control of Transboundary Movements of Hazardous Waste. In addition to the categories defined by the US EPA, the United Nations adds radioactive, infectious, and mutagenic (causing cancer and birth defects) wastes. In the US, radioactive wastes are regulated at the federal level, but separately from hazardous wastes. Infectious wastes are regulated at the state level. Mutagenic wastes are regulated through disposal requirements in the Resource Conservation and Recovery Act (RCRA) and the Toxic Substances Control Act (TSCA).

Exclusions to solid waste and hazardous wastes

In the US, a wide variety of types of waste are excluded from the definitions of solid waste and hazardous waste. Some exclusions are to avoid redundant regulation. For example, the EPA does not address radioactive waste as a hazardous waste because it is regulated by the Atomic Energy Act, rather than by RCRA or TSCA. Similarly, domestic sewage is regulated under the Clean Water Act. Some substances are unregulated when they are emitted in small amounts by municipal users, but are regulated when emitted by industry. These exceptions recognize both the difficulty of regulating a large number of users and the practicality of not regulating users that, individually, cause little harm. Recycling streams are often excluded, to encourage recycling. Finally, in some cases, evidence is not yet available to determine the level of harm associated with a given substance.

Knowledge Check

Take a moment to complete the short quiz below to assess your understanding of this section. Read each question carefully and refer to the section content as needed. This quiz is not graded – it’s simply an opportunity for you to reflect on what you’ve learned and reinforce key concepts.

Among the UN Sustainable Development Goals, issues of waste are covered best by the goal for responsible consumption and production. But the impacts associated with waste are relevant to several other goals, including good health and well-being and sustainable cities and communities. Waste is not directly identified in any of the planetary boundaries, but plastics – synthetic materials – contribute importantly to waste and are considered a novel entity, which is a planetary boundary at high risk. A number of hazardous wastes are or contain synthetic chemicals and so are also novel entities.

Trends in waste production

During 1960-2018, the period for which data are available, US municipal solid waste increased more than three-fold (Fig 1). Some of this increase (from 2017-2018) was mainly due to a change in the way food waste was measured.

Figure 1. Total municipal solid waste and per capita waste from 1960-2018. US EPA. Public domain.

Globally, solid waste generation varies considerably among regions, both in total volume generated and in per capita generation (Fig 2). Wealthier countries tend to produce more waste per person. Overall, predicted waste generation continues to increase through 2050 and to the end of the 21st century.

Figure 2. Total observed and projected waste generation and waste generation per capita by world region. Values encompass residential, commercial, and institutional waste. World Bank. CC BY 3.0 IGO.

Environmental impacts of waste

Life-cycle analysis or assessment is an approach to understanding the impacts of a product throughout its existence – the phrase “cradle-to-grave” is often used. Waste impacts and impacts related to processing can occur in the acquisition of raw materials, production, use, and disposal of a product. For consumers to understand the true impacts of their purchases, they need information not only on disposability and recycling options, but on the full suite of impacts that occur during the life of a product.

Waste that is not disposed of – what we would call “litter” or “leaks” – is a form of pollution. Depending on the nature of the waste, it may pollute air, soil, and/or water. It can harm people and wildlife, and can interfere with the proper functioning of ecosystems. If it fouls waterways, it may also result in economic impacts to businesses and utilities and reduce water availability.

Uncontrolled dumping and burning are usually associated with the greatest environmental harms from waste. Open dumps produce unpleasant odors and attract pests. As hazardous materials leach out or are dissolved out by rain, the leachate percolates through the soil, polluting soil, groundwater and surface water. Trash can be blown out or carried out by scavengers, spreading toxins and infectious material over the landscape. Open burning in dumps creates greenhouse gases and other air pollution, creating new toxins by burning plastics and other synthetic materials that can carry far from the burn site. Children and others seeking useful items in the dump suffer from physical injury and exposure to air-borne and water-borne toxins including carcinogens and other harmful materials.

In more controlled settings, even well-constructed landfills may leak, especially with age, causing soil and water pollution; if they are not well constructed, then the extent of soil and water pollution will obviously be worse. All landfills are sources of methane from the decomposition of organic material. Methane is a potent greenhouse gas, which may be captured for energy or heat, or may be released to the atmosphere. In the US, in 2022, landfills were the third largest contributor of methane emissions. Landfills take up valuable space, although, if well managed after they close, the space may become available again, for some uses.

Incinerators produce substantial greenhouse-gas emissions even when well-regulated. If they are used for energy and heating, some of the emissions may be offset, but even so, they are significant contributors to global warming. Incineration is sometimes listed as a “green” waste management approach, but its global warming impacts are close to those for fossil-fuel plants, even if energy recovery is in place.

Without careful monitoring and maintenance, pollution beyond GHGs can be released. Incinerator fly ash – the solid waste that is captured from the flue emissions of the incinerator – is hazardous waste that can cause environmental harm on its own, if not properly treated and disposed of. With best-available technology, most harmful particulates and non-GHG gases can be captured – in fact, incineration is a preferred technique for disposing of many hazardous wastes.

Environmental impacts and regulation of international (transboundary) movement of hazardous waste

Concern about transboundary movement of wastes dates back to the 1980s when perhaps 10% of hazardous wastes were estimated to be shipped internationally, often from developed countries with stricter environmental controls to less-developed countries with looser environmental regulations, such as countries of Eastern Europe, Africa, and Asia. Less regulated disposal of hazardous wastes, particularly dumping in open dumps or in streams or open-air burning leads to contamination of air, soil, and water, with attendant impacts to human health and ecosystems.

Even legal transboundary movements, accompanied by payments to the receiving countries, can lead to environmental impacts if the receiving countries lack the capacity for proper disposal. Illegal movements are even more likely to lead to environmental harm. Because nations and industries may seek to reduce costs for disposal of dangerous wastes, and because some wastes include valuable, recoverable substances, so-called waste trafficking is estimated to involve billions of dollars of illegal trade.

In response to concerns about international movement of wastes, 187 nations met and developed the Basel Convention on the Control of Transboundary Movements of Hazardous Wastes and Their Disposal, which was adopted in 1989 and came into force in 1992. The Convention requires that movements of hazardous wastes occur only where they are permitted by relevant national laws, only when appropriate disposal facilities are available to process the waste, and only when prior informed, written consent has been obtained for the transfer. The need for appropriate facilities and prior, informed consent set standards that allow for easier policing of waste shipments.

The Convention includes the establishment of training centers to improve national capacity for appropriate disposal of hazardous wastes, as well as support for nations entering into regional and bilateral agreements for transboundary movement of waste. Amendments have been added over time, including agreements to require that movement of mixed, contaminated or hard-to-recycle plastics meet the same standards as previously defined hazardous wastes (passed in 2019 and effective in 2021).

Waste management

Figure 3. Methods of waste management from most preferred to least preferred. Note that in the open burning and dumping, the very least preferred disposal method, is not included here, because it is illegal in the US. US EPA. Public domain.

There is wide agreement about the preferred options for waste management (Fig 3). It is easiest to manage waste that is not produced, so reducing waste generation through reduced consumption and increased reuse is the most preferred option. Frequently mentioned methods to reduce waste include reducing use of “single-use” plastics including much packaging material, avoiding “fast fashion” clothing in favor of longer-lasting items, and working to reduce food waste. We have seen in Figure 2 that, however much reduction and reuse may occur in the near future, it is not expected to halt the increase in waste generation.

Recovery of materials or nutrients through recycling and composting is the second most preferred option, followed by energy recovery through incineration combined with heat or electricity generation and gas generation through anaerobic digestion (usually in landfills) with recovery of methane. Regulated landfills and incineration without energy capture would be less preferred. Finally, open dumping and open-air burning, such as still occur in many developing nations, are the most harmful and least preferred methods of disposal.

The Environmental Performance Index, housed in Yale’s Center for Environmental Law and Policy in the US creates an annual product that includes a Waste Management subcategory (Fig 4). The Waste Management index is computed for each nation based on volume of municipal waste generated per person (40%), recovery of energy and materials from waste (40%), and proportion of waste that treated through recycling, regulated landfill, and regulated incineration (20%). Although few countries are in the top performance category, few are in the bottom category, as well.

Figure 4. National scores on the the 2024 Environmental Performance Index subindex for solid waste management. K. Buchholz, Statista. CC BY ND 3.0.

The maps of Figure 5 show the anticipated change in per capita solid waste generation between 2015 and 2050 – positive for almost all countries. In addition, the left-hand bars show types of waste and the right-hand bars waste treatment, for 7 countries around the world, for both years. We see high variability in reliance on uncontrolled dumps for waste management in 2015; this approach diminishes in 2050 as a proportion of waste management, although use of landfills tends to grow more than use of recycling and composting. The palette of waste management approaches is most stable over time in the US and in Europe. The anticipated proportion of organic material (from food waste and other “green” sources such as yard waste, site clearing, and landscaping) drops over time, with other wastes increasing, particularly paper. Plastics increase as a proportion of municipal waste in all the countries shown.

Figure 5. Global waste generation and treatment, 2015 and 2050. Chen et al. 2020. The world’s growing municipal solid waste: trends and impacts. Environmental Research Letters 15. CC BY 4.0.

Different kinds of waste are predicted to increase in quantity differently over the 21st century (Fig 6), and the methods of treating them are predicted to change over time. Production of organic wastes, which usually include a large component of food waste, is predicted to flatten out by the end of the century, as population flattens and food demand plateaus. The share of organic waste going to dumps declines, and incineration and composting increase somewhat. Paper, plastic, glass, and metal wastes are all predicted to increase throughout the 21st century, with increasing amounts of recycling. However, recycling is not predicted to become the primary means of treatment for any of these wastes, and only clearly rises above 30% for metal. Overall, landfills remain the primary means of treatment throughout the century.

Figure 6. Global yearly waste production by type, 1970-2100. Chen et al. 2020. The world’s growing municipal solid waste: trends and impacts. Environmental Research Letters 15. CC BY 4.0.

Note that Chen et al. (2020) did not quantify waste treated by open burning.

Plastic waste

Plastic wastes are of particular concern in the waste stream because they do not decompose, they produce toxic chemicals when burned, they leach toxic chemicals when they are repeatedly wetted in uncontrolled dumps. As they degrade over time, they produce microplastics, which are everywhere in the environment and in all kinds of organisms – able to cross the placental barrier, the blood-brain barrier, and enter circulatory systems through gas-exchange systems (e.g., in the lungs, in humans) and through food intake (e.g., in plankton). Discarded plastic items clog waterways, are plowed into fields, harm wildlife, and eventually degrade to microplastics that cause additional harm. UN member states agreed, in 2022, to create an international agreement to reduce plastic waste. However, efforts to do so have failed. The most recent attempt, in the summer of 2025 failed due to the insistence of nations with large fossil-fuel and plastics industries (fossil fuels are feedstocks for plastic production) that there be no specific goals to reduce production of plastics.

For information on the oceanic “garbage patches” composed primarily of plastic pollution, but probably not the plastics you thought, visit [click] here.

For the United Nations’ Environment Programme site for plastic pollution, visit [click] here.

Plastic recycling is covered in section 5.4.

E-waste

E-wastes are electronic wastes – cell phones, monitors, computers, televisions, refrigerators, etc. The UN’s 2024 Global E-waste Monitor reported that in 2022, 62 million tonnes of e-waste was produced – the equivalent of 7.8 kg (about 17 pounds) per person for the world – and predicted that figure would rise to 82 million tonnes by 2030.

E-wastes are difficult wastes because they are mixtures of dangerous and valuable components, and they are seldom constructed to allow easy separation or recycling of those components. In the past, the developed world tended to dump e-wastes on the less-developed world, where low labor costs often made recovery of valuable components worthwhile. However, as the potential harm from uncontrolled dumping of e-wastes has become more apparent, many nations have closed their borders to e-waste. E-wastes often include toxic components such as lead and mercury, and also often include plastics. Leaching of these materials in open dumps pollutes soil, groundwater and surface water. Open burning of e-wastes creates toxins from the plastics and other components and releases lead and mercury into the atmosphere. Informal processing of e-waste with acids to recover valuable components such as gold results in soil and water pollution. Where informal recovery of valuable components is conducted in the developing world, child labor is often involved. However, so little recycling of any kind is undertaken that the Global E-waste Monitor report indicates that recycling meets only 1% of the demand for critical minerals, leading to ongoing mining and its attendant environmental harms. The report estimates that unrecovered resources in e-waste are worth US$ 62 billion.

For the World Health Organization site on e-wastes and other links, visit [click] here.

Landfills

Read here to understand how US regulated landfills are constructed and operated to minimize environmental impacts. Landfills receive many kinds of waste, including hazardous waste, and appropriate monitoring and maintenance are needed to ensure their safe use.

Incineration

Watch here to learn about the pros and cons of incineration and its place in waste management, overall. Like landfills, incinerators receive many kinds of waste, including hazardous waste, and appropriate monitoring and maintenance are needed to ensure their safe use.

Composting

Read here to understand the pros and cons of and alternatives to using composting to deal with food waste

Recycling

Recycling is discussed in section 5.4.

Knowledge Check

Take a moment to complete the short quiz below to assess your understanding of this section. Read each question carefully and refer to the section content as needed. This quiz is not graded – it’s simply an opportunity for you to reflect on what you’ve learned and reinforce key concepts.

Brownfields are a case of land as waste. They occur where pollution has made land unusable until hazardous materials are removed. In the following video, Dr. Jeff Adams introduces the field of brownfield management and remediation.

Recycling requirements and limitations

Recycling, regardless of the material involved, has some basic requirements to succeed. The public and/or industries must be willing to contribute appropriate materials to the recycling process, which may involve some cleaning and sorting. A system must exist for collecting recyclable waste and conveying it to recycling facilities. Economically reasonable methods of recycling must exist. Finally, a market must exist for the recycled materials.

A significant barrier to recycling of some materials is that creation of new materials (fresh wood, newly mined and smelted metals, freshly produced glass, etc.) may be cheaper than production of recycled materials, particularly if externalities are permitted to occur in the production of new materials, for example through unsustainable timber and mining activities. Recycling often involves chemicals, equipment, and energy – it can be expensive in multiple ways. Just sorting out the components of a refrigerator, a truck, a railway engine, or a cellphone may require considerable time and labor, or sophisticated machinery. Energy requirements for recycling may be high, potentially leading to high greenhouse gas production, and where energy requirements are high, water requirements are also often high.

Contamination is also an issue in all recycling streams. Pizza cheese on cardboard, or oils on metal or glass can degrade an entire load of recyclable material, reducing its value. Sorting to remove contaminated materials increases the cost of the process. The “feel-good” aspect of recycling can lead to what has been called “aspirational recycling,” which increases contamination and can significantly reduce the economic viability of the recycling industry. You can read more [click] here.

The present limits of recycling are apparent in the record of US recycling of municipal waste (Fig 1). Paper dominates throughout, comprising more than 60% of the recycling tonnage in 2018, despite comprising less than 25% of total municipal waste (Fig 2). The second largest recycled material, metals, reaches less than a quarter of paper’s tonnage, in 2018. Overall, the total recycling was on the order of 70 million tons in 2018, but total municipal waste production approached 300 million tons in that year.

Figure 1. Composition of US recycling tonnage from municipal waste, 1960-2018. US EPA. Public domain.

Figure 2. US municipal waste generation, 1960-2018. US EPA. Public domain.

Recycling specifics

Paper

Paper users and the paper industry have improved sustainability of the industry even before recycling, with reduction in use. Some paper products are lighter than in the past. Newspapers have reduced page size and margins, to use less paper. Not all reductions in use improve sustainability: in some cases, paper use has been reduced by strengthening the paper with coatings of wax or plastic. Although this reduces paper use, the resulting product is typically not recyclable. Shredded paper tends to tangle in machinery used in paper recycling; as a result, many facilities will not accept shredded paper.

Watch a quick, simple [click here to watch the] video on the paper recycling process. Breaking paper down into pulp often uses fairly strong, caustic chemicals; de-inking and brightening use solvents and bleaching agents.

Each successive recycling shortens the fibers in the paper, and this reduces the kinds of uses for which the recycled product is suitable. As a result, the market for the recycled product is reduced.

Despite drawbacks in paper recycling, this is the waste stream with the highest rate of recycling among municipal wastes.

Metal

Worldwide, mining of metal ores increased almost four-fold from 1970 to 2022. About 70% of this activity is related to international supply chains. Mining may pollute air, water, and soil, cause habitat loss and reduce biodiversity, and consume also needed for other uses. Recycling of metals is an obvious way to limit these harms.

Whereas the paper stream comprises, well, paper (also, often, paperboard, but not cardboard), the metal stream comprises many different metals, with different recycling processes and markets. Metals are infinitely recyclable; they do not degrade in the recycling process. Different metals have widely varying recycling rates; here, we look at some of the most recycled metals.

Steel is the most recycled material by weight, overall, in the world. Steel is about 97% iron, with much smaller amounts of carbon and other additives, including other metals. In the US, a 2021 report from an industry trade association estimated that approximately 70% of steel was recycled. A 2019 (approximately) report indicates over 90% of end-of-life steel products are recycled in the EU. Globally, scrap accounted for about 30% of the metals used to make steel in 2022.

Smelting new steel still uses coal for production, resulting in high greenhouse gas emissions. Recycling uses only about 28% of the energy of smelting and saves 1.67 tonnes of CO₂ per ton of steel. Use of scrap steel needs to rise by perhaps 50% to meet climate-change targets. But recycled steel is a potentially limiting resource, due to the lifespan of steel products. Scrap steel is a subject of international trade as well as trade barriers, resulting in an uneven availability of the metal for recycling.

You can watch here to see how scrap metal is recovered from items such as car bodies and sorted to produce useful recycling streams.

Aluminum is the second most recycled metal, by weight, in the US.  Aluminum is largely used as alloys with small amounts of other metals. It is used in transportation, construction and other industries, packaging, household items, and in electrical lines. Recycling information for aluminum is uneven among sectors, with good reporting on recycling of aluminum cans but indifferent reporting elsewhere. For 2018, the US EPA estimated overall aluminum recycling at 35%, with aluminum cans at 50%. For the same year, the EU recovered 90% of aluminum from construction and transportation (65% of aluminum by use).

A 2022 analysis of the slower US recycling rates pinpointed issues with aluminum-can recycling involving consumers unwilling or unable to separate waste for recycling (accounting for 80% of aluminum lost to recycling), municipal recycling facilities that landfill the can they collect to save money, and problems with older sorting facilities that misidentify cans. With larger scrap, scrap yards tend to prioritize higher-value components, so that aluminum is often not well separated from the rest of the waste. The mix of low-value material is exported to developing nations where low-cost labor is available to sort the material, rather than being recycled domestically in the US.  The EU uses a deposit on cans, which creates an incentive to recycle. Incentives could also motivate scrap yards to do a more thorough sorting of waste so that higher-quality aluminum scrap would become available.

Emissions from aluminum scrap are 92% lower than emissions from smelting aluminum from bauxite ore. Globally, production using recycled aluminum is twice the volume of production of aluminum from bauxite. It’s not surprising, then, that demand for scrap is expected to increase by 50% by 2050 in the EU over demand in 2019.

Lead is used most commonly in car batteries (approximately 86% of lead used in the US is in car batteries); in the US these are considered hazardous waste, not only for the heavy metal toxicity of the lead, but also because of the corrosive nature of the acid. As a result of the care required to dispose of car batteries, lead recycling rates are quite high and new car batteries are largely made of recycled material. Overall, 76% of US lead is recycled.

However, despite the high recycling rate, aspects of lead recycling are less sustainable. Because of the market for recycled lead, and the low melting point of the metal, informal, and often illegal recycling of lead is common, for example, in China. Because of lead’s toxicity, poor recycling practices create public health risks.

Copper, globally, is used most in electricity generation and distribution, as well as in construction, appliances, and transportation. Between 2009 and 2018, 32% of the copper used came from recycled sources, worldwide. In the EU, which has low copper reserves, 44% of copper comes from recycled metal, and 70% of end-of-life copper is recycled.

Because copper stays in service for long periods, lots of the already-smelted copper is bound up in use and is not available for recycling, driving demand for new metal. In addition, copper is often used in complex electronics applications that do not lend themselves well to material recovery. However, copper ores are declining in quality, and new mines are only opening slowly, increasing the pressure to improve recycling rates.

As with lead, informal recovery of copper, particularly from copper cable used in electrical cables or in electronics causes health hazards. Open burning is a common approach to removing the plastic insulation on such cables, releasing dioxins, mercury, and other hazardous chemicals into the air, where they can be absorbed through the lungs and skin and can be adsorbed onto soil and washed into ground water and surface water.

Glass

Glass, like plastic, is a forever substance, able to survive for millennia without breaking down. At its simplest, glass is simply sand (SiO₂) that has been heated until it is molten and then cooled. It is infinitely recyclable because it does not degrade during recycling, which remelts the glass and reforms it. However, most glass recycling is limited to fairly basic forms of glass. Other kinds of glass – coated and laminated glass, for example – can be recycled, but at higher expense and potentially lower recovery rates. In 2019, the US glass recycling rate was 33%, compared to 90% for some European nations. The recycling stream in the US is often a single stream of co-mingled material because this is cheaper for municipalities to handle. Often, that thoroughly mixed set of recyclable materials is contaminated with trash and food wastes, which further complicates recycling of all the components, not only glass. US transport distances are often farther than in Europe, and the US does not have Europe’s high landfill costs, which encourage reducing trash output and favor recycling. The US states in which consumers pay a deposit on bottles have rates of bottle recycling on a par with Europe’s.

Use of recycled glass reduces energy costs of glass production. And for every 6 tons of recycled glass, one fewer ton of CO₂ emissions is produced. The heat needed to melt glass limits recycling to industrial facilities, so glass recycling is not connected to the health hazards of metal recycling. However, glass is heavy, relative to plastic and even metal, which increases transportation costs both to get used glass to recyclers and to put recycled glass back into the hands of consumers. And sand is in increasingly short supply. It is used in many other applications, especially road building and cement manufacture; mining it causes the same kind of habitat destruction as most forms of mining, in addition to creating dust that causes the respiratory disease silicosis.

Plastic

We saw, in section 5.2, the never-ending environmental problems associated with plastics. Some plastics can be recycled. However, no reasonable level of recycling could keep up with the volume of plastics being produced, let alone the volume anticipated in the future.

Begin with this “How Simple Things Work”video about the plastic recycling process and some of its limitations.

Then read or listen here for a discussion of how the vast plastic recycling business came to be, in the face of an abject failure to accomplish recycling.

Critical minerals

We have seen some examples of real progress in recycling, some halting progress in recycling, and an example of almost mythical recycling. Critical minerals recycling is both a young and an old field that shares some of the problems associated with copper recycling because critical minerals co-occur with copper in many examples of modern technology and electronics. It’s not surprising, then, that recycling rates for other critical minerals rise no higher than copper (Fig 3).

Critical minerals are minerals that one or more nations consider to be important for key industrial and national security applications that are also rare or associated with potential supply-chain problems. The US Geological Survey provides this extended definition.

The Energy Act of 2020 defined critical minerals as those that are essential to the economic or national security of the United States; have a supply chain that is vulnerable to disruption; and serve an essential function in the manufacturing of a product, the absence of which would have significant consequences for the economic or national security of the U.S. The act further specified that critical minerals do not include fuel minerals; water, ice, or snow; or common varieties of sand, gravel, stone, pumice, cinders, and clay.

Mineral criticality is not static, but changes over time as supply and demand dynamics evolve, import reliance changes, and new technologies are developed.

Critical mineral recycling is as old as hoary (and also quite new) tales of grave robbers digging in graveyards to steal gold fillings and jewelry. Gold is a critical mineral. It conducts electricity very well and resists corrosion, making it a common component, in small quantities, in electronics. We will discuss other aspects of critical minerals in Section 6.3 in the Energy chapter, because of the importance of critical minerals in renewable energy components such as wind turbines and solar panels. Our attention here is on recycling.

Figure 3. Global recycling of some critical minerals, 2015-2023. International Energy Agency. CC BY 4.0.

Recycling of critical minerals is not only important for economic and environmental sustainability but also, given their nature, for national security and such peace as the world can manage. Lithium-ion batteries, which power electric vehicles, computers, cell phones, and home appliances are a clear target for improved recycling, given the lithium recycling rates in evidence in Figure 3. The more commonplace lead-acid batteries used in automobiles have a 99% recycling rate in the US, where recycling is required by law. Many fewer lithium mines will be needed if the same rate can be achieved with this newer battery type, despite the sometimes more difficult process of accessing them. Traditional means of recycling copper can be scaled up to improve rates of recycling of this more common but still critical mineral. For rarer minerals, a considerable improvement in recycling of e-waste will be important. EV motors and wind turbines are related technologies that account for rare-earth minerals as well as copper. Each wind turbine contains several tons of copper, depending on its rating. As we saw in section 5.2, not only will recycling rates need to increase, but regulation and oversight of disposal and recovery will also need to improve, to reduce health risks presently associated with the processes.

Knowledge Check

Take a moment to complete the short quiz below to assess your understanding of this section. Read each question carefully and refer to the section content as needed. This quiz is not graded – it’s simply an opportunity for you to reflect on what you’ve learned and reinforce key concepts.

Circular economies as a framework for reducing and eliminating waste

The foundational idea for a circular economy is that, in the long run, all non-renewable resources should be continuously recycled, re-entering the economy in a new, valuable form after each use, and all renewable resources should be disposed of in positive ways such as in compost that produces nutrients that can be reused. Product design should ensure that finite resources can be recovered and recycled without loss of value. Such design is the antithesis – the opposite – of planned obsolescence, in which products are designed to be replaced in order to keep consumers buying new items and throwing away old ones.

Life-cycle analysis or assessment is an approach to understanding the impacts of a product throughout its existence – the phrase “cradle-to-grave” is often used. Waste impacts and impacts related to processing can occur in the acquisition of raw materials, production, use, and disposal of a product. For consumers to understand the true impacts of their purchases, they need information not only on disposability and recycling options, but on the full suite of impacts that occur during the life of a product. It can be a useful tool for assessing product design and determining where opportunities exist to move close to a circular economy.

Many of the processes by which products are made and disposed of create opportunities for externalities. To eliminate externalities, industries would need to be made completely liable for eliminating impacts during raw-material acquisition, manufacturing, and disposal, and users would need to be made completely liable for eliminating impacts during use. Product prices would rise accordingly, but environmental and other impacts could be controlled, and costs would be borne by consumers of specific products, not by the public in general, through taxes.

Changes needed to reduce externalities and move toward a circular economy cannot occur overnight. Policies and regulation can be used to guide industries to use production methods that minimize waste and other impacts; governments or industries can create means for recycling and composting. But standards must continually improved, and penalties must continue to be applied, so that progress occurs.

Ratcheting mechanisms in environmental policies, such as we saw with pollution reduction, are useful in incremental (stepwise) approaches to addressing externalities. Ratcheting mechanisms allow policies to continue to improve sustainability without the need for continual amendment or new policies, ensuring ongoing changes to standards over time. Changes can be specified in the policy (e.g., 5% reduction every 3 years), or the policy can delegate authority for future changes to specific agencies or to collaborative groups of diverse stakeholders to permit greater flexibility (but perhaps, also, greater procrastination).

Regulation of waste and recycling in the US

The Resource Conservation and Recovery Act (RCRA) of 1976 is the primary means by which the US regulates management of solid and hazardous waste; it is administered by the US EPA. RCRA sets out a national system for control of solid waste and establishes cradle-grave control of hazardous wastes. Open dumping is banned, and minimum standards are set for landfills. States implement the regulations for non-hazardous waste and may develop tighter standards than set by the EPA. States may develop programs to handle hazardous wastes; if they do not do so, then EPA implements the regulations within the state. Local governments may also create regulations related to waste, including bans on plastic bags, requirements for recycling, bans of recyclable material in landfills, deposits on cans and bottles, etc. No federal statute requires recycling, but state and local requires exist. Food waste is not addressed at the federal level, but some states require composting or recycling of organic wastes, generally, and New York and California require some businesses and institutions to donate food that is still edible to food banks and similar organizations.

A variety of additional laws govern specific substances or situations. Some examples follow.

The Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA, sometimes called Superfund) of 1980 establishes a tax on chemical and petroleum industries to create a trust fund (the Superfund) to clean up abandoned heavily contaminated lands (Superfund sites) and holds responsible extant companies that are found responsible for such contamination.

The Toxic Substances Control Act (TOSCA) of 1976 gives EPA the authority to require information about toxic substances from manufacturers and to require testing to understand the risks of toxic substances before they can be produced for market. TOSCA allows EPA to ban substances deemed to dangerous to produce.

The Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA)  of 1947 directs EPA to oversee the sale and use of these substances to protect public health. Pesticides must be registered before they can be sold; uses that do not follow labeled instructions are illegal. Regulations distinguish between uses associated with food and food crops and other uses.

The Ocean Dumping Act of 1972 is part of the Marine Protection, Research, and Sanctuaries Act. It prevents or severely limits dumping beyond “territorial waters” of the ocean – beyond 3 nautical miles (3.5 statute miles; 5.6 kilometers).

The US makes only sparing use of the “polluter pays” approach at the federal level. Superfund sites are the responsibility of the parties – usually industries – that created the contamination. States may add regulations that require producers to address the waste they produce. For example, New York’s state Electronic Equipment Recycling and Reuse Act requires electronics manufacturers to manage collection and recycling of e-waste.

Similarly, no incentives for waste reduction or recycling exist at the federal level, but state and federal requirements exist.

International regulations and guidelines for waste and recycling

The EU Waste Framework Directive is part of a suite of measures associated with establishing a circular economy. It lays out a legal framework for dealing with waste. It uses the upside-down pyramid of preferred approaches for waste from Section 5.1 to establish grounds for its priorities. It creates a system in which the original waste producer is expected to pay for the cost of their waste (“polluter pays”). It establishes recycling and recovery targets for 2020 of 50% for household waste and 70% for construction and demolition waste and ratchets the targets down at 5-year intervals, adding new targets such as having bio-waste collected separately or recycled onsite by the end of 2023. In the area of waste and recycling, the EU uses ratcheting mechanisms more consistently than the US.

A variety of EU statutes deal with specific kinds of waste, including packaging waste, waste containing persistent organic pollutants, and wastes associated with electrical and electronic equipment. Regulations place the burden of cost on producers and require member states to ensure wastes are collected and that wastes such as electric and electronic equipment are collected free by producers or their agents.

The World Health Organization and other UN bodies also provide guidance on waste disposal and recycling. Their recommendations are grouped together in a compendium on health and the environment. The waste chapter addresses issues such as improving waste collection, monitoring of waste streams and health risks, child labor, and providing incentives for waste reduction and recycling.

The UN also coordinates the attempts to develop an international treaty to address plastic pollution. As mentioned earlier, as of 2025, those efforts have failed primarily due to obstacles created by nations with large fossil-fuel sectors.

Knowledge Check

Take a moment to complete the short quiz below to assess your understanding of this section. Read each question carefully and refer to the section content as needed. This quiz is not graded – it’s simply an opportunity for you to reflect on what you’ve learned and reinforce key concepts.

This chapter’s case study is from the peer-reviewed technical literature – an article [click] here. It contains some technical language that you do not need to understand in order to see the many interwoven sustainability themes. Read for an understanding of the reasons this problem exists in many places in the world, the kinds of environmental harm that result, and some of the solutions proposed (without getting into the technical details).

In the world of regulated gold mining, the chemicals used in extracting gold are extensively reused and disposed of as hazardous wastes. In the world of this article, that doesn’t happen.

Fossil fuels

Although we think of them very differently, fossil fuels, like wind and solar energy began with the sun. Fossil fuels come from the organic matter of plants, algae, and cyanobacteria that were buried, heated, and compressed under high pressure over millions of years. The process transformed the biomass of those organisms into three types of fossil fuels: oil, coal, and natural gas. Different types of coal, oil, and gas are a result of differences in heat, pressure, and time, which affect the energy content of the fuel.

The energy content or energy density of fossil fuels is a measure of the amount of energy provided by each unit of fuel. Natural gas has the highest energy density, followed by oil and then coal. In a world in which climate change is an urgent concern, we can also discuss the carbon intensity of these fuels, which is related to how much CO₂ is produced when we produce a set amount of energy (different from burning a fixed amount of the fuel). Coal produces the highest volume of CO₂ for the energy produced (96 kilograms of CO₂ per million British thermal units – kg/BTU), , followed by oil and gasoline (mid-70 kg/BTU) and then natural gas (53 kg/BTU).

Mineral resources occur in varying concentrations and purities. Any occurrence is termed a resource, whereas a resource that is technically, economically, legally, and socially recoverable is termed a reserve. Many billions of tons of coal, oil, and gas will never by mined from the Earth due to limitations in one or more of these criteria.

Coal

Coal is an abundant resource that is relatively inexpensive to produce, particularly if the impacts of mining are not well regulated. Harder varieties – anthracite and bituminous coal – have a higher energy density. Soft coal, subbituminous (harder) and lignite (softer), contains lower amounts of energy, and therefore more needs to be burned to get the equivalent amount of energy. All coal contains some sulfur – it was part of the proteins of the living things that became fossil fuel. When burned, this becomes SO₂, an air pollutant that becomes part of acid rain. Soft coal has higher levels of sulfur than hard coal. Coal is primarily used to generate electricity.

Hard coal reserves are greatest in the US, China, Russia, and India, in that order; soft coal reserves are greatest in Russia, Australia, Germany, and the US. Coal use has flattened in recent years; at current rates of use, coal reserves can last for centuries. However, distribution of coal reserves among countries is very uneven (Fig 1).

Figure 1. Confirmed (proved) reserves of coal in the world.

Although coal consumption in the US has significantly declined in the last couple of decades, globally its consumption continues to grow, roughly doubling in the last few decades from 4 to 8 gigatonnes per year, dominated by growth in Asia. (Fig 2). China leads the world in the pace of its build-out of renewable energy, but it also burns the largest share of coal.

Figure 2. Global coal consumption 2022-2026. From Coal 2024: analysis and forecast to 2027. International Energy Association (IEA) CC BY.

Coal is plentiful and inexpensive when looking only at the market cost relative to the cost of other sources of electricity, but its extraction, transportation, and use produces  environmental impacts that the market cost does not truly represent. Coal is mined from surface, open-pit mines, from underground mines, and by mountain-top removal (practiced only in the US).

Coal mining, if not carefully regulated, acidifies and otherwise pollutes surface water, groundwater, and soils. Mountain-top removal buries streams under large volumes of earth, and pollutes streams that flow from the burial sites. Burning of coal contributes to global warming and also releases sulfur dioxide, nitrogen oxide, and mercury, which are linked to acid rain, smog, and health issues. Coal combustion byproducts include large volumes of solid ash that can be rich in toxic materials such as metals and organic compounds that can pollute air and water. Mining of coal in countries with fewer safety regulations continues to claim lives, often due to explosions that trap miners underground, and black-lung disease, caused by inhaling coal dust over long periods, is a threat, as well.

Oil 

Petroleum or crude oil is a liquid energy resource, mostly derived from marine algae. Crude oil is a complex mixture of organic chemicals and is refined into a series of products, but because of its high energy density and ease of handling, it is most used as a transportation fuel. As with coal, many billion tons of oil resources will never be recovered.

Oil reserves are more evenly distributed geographically than coal resources, but many countries lack both resources (Fig 3). In countries with rich oil reserves, such as Saudi Arabia, oil flows to the surface, making extraction very inexpensive. However, most oil is pumped from wells 1-2 miles below the Earth’s surface (0.6 – 1.2 km). Oils range widely in quantity and ease of production, ranging from thin, light, “sweet” oils that are easy to produce and refine to thick, heavy, sour (sulfur rich) crudes that are expensive to produce and refine and that cause more air pollution. Refining uses energy, and which contributes to the carbon intensity of the resulting fuel.

Oil is also found in oil sands, tar sands, and shales; these are all considered unconventional sources, and are proportionally much more expensive to produce. Energy analysts are increasingly skeptical of the accuracy of reported oil and gas reserves because of the economic and political implications of the data and the incentives for countries to misreport their holdings.

Figure 3. Oil reserves around the world. OurWorldinData.org. CC BY.

During oil production, leakage during drilling and pumping can contaminate soil and water; spills cause similar damage at a larger scale. Often, wells are drilled through aquifers that contain salt water, leading to salt water being carried to the surface, along with oil. Usually, saltwater is reinjected back into the aquifers. If freshwater aquifers overlie the salt-water aquifers, they may be contaminated by both oil and salt water. Disposal of produced saline water into surface water can pollute surface and groundwater, as well as soil.

Oil and tar sands are mined or extracted by heating the oil in situ, rather than drilled. Extraction produces more greenhouse gases than conventional oil drilling, and produces large volumes of contaminated water. Burning oil results in air pollution from SO₂, NOx, VOCs, and particulate matter, and global warming from CO₂ and methane.

Fracking – hydraulic fracturing – is regularly used to increase production of oil and gas in deposits in which oil and gas are trapped in rock. Perforated pipes are placed into the rocks that hold the oil and gas reservoir, and a mixture of water, sand, and a suite of proprietary chemicals (including the forever chemicals PFAS) is injected into the reservoir under high pressure to crack the rock and release the oil and gas (Fig 4).

Fracking consumes large volumes of water, often 10 million gallons (3.8 million liters) or more, and also produces large volumes of polluted water, because much of the water-chemical mix used to pressurize the oil and gas deposits returns to the surface along with the oil and gas. If wastewater escapes or contaminates overlying freshwater aquifers, then surface and groundwater contamination can result. Recycling of wastewater is common in areas with limited freshwater availability, and this can reduce water use in an oil field – especially important in arid areas. Methane, one of the major GHGs, is released during fracking. A variety of public health hazards have been reported, as well.

Figure 4. Diagram of hydraulic fracking machinery and process. Emiliawilkinson from Wikimedia Commons, CC BY-SA.

Natural gas is a suite of gases mainly composed of methane (CH₄) and is a very potent greenhouse gas. Thermogenic gas was formed in the past, from the compression of deeply buried, ancient organic matter with deep heat, underground. Thermogenic gas is found with petroleum in reservoir rocks and with coal deposits, and these fossil fuels may be extracted together. Biogenic gas is found at shallow depths and arises from present-day bacterial decay of organic matter where oxygen is unavailable, as in landfill gas or swamp gas. Most plastics are made from natural gases (e.g., ethylene, C₂H₄).

As with coal and oil, the global distribution of natural gas reserves varies geographically. Up until the first decade of the 21st century, all natural gas was produced in the same manner as oil, by the drilling of vertical wells that produced a flow of gas to the surface, powered by its own pressure. With the advent of two new technologies in the US, the drilling of wells into reservoirs horizontally and the fracturing of reservoirs to enhance their productivity, vast new reserves of natural gas were established. This radically changed the energy profile of the US. The major decline in the consumption of coal for the generation of electricity was made possible by large increase in the production of natural gas which is now the predominate source energy for generation. Natural gas has a lower carbon density than coal, so US production of GHG decreased as a result of this switch.

Approximately 35% of methane released to the atmosphere during 2010-2019 was produced naturally, particularly from wetlands. Our attention here is to anthropogenic natural gas.

Approximately 30% of anthropogenic natural gas is produced by fossil fuel production and use. Reported leakage during production is relatively low – less than 1% of US production in 2023, but this accounted for approximately 30% of overall US methane emissions, and not all leakage is reported. As we learned in Chapter 2, methane is a potent GHG. It often leaks from all phases of production, transportation, and consumption. This leaked or “fugitive” methane is now being recognized as a significant contributor to climate change, but whose abatement is moderately easy to accomplish.

When natural gas is produced but cannot be captured and transported economically – as when it comes up from oil wells equipped for liquid oil capture rather than gas capture – it is “flared” or burned at well sites, which converts most of it to CO₂. This is considered safer and better than releasing methane into the atmosphere because CO₂ is a less potent greenhouse gas than methane.

When natural gas comes up from a well, other gases may also be present, including hydrogen sulfide, H₂S, which is toxic. Gas with H₂S can be flared, producing SO₂, or treated to remove the H₂S, or released directly into the atmosphere where it is a health hazard first as H₂S and eventually as SO₂. Like oil, natural gas is produced by drilling, which may be accompanied by fracking. As a result, the environmental impacts of production are similar to those for oil. When burned, natural gas produces less CO₂ for a given amount of energy (heat) produced than oil or coal, making it the preferred fossil fuel. However, the methane that can be released during production of natural gas can cause it to have GHG effects similar to coal unless the methane is addressed. Natural gas produces less SO₂ and less NOx than coal.

Nuclear energy

Nuclear power is energy released from the radioactive decay of elements, such as uranium, which releases large amounts of energy as heat. Unlike fossil fuel power plants, because of the inherent dangers associated with the radioactive material that is used as fuel, nuclear power plants are highly complex and have many systems that other power plants don’t require to ensure the safe containment of the fuel. In contrast with the use of other energy fuels, nuclear power plants produce no carbon dioxide and nuclear fuels are often considered alternative fuels (fuels other than fossil fuels). A 2025 report from International Energy Association indicates that the proportion of world electricity provided by nuclear power peaked at the end of the 1990s, at approximately 18%; currently, nuclear energy provides about 9% of global electricity generation. Some reactors are also used for heat generation – a form of co-generation that uses the heat of the nuclear fission reaction that creates the power for electricity.

Many nuclear reactors are near the end of their original licensing periods. At the end of 2023, the average nuclear reactor in the developed world was 36 years, whereas reactors in emerging and developing countries averaged less than 18 years. However, the demand for electricity, ballooning with the huge energy needs of artificial intelligence, and growing even as climate change impacts are increasingly visible, is renewing interest in and plans for nuclear energy.

Mining and refining uranium ore and making reactor fuel demand a lot of energy. Large-scale nuclear power plants are very expensive and require large amounts of metal, concrete, and energy to build. However, small modular reactors are being designed, with lower construction costs and times, and faster times to profitability; the reduced costs make financing easier, and enable more players to be involved. The first of the so-called SMRs are due online in 2030.

Nuclear reactors generate considerable heat during operation and are often water-cooled. However, most of the water is returned to the source. In countries with relevant regulation, plants are limited in the amount of warming they can cause in receiving bodies of water, and may employ cooling towers or other methods of reducing the temperature of the water leaving the facility. Under global warming, as the temperature of the incoming water warms, cooling processes for the plant become less effective.

Although nuclear power requires mining of uranium, and is associated with the usual environmental impacts of mining, the main environmental challenge for nuclear power is waste. By volume, the waste produced from mining uranium, called uranium mill tailings, is the largest volume of waste and contains the radioactive element radium (Ra), which decays to produce radon (Rn), a radioactive gas with a half-life of over 1,000 years. The half-life of a radioactive element is the time it takes for 50% of the material to decay radioactively, and is one measure of how long wastes containing the element will be dangerous.

Waste from nuclear fuel rods – small uranium fuel pellets in long metal tubes – is considered high-level radioactive waste. Decay of radioactive uranium in nuclear fuel rods produces several different decay products, some with half-lives as short as 30 years, others with half-lives of thousands of years. Practically speaking, then, radioactive wastes from nuclear power can be considered another “forever” pollutant. Only Finland and Sweden have started construction on long-term storage facilities for nuclear waste; Finland’s facility is due to open in 2026. For now, everywhere in the world, spent nuclear fuel rods are first held in “temporary” storage pools, where water continuously cools their ongoing, but energetically less useful, radioactive decay, and later are held in outdoor steel or concrete containers cooled by air.

The potential for significant harm in the event of a failure at a nuclear plant is not a hypothetical issue. The 1986 failure at the Chernobyl nuclear reactor in Ukraine led to the evacuation and resettlement of over 200,000 people from three nations. Thousands of cases of thyroid cancer were linked to the radioactive fall-out, and economic loss and hardship occurred across a wide area. Thousands of square kilometers around the site are uninhabitable for thousands of years.

The 2011 accident at the Fukushima Daiichi Nuclear Power Station in Japan was caused by an earthquake and tsunami that overtopped the protective seawall and greatly complicated relief efforts. Radioactive material was spread both through the atmosphere and in flood waters. Initially, over 100,000 people left the area, with 79,000 still considered evacuees in 2017. Clean-up continues to the present day.

A unique hazard associated with nuclear energy is the potential for this energy resource to be weaponized. Some 31 countries operate nuclear power plants today. Securing nuclear power stations and the waste they produce requires a high degree of technology, which does not necessarily accompany the technology for building nuclear power stations. These additional risks associated with nuclear energy bring an additional level of ethical consideration to its use. However, despite the demonstrated potential for harm, interest in nuclear energy is returning after a prolonged decline. Decarbonization of the world’s energy supply through renewable energy has not advanced quickly enough to slow, much less halt, climate change; nuclear energy is seen as an alternative means of meeting growing demands for clean energy. The smaller, modular reactors may meet less resistance from voters despite the risks accompanying their use.

Closing and decommissioning traditional-fuel and nuclear production facilities

Coal, oil and gas mining operations must eventually close when the target resources are reduced below economically recoverable levels. In many countries, mining permits include payment of a deposit to ensure reclamation or safe capping and clean-up; reclamation and clean-up of operations that started before such regulations were imposed and sites that have been abandoned generally become the responsibility of the state, and may never be undertaken, due to lack of funds.

Coal

The goal of reclamation of surface coal mines is to return the land to beneficial uses and minimize impacts from the site. Reclamation typically involves recontouring the site so that it matches the surrounding landscape and, as nearly as possible, the original topography of the site. Waste materials that contain recoverable energy may be removed or burned for energy. Methods of reducing acid mine drainage in the long term should be put in place to avoid contaminating groundwater and surface waters that receive percolation or runoff from the mine. A variety of treatment approaches is available but this phase of reclamation can be particularly costly. Unless the site is to be developed immediately, some form of revegetation is appropriate to reduce erosion. Reclaimed sites have very different soils from the original soils on the site because of the highly disturbed nature of the subsoil and, often, the presence of a clay cap to reduce water percolation into the mined material.  As a result, revegetation requires use of plant species that can tolerate the new soil conditions. If the price of coal rises substantially, deposits that were left behind for financial reasons may become attractive again, and reclaimed sites may be reopened and subject to additional mining, particularly if they were reclaimed to wildlife habitat, agriculture, or recreational uses.

Underground coal mines should also reclamation, which employs many of the same processes as surface-mine reclamation – recontouring, addressing acid-mine and other drainage issues, and revegetating. An important difference is the need to backfill or stabilize the underground area of the mine to prevent formation of sinkholes and subsidence of the ground. When large volumes of material have been removed, stabilization – often undertaken by injecting specialized “grouts” into the empty (or void) spaces – can be costly.

Because coal mining began long before relevant regulations were in place anywhere in the world, abandoned coal mines and their acid-mine wastes are a reality in most places that have (or had) easily mined coal deposits. They continue to acidify local waters and the lands they occupy cannot be made useful without some form of reclamation.

Oil and gas

Oil and gas drilling sites also need reclamation to ensure safety and to minimize environmental impacts. Wells should be capped and infrastructure removed. Spills of toxic materials should be remediated. Salt water that may come to the surface in large volumes along with the recovered oil and gas should be disposed of, often by injecting it back into deep rock formation. Where fracking was employed, reclamation should include a search for methane leaks and assessment of impacts to local hydrology, and treatment of any that are found. Recontouring and revegetation may be undertaken, as needed depending on intended future uses of the land. Regulations that require some form of deposit or bond to cover these costs can help to ensure that reclamation occurs to a good standard.   

Nuclear

The goal of decommissioning a nuclear-energy facility is to safely return the site to alternative uses such as agriculture, industry, or housing. Disposal of the radioactive fuel is an ongoing part of operation of the facility, and is discussed above. However, parts of the facility itself are also radioactive, at lower levels. The fastest route to decommissioning is to remove and dispose of the radioactive components and then dispose of the rest of the facility – this approach can be completed in less than a decade but is still costly. Nuclear facilities can also allowed to sit, unused, with appropriate monitoring, for a long enough period of time to reduce radiation levels to the point that special disposal is not required, which takes decades. Lastly, facilities can be permanently entombed in concrete, as was done with the Chernobyl facility.  Decommissioning will eventually be a concern for the new, modular nuclear reactors planned for 2030 and later. Although these will be smaller than existing reactors, they will contain similar components.

Knowledge Check

Take a moment to complete the short quiz below to assess your understanding of this section. Read each question carefully and refer to the section content as needed. This quiz is not graded – it’s simply an opportunity for you to reflect on what you’ve learned and reinforce key concepts.

A wide variety of renewable energy sources are available to provide energy for electricity, heating, and transportation. Renewable does not mean that these energy sources have no environmental impacts. Some, such as animal dung, can create significant health hazards and contribute to climate change. Here, we explore the major types of renewable energy and their environmental impacts including impacts on the sustainability of resources needed for their use.

Biomass

No standard definition exists for biomass, but typically it includes wood and wood waste, animal and human manure, crop residues, and natural material in municipal waste, including paper products, food waste,  yard waste, and compost. Usually, biomass is burned to generate heat. Processes also exist to generate liquid fuels such as alcohol and biodiesel, or gases such as methane – biofuels (next section).

The use of animal dung and charcoal as fuel in lower-income areas is often associated with significant particulate air pollution. Soot is a common byproduct, and not only creates a health hazard but also reduces albedo and contributes to climate change. Biomass burning in mountainous regions can hasten melt of snow and glaciers because the soot darkens the snow and ice.

Biomass is considered a clean fuel because it burns organic material (creating GHG) that would have decomposed naturally, also creating GHG. Thus, burning of biomass does not add more C to the atmosphere. In contrast, burning fossil fuel takes a carbon source that was buried deep in the earth and converts it to carbon in the aboveground carbon cycle, thereby increasing GHG.

Biofuel

Liquid (ethanol, biodiesel) and gas (methane) fuels derived from biomass are considered to be biofuels. Biofuel researchers describe 4 “generations” of biofuels, but the world uses only the first two. First-generation biofuels are made from the edible portion of food crops – for example, corn and soybeans. Second-generation biofuels are made from non-food plant biomass, including the inedible portion of food crops (for example, the stalk and leaves of corn, called corn stover, or the grains left over after fermentation for grain alcohols, called spent grain), forest wastes, and some non-food energy crops, often grown on degraded or marginal farmland. Both first- and second-generation biofuel require agricultural work to produce the crop and processing to produce fuel, both of which require energy.

Almost all of the present-day biofuels are first-generation biofuels (primarily corn, soy, and palm oil), and represent food that could feed people or food crops that are planted in areas that could be producing more useful food for people. This “food or fuel” conflict is acknowledged in the biofuels world and is one reason that second-generation fuels are in use. We will see more of this when we look at sustainable agriculture and food security.

Because of the market for sustainable energy, large areas have been converted from natural habitat to production of food crops to be used for biofuel. Because of the loss of natural vegetation and the impact of soil disturbance, these potentially renewable fuels often incur a carbon debt. More GHGs are produced in the process of clearing land and undertaking agricultural work to grow them than is saved by using them, and carbon that may have been in forms that will not enter the atmosphere soon (trees that may grow old, soil that may lock the carbon into unavailable forms) is moved into forms that will enter the atmosphere quickly. Some crops in some settings can “repay” their carbon debt in a few years, because much of the initial carbon debt is a one-time cost and the renewable nature of the fuel offsets it quickly. But carbon debt resulting from destruction of rainforest, particularly rainforest on carbon-rich, peat soil, may require many decades to offset.

Figure 1. Palm plantation for palm oil, next to rainforest. AdobeStock by cn0ra. Used with permission.

Corn (for corn oil for corn ethanol), soy (for soybean oil for biodiesel), and palm fruit (for palm oil for biodiesel) have all been associated with major campaigns to open up new ground for their production, beginning with the early corn-ethanol boom in the midwestern US in 2007-2012, then increased clearing of Southeast Asian rainforest for palm oil (Fig 1), and, slightly more recently, destruction of the cerrado savanna of Brazil for soybean fields. All three of these biofuels are still used extensively.

Note that liquid biofuels (ethanol and biodiesel) are needed primarily for transportation, and could be replaced by electric vehicles powered with renewable energy.

Hydropower

Hydropower  is considered a clean and renewable energy source because it does not directly produce pollutants and because the power source is regenerated. Read here to learn more about the kinds of hydropower installations. Essentially all hydropower is used to generate electricity. Hydropower produces a reliable, consistent supply of electricity, in contrast to wind and solar sources which generate electricity intermittently.

Hydropower depends on a reliable supply of sufficient volumes of water for the energy needs. Depending on the amount of power to be generated, hydropower installations are also limited by the elevational gradient of the river flow and the geology of the region. Large dams require a geology that will support construction of a high dam that restrains a large volume of water to create a head – an elevation of water above the turbines – that will generate considerable force to turn turbines. Major hydropower dams typically have heads of at least 100 m (330 ft); pumped hydropower settings provide 100-200 m of elevation. By contrast, run-of-river dams may operate with only 1-2 m (3-6 ft) of head, or even less for micro-hydropower that provides small amounts of power for local uses.

Figure 2. A run-of-river micro-hydropower system. US Department of Energy. Public domain.

Run-of-the-river systems, which do not require large storage reservoirs, are often used for micro-hydro and sometimes small-scale hydro projects. For run-of-the-river hydro projects, a portion of a river’s water is diverted to a channel, pipeline, or pressurized pipeline (penstock) that delivers it to a waterwheel or turbine

Although hydropower is considered a clean energy source, it does have a carbon footprint. Hydropower infrastructure depends on concrete, often in large volumes, which, in turn, requires cement which takes a lot of energy to create, producing significant volumes of GHGs. When reservoirs are created, they submerge vegetation that generates methane as it decomposes, and lakes are usually ongoing sources of lesser amounts of methane. Reservoir creation also displaces people living along the river, downstream of the dam site, causing economic and social hardship. China’s Three Gorges Dam, operational since 2012, displaced over 1 million people.

All hydropower use modifies river environments to some extent. Dam-and-reservoir systems create the largest changes, blocking movement of aquatic organisms and sediment, creating GHG emissions, and affecting water volume, temperature, quality, and flow regimes (how the flow changes over a year) downstream. Depending on the extent of the changes, most aquatic species may be lost from the downstream waters for a considerable distance below the dam. Many fish species, including salmon, trout, and sturgeon, have been eliminated from rivers or had their populations severely reduced as a result of dam construction (not always for hydropower) that cuts them off from spawning and/or foraging grounds and may so modify the river environment that it no longer provides suitable habitat. Creation of the reservoir destroys terrestrial habitats as it creates aquatic ones.

Upstream of the dam, the reservoir soon resembles a lake more than a river, with limited flow and an increasingly sediment-laden bottom. Species that require flowing, well-oxygenated waters and gravel river bottoms are lost, and lake species and invasive species that can tolerate less oxygenated water and higher nutrient levels can become established.

Run-of-river dams can have all of the major impacts of hydropower dam – significant dewatering of portions of the river, changes in the natural flood regimes and sediment transport, emissions of GHG, and disruption to fisheries. Impacts vary depending on the proportion of flow extracted from the river and the nature of the instream construction.

In addition to environmental impacts, dam and run-of-river hydropower can displace communities and destroy livelihoods. The Three Gorges Dam in China displaced more than 1 million people. Associated agricultural, industrial, and municipal lands may be flooded, and fisheries may be sharply reduced.

Dams have lifetimes – major dams are expected to continue in operation for 50-100 years. Decommissioning a dam responsibly involves removing the dam and restoring the river to as natural a state as can be achieved. Initially, all the sediment that has built up behind the dam will enter the river, potentially causing considerable ecological harm, at least in the short term. Water volume and flow regime will change again, along with water temperature and water quality. The total cost of removal and restoration of two major dams on the Elwha River in the state of Washington in the US, between 2011 and 2014, was estimated at $350 million. Many dams in the world are operating past their intended lifetimes. It’s also possible to replace major dams, but, to date, this has seldom happened (many major dams are still within their operating lifetimes). The 220′ (67 m) replacement for the Calaveras Dam in California, in the US, completed in 2018, in order to improve safety in an earthquake zone, cost $823 million.

Geothermal energy

Geothermal energy uses the heat of Earth’s subsurface to provide endless energy. Heat from the planet’s molten core spreads upwards through the planet’s mass and heats rock and deep aquifer. Where tectonic plates come together, heat may come rather close to the earth’s surface, but everywhere on the planet, deeper layers are warmer.

High-temperature geothermal systems may use naturally occurring steam, directly, to turn turbines. But because this natural water is often rich with minerals and salts, which corrode equipment and block pipes, other geothermal systems pipe secondary liquids with lower boiling points very close to naturally heated water to create vapor to turn turbines. Geothermal systems are most common near tectonic plate boundaries and areas of volcanic activity – they are constrained by the natural availability of very hot rocks and/or water found at shallow depths.

Low-temperature geothermal systems, such as heat pumps, can be employed anywhere. A field of pipes is laid at a depth of 4-6 ft (1.2-1.8 m), where the ground temperature is constant, year-round and water is circulated in them. Heat pumps use the constant temperature at that depth as a heat source (in the winter) or a heat sink (in the summer). The environmental impact of geothermal energy depends on how it is being used. Direct use and heating applications have minimal impacts on the environment.

Clean hydrogen

Studies of how to achieve 100% clean energy typically include clean hydrogen in the mix of solutions. Hydrogen is a clean-burning gas – it creates water vapor when burned. It is created by electrolysis of water – splitting water molecules with electricity – which produces hydrogen gas and oxygen gas. It can be used to fuel vehicles or, like natural gas, to generate electricity. Because hydrogen is readily stored and transported, it can be a useful fuel to fill in the gaps of availability of solar and wind energy and to even out energy supply among regions.

For utility-scale power, hydrogen gas must be created, which takes power. Clean hydrogen is created using another clean energy source, essentially converting one power source to another. The IPCC suggests that use of clean hydrogen should be primarily for electricity generation, as electricity is becoming the fuel of choice for transportation, and heat pumps are a more obvious choice for heating.

Solar energy

Watch this video for a quick introduction to photovoltaic (PV) solar energy. It’s over-technical for 2 minutes but has good basic information on limitations. Figure 3 shows the distribution of solar energy. Areas near the tropics have an obvious advantage in relatively constant day lengths, but moist tropical areas may often have cloud cover, which reduces solar availability. Drier areas, even into the temperate zones, also have high potential for solar installations.

Then watch click here for video for additional information on concentrated solar energy and issues associated with energy storage and transmission.

PV solar energy uses crystalline silicon, which is created using energy, but silicon is extremely abundant and easy to obtain. Copper and silver are less common, and are considered to be critical minerals – important minerals whose quantities are, or may become, limiting (see the next section). Higher-efficiency solar cells may require additional critical minerals. All of the minerals are mined, so solar energy is linked to environmental impacts of mining, but mining for renewable energy, overall, is anticipated to be less than mining for fossil fuels. Recycling of solar panels is required in the European Union and includes recovery of critical minerals.

Solar panels are increasingly deployed in large solar farms, both as PV solar and as concentrated solar. In this format, solar energy represents habitat loss for many species and can reduce land available for other uses, particularly agriculture. Agrivoltaics, solar panels that share the land with agriculture or grazing, and floatovoltaics, solar panels deployed as floating units on water, are recent developments that seek to reduce the land footprint of solar energy.

Panels in solar installations need to be washed (or rained on) to maintain their efficiency. More than 10 billion gallons of water are used each year to clean solar panels, equivalent to drinking water for 2 million people. Less water-intensive measures are being developed.

After their useful lives, photovoltaic panels become waste. Presently, only about 10% are being recycled. By one estimate, China, where the largest number of panels is deployed, could have 13-20 million tons of PV-associated waste by 2050. Regulations in the EU have led to PV-specific recycling centers, but even there, capacity lags behind need.

Solar panels deployed by individuals and small businesses can often be connected to energy grids fairly easily. But large-scale installations require construction of infrastructure, with associated impacts. If battery storage is to be added to improve the stability of the energy supply, additional impacts will occur, including impacts associated with supply chains and waste handling for the batteries. Alternative means of storing energy, including pumped hydroelectric and compressed air storage, offer cleaner solutions to grid stability but are not yet as widely deployed.

Wind energy

Figure 4. Areas with abundant and reliable wind power. Antonini, E.G.A., Virgüez, E., Ashfaq, S. et al., Fig 2. https://doi.org/10.1038/s43247-024-01260-7, licensed under CC BY 4.0

 

Because a wind turbine has a small physical footprint relative to the amount of electricity it produces, many windfarms are located on crop and pasture land. They contribute to economic sustainability by providing extra income to farmers and ranchers, allowing them to stay in business and keep their property from being developed for other uses. However, wind farms are prohibited in some areas for a variety of reasons including noise, aesthetics, safety, impact to property values, and military security.

Collisions with wind turbines kill large numbers of birds and bats that use wind for foraging and migration. In the US, they are contributing to the endangerment of three species of bats that were otherwise considered numerous and safe. Research is ongoing to find ways to reduce mortality, including coloring turbine blades, adding sound or light effects. Some mortality can be avoided by siting turbines away from key locations and routes, but these are often in high wind corridors – high-quality locations for wind energy. During migration season, significant mortality reduction can be achieved by programming the turbine to spin only at higher wind speeds. Birds and bats face lower mortality from stationary blades, and much less power is generated at low wind speed, so the mortality reductions do not cause high economic losses. However, wildlife safety measures are not required by law, and numbers of deployed wind turbines continue to increase. Of course, if wind turbines are not deployed, and global temperatures continue to rise, many wildlife species will face serious declines due to climate change.  

Offshore wind turbines have impacts to marine species and to seabirds, both positive and negative. Many species avoid offshore wind farms, which results in habitat loss. Sea birds die through collisions. Noise may interfere with marine-mammal communication. Evolving technologies  can dampen some of the construction and operation noise; these are required in the EU, but not universally. Once installed, offshore turbines can offer habitat for some species, and mitigation measures can reduce impacts to birds. Nevertheless, a recent review of impacts to migratory marine species (excludes birds) found that the present, limited knowledge suggests a net negative impact. More research on both impacts and mitigation is needed.

Wind technology employs several critical minerals for the most common turbine generator. As a result, wind relies on mining of critical minerals with the resulting environmental impacts, although, as noted above, less mining is anticipated to be needed for renewables than for fossil fuels. Many wind turbine parts can be recycled, and turbine blades can be made of recycled materials. Some European countries have targets for recycling of wind-power components, but no larger commitments to recycling exist at present.

As with solar energy (see above), connecting wind farms to existing power grids also involves development and environmental impacts. Choice of energy-storage technology, if it is used, affects overall environmental impacts.

Electric vehicles

Electric vehicles obviously aren’t a power source. But they are a means of transitioning away from liquid fossil fuels in the major sector for those – the transportation sector. The use of biofuels to replace gasoline and diesel fuels remains problematical because of arguments over how to assess emissions and because the vast majority of the volume of transportation biofuels comprises first-generation fuels that are also food crops. Use of these fuels can affect food prices and food security. Electric vehicles eliminate the need for liquid fuels, and can be powered by renewable energy, if that is available. New options for fast charging and increasing availability of charging stations improve affordability and accessibility. Although electric passenger cars remain a small proportion (<10%) of the world fleet, in China, in 2024, almost half of car sales were of electric vehicles, accounting for over 60% of such sales, worldwide.

The clean energy spatial footprint – an assessment for the US

The US National Renewable Energy Lab issued a report in 2023 examining options for achieving 100% clean energy in the US by 2035. They examined four scenarios that varied in their assumptions regarding changing cost and performance; advances in transmission technology; land availability for wind, solar, and biomass; transportation and storage costs; etc. Figure 5 shows the anticipated land needed, for the 4 scenarios, for solar energy, for direct, land-based wind energy (the footprint of the turbines), for land for wind (the footprint of the windfarms), and for transmission rights-of-way. The estimated land needed for solar energy is much less than the land currently used for biofuel corn ethanol, whereas the land needed for wind farms is greater, depending on the scenario in question. However, given that wind farms are a mixed-use land use, much of the footprint needed for wind energy could be on the land presently used for corn ethanol, or for livestock grazing and feed production. Although solar and wind have considerable land footprints, they are by no means impossible to accommodate.

Figure 5. For scenarios to achieve 100% clean energy in the US by 2035, the land needed for wind, solar, and transmission rights-of-way, compared to other major land uses. US National Renewable Energy Laboratory. Public Domain.

The colored lines around the utility-scale solar, wind (direct and spacing), and transmission RoW boxes show the box size for the four scenarios. The ADE demand case assumes that demand for electricity will accelerate during the clean-energy transmission as processes previously fueled by fossil fuels convert to clean electricity. Rights of way and wind farms include land that is available for other uses – this is shown by the dashed lines around the related boxes. Although solar farms can also, in theory, include additional land uses, these share a footprint with solar panels, and utility solar is not considered a mixed-use land use for purposes of this graphic.

Knowledge Check

Take a moment to complete the short quiz below to assess your understanding of this section. Read each question carefully and refer to the section content as needed. This quiz is not graded – it’s simply an opportunity for you to reflect on what you’ve learned and reinforce key concepts.

An interactive H5P element has been excluded from this version of the text. You can view it online here:
https://iu.pressbooks.pub/environmentalsustainabilityscience/?p=194#h5p-25

Current energy portfolios

Historically, of course, renewable fuels – wood, animal dung, animal fat, and plant oils – were the only fuels available. Fossil fuels didn’t really take off until the 20th century, with coal coming on at the beginning of the century, oil after World War II, and natural gas mostly in the last quarter of the century. However, with their high energy density, fossil fuels have dominated ever since, producing electricity and powering transportation (Fig 1). In 2023, renewables made up only about 14% of global primary energy consumptions.

Figure 1. Global primary energy consumption by source, 1800-2023. OurWorldinData CC BY.

Primary energy is energy that is produced from raw fuel stocks or generated from natural energy sources. This is in contrast to secondary energy that is generated from primary sources, stored, and used – for example, electricity that is produced from natural gas or solar energy, stored in batteries, and then used to power electric cars.

In this data set, traditional biomass comprises wood, charcoal, animal dung, and crop residues.

The US, China, and India lead energy consumption worldwide (Fig 2). Several large economies – Japan, and several European nations, for example – use less primary energy. Overall, the global North dominates energy consumption. In the coming years, US energy consumption is anticipated to increase, with electricity demands expected to as a result of growth in AI, increase in production of hydrogen fuels, electrification generally and in the transportation sector, and increase in in-country manufacturing. Between 2000 and 2006, industrial and commercial energy use is forecast to rise 2.1 and 2.7%, respectively. Data center electricity needs are anticipated to increase 13-27% between 2023 and 2028. However, the world energy forecast shows a global decline in energy demand, as increases in energy efficiency continue to take effect.

Figure 2. Energy consumption by nation, 2023. OurWorldinData. CC BY.

US energy consumption 

Presently, most of the energy used in the US is generated by petroleum (primarily liquid fuels – transportation and industry) and natural gas (primarily energy generation), with coal, nuclear energy and renewables each providing about 9% (Fig 3). Among renewables, biomass provides the majority of energy, mostly from biofuels (primarily corn ethanol) and wood (including cord wood, wood waste, and wood pellets).

Figure 3. US primary energy consumption by energy source, 2023. US Energy Information Administration. Public domain.

One quadrillion BTUs (a common US energy unit) is approximately 293 terrawatt-hours (a common international energy unit).

Current GHG emissions

As of 2023, world CO₂ emissions continue to increase, led by China and the US. Nations are not meeting their pledges to reduce emissions, made under the UN Framework Convention on Climate Change’s Paris agreement. Even if they were to meet pledges, the pledges, so far, do not reach to zero emissions.

Figure 4. Annual CO₂ emissions by world region from 1750-2023. OurWorldinData.org CCBY.

Note that emissions here are direct emissions from burning of fossil fuels and industry. Emissions resulting from deforestation and other land use change are not included.

Current warming trend

As we saw in the first chapter, the planetary boundaries of CO₂ concentration and radiative forcing (the total increase in atmospheric heat retention) – the planetary boundaries associated with global warming – have been crossed, and the planet is no longer in a safe operating space with respective to temperature. The 2023 and 2024 planetary temperatures, boosted somewhat due to the El Niño cycle, but still unexpectedly high, increase the slope of the temperature data (Fig 5). If that trend continues, the planet is on track to warm beyond 1.5°C beyond pre-industrial temperatures within the 2030s. Most recent climate agreements are designed to limit the likelihood of crossing the 1.5°C threshold, or to limit time spent above that threshold. But as we have seen, nations have not delivered on their promises, which were, in any event, insufficient to limit warming to 1.5°C. The average anomaly in 2024 was +1.28°C, according to the US agency NASA.

Figure 5. Global temperatures from 1880 to 2024, with trend line. US NASA. Public Domain.

Knowledge Check

Take a moment to complete the short quiz below to assess your understanding of this section. Read each question carefully and refer back to the content as needed. This quiz is not graded – it’s simply an opportunity for you to reflect on what you’ve learned and reinforce key concepts.

Critical minerals are minerals that one or more nations consider to be important for key industrial and national security applications that are also rare or associated with potential supply-chain problems. The US Geological Survey provides this extended definition.

The Energy Act of 2020 defined critical minerals as those that are essential to the economic or national security of the United States; have a supply chain that is vulnerable to disruption; and serve an essential function in the manufacturing of a product, the absence of which would have significant consequences for the economic or national security of the U.S. The act further specified that critical minerals do not include fuel minerals; water, ice, or snow; or common varieties of sand, gravel, stone, pumice, cinders, and clay.

Mineral criticality is not static, but changes over time as supply and demand dynamics evolve, import reliance changes, and new technologies are developed.

Several critical minerals are involved in clean-energy technology. This video (click here to watch) explores critical minerals and issues related to supply and demand that can speed or hamper progress in the transition to clean energy. 

Known deposits and mines of critical minerals, like energy resources, are unevenly distributed around the planet (Fig 1). A map of known deposits and mines is not the same as a map of all deposits. Countries often do not share complete information related to high-value resources, and surveys vary in methods and level of investments from country to country. An additional facet of mineral supply is where critical minerals are processed. Currently many minerals that the US considers to be critical are processed exclusively in China which creates a supply vulnerability, as the video points out. Still, research suggests that active US metal mines currently generate, as unrecovered byproducts, a large proportion of needed critical minerals. In some cases, transitioning a mine to focus on these byproducts might generate more revenue that the current target of mining, but will require additional research and technology development.

Figure 1. Global distribution of selected mines, deposits, and districts of critical minerals. US Geological Survey. Public domain.

Knowledge Check

Take a moment to complete the short quiz below to assess your understanding of this section. Read each question carefully and refer to the section content as needed. This quiz is not graded – it’s simply an opportunity for you to reflect on what you’ve learned and reinforce key concepts.

Is it feasible to eliminate GHG emissions – to reach net zero emissions?

Plans exist that define emissions reduction strategies to avoid global warming beyond 1.5°C above pre-industrial temperatures or to return quickly to temperatures less than 1.5°C above historic temperatures. However, the longer progress towards sustainable energy use is delayed, the greater the levels of global warming that will be reached. As a practical matter, we have seen that nations have neither pledged nor made sufficient emissions reductions to avoid increasing their GHG emissions and therefore increasing global warming. Following the 2024 presidential elections, the US has made and committed to making steps backwards away from a goal of net zero emissions.

The IPCC’s 6th Assessment Report provided extensive guidance, particularly in the Working Group III report on climate mitigation, for actions to reduce emissions and to remove CO₂ from the atmosphere. The International Energy Agency’s (IEA) Net Zero by 2050 plan provides a roadmap with many specific milestones for reaching net zero emissions. For the US, the National Renewable Energy Laboratory developed four separate scenarios for achieving 100% clean electricity by 2035.

The IEA breaks the path to decarbonization into 7 processes, with changing importance over time (Fig 1).

• Energy efficiency – decreasing in importance over time as efficiency gains become permanent
• Behavioral change and avoided demand – increasing in importance as technology helps to reduce energy demand
• Electrification – increasing in importance to move energy demand to energy forms that can be supplied cleanly
• Renewables – supply approximately half of emissions reductions until 2030, and supply 90% of electricity by 2050.
• Hydrogen and hydrogen-based fuels – initially a transition fuel to avoid transmission and distribution needs, then a mainstay for flexibility in electricity and for transportation.
• Bioenergy – traditional biomass for cooking ceases by 2030, solid bioenergy provides flexible fuel, heat, CO₂ removal. Liquid biofuel becomes important for aviation.
• Carbon capture, utilization, and storage – offsets emissions during transition, especially in emerging and developing economies, removes CO₂ while permitting use of natural gas.

Figure 1. Decarbonization pathway for IEA Net Zero by 2050 plan. Emissions increase between 2020 and 2030, offset by decarbonization processes. Emissions increase from 2030 to 2050, offset by a different balance of decarbonization processes to reach zero emissions by 2050. International Energy Agency. CC BY.

Several of steps needed to reach zero emissions and beyond require technology and infrastructure not currently available. Carbon capture and storage, hydrogen technologies, and enhanced energy transmission systems will all need to be improved and extensively deployed to halt and eventually reverse global warming. It is not enough to create clean energy – it must be stored so that supply can meet demand, it must be transported from production sites to end users, and new energy types must be integrated into the existing power grid so that control of the power supply remains smooth.

Some processes that currently exist, particularly in carbon uptake by ecosystems and agricultural soils still need extensive study. As a result of ongoing developments and research needs, all scenarios to halt and reverse global warming have considerable uncertainty built into them. Some aspects may go faster than anticipated, but so far, where large uncertainties exist, deployment has been very modest.

Reaching net zero emissions will neither immediately halt nor reverse global warming

As noted in Chapter 2, research indicates that if all anthropogenic GHG emissions were to cease, global temperatures would level off in a few decades – not immediately, but perhaps within a lifetime. But a return to pre-industrial temperatures – a decrease in planetary temperature – will require many centuries. Partly this is due to the lifetime of greenhouse gases in the atmosphere, but CO₂ and heat have also been absorbed by the ocean, including the deep ocean, and all of that heat and CO₂ must be purged to return the planet to pre-industrial temperatures.

The IPCC scenarios that result in global warming that remains below 1.5°C above historic levels (or quickly returns to levels below 1.5°C following an overshoot) all combine zero emissions with removal of GHG from the atmosphere. Carbon reduction strategies are a suite of practices, techniques, and associated technologies that manage carbon either by reducing emissions of GHG or by removing GHG already present in the atmosphere. Carbon capture and storage is one technique that involves capturing carbon emissions at the source – at power plants and industrial sites – and redirecting them into storage in the deep subsurface, or removing CO₂ directly from the atmosphere through a variety of means and also storing them geologically. Some emissions may be used to create carbon-based products, in carbon capture, use, and storage (Fig 2).

Figure 2. A generic carbon capture, use, and storage system. International Energy Agency CC BY.

Although deployment of carbon capture and storage is increasing, it still falls short of levels considered necessary to fulfill its role as part of carbon management strategies needed to hold warming at or below 1.5°C (Fig 3). Technology for carbon capture at emissions sites is still improving; support for development and implementation can speed progress, but political and economic barriers may exist.

Figure 3. Carbon capture capacity compared to needed capacity for the International Energy Agency’s net zero emissions (NZE) scenario, 2020-2030. International Energy Agency. CC BY.

NZE = Net Zero Emissions by 2050 Scenario. Includes large-scale projects with a capture capacity over 100 000 t per year (1 000 t per year for DAC). Capture projects for CO₂ use are included as long as CO₂ is used in fuels, chemicals, polymers, building materials, or for yield boosting. Within planned carbon-capture-use-storage (CCUS) industrial hubs, only identified CO₂ capture projects are included (not the full potential capture capacity of industrial hubs for which capture sources are not specified).

Nature-based climate solutions are also available for carbon capture and storage. Plants take up CO₂ during photosynthesis and store the resulting carbohydrates in biomass. Soil can sequester carbon from organic material. Some organic material, such as woody tissue, is naturally resistant to decomposition. But organic material can also become sequestered within soil particles, and bound to the surface of soil particles, holding it in the soil without decomposition to CO₂.

Land management practices of ecosystems including halting deforestation, restoring naturally occurring forests (reforestation), planting forests where forests did not previously exist (afforestation), restoration of ecosystems with high plant biomass and high-carbon soils such as occur in wetlands, some tropical forests, and boreal forests can increase CO₂ removal from the atmosphere and retention in ecosystems. Regenerative agriculture that seeks to maintain and improve soil health is similarly useful. We will see more about these practices in later chapters.

Barriers to a transition to clean energy

Complex energy systems

Energy contributes to quality of life for individuals and drives economies of nations. Traditional energy systems are over a century old and worked well to advance society into the modern age through the use of high-density energy resources – fossil fuels. However, the greenhouse gases associated with that progress are an outstanding example of an externality that was never calculated into the overall expense of the energy systems, and the cost of addressing that externality has become extreme, and, unsurprisingly, unwelcome. Cost reductions have helped to ease the transition, but as energy demands continue to increase, the overall share of cleaner energy sources in the world energy portfolio has increased rather little.

Wholesale change in energy portfolios is therefore something to approach with care. Science can describe a path to clean energy, but implementing such large changes involves a host of other actors in policy, politics, communication and other social and economic areas.

National energy grids are typically complicated and highly regulated. Energy is subject to regulation at local, state, and federal levels, in the US, and even changes associated with conventional energy can be mired in delays.  Watch video click here for a discussion of the complexities associated with the US power grid and the increasing power demands associated with electric cars (if you like, stop at time code 11:13). Note that most of the relevant elements of the Inflation Reduction Act in the US, referenced in the video, have been halted by the subsequent US administration.

Social, political, and economic forces

From the earliest stages of global warming, scientific information has been met with distrust and disbelief. As global warming progresses and scientific certainty grows, along with loss of life and property, distrust and disbelief persist. Considerations of financial and political power have led to persistent misinformation and opposition. These, in turn, are sometimes offset by the economic and social advantages associated with reducing air and water pollution and climate change.

In the past, new technologies that were in the best interests of society often received support from governments in the form of direct subsidies, tax exemptions, protection of patents, etc. As technologies mature, industry begins to take over support in order to make profits, and prices begin to become competitive with existing technologies, so that market forces begin to encourage adoption of the new technology. The more consistent the support, and the more considered the transition from government support to private support to widespread adoption, the more straightforward the process. Development of both technology and policy to halt and reverse climate change has often lacked support and consistency. The changes envisioned are large, which creates the potential for economic and political winners and losers. The US, in particular has seen see-sawing support for climate-related actions and policies. Changes in financial incentives for climate-related actions have global repercussions; supply chains for climate-related technologies, including for critical minerals, have global reaches. The result has been slower progress, but not halted progress, in combating global warming.

For a discussion of the global processes related just to carbon capture and sections, watch this video that discusses the status, trends, and barriers to carbon capture and sequestration as a global mechanism to reduce and reverse carbon emissions. From the The 2nd High-Level Roundtable on Carbon Management Technologies, held in Riyadh, Saudi Arabia in February 2023. Note that most of the relevant elements of the Inflation Reduction Act in the US, referenced in the video, have been halted by the subsequent US administration, whose intransigence in matters of energy production contributes significantly to the barriers to decarbonization.

Geoengineering: an emergency intervention for climate change

Geoengineering involves major interventions into Earth’s energy processes in order to reduce or reverse climate change. Proponents suggest such interventions as emergency measures to avoid the worst impacts from climate change. However, safe testing procedures would require a test planet, and ours is the only one available. The balance between the documented evident harms from present and future climate change on one side and the potential harm of impacts from geoengineering is very hard to estimate, given the large number of unknowns on both sides.

Solar radiation modifications

The group of geoengineering interventions grouped together as solar radiation modifications seek to increase planetary reflectance or albedo to reduce the warming impacts of solar radiation.

• Injecting sulfur dioxide into the stratosphere to increase albedo. As we learned in chapter 2, atmospheric sulfates are associated with acid rain, which can cause severe air and water pollution and significant damage to terrestrial and aquatic ecosystems. Modeling work on sulfate geoengineering suggests that the approach is unlikely to cause catastrophic harm, but would change the location of acid rain from industrial regions to less disturbed parts of the planet where sensitive ecosystems may be at risk. Sulfate injections are achievable with current technology and would last 1-3 years in the atmosphere, requiring ongoing injections.
• Spraying aerosols of sea salt (basically, seawater) into low-elevation marine clouds to enhance cloud cover and associated reflectivity. This practice is achievable with current technology. Effects would be short-lived, so it would be easy to stop impacts. A recent review of research needed to understand this approach emphasizes that the effectiveness of the practice is still unclear, and that risks of changes to regional temperature and rainfall and resulting impacts to human and natural systems are also unclear.
• Orbiting mirrors or sunshades. This approach would carry large blocking, reflecting, or solar-panel-covered bodies into a point in space where they would be stable and would intercept solar radiation. This approach is currently only hypothetical as the fleet of spacecraft needed for the operation and the engineering technology needed for the sunshades do not exist at this time. Proponents point out that deploying sunshades avoids actions on Earth, can be undone at need (if you have the spacecraft to deploy them, theoretically, you have the spacecraft to move them out of line with the sun to return incoming radiation), and could, potentially, supply additional energy to Earth if the sunshades were designed to collect or focus solar energy to terrestrial energy facilities.

Carbon dioxide removal

A variety of geoengineering techniques are suggested that can remove carbon dioxide from the atmosphere using biological and geological approaches. Four of the most common suggestions follow.

• Direct-air capture removes carbon dioxide directly from the atmosphere and then injects it deep underground to sequester it there, permanently. In 2024,  the International Energy Agency reported that several nations had plans to implement carbon removal in the coming years, but the process is expensive and the volumes of C to be removed are still small relatively to the overall need.
• Nature-based climate solutions facilitate carbon uptake by plants and soil in natural ecosystems and also in agro-ecosystems. In addition, some solutions seek to maintain carbon in natural carbon sinks, particularly in organic soils in tundra, wetlands, and peatlands, including tropical rainforest peatlands. Our understanding of the details of carbon processes in natural ecosystems and agroecosystems is still incomplete, but researchers estimate that land systems presently absorb perhaps 30% of current emissions. Reforestation and protection of existing forests, wetland restoration, and supporting large grazer populations on tundra are some of the recommended approaches for increasing carbon sequestration in natural systems. Chapter 7 describes some of these approaches for using agroecosystems for carbon sequestration. 
• Ocean fertilization uses iron additions to the ocean to increase algal biomass in less productive parts of the ocean. The algae take up CO₂ during their lifetimes, during photosynthesis and incorporate it into their biomass. When they die, some of the biomass sinks to the ocean floor, which is a global C sink. Ocean fertilization experiments have been performed, and they do increase uptake of C by algae. However, these experiments also have the potential to cause ecosystem-level changes that propagate through the ocean, reducing productivity by using up nutrients to produce algae that would otherwise be used elsewhere in the ocean. Models of fertilizing the Southern Ocean suggest significant impacts to tropical fisheries, demonstrating the potential for far-reaching ripple effects from the practice.
• Enhanced weathering of rock formations to absorb CO₂ during weathering – CO₂ from the atmosphere is chemically fixed into the minerals that form during weathering. In these approaches, ground rock would be applied to oceans or land. Powdering the rock (olivine is one suggested mineral to use) greatly increases its surface area, providing more surface for weathering reactions that transform CO₂ into rock. The chemistry is not always straightforward and rock with the wrong composition could actually increase GHG emissions. But the largest problem with enhanced weathering is the cost and energy use of mining, pulverizing and transporting rock.

Just as the use of natural gas was put forward as a means of transitioning from coal to decarbonized energy sources – a way to buy time while technology was developing and society was adapting – geoengineering is suggested as a means of offsetting carbon emissions while society continues to make progress towards decarbonizing. One major criticism of geoengineering is the same as a major criticism leveled against use of natural gas as a transition fuel: those who profit from slowing progress towards decarbonization will use the breathing room gained from geoengineering to continue business as usual with fossil fuels, slowing progress and dulling public perception of the urgency of the climate crisis.

However, as mentioned above in this section, all modeled climate futures that avoid global warming above 1.5 or 2°C require some kind of removal of carbon dioxide or faster-than-decarbonization-only reduction in GHG emissions. To date, carbon capture and storage is the primary means at hand.

The United Nations Convention on Biodiversity created an embargo on large-scale geoengineering in 2010, pending strong scientific justification for its use and full understanding of potential impact. Some have noted that concerns about impacts from geoengineering have mostly successfully stymied work to determine feasibility of some approaches. In the face of growing concerns about climate-change impacts and growing pressure to slow climate change, perceptions of nuclear energy have changed considerably. If the pace of decarbonization proves unequal to the task of significantly slowing climate change, perceptions of geoengineering may similarly change.

Energy sustainability is not only about global warming

Global warming is probably the most urgent aspect of energy sustainability at this time, because the resulting climate change affects everyone, everywhere, with increasing force. Climate change touches directly on Sustainable Development Goals 7 (affordable and clean energy) and 13 (climate action), but also on many others, including Goal 3 (good health and well-being), 12 (responsible consumption and production), 14 (life below water) and 15 (life on land).

Increasingly, energy sustainability is also associated with additional social goals, including 1 (no poverty), 2 (zero hunger), 4 (quality education), 5 (gender equality) and 11 (sustainable communities). While developed countries are learning to integrate new, advanced technologies into their energy grids in order to reduce GHG emissions, many areas of the world still have limited access to clean fuels for cooking (Fig 3). The burden of gathering fuel for cooking fires in developing countries falls largely on women and children (particularly girls), contributing to inequality and lack of education. Similar issues related to unequal impacts arise with mining activities associated with energy. Unequal impacts of energy generation include impacts from climate-change, air and water pollution, and land-use change. Some of these sustainability issues will be improved as efforts to reduce climate change continue. Legacy problems associated with prolonged, unequal environmental impacts and related social and economic impacts, will require more focused attention.

Figure 3. Proportion of the population with access to clean fuels for cooking, in 2021. OurWorldinData.org. CC BY.

Knowledge Check

Take a moment to complete the short quiz below to assess your understanding of this section. Read each question carefully and refer to the section content as needed. This quiz is not graded – it’s simply an opportunity for you to reflect on what you’ve learned and reinforce key concepts.

The data centers that power AI have grown in energy consumption at about 12% annually since 2017, which is four times faster than total energy consumption over that period. Data centers currently consume approximately 25% of the power generated in the state of Virginia in the US (the state with the highest level of data-center energy use). And forecasts suggest the pace of growth will accelerate. At the same time, AI excels at optimization and can generate savings throughout the sector. The case study here looks at the current understanding of AI’s role in energy, including data gaps that are to be expected in a largely proprietary landscape: AI developments are largely in the private sector.

First, consider the Executive Summary of the International Energy Association’s 2025 World Energy Outlook Special Report on Energy and AI. The executive summary avoids more technical language and provides a good synthesis of the report findings. For specifics, read on, into the body of the report itself.

Second is an episode of Inside Climate News Sunday Morning, titled AI’s Massive Energy Demands, with a conversation between executive editor Vernon Loeb and energy reporter Dan Gearino that helps clarify some of the issues discussed in the report.

Providing food for more than 8 billion people cannot be done without environmental impacts, any more than the mere existence of 8 billion people can occur without environmental impacts (Fig 1). Rather, sustainability in agriculture typically seeks to feed the world while maintaining or improving the productivity of soil and minimizing impacts on air and water quality, water availability, and biodiversity. Issues of safe storage and equitable distribution of food are separate from agriculture and will be addressed at the end of this chapter.

This chapter relates most closely to UN Sustainable Goal 2 on zero hunger and Goal 12 on Responsible Consumption and Production, but because our focus is on environmental impacts, goals for clean water, life on land, and life on water are also relevant. As we will see, agriculture affects the status of most of the 9 planetary boundaries of Earth, including those for biogeochemical flows (nitrogen and phosphorus), biosphere integrity (especially functional integrity of soil and water), land system change (deforestation), freshwater change (irrigation), and novel entities (synthetic fertilizers and pesticides.

Figure 1. Environmental impacts of food and agriculture. H. Ritchie, OurWorldinData. CC BY 

Some definitions relevant to agriculture

The international Food and Agriculture Organization includes within its definition of agriculture the raising of crops in soil for food and fiber, husbandry of livestock for food and other products, aquaculture of aquatic species, fisheries management, and silviculture of forests and plantations. In the US, most of these are under the purview of the US Department of Agriculture, including

• the Natural Resource Conservation Service, which has a primary focus on soil conservation on private lands under a variety of land uses including farming, grazing, and forestry and
• the US Forest Service, which manages the federal public lands of the national forests and grasslands for multiple uses include timber harvest, watershed management, grazing of privately owned livestock, hunting, fishing, biodiversity conservation, recreation, and extraction of fossil fuel and mineral resources.

Farming may seem like a narrower term, and it is. But, technically, farming encompasses growing of crops as well as animal husbandry, including aquaculture. Agronomy is the area of science related to soil management and crop production. Horticulturalists study and grow plants in managed areas, but these can include turf grasses, ornamental plants, and many food plants such as fruits, nuts, herbs, and spices, usually grown at smaller scales. Horticulture does not extend to so-called row crops, which are planted mechanically or by hand, in rows. Most major staple crops are row crops – corn, soybeans, wheat, rice – but vegetables can also be grown as row crops.

In less formal use of the term, “farming” is often limited to growing of non-woody food and fiber crops. This chapter deals primarily with this aspect of farming, and briefly with issues related to livestock production and aquaculture. Wildlife, fisheries, and forest management are addressed in later chapters.

The following terms describe agricultural and related systems, and may be used later in the chapter. Note that terms such as conventional farming, organic farming, and regenerative agriculture are stereotypical descriptions that can become rather like caricatures rather than serving a useful purpose in understanding agricultural practices. It may be easier to think of terms like these as positions along gradients of farming practices – some fields and some farmers (or producers) may use some, none, or all of the practices described here.

Conventional farming or industrial farming or intensive farming describes farming at the far end of the technical gradient that involves mechanization in soil preparation, planting, harvesting of crops, and treatment of crop residues. Most fields contain a single crop – a monoculture. Typically, high levels of inputs are used, including synthetic fertilizers and pesticides in order to maximize outputs (yields and/or profits). Genetically modified crops may also be used to maximize yields and profits. Fields managed with conventional farming can be quite large, and row crops are the most common crops. In drier areas, some form of irrigation may be used.

Organic farming occupies a different position from conventional farming with respect to synthetic fertilizers, pesticides and genetically modified crops, which are avoided. However, it may resemble conventional farming with respect to mechanization – particularly in organic farming of row crops – and in levels of soil disturbance. In many countries “organic produce” is a term defined by law, which therefore controls some of aspects of organic farming.

Regenerative agriculture is an approach to sustainable agriculture that seeks to improve resilience of agricultural systems,  maintain or improve soil health and productivity, support biodiversity, and contribute to the health and integrity of surrounding ecosystems. It incorporates a holistic view of the agricultural environment and acknowledges that the natural world can both benefit and benefit from agriculture. Conventional farms and organic may both use practices considered regenerative (for example, cover crops) without using only practices considered regenerative. Like permaculture (see below) and sustainable agriculture, generally, regenerative agriculture is defined more by the intent or aspirations of the farmers, rather than by specific practices or laws.

Precision agriculture involves the use of spatial technology, linking agricultural equipment to geolocating information to allow variable delivery of water, fertilizers and pesticides according to the precise needs of the crop, at the scale of feet or meters. Typically, precision agriculture leads to lower use of inputs and thus can reduce environmental impacts.

Climate-smart agriculture seeks to maintain or increase production under climate-change stresses while also decreasing contributions to climate change by decreasing GHG emissions from agricultural systems.

Agroforestry intentionally integrates useful trees and shrubs with crops and livestock pastures to diversify production, increase outputs, and improve environmental outcomes.

Subsistence agriculture is any kind of farming that is undertaken to feed a family or community, rather than for profit.

Shifting cultivation or swidden agriculture or slash-and-burn is a traditional farming system practiced in forested areas, especially in the tropics where soil fertility is low. Farmers clear an area of forest and burn the forest vegetation, producing mineral-rich ash that fertilizes the resulting fields for a short period. After one to a few years, a new field is cleared and the previous field is left to regrow for a longer period. Sustainable under low population densities, shifting cultivation can become unsustainable at higher population densities where forested areas are not large enough to allow forest to continue to exist under the constant pressure to clear land.

Permaculture is presently a movement, as much as, or more than, a form of farming. Because it sounds like a form of agriculture, it is included here. The World Permaculture Association defines permaculture this way.

Permaculture is an ecological design philosophy that emphasizes sustainable and harmonious integration between human systems and natural ecosystems. It goes beyond traditional gardening or farming; it’s a holistic approach that involves observing and mimicking the patterns found in nature. By understanding and working with these patterns, permaculture aims to create resilient and self-sustaining ecosystems that benefit both people and the environment.

Knowledge Check

Take a moment to complete the short quiz below to assess your understanding of this section. Read each question carefully and refer to the section content as needed. This quiz is not graded – it’s simply an opportunity for you to reflect on what you’ve learned and reinforce key concepts.

The vast majority of crop production relies on soil as the medium of growth. Review the linked PowerPoint for basic soil information relevant both to sustainable agriculture and to conserving natural ecosystems.

• An Introduction to Soil (PDF)

Knowledge Check

Take a moment to complete the short quiz below to assess your understanding of this section. Read each question carefully and refer to the section content as needed. This quiz is not graded – it’s simply an opportunity for you to reflect on what you’ve learned and reinforce key concepts.

Benefits

Conventional farming developed because it does a good job of producing crops and profits. In the aftermath of World War II, soil processes were poorly understood, the agricultural areas of the world had deep, rich, organic layers, and technology was leading to progress in many areas. Many developments and expansions of that era resulted in unintended environmental consequences due to limited understanding of environmental processes – DDT, acid rain, cheap gasoline, cheap fertilizer … Most started with benefits. The Green Revolution of the latter part of the 20th century dramatically reduced hunger around the world. By increasing yields on productive land, it prevented large areas of less productive land from being put into production to feed the world’s growing population, also staving off related GHG production, biodiversity loss, and other environmental impacts. Fewer farm workers were needed to produce food, allowing more people to seek higher paying jobs.

Figure 1. Tillage after corn harvest in Wisconsin, USA. Photo by Wikideas1. Wikimedia Commons. CC0. Video here.

Tillage

Conventional farming is economically efficient if conservation of the soil resource is not part of the calculation. Tillage, the process of plowing and turning the soil in spring prior to planting seeds, dries and warms the soil, speeding initial crop growth, and killing weeds without the use of herbicides (Fig 1). In colder climates and on compacted soils, tillage is particularly good at improving yields.

Drainage and irrigation

Figure 2. Agricultural drainage pipe draining a field and discharging water into a ditch. Control structures are often not present. When they are, they can control the water level in the field at levels higher than the drainage pipe. This can be useful for optimizing water to crop roots and to reduce nutrient discharge into ditches and local waterways. US Natural Resource Conservation Service. Public domain.

Wetland soils are typically rich in nutrients, and such soils are often drained to allow access for agriculture. Waterlogged soils are only conducive to a few crops (primarily rice), and cannot be worked by usual farm machinery, which requires solid ground.

In the US, tile drainage (perforated pipes) is placed underground throughout fields to carry excess water to drainage ditches at the edges of fields (Fig 2). In the state with highest use, Iowa, approximately 53% of cropland is drained in this way. Tile drainage is used extensively in the US, but most intensively in the Midwest (Fig 3). Artificial drainage is used worldwide in areas of intensive agriculture.

Figure 3. County-level tile drainage area in hectares based on the US Department of Agriculture 2017 Census of Agriculture. Valayamkunnath et al. 2020. CC BY.

Figure 5. Satellite image of part of Kansas in the US, showing high-density use of center-pivot irrigation. US National Aeronautics and Space Administration. Public domain.

Irrigation is used to supplement water to crops, throughout the world (Fig 4). Gravity-fed methods run water downhill without mechanical assistance and are used to flood entire fields, for example for rice, or to run water into furrows. Mechanization allows sprinklers to be fixed or that can move using electrical power  either in a line that “walks” through the field or from a central water supply in central-pivot irrigation (cheaper than linear systems; creates a circle of irrigated area;  Fig 5). Drip irrigation is most expensive to employ, but allows precise delivery both in quantity and in location. Irrigation relies on local surface or groundwater. Irrigation increases the total area available for agriculture and also changes the amount of land that is suitable for specific crops.

Figure 4. Global agriculture and irrigation. Areas with color different from the background are cropland. The nature of the color shows the proportion of cropland that is irrigated. Tian F et al. 2025. CC BY.

Drawbacks 

Tillage

Figure 6. Soil carried in runoff from a cornfield after rain. AdobeStock NokHoOkNoi.

Among the soil processes unknown as conventional agriculture was taking over industrial food production was the link between mycorrhizal fungi and plants of many kinds, including crops. Section 7.2 introduces the importance of these fungal-plant partnerships.

The form of mycorrhizal fungi in the soil – a dense, fine network or mycelium of threadlike hyphae – as much as 100 m per cubic centimeter of soil (a football field length in a quarter teaspoon) – makes them vulnerable to plowing. Plow blades shred the networks that bring water and nutrients to plants, making fertilizers and irrigation more necessary and reducing resilience.

Tilling also destroys the crumb-like aggregate texture of good agricultural soil that facilitates good root growth,  aids water percolation into the soil and allows oxygen to penetrate to deeper roots. It kills beneficial soil organisms such as earthworms (which also contribute bioglues that create good aggregate structure). Although it fluffs up the upper layer of soil, tilling creates a layer of compaction below the plow-blade depth, reducing root penetration and water percolation to greater depths in the soil, where water might remain available during droughts. The drying effect that allows soils to warm earlier in the growing season reduces water percolation and availability, potentially hastening agricultural drought during dry seasons. And by reducing deep infiltration, killing fungi, and reducing the amount of bioglue in the soil, tilling substantially increases erosion, even on comparatively flat ground (Figu 6; note the ditch in the foreground).

Erosion carries soil into nearby receiving waters, but not only soil. Runoff carries dissolved nutrients, contributing to eutrophication and reducing dissolved oxygen. Sediment builds up in streams, burying stream bottoms in muck and destroying spawning habitat for fish and invertebrates. Soil particles and algae reduce water clarity. Runoff also carries herbicides, insecticides and other pesticides in water and adsorbed to soil particles, potentially poisoning aquatic life and humans and wildlife that drink the water.

Finally, tillage exposes deeper soil to oxygen, hastening the decomposition of organic material and decreasing organic material and thus C stored in the soil. Organic material contributes to nutrient- and water-holding capacity of soil, and sequesters carbon, as well as providing nutrients for bacteria and fungi. By reducing C in soil, tillage contributes to climate change.

Drainage and irrigation

Drainage water coming off of agricultural fields carries with it some of the various substances applied to the fields, as described above. Drainage may help to conserve soil by minimizing runoff from across the top of the soil, but dissolved substances will percolate into the drainage system and contribute to water pollution, including eutrophication. Depending on how well the amount of drainage is controlled, it can also cause fields to become dry sooner than necessary, during a drought.

Irrigation expands the footprint of agriculture, increases yields of high-value crops and confers resilience against drought. But it can only do so as long as water remains available. Because irrigation can increase productivity and value in agriculture, it can also be associated with depleting local water resources, particularly groundwater in confined aquifers that cannot be recharged. Aquifers on every continent that supports agriculture are included in the list of such aquifers, as we saw in section 4.2 of the Water Availability chapter.

Fertilizers

Both natural and synthetics fertilizers are used in conventional agriculture. Manure from livestock is a common natural fertilizer rich in many nutrients, particularly nitrogen and phosphorus. By far the largest amounts of synthetic fertilizers are bioavailable forms of nitrogen – ammonium nitrate, ammonia, and urea – and bioavailable forms of phosphorus – phosphates. Additional nutrients may be added depending on local soil makeup and crop needs.

Fertilizers support early and rapid growth of crops; some commercial crops are bred or engineered to use nutrients rapidly. In the midwestern US, fall application of fertilizers, months ahead of crop emergence, is common. Drier fields in fall are easier and safer to access, and moving fertilizing to fall reduces the number of field entries in spring. However, leaving fertilizers in the field over winter increases the opportunity for nutrients to leach from soils into local waterways. Because fertilizers are relatively inexpensive in the US, the loss is economically supportable, but environmentally harmful. For the same reason, farmers often overapply fertilizers, increasing leaching.

Fertilizer run-off is a major source of nutrient pollution (see Chapter 3). Nitrates are highly soluble and nitrogen fertilizers are often applied heavily. Some common nitrogen fertilizers acidify soils, and acidification reduces soil ability to bind nutrients, increasing the tendency for nitrates to be washed from fields and into rivers and lakes. Nutrient pollution leads to “dead zones” in estuaries that drain agricultural land. In these areas, both commercial and recreational fisheries suffer economic losses as marine organisms leave the area or die from lack of oxygen. In severe cases of freshwater pollution by nitrates, infants may suffer from blue-baby syndrome, which can be lethal.

Limits on nutrients in farm runoff are managed by states in the US, because runoff is a nonpoint process – not all state impose specific limits. Where they exist, nutrient limits can be difficult to enforce because nutrients are everywhere and all farmers apply them. Nutrient pollution is widespread, as a result.

Fertilizer use is not high everywhere. Africa, particularly  sub-Saharan Africa, uses approximately 20 kg/ha of fertilizer (17.8 lbs/acre), whereas the global average is approximately 135 kg/ha (120 lbs/acre). Fertilizer is largely imported, much of it, previously, from Ukraine and Russia, so that supply chains have been disrupted and prices have increased since Russia invaded Ukraine. In addition, distribution and delivery are hampered by infrastructural problems. Reduced availability of fertilizer is one of the reasons for high food insecurity in Africa.

Knowledge Check

Take a moment to complete the short quiz below to assess your understanding of this section. Read each question carefully and refer to the section content as needed. This quiz is not graded – it’s simply an opportunity for you to reflect on what you’ve learned and reinforce key concepts.

Conservation tillage

Conservation tillage is an umbrella term that covers a range of reduced-tillage and no-till systems. A key aspect of conservation tillage systems is to leave sufficient crop residue in the field to substantially reduce erosion potential of rain and wind. No-till leaves all residue in place. Some crop residues decompose quickly and provide less protection than others. Under a no-till system, seeds are planted with a seed drill or using a plow attachment that cuts a shallow furrow through the crop residue but only very shallowly into the soil. Rates of adoption of no-till are variable in the US, with the areas of highest adoption above 40% (Fig 1). Where year-round growing seasons occur, conservation tillage may also be an important means of increasing soil nutrients.

Figure 1. US no-till acres as a percent of total cropland acres, by county, 2022. US Department of Agriculture National Agricultural Statistics Service, from the 2022 Census of Agriculture. Public domain.

Other conservation tillage systems employ some kind of soil-disturbing process, but limit the depth and width of the disturbed area, to leave as much undisturbed soil as possible. The soil is often only sliced rather than sliced and overturned. Strip tillage usually combines strips of reduced-depth tillage with untilled strips, to minimize soil disturbance and retain some of the benefits of the crop residues. These intermediate measures allow farmers to get some of the benefits of tillage while minimizing erosion and loss of beneficial soil organisms and soil characteristics.

Conservation tillage may require that farmers re-tool to handle planting, fertilizing, and pest management while protecting crop residues and minimizing soil disturbance. Old equipment cannot be modified for the new techniques. The expense of farm equipment means that such changes in farm practice take significant investment and result in sunk costs – investments that cannot easily be recovered in the event of a change of plans. Farmers are understandably reluctant to undertake such changes without clear evidence that the results will be worthwhile.

Conservation tillage has limitations. In wet or compacted soils, the limited disturbance leaves soils poorly prepared for planting. In cold and wet conditions, crop residues may harbor more disease organisms and harmful molds, leading to crop loss. Without the soil turnover of standard tillage, weed loads are much higher, requiring additional expense to eliminate. Long-term conservation tilling also risks compaction in subsoil from heavy machinery use to plant and harvest crops. In the future, light, autonomous (self-driving) farm equipment may reduce compaction.

Cover crops

Cover crops are crops grown after the primary crop is harvested, to protect the soil, reduce weeds, aid in carbon sequestration in the soil, and, in some cases, to add an additional crop. Some cover crops can be used to take up excess nutrients in the soil to reduce nutrients in field run-off and protect water quality. Other crops, particularly legume crops (clover, vetch, pea) can be used to add N to fields as natural fertilizers, to replace synthetic fertilizer. Cover crops are typically used used in areas with a cold or a dry season and must be able to tolerate the harsher conditions between crops. Rates of cover-crop adoption are still low in the US (Fig 2).

Figure 2. US cover crop use as a percent of total cropland, by county, 2022. US Department of Agriculture Economic Research Service from 2022 Census of Agriculture. Public domain.

All cover crops protect soil, but their ability to reduce weeds depends on the nature of the problem weeds in the area. Some weeds, particularly perennial weeds, can compete well against cover crops. Some cover crops have an allelopathic effect – they are harmful to other plants. Usually, these effects work better against annual weeds than perennial weeds. And farmers must be careful not to choose cover crops or cover-crop schedules that may expose crops to allelopathic effects that could reduce growth and yield.

Cover crops protect water quality by limiting erosion, but they also take up excess nutrients from the soil. In a field that is left fallow during the non-growing season (the cold season or the dry season), any fertilizer residues or nutrients from decomposing crop residues can leach into nearby waters. Cover crops reduce the likelihood of leaching and reduce eutrophication.

After a harvest, mycorrhizal fungi lose their plant partners. A cover crop provides plant roots to continue to support the fungi. In one study, the soybean crop planted in the growing season after the cover crop grew larger and took up more P than the crop planted after the field was fallowed.

Figure 3. Tractor with a roller crimping a rye-grass cover crop in spring, to allow planting of a commercial crop. Ted Kornecki, Agricultural Research Service, US Department of Agriculture. Public domain.

When it is time to plant the new primary crop, cover crops must be killed, unless the primary crop is strong enough to grow through the cover crop and shade it out or outcompete it. Broad-spectrum herbicides are often used to kill cover crops and any weeds. Such herbicides may drift, if applied aerially, or run off into streams, and can kill native plants and harm beneficial insects and animals of other kinds. Crimping – using a heavy roller to flatten the cover crop – can be used to harm the crop mechanically, effectively killing it and reduce its ability to block light to seedlings (Fig 3). Crimping creates a mulch layer – a layer of organic material over the soil – that reduces weeds and protects soil moisture. It also helps to protect against erosion until the crop is established and can provide that protection.

Green manures are a variation of cover crops that are grown specifically to be tilled back into the soil before the commercial crop is sown. The tilled-in vegetation breaks down and provides nutrients for the new crop while also improving aeration and water storage. The green-manure crop can take up excess nutrients after the commercial harvest, turn those nutrients into plant material over the winter, which then decomposes after it’s tilled in. Legumes are common green manure crops.

Knowledge Check

Take a moment to complete the short quiz below to assess your understanding of this section. Read each question carefully and refer to the section content as needed. This quiz is not graded – it’s simply an opportunity for you to reflect on what you’ve learned and reinforce key concepts.

Pests are organisms that occur where they are not wanted or that cause damage to crops or humans, or other animals. Thus, the term “pest” is highly subjective. Researchers estimate that crop losses to pests and diseases for major crops average 17-30%.

A pesticide is a term for any substance intended for preventing, destroying, repelling, or mitigating any pest. Though often misunderstood to refer only to insecticides, pesticides also apply to herbicides, fungicides, nematicides, and other substances used to control pests. By their very nature, most pesticides create some risk of harm—pesticides can cause harm to humans, animals, and/or the environment because they are designed to kill or otherwise adversely affect living things. At the same time, pesticides are useful to society because they can kill potential disease-causing organisms and control insects, weeds, worms, and fungi.

“Pesticide” is a general term; more specific terms relate to the specific kind of target pest: herbicides, insecticides, nematicides (nematodes), ovicides (eggs of pests), etc. Pesticides vary in their level of specificity, and many of them kill beneficial species as well as pest species. The term “broad-spectrum” refers to a pesticide that kills a wide range of species.

Pesticide use is common in agriculture

Conventional agriculture almost always plants monocultures – fields or entire landscapes comprising a single crop. Large populations of a single crop can support large populations of pests that specialize on that crop. Once established, pest populations can be difficult to control or eradicate. As a result, pest loads in conventional agriculture can be high, and pesticide use can be similarly high.

Chemical pest management has helped reduce losses in agriculture and increase food and fiber production. Chemical pesticides can be effective, fast acting, and adaptable to a variety of crops and situations. When first applied, pesticides often result in impressive production gains. However, despite these initial gains, excessive use of pesticides can be ecologically unsound, leading to the destruction of natural pest enemies, increased pesticide resistance, and outbreaks of secondary pests. Impacts to human health, including health of agricultural workers applying the chemicals, are also of concern. Most synthetic pesticides persist in the environment, undergo bioaccumulation and biomagnification, and are classified as “forever chemicals.”

Insecticides and genetically modified (GM) plants

Synthetic chemical insecticides began with DDT, developed in the 1940s to control malaria and other insect-borne diseases. Its use was then supported by the US government and by industry in agriculture and households. The resulting loss of beneficial insects and birds and harm to human health led Rachel Carson to write Silent Spring, a book contributed significantly to the environmental movement of the 1960s. DDT was banned in 1972, but it, like succeeding synthetic pesticides, is very slow to break down in the environment, and it bioaccumulates. One of the breakdown products, of DDT, DDE, is also harmful, continuing the damage caused by the initial use. The international treaty called the Stockholm Convention on Persistent Organic Pollutants bans use of DDT in agriculture, but because of its effectiveness against mosquitoes, the treaty makes an exception for DDT use for public health, primarily against malaria (mostly indoor applications), so long as that use falls within World Health Organization guidelines.

Insecticides that have followed DDT have included chemicals with shorter-acting lifetimes such as organophosphates and carbamates, which are active for less than a season. However, PFAS are becoming more common in insecticides, as inert ingredients (for example, to reduce drift or facilitate uptake on stems and leaves). In the US, manufacturers do not need to disclose toxicity information for inert ingredients, making it harder to detect these forever chemicals in agricultural products. Fluorination is also being used with active ingredients (thus moving them into the PFAS category), to extend active lifespan, creating more forever chemicals and chemicals that can have unwanted side effects and direct effects in the environment for longer.

Figure 1. Spray application of pesticides from a tractor. AdobeStock – Vesna.

Fertilizers and pesticides (for plants, insects, nematodes, etc.) may be applied as sprays. Application may be by backpack sprayer in smaller areas, by tractor (above), or by light aircraft.

A particularly problematical class of insecticides – nicotine-like, synthetic chemicals called neonicotinoids or neonics – have been widely used in agriculture in the US since the 1990s. Unlike other insecticides, which are applied as sprays (Fig 1), neonics are often applied as coatings on seeds, to protect the seeds from predators of seeds and seedlings. The seed coating is absorbed by the plant as it sprouts and grows, so that the toxins are incorporated into the growing plant, making it toxic to insects. During planting of coated seeds, neonic dust can be spread throughout the field and nearby areas, where it can be taken up by nontarget plants. Neonics are also applied by spraying). Once absorbed (seed coatings) or applied (sprays), the toxin is expressed in all plant parts as well as pollen and nectar, and affects the central nervous systems of insects of most kinds as well as other kinds of organisms including soil organisms. Neonicotinoids are implicated in severe declines of bees, including honey bees.

The European Union has banned use of three neonicotinoids since 2018, but permits use of others  that show less harm to bees. However, even neonicotinoids that cause lower mortality may have harmful side effects that affect mortality indirectly. The EU, in 2020, withdrew permission for thiacloprid, one of the neonicotinoids believed to be less harmful to bees, after researchers found it strongly suppressed immune responses in bees.  The US has no federal bans, but some states regulate some uses, as do some Canadian provinces.

Genetic modification has been used to enable plants to produce a toxin that is produced in nature by the bacterium Bacillus thuringiensis. Common Bt crops include cotton, canola or rapeseed, maize or corn, papaya, potato, soybean, and summer squash. Bt crops are preferable to broadcast spraying of insecticides because the toxins are contained within the plant. Following widespread use of Bt crops in the US, levels of insecticide use dropped, and this was seen as evidence of the success of the GMO approach in protecting crop yields. However, the toxicity of insecticides being employed has increased since the advent of Bt crops, offsetting the decrease in quantity applied. Toxicity has become more specific, with impacts to birds and mammals (including humans) declining. However, impacts to insects and plants have increased.

As the amount of Bt toxins has increased in the US, resistance in the targeted insect pests has also increased. The EPA requires that farmers using Bt crops plant 20% of their crop area in non-Bt varieties, so that selective pressure for resistance is reduced, and resistance develops more slowly. However, monitoring and enforcement of this requirement has been limited. Farmers resist the requirement to plant so-called refuges, concerned about yield decreases in the refuge. However, evidence suggests that , at least for corn, yields for non-Bt corn are similar to yields for the genetically engineered strains [note that most of the corn grown in the US is  field corn destined for livestock feed, not sweet corn destined for the dinner table].

Herbicides and GM plants

Figure 2. A monarch butterfly on a milkweed host plant. Jim Hudgins/US Fish and Wildlife Service. Public domain.

Prior to development of genetically modified crops, and accompanying increases in herbicide use in agriculture, milkweeds were common roadside plants in the US Midwest, and monarch butterflies were common. Increasing herbicide use led to the loss of approximately 70% of monarch-hosting capacity of milkweeds. Monarch butterflies have been proposed for “threatened” status on the US endangered species list, in part due to loss of milkweed.

In the 21st century, most herbicide applications in agriculture are of broad-spectrum herbicides that kill most plant species, regardless of whether or not they are pests. Herbicides can leave agricultural fields as drift during aerial application, and through run-off and leaching via water movement. As a result of this movement, they kill plants in areas beyond fields. Herbicides can also have impacts on wildlife and humans (Fig 2). Herbicide impacts include developmental disorders, endocrine disruption including reproductive problems, and damage to DNA.

Readers may notice the apparent contradiction of applying broad-spectrum herbicides to crops. How does the crop escape harm? Most of the major crop varieties now planted in conventional agriculture have been genetically modified to be resistant to one or more of the most commonly used herbicides.  With these crops in the fields, farmers can apply herbicides freely, knowing that only non-crop plants will be killed.

In contrast to insecticides use, which decreased dramatically after the advent of Bt crops, herbicide use has increased dramatically since the advent of genetically modified, herbicide-resistant crops. Use of glyphosate, the most commonly applied herbicide, increased by 1500% between 1996 and 2016. However, during this time, no-till and reduced-till agriculture also increased dramatically, requiring greater use of herbicides to compensate for loss of the weed control aspect of tillage. In addition, herbicides are being used for new purposes, such as to kill crops that need to dry before being harvested, for faster drying, rather than waiting for the plants to die naturally. And research shows that increases in herbicide use in GM crops was lower than in non-GM crops.

Resistance to herbicides since development of GM, herbicide-resistant crops has led to the reintroduction of older, more toxic herbicides that had been replaced by the glyphosate and other, newer herbicides. The fact that one of the reappearing herbicides, 2-4 D, was a component of the defoliant Agent Orange used during the Vietnam War that caused long-lasting environmental harm and harm to human health, added to the controversy surrounding modern herbicide use.

Use of genetically engineered crops and stacking of GM traits in crops

In 2024, the top 6 GM crops by area were soybean, maize, cotton, canola (rapeseed), alfalfa, and sugarbeet, and the top 6 countries in GM crop area were the US, Brazil, Argentina, Canada, and India. . Many countries have approved one or more GM crops for cultivation (Fig 3).

Figure 3. Countries that have approved use of genetically modified crops. This material is published by ISAAA (www.isaaa.org).

In the US, the leading country in acres of planted GM crops, most of the corn, soybean, and cotton that is planted is genetically modified (Fig 4). Several crops, particularly corn and cotton, carry stacked traits. That is, they carry more than one genetically modified trait. Traits may be stacked for a single purpose – most Bt crops carry genes for the expression of multiple toxins from Bacillus thuringiensis – or for multiple purposes – usually insecticide and herbicide traits. Countries generally approve GM crops by specific agricultural products, so early approvals may be of single-trait GM crops, and stacked-trait crop varieties would require additional approval.

Figure 4. Adoption of genetically modified crops in the US – 1996 – 2024. US Department of Agriculture, Economic Research Service. Public domain.

Pesticides are not the only solutions to pests

A recurring complaint about the increasing dependence of conventional agriculture on synthetic herbicides and other pesticides is that their use has led to a decrease in non-chemical approaches to pest management such as crop rotation and multi-pronged approaches such as integrated pest management for insect and invertebrate pests and crop rotation, cover cropping, intercropping, and integrated weed management for plant pests.

Crop rotation reduces insect and invertebrate pest loads and disease loads simply by depriving pest and disease populations of an ongoing food supply, so that crop-specific pests are regularly starved out and cannot build up in fields. In addition, crop rotation can reduce weed pests, increase resilience, and contribute to soil health and fertility. Crop rotation may require more equipment than a continuous single crop, to address the needs of planting and harvesting different crops. It requires more complicated understanding and monitoring of markets for multiple crops, and it prevents farmers from continually producing the most profitable crop, year after year.

The point of integrated pest management systems – for weeds, insects, or diseases – is that no single approach to pests provides a long-term solution. Whereas herbicides and pesticides are forms of artificial selection that lead to pests that are constantly harder to defeat, integrated pest management uses a variety of approaches, including chemical control; pests are less able to respond to several pressures and less likely to defeat multiple approaches.

Biological control organisms or biocontrols are another non-chemical means of combating primarily insect pests by using a natural predator of the pest. Natural predators can help to control crop pests, if they are not poisoned by pesticides and if habitat is provided for them. Mites, wasps, beetles, nematodes and other organisms can be purchased and released for one-time application or can occur naturally if strips of natural vegetation are left or planted with specific attractor plants, for habitat. Introduced biocontrols imported from other areas have also been used to control crop pests, but these are more problematical. Early efforts with imported species resulted in considerable harm to native species.

Figure 5. A luna moth. Ryan Hagerty, US Fish and Wildlife Service. Public domain.

Gypsy moths (now called spongy moths) were brought to North America in the 1600s in shipments of silk moths ordered by entrepreneurs interested in starting a silk industry in the US. They defoliate deciduous trees and rapidly became a problem. To control the moths, a parasitoid fly was introduced repeatedly between 1906 and 1986, and became established. Unfortunately, it was a rather generalist insect and has caused the steep decline of native moths in the same group, including luna, cecropia and polyphemus moths – large and beautiful examples of the taxon (Fig 5). As a result of this and other examples of introduced biocontrol agents gone awry, the US and other countries now impose severe restrictions on them and require extensive testing of biocontrol agents to seek to reduce the chance of harm.

Knowledge Check

Take a moment to complete the short quiz below to assess your understanding of this section. Read each question carefully and refer to the section content as needed. This quiz is not graded – it’s simply an opportunity for you to reflect on what you’ve learned and reinforce key concepts.

Alternatives to conventional agriculture

Before examining practices that are not used in conventional agriculture, recall that conventional agriculture is not a monolith. Practices such as cover cropping, precision agriculture, and no-till are increasingly widespread, and increase the sustainability of intensive agriculture. Some of the practices that follow can and are incorporated into so-called conventional agriculture.

Organic farming

The basics

Organic agriculture can be defined as an ecological production management system that promotes and enhances biodiversity, biological cycles, and soil biological activity. But it can also be defined by what is legally permitted in food labeled as organic in a given place. Organic meat, poultry, eggs, and dairy products come from animals without antibiotics or growth hormones. Organic food is produced without using most conventional pesticides, fertilizers made with synthetic ingredients or sewage sludge, or GMOs. The motivation behind organic production is often stated as prioritizing use of renewable resources, and conserving soil and water to enhance environmental quality for future generations, but this is obviously not part of a legal definition, and organic farming creates its own kinds of environmental impacts, some of which can be severe.

Organic farming replaces synthetic chemical fertilizers with naturally occurring materials. Animal manure and compost are often mainstay fertilizers. Additional nutrients may be added using bone meal, blood meal, and fish emulsion from animal processing; seaweeds and meals from plant processing (cottonseed meal, for example); or mineral-based material such as crushed limestone. Organic fertilizers can lead to eutrophication of nearby waters, just as synthetic fertilizers can. Manure, used in both conventional and organic farming, is particularly prone to leaching and can be heavily applied in either context, resulting in eutrophication of area waters.

Can we feed the world with organic farming?

The short answer is “no.” Presently, only a small fraction of world agriculture uses organic approaches, and converting all the rest of world agriculture is infeasible in the short term.

Presently, organic produce costs more than produce without that label, due to higher labor costs, lower yields (on average), costs associated with certification, smaller supply chains and markets, and the economics of smaller farms. The price differential for the “organic” label is needed to support farms that grow organic produce, but the price differential also puts much organic produce out of the reach of less affluent individuals. Farmers using non-organic production techniques have significant income “sunk” into their equipment and land; they could not quickly finance equipment needed for organic production techniques, even if sufficient equipment could be produced on short notice. Conventional farmers are not trained in organic practices and would need education. The land would need time to build resources – even no-till, which is increasingly widely adopted in conventional agriculture, needs several years before the benefits of no-till are well established and yields reach a new equilibrium.

A slower-paced conversion to organic practices might be more practical, but would not address the yield differential between organic and conventional farming that is the result of organic farming’s avoidance of synthetic fertilizers and pesticides and genetically modified organisms. If existing agricultural land were all converted to organic production, an additional 18% of land (approximately) would be needed to address the yield gap between conventional and organic approaches (18% on average across crop types, locations, and climates). It’s not clear where that land would come from; it’s not likely new agricultural lands would be highly productive because most highly productive land is already in service to agriculture. New land might not be in places where organic requirements could be met or monitored.

Although we cannot presently feed the world with organic farming, that does not mean we cannot transition to less harmful agricultural practices, including lessons learned from organic farming. We already see such transitions in the use of no-till, crop rotation, and cover crops in conventional farming and the interest in creating crop varieties that will respond well to organic farming and reduce the yield gap with conventional farming.

Integrated pest management

Integrated pest management (IPM) refers to a mix of approaches to control of weeds and plant pests to reduce the occurrence of these agricultural problems and to address them when they reach levels of concern. IPM is used across the types of conventional and organic farming, but in conventional approaches, IPM includes synthetic control chemicals, whereas in organic farming, naturally occurring controls are used, including fairly toxic compounds such as pyrethrins (developed from chrysanthemums), Bt proteins (developed from a bacterium), neem oil (from the neem tree), and naturally occurring copper and sulfur compounds.

The first effort in IPM is to avoid high concentrations of weeds and pests so that control of outbreaks with toxins is less necessary. Crop rotation – planting different crops in the same field over time – breaks the cycle of crop-specific pests so that they and their eggs or young do not build up in soil or crop wastes. It may require farmers to have equipment for different crops on hand, simultaneously, which can increase costs, but may be balanced by higher yields with less effort. It also requires that farmers not continuously plant the most economically valuable crop in their repertory, which decreases income but may eliminate the chance that crop-specific pests become resistant to methods used to suppress them. But until resistance appears, it can be hard to convince producers to forego the income associated with continuous planting of a single, valuable crop.

Breeding of crop varieties that are resistant to harm from pests and that compete well with weed species is also helpful, as we saw with GMO varieties of major crop plants that incorporate Bt genes. The use of native biocontrols – breeding and deployment of large numbers of so-called “good bugs” and planting of habitat to support them is another means of reducing use of toxic chemicals, as we saw in the previous section.

Multiple crops in sequence – crop rotation

As we have just seen, crop rotation can be useful to break pest cycles and reduce the need for pesticides. In the midwestern US, corn is often grown in rotation with soybeans, to reduce damage from corn borers, which can cause considerable damage if corn is grown continuously. Another benefit of crop rotations that include a nitrogen-fixing legume such as soybeans is that legumes are able to fix nitrogen from the atmosphere, with the assistance of bacteria that they house in root nodules. Legume crops reduce the need for synthetic fertilizers. Clover and vetch are common examples of legumes that can be grown as a cover crops or as a hay crop, alternating with commercial food crops.

Any use of multiple crops, whether for cover crops, in crop rotation, or in simultaneous crops (see below) requires either equipment or labor to deal with multiple crops. To be profitable, the advantages of multiple crops must outweigh these additional costs.

Polyculture – multiple simultaneous crops 

Whereas crop rotation uses multiple crops in sequence, polyculture uses multiple crops simultaneously, and may also incorporate livestock. Where multiple crops are grown, the process of planting and harvesting becomes more complex. Crop rotation required the use of different equipment in different planting rotations. In contrast, polyculture requires different approaches for each product, and at the same time. The simplest systems still permit use of machines, but more complex systems may be worked primarily by hand labor. Farm labor is unequally available among nations. In addition, the ability to work outdoors is increasingly affected by climate change, which is making many tropical areas too hot for safe outdoor work during the hottest part of the day.

Intercropping

Figure 1. Intercropping alyssum (the flowering plants shown center left) with organic romaine lettuce. Alyssum attracts predatory insects that can reduce aphid damage to the lettuce. Stephen Ausmus, US Department of Agriculture Agricultural Research Service. Public domain.

Intercropping is a form of polyculture. In intercropping, two or more crops are grown in close proximity to each other during part or all of their life cycles to promote soil improvement, biodiversity, and pest management. Incorporating intercropping principles into an agricultural operation increases diversity in soil, crops, and insects and other invertebrates (Fig 1). Different crops may have roots that reach nutrients and water at different depths; they may attract or repel different insects, and they may support soil organisms differently. Crops grown in intercropping are subject to fewer pest outbreaks,  and can improve nutrient cycling and crop nutrient uptake, and increase productivity. This approach can be particularly useful in more arid settings.

Agroforestry

Figure 2. Shade-grown coffee plantation in Colombia. Brian Smith, American Bird Conservancy. CC BY.

Agroforestry is an intercropping system that includes woody plants. Shade-grown coffee is an example that many have heard of. Coffee originated in Ethiopia and was not originally a sun-tolerant plant; growing the coffee trees in the shade – either of a existing forest or together with other, taller crop-producing trees that can provide shade – was required. More recently, sun-grown varieties have been created to increase yield, but these require clearcutting of existing forests and more synthetics pesticides and fertilizers.

Shade-grown coffee systems are more forest-like, which retains more soil moisture, lowers temperature, supports greater biodiversity (including birds that eat coffee borer beetles) and, according to coffee lovers, improves the flavor of the coffee. The beans are considered to be of a higher quality and can be marketed for higher prices, although the balance of economic benefits and costs (from reduced yields) is not completely clear.

Agroforestry has considerable flexibility and can include woody plants grown for crops or for timber, non-woody crops, and may also include animals that graze or browse in the vegetation. Woody plants may form a somewhat continuous overstory or may be islands in a sea of non-woody plants (and animals). Subsistence food, fodder for livestock, building materials, fuel for cooking fires, and marketable products may all be produced. Agroforestry is often used by small-holders, and is considered important for food security in rural and developing areas. Nitrogen-fixing plants, including leguminous trees, can be included, and are a natural source of increased soil fertility, particularly important where financial concerns may limit purchase of fertilizers.

Agroforestry systems may be as simple as planted lines of trees and shrubs (called alley cropping) or as complex as mixed trees, agricultural crops, and forage crops, intermingled in less orderly ways (Fig 3). The more complex the arrangement of resources, the less likely it is that automation and technology can assist with any phase of growing or harvesting. This is another reason agroforestry is more common on small holdings and in areas where human labor is readily available.

Figure 3. Agroforestry in Bahia, Brazil. In addition to cocoa trees, avocado and citrus trees also grow in this mixed culture, topped by Centrolobium tomentosum (a timber species), Pouteria caimito (a fruit tree), Eugenia brasiliensis (a fruit tree) and other plants on the Fazenda Olhos d’Água in Piraí do Norte, Bahia, Brazil. The fruits and precious woods are commercialized. The soil is covered on site with prunings and leaves. Soil organisms build nutrient-rich humus from this so that no artificial fertilizers need to be brought in. Photo by Ilanagotsch. Wikimedia. CC BY-SA.

Knowledge Check

Take a moment to complete the short quiz below to assess your understanding of this section. Read each question carefully and refer to the section content as needed. This quiz is not graded – it’s simply an opportunity for you to reflect on what you’ve learned and reinforce key concepts.

Livestock

Livestock comprise 62% of mammal biomass (humans are 34%) and 71% of bird biomass in the world. Among mammals, cattle account for 35% of world biomass (Fig 1). Meat is an energy-dense, and nutrient-dense food in a world in which approximately a quarter of children under the age of 5 still exhibit stunting due to malnutrition. But meat’s role in diets and the role of livestock in environmental impacts are complex.

Figure 1. Distribution of mammals on Earth among types of species, by biomass. Each square corresponds to 1% of global mammal biomass. Note: buffalo are domestic water buffalo. Hannah Ritchie and Klara Auerbach, OurWorldinData.org. CC BY.

Livestock production and impacts

Global meat production has more than tripled since 1961, with the largest increase occurring in Asia (Fig 2). In 2022, the average meat supply per person per year, globally, not counting fish, was over 40 kg per person (97 lbs), with Mongolia, a country with a strong pastoral tradition leading, and the US not far behind (Fig 3). In general, wealthier nations produce more meat per person. In the developing world, livestock are important for food security and income, particularly for those living in poverty.

Figure 2. Global meat production, 1961-2023, by geographic region. OurWorldinData.org. CC BY.

Figure 3. Meat supply per person per year in 2022, excluding fish and seafood. OurWorldinData. CC BY.

Life-cycle analyses of meat production reveal a number of processes associated with impacts. Agricultural impacts from production of animal feed, methane production during digestion, impacts associated with wastes from pastures and feedlots, transportation for processing and to market, and packaging all have related impacts. Impacts vary significantly both between subsistence and industrial production and by location.

Currently, livestock production uses approximately 30% of Earth’s land area and 70% of overall agricultural area is used for grazing and livestock feed production. Approximately 60% of harvested biomass used by humans goes to livestock. In 2022, approximately 40% of cereal production went to livestock (Fig 4), and about 75% of soybeans, by weight, are fed to livestock directly or in processed feeds.

Figure 4. Share of cereals allocated to animal feed in 2022. OurWorldinData.org. CC BY. Compare to the previous map.

The leading cause of deforestation for commodities during 2001-2015 was land for livestock pasture.. In the US, the EPA estimates that burping livestock (primarily from ruminants – cattle, sheep, goats; sometimes classified as “enteric fermentation”) account for approximately 25% of agricultural GHG emissions, and manure management accounts for another 14%. Overall, the agricultural sector was responsible for 10% of US emissions in 2022. Burping livestock produced 36.6% of total methane emissions across all economic sectors in that year.

Animal feeding operations

In the developed world, most livestock are raised in confined-animal feeding operations (CAFOs), also called, less formally, factory farms or megafarms. Dairy cattle, pigs, and chickens may spend most of their lives in such operations. Beef cattle typically are on pasture for their earlier life and on CAFOs for 6-12 months before they are slaughtered. Because many animals can be raised in a small, carefully monitored space, with controlled access to food, CAFOs are efficient, potentially reducing costs for meat, eggs, and dairy products. They reduce the direct footprint of livestock agriculture. In 2020, the EPA listed over 21,000 CAFOs in the US.

However, CAFOs have significant drawbacks, as well, in terms of sustainability. for all three pillars of sustainability. Although the confined nature of the operations means that animal wastes are concentrated and can be managed efficiently, water pollution, particularly nutrient pollution such as nitrates, is a constant problem associated with the operations. Although the manure can be a welcome source of organic fertilizer, the feeding operations are often not located near appropriate fields, which increases transportation costs and GHG emissions. When manure is located close to fields, it may be overapplied in order to dispose of it, although laws can limit application rates. During rainfall events, waste collection areas may flood, washing wastes, pathogens, and associated pollutants including livestock pharmaceuticals into groundwater and surface waters. In the US, CAFOs are not covered by point-source, water-quality regulations, despite their concentrated nature. In the EU, they fall under the Industrial Emissions Directive and must meet the relevant standards.

Ammonia fumes from concentrated waste collection areas on CAFOs can exceed normal levels by 40-fold and can cause respiratory and other health issues.  Odor, generally, is also an issue, and one that is less regularly regulated. Greenhouse gas emissions from livestock operations (see above) are also unregulated in the US; the EU covers them under the Industrial and Livestock Rearing Emissions Directive, requiring best practices, but does not set numerical limits per operation.

If manure can be contained, then it can be used to generate methane (natural gas) as an energy source. Anaerobic digesters are used to capture the gas, and can also be used with food waste and solids from sewage. However, the nutrient-rich material that remains after methane production must still be disposed of.

Animal welfare and related ethical concerns are also an issue with confined animal feeding operations. Animals experience higher levels of stress and injury than in more naturalistic settings and incidents of abuse have been documented.

Grazing operations

Grazing of livestock herds is a practice that dates back approximately 10,000 years, both nomadically and with fenced enclosures. Grazing is a good means of producing food on land that is marginal for agriculture. These rangelands (comprising mostly dry grasslands, shrublands, woodlands, and deserts) cover 50% of the land surface of the planet, however, production on them is low and can only support low population densities of humans – they produce an estimated 16% of food, globally. Of course, livestock can also be pastured on more productive lands, but these typically can produce more food under intensive agriculture than under grazing. In the developed world, smaller operations including some family farms, organic farms, and parts of beef cattle operations using grazing systems. In the developing world, grazing is of particular importance to the  poorest inhabitants, providing food and income.

Although animal densities are lower in grazing systems than on CAFOs, animal wastes can still contribute to water pollution, particularly if animals are allowed direct access to water bodies and can deposit urine and manure directly into them. Overgrazing degrades land, resulting in soil compaction and erosion, further contributing to water pollution. In 2021, the FAO estimated that perhaps 35% of grasslands are at risk of degradation and other rangelands up to 26-27%.

Sustainability of meat-based diets 

In Chapter 1, we learned the basics of food webs – that each successive level of a food web has available to it only about 10% of the level below it. There is a lot of variability around that 10% figure, but the general lesson of decreasing energy at successfully higher levels of a food web is inescapable.

By feeding food to animals to create food for people, we are pushing at least part of the human diet up the food web, and strongly limiting the quantity of food that can be produced, in the process. The American Soybean Association states that more than 90% of US soybeans are fed to animals. Figure 4 above shows us that the developed world feeds over half its cereal to livestock. These are food resources from which we will see only a small fraction emerge as food for people, because of food-web realities.

Livestock researchers study food conversion ratios – how much meat a given quantity of food produces when fed to livestock. Commercially bred chickens are most efficient, with food conversions ratios around 1.5-1.9: it takes between 1.5 and 1.9 kilograms of feed to produce a kilogram of chicken. Food conversion ratios aren’t the most accurate instrument – ratios vary with the specifics of the feed, and it’s important to know how much of the chicken is edible. But they are the instrument in use at the moment. The conversion ratio for pigs averages about 3.3, and for cows it’s 4.5-7.5.

Beyond the inefficiency of meat-based diets in using agricultural products to feed the world, are a suite of other environmental impacts. In a 2018 study, across GHG, land use, acidification of land, eutrophication of water resources, and freshwater use, even the animal-based foods with the lowest impacts had more impacts than almost all of the vegetable products examined. Land-use comparisons are illustrative; beef from cattle raised in a beef herd used almost 100 times as much land as tofu, a complete, plant-based protein (Fig 5).

Figure 5. Land use of foods per 1000 kcal, measured in square meters needed for production. OurWorldinData.org CC BY.

The FAO estimated that meat production produces 12% of global GHG emissions. Cattle production, alone, is estimated to drive 36% of tree loss associated with agriculture. Meat-based diets cause considerable environmental harm, as well as being linked to clear risks to human health.

Aquaculture

The raising of aquatic plants and animals for food, like grazing, is an ancient practice dating back thousands of years. Its use has increased in intensity and in overall use from perhaps 2 million tons in 1960 to over 100 million tons in 2022 (Fig 5); Asia’s contributions to its growth are much larger than its contributions to the growth of land-based meat production. Aquaculture surpassed so-called capture fisheries – catches of wild fish – in the early 2010s (Fig 6). Aquaculture relieves pressure on wild fisheries and shellfisheries, but not all aquatic species are readily domesticated, so relief varies among target species.

Figure 5. Aquaculture production of aquatic plants and animals, 1960-2022, by continent. OurWorldinData.org. CC BY

Figure 6. Seafood production of aquaculture (including aquatic plants) and capture fisheries, 1960-2022. OurWorldinData.org. CC BY.

Figure 7. Shrimp aquaculture in salt-water coastal ponds in South Korea. US NOAA. Public domain.

Aquaculture takes a variety of forms, with species raised indoors, in inland ponds or raceways, in coastal salt-water or brackish-water lagoons, and in netted enclosures in open water (Figs 7, 8). For some species, different systems are used at different life stages. Freshwater species can be raised in ponds created for that purpose or in enclosures in rivers or lakes. Shrimp are often raised in coastal lagoons (Fig 7). Marine fish are often raised in coastal enclosures in open water (Fig 8), although intensive operations may use indoor facilitites. More vulnerable life stages and more valuable species are most likely to be raised in the most protective settings.

Figure 8. Mariculture of marine fish off the coast of Amarynthos, Greece. By Jebulon, edited by Bamesk. CC0.

Food-web issues in aquaculture

Fish species vary in how efficiently they can be produced through aquaculture. In the worst cases, wild-caught fish that are less valuable as food for humans (but often still eaten by humans – anchovies, for example) are reduced to fish meal and fed to fish that are more attractive to humans, such as salmon or swordfish, with an overall loss in protein and biomass during production. Whereas chickens and cows are fed plant food, and so occupy one level up from plants, fish that are fed other fish are feeding at least one additional level up the food web. Such losses are mostly characteristic of predator fishes and early use of these species in aquaculture. More sustainable approaches use fish that can be fed vegetable feeds, keeping them down on the first level above plants.

Environmental impacts of aquaculture

In general, aquaculture has not resulted in the kind of forest loss that occurs when land is converted to agricultural use. However, coastal mangrove forests have been targeted for aquaculture ponds for marine species. Shrimp farming, in particular, was responsible for clearing of mangroves throughout the Asia-Pacific region during 1980-2000s. During this time, over a third of world mangrove stands were lost, with approximately half of this owing to aquaculture. Strict regulations against destroying mangroves have reduced loss of this important coastal barrier to sea-level rise and storm waves, but regulations have not spurred restoration efforts. Mangrove clearing continues, at a much slower pace, but mangrove-aquaculture systems are also being introduced, which provide some progress towards restoration.

Significant aquaculture impacts stem from wastes – wasted food and bodily wastes of farmed organisms; escape of other inputs – antibiotics, pesticides, pharmaceuticals; and escape of the target organisms, themselves. These issues are least severe with indoor and inland-pond operations, where access to natural water systems is reduced. However, indoor operations must dispose of wastes and polluted water, and inland ponds can be flooded during storms, sending polluted water into nearby receiving waters. Polluted water from coastal ponds and lagoons is more likely to reach nearby water bodies. Finally, caged operations in freshwater or marine locations exchange polluted water freely with rivers, lakes, or seas and generally have no means of recovering pollutants.

Farmed fish, themselves, can harm wild fish populations. Due to high densities of fish and nutrients, floating fish farms support diseases organisms and parasites in high numbers, which can also attack wild fish. Sea lice levels in wild salmon, attributed to salmon farms, are associated with an 18% increase in mortality of the wild fish. In addition, farmed fish, which are bred to grow quickly and efficiently in captivity, can escape from their netted enclosures during storms or due to damage to the nets. When they interbreed with wild salmon, the resulting offspring are less well adapted to the wild.

A study of hatchling salmon found decreases in survival over 2 years of 49% and 70% in two year-classes for which both interbred (introgressed) and pure wild individuals were available. To date, there are no data on impacts of genetically modified aquaculture fish. Modified salmon were in production in the US and Canada from 2021-2023, but major retail chains declined to offer them for sale, and the company closed in 2024.

Knowledge Check

Take a moment to complete the short quiz below to assess your understanding of this section. Read each question carefully and refer to the section content as needed. This quiz is not graded – it’s simply an opportunity for you to reflect on what you’ve learned and reinforce key concepts.

Climate-change impacts on agriculture

Overall, climate change has reduced growth in agricultural productivity world-wide, although cropland area has increased (Fig 1). As one might expect, reductions in productivity growth and increases in cropland area occur together, particularly in South America and Africa. In the absence of climate change, one study suggests cropland area might have decreased over 1992-2020. Instead, climate change is a significant factor in reduced productivity growth, which is reducing progress towards food security goals, globally.

Figure 1. Proportion of stable cropland, cropland expansion and cropland reduction from 2003-20019, using Landsat 30-m data. Potapov et al. 2021 https://doi.org/10.1038/s43016-021-00429-z. CC BY.

Impacts on productivity – changes in average weather variables

Average changes in temperature, water availability, and carbon dioxide have major impacts on plant productivity generally. Plants have preferred temperatures, like other organisms. Lacking strong metabolic processes, they are dependent on atmospheric temperatures to function. In addition, when water is frozen, plants cannot function, so that sets a firm bottom temperature for production, although many plants are not adapted even to survive at temperatures near freezing. Warming temperatures create a longer growing season in temperate and polar regions, but the tropics do not get this advantage, as that region already has growing temperatures year-round. Warmer temperatures will be advantageous for some species, but not for others; many areas that grow corn are already near the upper limit of temperature for that crop, whereas areas that grow wheat still have some room for warming. Agricultural suppliers are breeding and engineering crop varieties adapted to warmer temperatures to allow crops to continue to be grown in areas where they have historically produced well.

Shifts in average precipitation are better predicted than shifts in average temperatures. In temperate areas, rainfall is increasing overall, but is shifting into the non-growing season, leaving a higher risk of drought during the growing season.

In mountainous areas, more precipitation is falling as rain, rather than as snow. Snowpack is a form of water storage that releases water downhill and downstream long after the snow falls, potentially delivering water throughout the summer season. Rainfall has no such delaying mechanisms and quickly moves downstream or evaporates, reducing water supplies during the growing season. Warmer air and warmer soils lead to faster evaporation, so increases in rainfall during the growing season, when warmer temperatures are most in evidence, are often offset by increased evaporation.

The increase in CO₂ that is causing warming has the potential to provide a “fertilizing effect” in agriculture. For plants, CO₂ is a raw material used, along with water and sunlight, to create sugars that are the basis for the world’s food webs. More CO₂ could translate to more food. However, the negative effects of warming, drying, and changes in nutrient cycling generally cancel the fertilization effects except in wet areas, leaving overall negative impacts. The more severe climate-change scenarios result in greater losses of production.

Indirect effects of climate change on productivity can arise through changes in invasive species, pest and disease loads. Outside of the tropics, warming typically increases numbers and densities of invasive species, crop pests, and disease organisms. These reduce agricultural productivity. Breeding and engineering of new crop varieties and consideration of new crop species are constantly working to counter these new problems. As we have seen, adoption of new practices is slow in agriculture; new crop species will take longer to catch on than new varieties of existing crops. Other indirect effects are occurring through increased energy needs for fertilizer and pesticide production, irrigation, and other aspects of agricultural practice needed to cope with changes in weather and other crop threats.

Modeling climate-change impacts on agricultural productivity – limitations

Note the number of ways that climate change can affect agriculture. The models used to predict future climate and future agricultural productivity cannot reflect all of the latest science. Some findings are still insufficiently precise to permit mathematics to be developed to incorporate them into models. Other findings are too recent to permit time to incorporate them into models. As a result, crop-production predictions are fuzzier (lower precision of included variables) and less accurate (missing variables) than they may become in the future. Decisions often cannot wait on hoped-for future predictions, but must proceed with an understanding of the potential weaknesses of models and the areas in which research is still needed.

Impacts on productivity – changes in weather variability and extremes

Temperature and precipitation averages are changing, but variability is also increasing, which decreases predictability for producers and increases stress on plants. Crops tend to do best with intermediate temperatures and moisture. Increased variability means more time away from intermediate values and also increasing extremes, particularly hotter heatwaves, more intense rainfall and hail, larger floods, and longer droughts. Weather variability both reduces agricultural productivity and increases variability in yields. Unpredictable food supplies in turn reduce food security.

Extreme weather events do not merely stress plants but may destroy crops locally or over large areas. One late freeze that kills flower buds is enough to wipe out a fruit crop for a year. A hailstorm, driving rain event, or flood just before harvest can similarly eliminate production for a season.

Mongolia has a weather pattern called a “white dzud” in which a summer drought is followed by a severe winter with heavy snows, resulting in starvation of livestock. In 2024, an especially severe dzud resulted in the deaths of more than 7 million animals, more than 10% of the country’s livestock.

Changes in the severity of extreme weather events are harder to predict than changes in averages simply because of sample size. All the weather in a year contributes to our understanding of the average temperature or precipitation. But only the highest temperature or the total rainfall or the maximum rainfall per hour provides our understanding for the maxima for that year. Because extreme events occur rarely, we have fewer data from which to predict their behavior. Yet extremes have a greater chance of causing great harm – of killing 10% of livestock or destroying an entire harvest. So, as climate change increases weather variability, it also decreases our ability to accurately predict the occurrence and outcome of extreme events.

Increasingly, severe weather events are entirely outside of recorded weather – they represent new highs or lows, or combinations of events. Science cannot prove that climate change caused a particular severe-weather event. However, it can document trends in weather severity since climate change began, and, where sufficient data are available, science can determine how much climate change raised the odds of a severe event occurring. For example, researchers have calculated that extreme weather conducive to forest fires has become 88% to 152% more likely across the world, in forested areas, compared to the 1851-1900 climate.

Finally, extreme weather events can co-occur, bringing more than one severe stressor into play, simultaneously. Co-occurrence of heat and extremes in precipitation (both drought and flood) can cause more harm than either stressor alone, but in some cases, co-occurrence may balance one stress against the other. More frequent occurrence of these compound events challenges farm managers to understand crop responses – responses that work for a single stressor may be counterproductive for compound events.

Adaptation

Adaptation to climate change is already going on in agriculture, with mixed effects. For example, farmers in northern China have switched from soybeans to more valuable maize, which can now grow there. However, China imports more than 90% of its soybeans, presently, so the move does not contribute to food security.

A study of 6 major staple crops suggests that adaptation can help to mitigate about 23% of worldwide production losses by 2050, under the most pessimistic (but perhaps most realistic) climate-change scenario. But climate change impacts are projected to be large in current producing regions for staple crops, and adaptation has been slow to occur in part because these areas are experiencing relatively moderate climate-change impacts, to date. Maize is projected to decline by 40% in temperate zones, but only by 15% closer to the tropics, where precipitation will be greater and more reliable. Soybeans decline by 50% in the US but gain 20% in the wet tropics of Brazil.

A number of methods are available for adapting agricultural techniques to climate change, and these are typically grouped under the heading of climate-smart agriculture. As a group, these approaches seek to increase crop and soil resilience, maintain or increase productivity under climate change, and reduce emissions from agriculture to diminish harm from climate change.

Diversification can help to reduce losses from pests and extreme events. In a good year, farmers could lose money by not having all their fields in the most valuable crop. But in a bad year, diversification could provide yields that would not otherwise be available. Crop insurance, widely available throughout the agricultural world, even in developing countries, can work against diversification if insurers won’t write policies for diverse crops or new crops. Policies that support diversity and innovation are becoming more common, and could help promote these adaptive practices.

Precision agriculture can help to optimize resource use – water, fertilizer, pesticides, etc. The combination of drone imagery and GPS locations can pinpoint use of these resources. Drip irrigation can deliver water in smaller quantities, more precisely. These are high-tech solutions, but approximations might use lower technology. Training could help farmers understand the significance of variation in crop height, leaf color, and other variables, and a drone shared within a community could provide the subtler information available with advanced sensors. In smaller fields, precision use of fertilizer and pesticides might mean walking to the right location.

Regenerative agriculture provides a suite of practices designed to improve farm resilience by increasing carbon uptake, improving soil conditions, benefiting natural pest predators and pollinators, balancing soil drainage with soil water-holding capacity to best fit local water conditions, etc. Although not all of these are aimed specifically at climate change, some are, and plants that have the strong, general support systems are more likely to withstand all stressors, including climate change.

Getting the greatest food yield out of existing land will reduce carbon costs and biodiversity loss of clearing yet more land. Food-bearing cover crops, crop rotations, and intercropping can be used where possible. High-yield varieties are useful if they are designed for local conditions. These are forms of diversification – another common recommendation (see above).

Breeding/engineering resilient crops suited to specific conditions will improve producers’ ability to maximize yields. We have already seen that organic farmers lack crops bred to thrive under organic practices. Similar problems exist for agroforestry as well as for producers with soils or climates that vary from the most common conditions. Agricultural companies are breeding and engineering crops to withstand climate change and increased climate variability, but the wider range of soil and climate conditions and agricultural approaches is not yet being addressed well.

One drawback to private development of crop varieties is that they are legally protected in the way any privately developed product might be. Farmers that buy hybrid and engineered seeds are legally prevented from keeping seed of the resulting crops, so that they are locked in to buying seed every year. In the developed world, this is standard practice, but in the developing world, farmers often hold back some seed from a harvested crop to sow for the next year. With privately-developed crop varieties, not only are they prevented from sowing seeds from their crops, but offspring of hybrid crops do not retain the characteristics of the parent plants (although offspring of GMO crops will retain GMO characteristics), so that the benefits of breeding are lost.

Resilience can be extended to infrastructure, supply chains, and design of farm equipment and structures. All the parts of agrifood systems can be improved to survive increases in variability and extreme conditions. Simple improvements like windbreaks or sturdier design of irrigation systems can help, but more and better shelters for farm equipment, food storage areas, transportation systems for moving food towards processing and market are also appropriate.

Education, training, knowledge sharing and networking of farmers, support services, and policy-makers can increase adoption of successful practices, increase food yields, and encourage development of useful agricultural policies such as the crop-insurance policies discussed above. Farmers are typically described as conservative in their approaches to crop choices and practices because they face many risks and often have most of their money sunk into equipment they cannot sell if the agricultural fashions change. Both the farmers and the policy-makers can benefit from frequently updated information about new equipment, new varieties, better practices, and demonstrated successes in their regions in order to adapt to the rapidly changing conditions they face.

The food-water-energy nexus 

Agriculture, water, and energy come together in several ways. Obviously, undertaking mechanized agriculture or using synthetic fertilizers and pesticides requires all three, as we have already seen. In addition, climate-change impacts on agriculture are increased by the current use of food crops for first-generation biofuel (Chapter 6), which subtracts agricultural land from food production in favor of using it for energy production. Water use for biofuel also subtracts resources from food production, where water is limiting. Water to cool energy facilities also subtracts water from food production, although some cooling water is returned to its source in relatively high proportion. Renewable energy reduces water use and the need for biofuels, but will not be a majority share of energy production for many years.

Agricultural impacts on climate change

Agriculture presently accounts for 10-15% of GHG emissions, second only to energy production. Including deforestation, transportation, and other aspects of food systems brings the proportion up to about 30% – almost a third of world GHG emissions (Fig 2). Regionally, emissions totals are highest in Asia, followed by the Americas (Fig 3), but Indonesia and Brazil lead the world in emissions per dollar of product. Their lead is clear in farm-to-gate emissions. But in emissions due to land-use change – largely from deforestation, agriculture in these two nations accounts for the vast majority of such emissions, globally (Fig 4). Note that Figures 2 and 3 include all aspects of agrifood systems, including pre-and-post production, which is outside the scope of agriculture for the discussion here.

Figure 2. Global agrifood systems emissions by component and share of agrifood systems emissions in total emissions. FAOSTAT CC BY.

Figure 3. Agrifood systems emissions by component and region. “Farm gate” represents on-farm emissions. FAOSTAT CC BY.

Figure 4. Emissions on agricultural land per value of production for the top 10 countries by agricultural value, 2022. “Farm gate” represents on-farm emissions. FAOSTAT CC BY.

Palm-oil agriculture for food oils and for biofuels is responsible for the conversion of large swaths of tropical forest, particularly peatland forests. Conversion, alone, of these forests is responsible for approximately 25% of Indonesia’s total GHG emissions, and agriculture on the remaining soils also contributes significantly. Tropical peatlands are  immensely carbon rich and their conversion to oil-palm plantations, primarily in Southeast Asia, causes annual GHG emissions similar in magnitude to the C that may be lost from boreal forests in the future – 70-117 tons of CO2-eq per hectare per year. Loss of these forests is also linked to extensive biodiversity loss and economic impacts to local farmers.

The magnitude of harm caused by converting peatland forests to palm-oil plantations created considerable controversy around palm oil beginning in the early 2000s, particularly as some of it is used (especially in Europe) as a biofuel, which is supposed to reduce GHG emissions. A certification program – the Roundtable for Sustainable Palm Oil – was created to provide sustainably sourced palm oil, but after 20 years, it accounts for only 20% of available palm oil. Generating demand for sustainable palm oil has been more difficult than creating sustainable palm oil.

Agriculture promotes climate change by producing all three of the major greenhouse gases (Fig 5). Land conversion contributes most to carbon dioxide, as do some soil-amendment practices. The fact that some carbon is sequestered in agricultural land offsets a portion of these CO₂ emissions. The majority of methane from agriculture is from livestock, with manure, the moist or inundated soil of rice paddies, and burning of crop residues contributing the rest. Nitrous oxide comes primarily from use of nitrogen fertilizers, with a small proportion from use of manure and burning of crop residues. Note that nitrous oxide, which has the greatest global warming potential and the longest lifespan in the atmosphere is produced in the largest quantity of the three GHG.

Figure 5. Sources of agricultural greenhouse gas emissions, 1991-2021. Resource for the Future using data from the EPA Greenhouse Gas Inventory Data Explorer. CC BY NC ND.

As Figure 5 makes clear, the greatest gains in GHG reductions from agriculture come from reducing nitrous oxide emissions from agricultural soils and methane burps from livestock. Not only are these two gases produced in larger amounts than CO₂, but they also have higher warming potentials.

The most common recommendation for reducing nitrous oxide emissions from agricultural soils is to reduce the use of nitrogen fertilizers and to reduce the conditions that allow fertilizers to become N₂O. Precision agriculture helps farmers to put the right amount of fertilizer in the right place at the right time, which can reduce use. Placing the fertilizer at the correct soil depth for plant roots can also be helpful. In the midwestern US, reducing fertilizer application in fall would allow more of it to be taken up by plants and less to be left where it can become N₂O.

Soil compaction can contribute to N₂O emissions by holding water on the field and creating the oxygen-poor conditions that support the bacteria that convert nitrogen fertilizer to N₂O. Methods to reduce soil compaction vary by soil type, but can include no-till and use of cover crops with stout roots (e.g., radishes) that can break up compacted layers.

The most obvious way to reduce methane emissions from livestock is to reduce the number of livestock. Feed suppliers are working on ways to reduce burping by modifying animal feeds and introducing feed additives. Researchers are working to produce livestock breeds with digestive systems that produce less methane. But reducing livestock numbers would be most effective.

Manure management also contributes to methane production. In larger operations such as CAFOs, methane can be captured from manure containment areas and used for fuel. Keeping manure dry and aerated also reduces methane production, but is difficult to do if manure is piled. Livestock that are out on pasture at relatively low density deposit manure in the field where it can dry and decompose naturally, much of the time, but as we have seen, only a small proportion of livestock, globally, are on pasture.

Agriculture’s contributions to climate-change mitigation

Climate-change mitigation efforts seek (1) to reduce GHG emissions and to also (2) to remove existing GHG from the atmosphere and sequester it away from the atmosphere. Both pathways work to slow climate change. A suite of nature-based solutions to climate change has been proposed to aid in this effort, including reforestation and afforestation.

Among agricultural activities, reduced land clearing (CO₂), protection of and increase in organic material in topsoil (CO₂), reduced farming in wet conditions (N₂O and CH₄), reduced and better targeted use of N fertilizers (N₂O) and other approaches discussed above are recognized ways of reducing GHG emissions from agriculture. However, efforts to actively sequester C in agricultural soils are less well understood.

Many agricultural soils exhibit a downward trend in C levels owing to the history of agricultural practices, so even if organic material is increased, the trend may only slow, rather than reversing – soils may continue to lose C but at slower rates. . This would contribute to reducing GHG emissions but would not reduce GHG in the atmosphere – no net C sequestration would occur. Even where C increases occur, it may take many years to restore the C that was once in the soil; still, any C sequestration will reduce GHG emissions. Recent modeling suggests, however, that attempts to use agricultural soils for climate-change mitigation – as a means of removing C from the atmosphere – rarely also improve crop production.

It’s important to understand C changes throughout the soil column to assess impacts of soil practices. Many soil studies sample only the top 10-30 cm (4-12 inches) of the soil column. It’s now clear that climate change is causing loss of C in deep soils, so that experiments that study changes in organic material in upper levels of soil are not able to report the overall picture of soil C levels, and cannot accurately determine whether overall gains in sequestering C are occurring. Our understanding of deep soil C is still in its infancy and recent reviews of the dynamics of soil C under climate change refrain from making definitive statements and call for additional research. While that aspect of agricultural impacts on C sequestration is being better studied, efforts to reduce GHG emissions are certainly still appropriate!

Climate change and the agricultural land base 

Figure 6. Soil organic carbon content in top 1 meter of soil (in metric tons per hectare) in areas of climate-driven agricultural commodity frontiers using the 
most severe IPCC climate projections ( in blues). Existing agricultural land cover is represented in light brown. Hannah et al. 2020.  https://doi.org/10.1371/journal.pone.0228305.g002 . CC BY.

Under climate change, climate in the boreal zones of Canada, Alaska, Russia, and northern China is creating weather conditions suitable for agriculture. Hundreds of millions of hectares of land currently suited to forestry is projected to become suitable for agriculture by 2100. In addition to being forested, much of this land also has peat soils, which are the largest natural carbon sink on Earth. The loss of the carbon in the tree biomass and the peat soils would significantly increase GHG levels at the global levels. As of 2018, only about 2% of the boreal peatlands – wet, boggy forests – had been drained for farming, leaving most of the region’s carbon stocks intact, but ongoing warming will make these areas increasingly attractive for food production. A 2020 study estimated that converting northern areas to farming could convert 177 Gt on C in peatland soils into CO₂ – an amount equal to more than 100 years of present US GHG emissions (Fig 6).

Peat soils are highly acidic in nature, as well as containing large amounts of organic material. Preparing these lands for agriculture requires extensive road building, deforestation, and draining. The acidic soils that result are low in nutrients and retain added nutrients poorly. To be productive, high fertilizer use will be needed, which is likely to result in nutrient pollution of surrounding waters. Weather extremes, particularly unseasonal freezing and flooding, are likely to decrease productivity.  Labor and supplies are generally less available. Lastly, many of these northern areas belong to, or are used by, indigenous people who rely on the existing forests for food, building materials, and cultural services.

At the same time that boreal lands are becoming more climatically suitable for farming, some tropical areas are becoming less suitable for both timber production and agriculture, as heatwaves, drought, and wildfires increase. Current predictions suggest that increases in agricultural yields in the northern land base will not replace what is lost from the tropics.

Knowledge Check

Take a moment to complete the short quiz below to assess your understanding of this section. Read each question carefully and refer to the section content as needed. This quiz is not graded – it’s simply an opportunity for you to reflect on what you’ve learned and reinforce key concepts.

Food security

Food security is typically divided into four components:

• Physical availability is an indication that food exists, somewhere.
• Access to food means that food is available to actual people in their households.
• Food utilization includes nutritional, cultural and social aspects that affect how people are able to benefit from the food they eat. It involves food storage, preparation, how and when it’s eaten, how it’s shared among members of the household.
• Stability measures the ongoing nature of the first three factors – whether political, economic, social, or environmental factors disrupt availability, access, or utilization.

Environmental issues are involved in all of these, in some way, but most of all in physical availability, by affecting food production and in stability, by affecting variability in food production.

The UN report on progress on SDG goal 2 – end hunger – indicates that hunger has increased since 2019, when 7.5% of the world was affected by hunger; 9.1% of the world’s population experienced hunger in 2023. ] Regionally, Africa has the highest rates of hunger, at 20.4%, but Asia has the highest number of people facing hunger: 384.5 million or more than half the world’s hungry.

In the US, the federal Department of Agriculture defines “low food security” as a matter of poor diet, rather than a problem of food quantity. “Very low food security” involves “multiple indications of disrupted eating patterns and reduced food intake.” Both measures have increased since 2021 (Fig 1).

Figure 1. Trends in food insecurity and very low food security in US households, 2001-2023. USDA Economic Research Service. Public domain.

Physical availability of food is affected by local weather and climate trends, as we saw in an earlier section, but also by availability of inputs – for example, fertilizer and energy prices – which in turn are affected by conflicts including the Ukraine-Russia conflict. Government policies can offset or increase the impacts of these factors. Going forward, the US decision to impose steep tariffs in much of the world will undoubtedly affect food availability; China’s decision to stop buying US soybeans is a response to US tariffs that will shift food prices, availability, and stability around the world.

Environmental factors such as weather events and pest outbreaks can affect food access, security and stability. Food safety is affected by contaminants which may enter food chains through air and water pollution, including contaminants and pathogens.

Some of the most important factors affecting food security are outside the scope of this section. Poverty, conflict, government policies, and historical legacies all play large roles in food security. Food loss and waste and other inefficiencies in the food-processing pipeline eliminate about one-third of produced food. The UN Environment Programme reports that “19 per cent of food available to consumers [is] being wasted, at the retail, food service and household levels. This is in addition to the estimated 13 per cent of the world’s food that is lost in the supply chain from post-harvest up to and excluding retail.”

Lack of data on food production, consumption, loss, and wastage makes it difficult to accurately understand food security. Industrial aspects of food production are relatively well reported, but small-scale agricultural activities are poorly tracked. Summary statistics on global food security are mostly produced by international organizations such as the FAO at infrequent intervals. In addition to a scarcity of accurate information on aspects of food security, we also lack good information linking food need in the future to the capacity to produce food in the future.

Agricultural sustainability – can we feed the world in 2050?

We’ve seen a number of unsustainable aspects of agricultural practice in this chapter, including the factors listed here.

• Loss of topsoil and reduction in soil health due to intensive agriculture and overgrazing
• Eutrophication of fresh water and coastal ecosystems
• Poisoning of native plants and insects, including pollinators and “good bugs”
• Spread of invasive species
• Creation of herbicide-resistant weeds, pesticide-resistant pests, and antibiotic-resistant pathogens
• Loss of significant food in food-web processes of livestock production
• Climate change reductions in agricultural productivity
• Loss of biodiversity as a result of all of these, including biodiversity important to agricultural productivity.

That we can feed the world in 2050 seems, perhaps surprisingly, not to be too much in question. But if we continue to produce food unsustainably, by 2050 we may have so damaged the productive capacity of agricultural lands that we will not be able to produce sufficient food for much longer beyond that.

A 2025 report quantifies the role of food systems in taking the planet towards and beyond the boundaries of the safe operating system of the world (planetary boundaries were introduced in Chapter 1). The food system currently produces more than 3 times the boundary amount of GHG. It is nearing the boundary of land conversion and amount of agricultural land that is the boundary for intact land. Food systems produce more than twice as much N₂0 as is consistent with healthy stratospheric ozone concentrations and they contribute 25% of the CO₂ emissions responsible for ocean acidification. Nitrogen levels are more than twice the food-system boundary value and phosphorus levels are 156% of boundary values. Water use is approaching boundary values. Synthetic chemicals released without adequate safety testing are >16 times higher than levels that pose low pollution risk. Food systems are a significant part of the threats that are taking the planet out of its safe operating space.

A World Resources Institute summary boils issues related to feeding the world in 2050 down into 3 gaps:

• a 56% gap in calories produced in 2010 and what will be needed in 2050 (not a straightforward quantity to estimate,)
• a gap of 593 million hectares between land needed for agriculture in 2010 and what may be needed in 2050 (an area about 3/4 the size of Australia), and
• an 11 gigaton GHG-emission gap between emissions predicted for 2050 and what is needed to hold global warming below a 2°C increase, to reduce harm.

The suggested remedies we have seen in this chapter are designed to close these gaps. For example, using more grain to feed people and less to feed livestock, alone, would put a big dent in the land gap. But the proposed remedies are not yet close to closing the gaps, and are not currently predicted to do so by 2050.

We have a solid understanding of how to proceed to reduce food-system impacts, generally, but some approaches take time to implement, and others are unpopular or, even if popular, affect powerful economic and political interests. Support for better practices will require reallocation of funds both to provide greater equality in access to seeds, inputs, and equipment and to offset sunk costs in equipment that may not suit current or future needs. Not only governments and industries must be persuaded to change. Farmers must also be convinced that these changes are needed, as must consumers.

Feeding Africa in 2050 – the extreme case

A major concern in feeding the world going forward is the tremendous inequality of growing population pressure and accompanying increases in food demand. Although Asia has the largest proportion of world population, Africa will contribute the most to population growth throughout the 21st century, with the sub-Saharan population predicted to nearly double between 2020 and 2050, adding more than half of all the additional population on the planet during that interval. Sub-Saharan Africa also leads the world in poverty, has led the world in violence by states and organizations against civilians since 2013, and trails the world in organic material and nutrients in soils (Fig 2).

Figure 2. Soil suitability for rainfed agriculture under low inputs. IIASA/FAO, 2012. Global agro‐ecological zones (GAEZ v3.0). IIASA, Laxenburg, Austria and FAO, Rome, Italy. CC BY.

Figure 2 shows suitability of world soils for rainfed (unirrigated) agriculture with only low inputs (fertilizer, pesticides). Africa lacks any of the large native grasslands that characterize the high-productivity agricultural areas of the North American prairies, Argentinian pampas, and Eurasian steppes (shown in dark green) and has a preponderance of soils in the medium and moderate categories.

As with the world food situation, recommendations for feeding Africa in 2050 are available, but they require even greater increases in training, investment, industry and agricultural practice than for the world at large. Not only country-level processes must change, but the continent as a whole would benefit immensely from a coordinated, sustainable, resilient supply chain for all aspects of food production, to eliminate the current dependencies on imports of food and agricultural inputs and on international food aid.

Knowledge Check

Take a moment to complete the short quiz below to assess your understanding of this section. Read each question carefully and refer to the section content as needed. This quiz is not graded – it’s simply an opportunity for you to reflect on what you’ve learned and reinforce key concepts.

Ending on a positive note, visit this World Bank blog on Greening the Rice We Eat.

Living things and the ecosystems they inhabit are affected by many environmental impacts, and contribute to many human benefits through ecosystem services. Food, fiber, building materials, medicinal materials, climate regulation, water purification, oxygen provision, links to physical and mental well-being, and the sacred – all of these things come to us from the planet’s living things and the ecosystems of which they are a part.  The UN Sustainable Development Goals for life on land and life on (fresh) water are the best direct matches for our topic here, but because of all the links to and from human activity, most of the other goals are also relevant.

Similarly, because the planetary boundaries are all different measures of the health of the planet, all of them are related to biodiversity and ecosystems – terrestrial, freshwater, and marine. Biosphere integrity is a specific match, as it gauges the overall health of organisms and their ecosystems. Other closely related boundaries include land use change, which monitors the modification of natural land cover – the plant component of ecosystems and the source of much of the habitat for the animal component. Freshwater change, modification of biogeochemical flows (especially nitrogen and phosphorus and the nutrient pollution they can cause), and ocean acidification all have direct effects on aquatic ecosystems. Climate change affects the other planetary boundaries and, as we will see, has increasingly harmful effects on biosphere integrity.

Vocabulary for biodiversity and ecosystems

A species is a particular kind of plant, animal, or other organism (species is both singular and plural). Definitions for “kind” vary, but one of the simplest is that a species comprises individuals of the same kind that can breed among themselves. This definition has a lot of gray area. Many animal species in North America are very closely related to species in Eurasia. They are physically separated and cannot breed with each other, but if they are brought together – for example by introduction of one group to the other’s continent – they may be able to breed successfully. But because they were unable to breed with each other, on their own, these would usually be considered different species. Some plant species are very difficult to differentiate because their breeding barriers are particularly poorly developed and, although they might have slightly different habitat preferences, when they meet each other, they hybridize along the zone of overlap.

Figure 1. An individual Cope’s gray treefrog on a leaf. Vicky Meretsky. CC0.

A single organism from any species is not called a species. It is called an organism or an individual. Thus, the frog in accompanying image (Figure 1) is not a species. Rather, it belongs to a species. It is an individual Cope’s gray treefrog. Similarly, the plant that it is sitting on is not a species, but an individual of a species – a mapleleaf viburnum.

A population is a group of organisms of the same species that occupy the same space and time. Some populations are easy to delineate. All the individuals of a single species of fish – let’s use sunfish – in a single pond are a population. The red-whiskered bulbuls (a classy songbird) in Hanoi might be rather well delineated, because they prefer urban and suburban areas, parks, and gardens, and they might be much less common outside the city and its immediate surroundings. But other populations are delineated more for management purposes than because there is any clear separation of the individuals from other, nearby individuals. For example, park managers might refer to the population of impala in their park. But the impala don’t recognize park boundaries and probably move in and out of the park, although the number in the park might be somewhat consistent over time.

A community is the individuals of the plant and animal species (and fungi and other microorganisms, if those are also of interest) that share space and time. Some species may be quite closely adapted to particular conditions – specialists. Others may be tolerant of a wide range of conditions – generalists. The species in a community don’t share their habitat requirements completely – they simply overlap in this space and time.

We most often talk about communities with relation to the ecosystem they inhabit. The ecosystem includes abiotic – nonliving – elements including soil and water as well as the processes necessary to keep the ecosystem running – photosynthesis, decomposition, nutrient cycles, disturbance regimes such as fire cycles, etc. A forest ecosystem comprises the plants and animals of the forest, but also the landscape in which they are embedded. A forest community only encompasses the plants and animals.

A biome is a category of ecosystems. If you live near a forest, then your local forest is an ecosystem that is a single example of the forest biome. In places where natural landscapes are undisturbed, a single ecosystem could be quite large. But it is still an example of a biome. The boreal forest biome includes the forests across the northern lands of North America and Eurasia. Any single boreal forest is a small subset of the boreal forest biome (Fig 2).

Figure 2. Distribution of the boreal forest (or taiga) biome, with an inset of a Norwegian boreal forest ecosystem. Map: Mark Baldwin-Smith. Inset: Øyvind Holmstad. Both CC BY-SA from Wikimedia.

No single biome classification is accepted by everyone, but many of the major classifications are standard across lists of biomes. Some users group tropical and subtropical forests. Others subdivide tropical forests into wet, seasonally dry, and dry tropical forests. But in all cases, these are global categories that include many local ecosystems, often on multiple continents (Fig 3).

Figure 3. Biomes of the world. Ville Koistinen, Wikimedia. CC BY-SA.

A watershed is a unit of land, and is described by topography and hydrology, not by species. A watershed is the land that drains to a single body of water – a river, a lake, or an ocean. The watershed of a small stream will be similarly small. Watersheds of large rivers are often called basins – for example, the Amazon basin. Figure 4 shows the major watersheds of the world, including watersheds that do not drain to an ocean but instead drain to continental interiors. The Great Salt Lake of North America, the Caspian Sea, Lakes Baikal and Balkhash of Central Eurasia are all examples of such land-locked basins.

Figure 4. Major watersheds of the world, including landlocked watersheds, in gray. By Citynoise in Wikimedia. Public domain.

In contrast to Figure 4, Figure 5 shows a specific level of watersheds in the US. If you look closely, you can see that the darker lines in Figure 5 include lines that follow the North American watershed lines shown in Figure 4. The US Geological Survey maps watersheds at many different scales in order to understand water movement and water availability both broadly and locally.

Figure 5. The US Geological Survey 4-digit watersheds of the contiguous United States. USGS. Public domain.

What is biodiversity and where does it come from?

Biodiversity is the diversity represented in living things. It can be considered in 3 separate ways.

• Species diversity is what most people think about when biodiversity is mentioned. The differences between a butterfly and a banana and a bat are easy to see.
• Ecosystem diversity is about the diversity of different kinds of collections of plants and animals and land resources: forests, wetlands, deserts, etc.
• Genetic diversity is about the diversity within a single species that comes about because of differences in genes: genes for digesting new foods, for running fast, for having spots or stripes, etc.

New genes happen rather quickly, through mutation, but most mutations are not helpful, and the mutations don’t persist as they are weeded out by natural selection. Natural selection occurs very simply, because individual organisms with genes better suited to a particular time and place have more offspring than less suited individuals. As a result, the genes that are better suited become more common, and the species becomes better adapted. Perfect adaptation never occurs because environmental conditions are never constant – something is always changing, and change means that new genes may become the better-adapted genes.

Species typically evolve fairly slowly, through natural selection. Occasionally, such evolution is more rapid, as when a flock of small birds survived a flight to the Galápagos Islands off the coast of Ecuador and, over time, became an entire suite of finch species, each adapted to feed on a different kind of food. These “more rapid” examples of species evolution still take time on the order of 1 or more millions of years – “rapid” is relative!

Ecosystems cannot evolve because they are collections of species and land conditions. But combinations of plants and animals can become more and less common. During an advancing Ice Age, as weather becomes colder, the combinations of plants and animals in a location changes, the precipitation regime probably changes, and the land itself changes as glaciers roll over it. When humans modify landscapes, similar changes may occur, for example after mining has disturbed the land.

Humans can cause the disappearance of populations – most of the wolf populations of the US have been gone for over a century. We can cause the extinction of species, and have apparently done so for thousands of years – the disappearance of many of the huge mammals of the last Ice Age – mammoths, dire wolves, and giant sloths – is believed to be due, in part at least, to overhunting by the humans of that time. We can eliminate local ecosystems by converting them to agriculture, building cities over them, flooding them with reservoirs, etc.

The biodiversity currently on the planet exists despite 5 major extinctions episodes in the planet’s past, including extinctions that are believed to have wiped out the majority of species on land and in the oceans. Recovery of planetary biodiversity from these extinctions took millions of years, and, in each case, the subsequent biodiversity was different from what came before. Researchers characterize our current time as beginning a 6th major extinction, due to the elevated extinction rates resulting from human activities including climate change.

Introduction to ecosystem services and valuation of ecosystem services

Just as humans define ecosystems, we define the benefits we derive from them. We value (or not) these services as a result of our own priorities. Ecosystem services are commonly divided into four categories.

• provisioning services provide us with goods – food, fiber, water, building materials, etc.
• regulating services help to maintain the world around us – trees that sequester carbon help to regulate climate; wetlands that can absorb waters help to reduce flooding.
• cultural services provide us with recreational opportunities, opportunities for reflection, inspiration for art, sacred places.
• supporting services underpin the other services – soil formation, nutrient cycling – they are the natural equivalents of keeping the lights on.

Obviously natural processes can provide services in more than one of these categories. In dry areas, springs may be sacred at the same time that they provide water. Trees provide building material and fiber, sequester carbon, provide recreational opportunities, and contribute to nutrient cycling.

Ensuring the continued provision of ecosystem services is often an aspect of conservation. It is complicated by the problem of valuing ecosystem services. In some cases, some measure of valuation is straightforward. The value of pollination to an almond producer might be measured in the market value of the almonds. But the same pollinators may also support other plant species that in turn support other pollinators and other wild animals – this additional value, if we deem it to be a value – is much harder to quantify.

We know, because the existence of support for nonprofit organizations demonstrates it, that people can value species and ecosystems they have never seen. This existence value is not fully measured by donations to nonprofit organization or tourism – many people may never act on their appreciation for this unseen nature. We can ask people how much they would be willing to spend to live closer to a national park or to have more hunting opportunities, or how far they would travel to have some experience with the natural world, but research tells us that these valuation methods often fail to produce useful results. Our inability to measure value for ecosystem services leads some to discount or dismiss their value entirely. In the case of common-pool resources (introduced in Chapter 1), which have no owners, difficulties with valuing ecosystem services may lead to their loss. Of course, common-pool resources of known value may also be lost, not because they are not valued but because they cannot be protected from those who would profit from their value.

A variety of approaches have been used to preserve biodiversity, ecosystems, and ecosystem services, with and without valuation. Recently, some governments have extended legal rights to Nature or to natural features. For example, in 2017, in the Te Awa Tupua (Whanganui River Claims Settlement) Act, the Parliament of New Zealand extended legal personhood to the Whanganui River. As with restorative agricultural practices (Chapter 7), however, the global balance is still towards loss of resilience, species, ecosystems, and ecosystem services.

Knowledge Check

Take a moment to complete the short quiz below to assess your understanding of this section. Read each question carefully and refer to the section content as needed. This quiz is not graded – it’s simply an opportunity for you to reflect on what you’ve learned and reinforce key concepts.

Read the two linked PowerPoints or their .pdf equivalents for an introduction to terrestrial biomes, their ecology, ecosystem services, and threats.

Knowledge Check

Take a moment to complete the short quiz below to assess your understanding of this section. Read each question carefully and refer to the section content as needed. This quiz is not graded – it’s simply an opportunity for you to reflect on what you’ve learned and reinforce key concepts.

A number of wetland and aquatic settings of the world are not on the scale of entire biomes. In terrestrial systems, we often see intermingled ecosystems in places with complex topography – a very tall mountain in the desert Southwest of the US might go from desert at the bottom, through grassland, woodland, dry forest, wetter forest, to alpine tundra. The large swaths of continuous vegetation that are biomes are also not completely continuous – they are dotted with wetlands, cut through by rivers, and where the soil changes or bedrock comes near the surface, areas of different vegetation types will appear – too small to be biomes in their own right.

Terrestrial wetland and aquatic ecosystems are almost never large enough to be considered biomes. Much of the boreal forest is wetland forest, but we label the biome boreal forest. Rivers, lakes, and ponds do not continuously cover the landscape in the way that the major grasslands, deserts, savannas, and forests do. For that reason, this section is about wet terrestrial ecosystems as well as about the aquatic biomes that occur in oceans, where continuously wet environments are the rule. But even in oceans, important features such as coral reefs are not continuous over large areas, and are best discussed as ecosystems.

Read the linked PowerPoint or its .pdf equivalent for an introduction to wetland and freshwater aquatic ecosystems and biomes, their ecology, ecosystem services, and threats.

Knowledge Check

Take a moment to complete the short quiz below to assess your understanding of this section. Read each question carefully and refer to the section content as needed. This quiz is not graded – it’s simply an opportunity for you to reflect on what you’ve learned and reinforce key concepts.

All living species are able to reproduce faster than needed to sustain constant population sizes. If they could not do so, they would not be able to recover from periods of increased mortality. The fact that populations can bounce back from disturbances demonstrates their ability to grow their populations.

Because all species can grow in numbers, it is, theoretically, possible for humans to harvest individuals without threatening the existence of the species involved. So long as the harvest leaves enough individuals to sustain the harvested populations, harvest can be sustainable.

It’s not a bad theory, and it holds true for a variety of plants and animals in a variety of settings. For example, waterfowl hunting in North America has supported both subsistence and recreational hunting across three countries fairly successfully for many decades. But humans have been involved in unsustainable harvest since at least the last Ice Age, when they were a driver of extinction of the large mammals of that period. We have not needed huge numbers of humans or advanced technology to develop unsustainable practices.

For any species in any area, there is a limit to the number of individuals the area can support sustainably, called the carrying capacity. Carrying capacity is not constant through time, but depends on the productivity of the area – usually affected by climate and nutrients available to plants – the nature and number of food species, predators, and competitors, and any disturbances, natural or anthropogenic.

It’s natural to think that management of harvested species would seek to maintain them at carrying capacity – it’s the maximum number of individuals one should manage for – but a population at carrying capacity is a population close to its limits. Food is just enough to sustain the population, leading to higher mortality of young and old individuals, and lower productivity of reproducing individuals. Instead, for harvested species, the goal of harvest regulations is typically to maintain the population well below carrying capacity, where the population is still large enough to withstand unexpected disturbances, but small enough that resources are relatively abundant and reproduction can be quite high, producing lots of “excess” individuals that would increase the population under natural conditions but can be harvested under managed conditions that seek to maintain the base population size.

This section discusses both terrestrial and marine species because many of the issues associated with harvest are similar.

Understanding sustainable harvest – fish and wildlife

We need detailed information about population characteristics in order to determine a sustainable level of harvest, regardless of the species involved. The larger the proportion of “extra” individuals we want to harvest, the greater the need for accurate information and care in managing the harvest.

For wildlife populations, starting information includes a good estimate of total population size, rate of increase of the population, and the age classes of reproductive individuals and their age-specific reproduction rate. If animals do not reproduce before they are 3 years old, we need to be sure that a good number of individuals reach that age. If only a few three-year-olds reproduce at all, and they all have a single offspring, but 4-year olds often have twins, and animals 5 years old or older might have three or more offspring, then we may want many individuals to reach at least 5 years of age.

But how are trappers, hunters, or fishers to know how old the individuals they target are? Few species are easy to age, accurately. In some mammals, males can be aged to some extent by size of horns or antlers, and size is correlated to age in some species, but not always closely. As a result, even if we have good information about age and reproduction, we may not be able to use it when harvesting.

Harvest limits on male mammals with horns and antlers may specify recognizable categories of those to try to approximate age classes. But for many species, it may be hard to do more than separate the very young from the not-very-young.  Harvest limits can be set under such conditions, but they have to factor in uncertainty about age and the chances of harvesting reproductively important individuals. Birds and mammals are typically caught one at a time, and are not subject to commercial harvest as fish are. Under these circumstances, it’s easier to document harvest, or, at least, legal harvest.

Harvest limits on fish that are caught on single-hooks lines often specify a required minimum size because size is easy to measure and determining age requires killing the fish. For fish that are caught in nets, the mesh size of the net can allow small individuals to escape, but those caught in the net are likely to be seriously injured or killed when the net is hauled in. It’s still possible to throw too-small individuals back overboard, if the catch is being handled at the level of individual fish while at sea, but this may only serve to feed sharks and other species that accompany fishing vessels.

An additional complication in understanding sustainable harvest in fisheries is the role of technology in assisting harvest. Advanced sonar systems on fishing boats not only locate schools of fish but can, in some instances, identify species. Gear to handle miles of nets and lines of baited hooks and “mother ships” that can receive the harvest of many fishing ships extend the reach of fishing effort and enable it to continue nonstop. This level of effort requires well-informed management to understand the potential for large harvests.

Impacts on non-target species

Nets are examples of unspecific harvesting gear – they catch whatever enters them that cannot escape through the mesh – jellyfish, sea turtles, sea birds, marine mammals, and many species of non-target fish. Drift nets, which are typically miles long, are now illegal in the territorial waters of many nations, but are still used on the high seas. When they become old and tattered, they are often cut loose from the ship and discarded into the ocean where they can continue to catch anything in their path, as “ghost nets.” Exclusionary devices are available for some kinds of nets and some kinds of non-target species, to allow escape of unwanted catch. They increase the cost of the nets and are not always popular, but in some areas, they are required.

Baited hooks, particularly if they are on longlines – kilometer-long fishing lines lined with hooks – also catch many non-target species. Non-target or wrong-size fish caught on longlines are unlikely to survive if released, due to exhaustion and injury. Birds, sea turtles, and marine mammals drown.

Nontarget fisheries catch is referred to as bycatch, and in some fisheries, it comprises the majority of the catch. A recent review of bycatch assessments reported bycatch rates ranging from <1% to >90% depending on the target species. Shrimp fisheries are consistently the worst for bycatch.

Among harvest methods for birds and mammals, nets (birds) and traps and snares (mammals) may catch and kill non-target animals. Nets are illegal as a means of trapping birds in most countries. Legal use of traps and snares is often for furs or to eliminate pest species, rather than for food. Nontarget species typically do not survive.

Note that the occurrence of bycatch does not necessarily compromise the sustainability of a particular method if sustainability is judged only by the status of the target species. Judgements of sustainability in such cases should properly consider the full range of impacts of the harvest, not only impacts to the target species.

Planning for disturbance

All species are subject to disturbances that may cause gradual or sudden reductions in population sizes – wildfire, drought, flood, disease, changes in predator or prey populations, etc. Such disturbances are not always detectable – for example in fish from the deep ocean or high seas, or terrestrial species that are secretive – yet such disturbances may significantly reduce populations of target species. When this occurs, previously safe harvest levels may threaten population viability.

Monitoring of harvested populations and harvest calculations that factor in the possibility of changes in population size after harvest levels are set can help to limit harm following unexpected mortality, but these require the additional expense of monitoring and conservative harvesting approaches that may be unpopular with stakeholder harvesting groups. Managers sometimes differentiate between maximum sustained yield that may not leave much margin for safety and optimum sustained yield that explicitly acknowledges possibilities of error in the understanding of population processes and the possibility of surprises. However, in some cases, calculations of maximum sustained yield are conservative and account for similar issues as optimum sustained yield – one has to examine the assumptions and calculations to know for sure.

Requirements for sustainable harvest

Sustainable harvest is more likely to be successful with species that reproduce at least somewhat quickly. Wildlife and fisheries experts differentiate between species that reproduce quickly and at fairly early ages, called r-selected species, and species that reproduce slowly and later in life, called K-selected species; of course, many species fall somewhere between these categories. As an extreme example of a K-selected species, Greenland sharks mature sexually around age 150 and have a gestation period of 8 to 18 years, after which they give birth to perhaps 10 young. Although a female may give birth to hundreds of offspring in her life, the slow rate of reproduction makes the species very vulnerable to overfishing – population recovery could take centuries, and any overestimate in the population size could easily lead to dangerous overfishing. Unfortunately, Greenland sharks are not the target of a species-specific fishery, but are more likely to be taken as bycatch, so no fisheries management accompanies their harvest.

The problem of nontarget harvest highlights another important aspect of sustainable harvest – it should be planned, and the vast majority of harvest should be legal and reported. Nontarget take is unmanaged and its sustainability cannot be guaranteed. Illegal take – poaching of plants and wildlife and illegal and unreported landings of fish – can reduce viability of populations but also threaten entire species with extinction. In the case of wildlife, poaching often targets individual species or groups of species, however illegal capture techniques such as wire snares can catch many species. Illegal and unreported landings of fish often involve nontarget fishing techniques which reduce fish available for subsistence and commercial harvest, render population estimates for legal fishing untrustworthy, and increase bycatch of fish, birds, marine mammals, sea turtles, and other groups.

Figure 1. A pangolin. Yu-Chih-Wei CC BY NC ND.

Obviously, we cannot accurately estimate the level of illegal harvest. The most trafficked mammal in the world is the pangolin – a group of 8 species of medium-sized mammals that look like a cross between an anteater and a pinecone (Fig 1). Estimates of illegal harvest range from hundreds of thousands to over a million, annually. They are taken for food in Africa and as a delicacy and for their scales, for medicinal purposes, in Asia. Trade in illegal natural-resource harvest is considered the fourth largest illegal market in the world. For fishing, a US agency report puts illegal and unreported fishing at 20% of global take, with national levels as high as 50%, but these are estimates with little data behind them.

Understanding sustainable harvest – trees

In general, tree populations are more easily understood than animal populations – trees stand still, and as a result, population monitoring is more straightforward. Ownership of plants and plant products can also be more straightforward. For harvest purposes, we typically deal with trees on the basis of their diameter, which is fairly easily measured, and their height, which can usually be estimated with some reasonable degree of accuracy.  

Just as we harvest wild fish and also have aquaculture, we harvest trees from natural forests and also have plantations. Silviculture, the tree equivalent of agriculture, is the process of managing trees for forest products, and involves practices for both natural forests and plantations.

We divided trees into conifers and broad-leaved trees when we studied terrestrial biomes. Timber producers make the same division, but refer to softwoods (conifers) and hardwoods (broad-leaved trees), although some hardwoods are noticeably harder than others. Softwoods are used for paper, cardboard, dimension lumber used in construction (2x4s and so forth), and engineered wood products such as plywood and strand board. Many of these products can be made from small-diameter trees, but large lumber beams require older trees. Hardwoods, in contrast, are used for furniture, flooring, veneer, and the pallets used in shipping. These tend to require larger trees. Both softwood and hardwood trees may be used to create wood pellets and other forms of biomass for energy. Wood of any size, including branches, and even shrubs, can be used for pellets.

Harvest from natural forests can involve harvest of selected individual trees or clusters of trees, leaving a thinned but recognizable forest; harvest of most of the trees, leaving some to provide a bit of shade and seeds to stock the next generation; or clearcutting of all the trees. If only certain species or sizes are desired, then so-called select cutting approaches will work, and leave behind a forest that will still provide habitat to many (but not all) forest species. However, if only the highest-quality trees are being taken out without any plan for continuing to increase the value of the forest – sometimes called highgrading – then forest health and economic value is likely to decrease over time. Select cutting is standard in natural hardwood stands. Hardwoods are also grown in plantations for commercial purposes and on private land, often for investment value.

Clearcutting – the removal of all trees on a site – can be controversial. Visually, the impact is overwhelming – one day a forest is there, and then there is a forest of stumps, with piles of removed branches and the marks of forest equipment on what was the forest floor. Clearcutting can increase erosion and impair water quality of nearby streams, in some cases for years. It completely removes forest habitat, reducing biodiversity significantly. But so can intense wildfires, ice storms, and windstorms.

The primary stated purpose of clearcutting is to regenerate a more desired forest stand. For purposes of timber production, natural forests contain unwanted tree species, and older trees may grow more slowly and are more likely to have diseases that reduce their value.

Clearcutting offers managers an opportunity to reset the forest to the desired species (one or more, but usually just one) and to take advantage of the faster growth rates of young trees. In addition, some tree species have young stages that need sunlight for growth, and those species grow poorly in the shade of a mature forest. However, arguments that clearcutting is not about profit but only about site preparation for the desired forest are disingenuous and ignore the fact that a clearcut of a natural mature or old-growth forest may produce the most valuable harvest the land will ever produce – a harvest more valuable than any subsequent, managed harvest, which will almost certainly involve younger, smaller trees.

On a site with relatively intact soil, a new forest will begin to grow almost immediately, or, in drier places, as soon as the rains begin. Wildlife species adapted to open areas will soon arrive, but the suite of species will differ substantially from the species that occupied the original forest. As more and more older forests are cut, the species adapted to mature and old-growth forests have declined, some of them precipitously, and the profit motives that accompany commercial timber harvest do not permit such forests to return. Managed forests may grow to the smaller end of the mature size classes, but often do not get even that old.

Timber managers use the term rotation length to describe the period of time between successive clear cuts. Forests grown for wood pellets (energy), paper pulp, and lower-value timber products – often conifer stands such as pine – may be managed on rotations as short as 10 years. These stands can be thought of as a slow version of corn. Sites are prepared, stands are fertilized, treated with pesticides and harvested. Value of short-rotation forests to wildlife is low but not absent. Some of the environmental impacts of agriculture (including fertilizer and pesticide runoff) occur, although usually at lower levels than for commercial agriculture.

Longer rotation lengths allow trees to grow larger, and may increase biodiversity, depending on how the stand is managed. Managed stands on public lands typically receive less intensive management and may support more biodiversity. However, such stands are still likely to be monocultures or near monocultures when they are replanted after harvest.

Figure 2. Coppiced poplar trees in the UK. By Incrediblehunk CC0.

Some energy tree crops are grown as coppices – fairly dense, planted stands of trees that are harvested by cutting back the stems every 2-5 years (Fig 2). Unlike pines and firs, trees grown in this way are usually broadleaf trees such as willow that are able to sprout back after their stems are cut to the ground. Some species can be cut 7 or more times before they must be dug up and replanted.

Plantations of all types are often sited on land that was originally natural forest, so they represent an overall long-term loss of natural forests and of biodiversity. Because plantations are heavily managed, many species of plants and animals are excluded. Plantations may also attract pest species and disease organisms, to the detriment of nearby natural forest.

Genetically modified trees are increasingly used for plantation stock. Such genetic engineering now involves a host of characteristics including growth rate (which affects rates of carbon sequestration), resistance to disease, resistance to herbicides (much as in agricultural crops, to allow herbicides to be used freely in plantations to suppress any competing plant growth), etc. Recently, genome editing was used to reduce the amount of lignin (a natural “woody” polymer) in poplar trees, which in turn reduced the chemicals needed to process the wood into engineered wood for structural building material. The resulting engineered product was stronger and longer lasting than previous products.

Use of engineered varies widely, with some countries having almost all plantations planted in engineered species (mostly poplar, eucalyptus, pines) and other countries banning them. The distinction between genome-editing, which only involves changes to the existing genome, and genetic modifications in which genes from other organisms are inserted into the tree genome, is sometimes important in defining whether the new varieties are legal.

Certifying sustainability of harvested natural resources

Sustainability certification programs seek to ensure that natural resources are harvested ethically and responsibly, in ways that protect biodiversity, environmental quality, and social values. In order to be viable, such programs must meet a social or economic demand, so that producers and consumers will support certification. Transparency and oversight are important to maintain trust in the system. Small-scale producers may not have access to such programs, may be unaware of them, or may not be able to afford to meet their requirements The Forest Sustainability Council, the most widely used forest-products certification organization, makes specific efforts to include smallholders in its program.

In addition to certification programs such as the Forest Sustainability Council (FSC) or the Marine Stewardship Council (MSC), nonprofit organizations provide some sustainability recommendations such as the US Monterey Bay Aquarium’s Seafood Watch guides that help consumers to make informed choices about seafood.

Knowledge Check

Take a moment to complete the short quiz below to assess your understanding of this section. Read each question carefully and refer to the section content as needed. This quiz is not graded – it’s simply an opportunity for you to reflect on what you’ve learned and reinforce key concepts.

The major threats to species and ecosystems cross lines of terrestrial, freshwater and marine. This section will be relevant to Chapter 9, on oceans, as well as to this chapter. Additional threats to marine species and ecosystems will be added in Chapter 9.

A history of threats to species and ecosystems

The threats to terrestrial biodiversity have changed dramatically over human history. In prehistory, overhunting of the enormous land birds, the moas, of New Zealand and of the giant mammals of the Ice Ages – wooly mammoths, dire wolves, giant sloths, etc. was the primary means by which humans reduced biodiversity.

During the so-called Age of Exploration, during which primarily European explorers traveled to places that were not Europe, we continue to see overhunting, partly to feed the explorers and partly to fuel demand for fur and other commodities at home. During this period, North America lost the great auk, Steller’s sea cow, the Labrador duck, and the Atlantic sea otter. Almost 100 bird species of Hawai’i are extinct, eliminated by avian malaria brought in by Captain Cook in mosquitoes in his water casks. Elsewhere, species losses are less well documented but include the dodo and various giant tortoise species.

Figure 1. A pair of thylacines or Tasmanian tigers – a marsupial predator – in captivity at the National Zoo, Washington, DC, USA, 1904. The species became extinct in 1936. Public domain.

As colonization occurred and unfamiliar forms of agriculture and livestock production were imposed on continents, hunting, poisons, and trapping were used to control predators and to secure commercially valuable wildlife and plant products. Predators were extirpated from many places, but outright extinctions were much fewer. The thylacine, or Tasmanian tiger was one loss during this period (Fig 1).

More recently, we are beginning to see losses due to massive changes in river systems brought on by damming. The baiji, a freshwater dolphin of the Yangtze River in China was last seen in 2002 and is considered extinct. Several salmon species have declined precipitously, although their cultural, recreational, and food value have led to recovery efforts that have staved off extinction, so far.

Ninety species of amphibians are believed to be extinct as a result of a fungal disease spread around the world by travelers, including, perhaps, researchers. Three species of North America bats have decline precipitously due to a fungal disease unintentionally brought over from Europe. Climate change is believed to have claimed the golden toad of Costa Rica. Three species of migratory bats in North America have declined precipitously since wind farms became common, although the industry has contributed to efforts to reduce losses.

Twenty years ago, it was relatively easy to outline the major threats to terrestrial biodiversity – habitat loss and degradation, invasive species (including diseases), illegal harvest and overharvest, secondary effects. It is becoming more difficult to rank the causes of biodiversity loss.

Certainly, habitat loss and degradation remains the most obvious source of blunt-force trauma. Species cannot persist without space in which to live. Massive losses to agriculture occurred over a century ago in North America and millennia ago in many other places, but continue in the present in places like Brazil and Indonesia, as we saw in Chapter 7. Urban sprawl also continues to eat land. These are forms of true habitat loss – the land is no longer available for habitat. In other cases, habitat may be degraded by pollution, overgrazing and other overuse, by fragmentation by roads, development, agriculture, and other land use.

Habitat fragmentation is a slightly less direct form of loss – theoretically, it can occur without any actual loss in acreage, although some loss is usually involved. Edges of fragments are different from core areas that still resemble original continuous habitat. Edges are open to humans, dogs, cats, trash, noise, sunlight, wind, invasive species and other disturbances, depending on the nature of the habitat. So, although the land may still be there, its value in terms of the original habitat is degraded. As a result, overall area of useable habitat is reduced for species that need characteristics of the original habitat. Some species need areas of a certain minimum size; fragmentation reduces the number of habitat patches that they can use. New species may benefit from the edge conditions, but they are often generalist species that were already common.

Figure 2. Burned area of the Sonoran desert near Phoenix, Arizona, USA with dead saguaro cactus. National Ecological Observation Network. Courtesy: Battelle. CC0.

Invasive species are often listed as the number 2 or number 3 threat to biodiversity. Most people don’t recognize non-native species, and they are so ubiquitous in many places that they go unnoticed. But their impacts can be devastating. Buffel grass was brought to the US Southwest in the 1930s as cattle forage for the desert. It was deliberately planted from the late 1930s until 1980, and it was noticeably invasive by the 1990s. It has spread throughout the Sonoran desert, where continuous vegetation was historically absent and wildfires were rare as a result. As buffelgrass has taken hold, it has supported the spread of wildfires over much larger areas than could previously support fire. Native cactus, notably including saguaro, are impressively vulnerable to fire and die when burned (Fig 2). By changing the fire regime of an entire biome, buffelgrass is threatening endemic native species and converting patchy Sonoran desert vegetation into a nonnative grassland.

Illegal harvest affects both plants and animals, from elephant ivory and rhino horn to rosewood and mahogany and a host of animal and plant parts used for medicinal and other purposes around the world. Some of this harvest is for personal use – some illegal pangolin harvest (see previous section) is for food, for example. But much of the illegal harvest is for trade, and natural resource trade is the one of the largest illegal markets in the world. Smuggling of illegal natural resource products can occur simultaneously with drug smuggling, leveraging profits from both kinds of trade. Overharvest is a related harm that can occur when harvest is unregulated or poorly regulated. Harvest of plants, fish, and invertebrates is often unregulated, leaving these species unprotected.

Secondary effects are unintended consequences of actions or substances intended for other purposes. In Chapter 7 we saw that use of neonicotinoid pesticides as seed coatings and genetic modification of crop plants with Bt genes have significantly increased the levels of pesticides in agricultural environments, leading to declines in pollinators including bees and butterflies. Bees and butterflies were not specific targets of either form of pesticide, but because both are broad-spectrum pesticides, they also affect “good bugs” and reduce biodiversity. The coal-fired powerplants that heated homes and powered industries in the last century and, in some places, in this century, were never intended to wipe out forests or render lakes devoid of most living things, but their emissions caused acid rain, which killed some organisms outright and degraded habitats to the extent they were unusable by many species. Such unintended consequences are sufficiently common and severe in their impacts that they rank among the leading causes of biodiversity loss.

This was the list of leading threats to biodiversity before climate change became a significant concern. These threats still exist, and all remain hugely important concerns. But climate change both exacerbates some of these threats – for example, heat waves and warming, generally, can make some otherwise suitable habitats unsuitable, contributing to habitat loss, and invasive species are spreading poleward and higher in elevation under warmer and sometimes wetter conditions – and adds to them.

Climate-change impacts on species and ecosystems

As climate change accelerates, its effects are increasingly clear. The news is not good, and reading about it is not uplifting. Losses have already occurred and more are inevitable. Most people find it difficult to face the details of these impacts, and it is important for readers to remember why they became interested in sustainability in the first place. We cannot solve all problems, but by seeking to solve those we can, and being ambitious about our abilities, we can limit overall harm. Understanding the drivers of loss is part of reducing loss.

Impacts to species

Direct climate-change impacts to species include impacts related to changes in average and extreme temperature and precipitation. All species have climatic conditions they can tolerate and conditions for which they are so unsuited that they will not settle in those areas. Researchers refer to the climate envelope of tolerable conditions for species, and model climate envelopes by studying the temperature and moisture regimes in the range of the species – the total area inhabited by all individuals of the species. As climate changes, the climatically suitable area for species also change. Species can move to newly climatically acceptable areas, adapt in place to changes in climate, or they can be extirpated from the unsuitable area. [Note: extirpation describes loss from a particular area. Extinction describes the complete loss of the species from the planet.]

Figure 3. Change in American lobster catch distribution in US waters from 1967 to 2014. The dark purple locations represent the highest number of lobsters caught. Credit: NOAA Climate.gov via data from Rutgers University OceanAdapt. Public domain.

A review of information on range shifts in a wide variety of plant and animal groups shows that some species and some groups are clearly shifting poleward or upward in elevation, apparently to better match their temperature preference or towards areas of newly appropriate moisture availability (which may sometimes be downslope, against the temperature change). However, the majority of species have not (perhaps not yet) shown such shifts. Highly mobile groups – birds, insects, and fish – were among the species groups to show significant poleward shifts, overall (Fig 3). This group includes a number of commercially important marine species.

Note that species that currently inhabit polar and high elevation locations have no colder places to move to and are limited to adapting in place or disappearing. Adding to the stress, polar and high-elevation locations are also warming faster than the planetary average – 4 times faster, for polar regions – narrowing the window for adaptation. Species adapted to using sea ice – penguins, polar bears, seals, and walruses, among others – are declining as this habitat disappears. Species adapted to living on or under snow, or foraging in snow fare much less well when warm spells bring rain that then freezes in thick layers, entombing ground-level vegetation in ice that hooves cannot break up and small mammals cannot burrow through, and collapsing polar bear dens on polar bear cubs that cannot stay warm when drenched and then frozen. The developing possibility that shipping lanes will develop in the so-called Northwest Passage north of the Northern Hemisphere will expose marine and coastal species through the region to the noise and pollution of international shipping along with the possibility of oil spills from damaged and grounded ships.

Less mobile species are limited in their ability to move in order to remain within their climate envelopes. Plant species with small seeds that are easily carried by wind or rain have a clear advantage over heavy-seeded species such as baobab, Brazil nut trees, and oaks. Amphibians, reptiles, smaller mammals, flightless insects, and many other less-mobile animal groups also have limited options.

Mobile species are not guaranteed success in finding climatically suitable areas simply because they are mobile. New diseases, parasites, predators, or competitors may make climatically suitable areas unavailable. For species near the edge of a biome, a move may take them into a different, and less suitable habitat type. Food plants or prey species may be absent in the potential new home, requiring a change in diet that may or may not work for the moving species.

Invasive species are among the species shifting in response to climate change, including pest species and diseases. The US Center for Disease Control notes that several diseases are already expanding in the US as a result of changes in temperature and precipitation (including flooding). Warmer, wetter conditions are hospitable to a wider range of species than colder, drier conditions, so more of the planet is becoming available to more species, and parasites and diseases spread with them. Climate refugeeism also contributes to the spread of disease.

Climate change combined with high human mobility is not only spreading wildlife diseases but also supporting the development of novel diseases as a result of close contact between humans and animals. Avian influenza was first detected in domestic geese in China in 1996, and spread through contact with wild birds. It was first observed in humans in 2020 and has not become dangerous or widely problematical. However, impacts on waterfowl, birds of prey, and wildlife species have been more severe. More than 20 endangered California condors died when the disease first reached their populations in 2023. In Canada, more than 25,000 gannets, a seabird, have died. And, of course, tallies of all mortality in all species are unavailable, because monitoring at that level does not occur. OneHealth is an initiative of the World Health Organization that works to provide an integrated framework for health information for humans, wildlife, and ecosystems.

Results of changes in climatic extremes can be less subtle than range shifts in response to changes in climate averages. A “blob” of unusually hot, nutrient-poor, unproductive marine water developed off the northwestern coast of North America during 2014-2016, in usually highly productive waters. In response to the lack of nutrients, the marine food web of the area collapsed, and an estimated 4 million seabirds – common murres (Fig 4) – were estimated to have died of starvation, the largest single reported wildlife mortality event in the modern record. Researchers are concerned that changes to the food web and to the murre population may persist.

Impacts to ecosystems

Ecosystems are collections of plant and animal species, along with abiotic elements including soil and water, plus the processes that bind them all together. As climate change leads to shuffling of species around lands and waters, species move, or fail to move, as single units, each responding to its own tolerances and needs, depending on its own mobility.

As a result of differential responses of species, the ecosystems that managers have become accustomed to managing are changing. In some cases, the changes are as simple as redrawing biome lines, as is occurring in some northern areas where woody vegetation is spreading into the tundra, converting it first to a more shrub-dominated system and then to boreal forest.

In other ecosystems, the biome type is maintained, at least for now. Familiar species may become less common or more common, and new species move in. New invasive species appear. Often, the ecosystem remains recognizable, with a slightly different cast of species.

As species assemblages are shifting, so, too, are disturbance regimes. Increased disturbance may increase mortality, generally, creating more holes in forest canopies or more bald spots in grasslands. Ecosystem services may be diminished and the higher disturbance level may favor invasive species, further diminishing ecosystem services. In some cases, novel ecosystems may form, unlike anything managers in the area have dealt with before.

In ecosystems where fires occur naturally with some regularity, a century of fire suppression has often already predisposed them to burn more intensely as trees and brush have accumulated in unnatural density. More frequent and more severe droughts, including flash droughts, can render vegetation so dry that even if fires have burned through fairly recently, remaining vegetation may still easily burn if an ignition source – lightning, a stray campfire, the catalytic converter of a vehicle pulled off on the side of the road – is available. In some once-forested areas, forest fires have become so frequent that forests can no longer persist – young trees are repeatedly killed back before they can establish a forest – and the areas are converting to shrublands: more shrub species can handle being burned back to their stumps than tree species can. These shrublands may represent new assemblages of species because of the new role of fire in shaping the ecosystem – these may be novel ecosystems.

If an extremely wet season in a usually drier environment leads to a rich, diverse forest or woodland understory in spring, then by summer it may become a layer of dry, fine fuel, easily ignited and easily blown around by fire updrafts. In this way, increased variability in weather, with unusual wetness followed by later dryness, can create fires of greater intensity and frequency.

Persistent or frequent drought, alone, can also convert areas from forested to unforested, because trees are also less able to manage extreme dryness than shrubs. Outbreaks of bark beetles occur naturally in the northern hemisphere, but are occurring more frequently and with greater severity, assisted by droughts that weaken tree defenses. The dead needles and branches of dying trees become fuel for wildfire, creating a triple threat to boreal forests. Pest species are often favored by warmer conditions, and invasive species pressure may also shape ecosystems affected by climate change.

All disturbance regimes have the potential to modify the ecosystems in which they occur, if changes are big enough. Rivers that were once lined with riparian wetlands may lose those wetlands if large floods regularly wash away the accumulated soil and rotting vegetation and scour the riverbed down to rock. Droughts severe enough to dry rivers back to isolated pools may extirpate fish species that cannot persist in small, warm pockets of water.

Managers seeking to conserve biodiversity must plan not only for movement of native species but also movement of invasives, implications of new combinations of species, and results of changes in the disturbance regimes of their areas. If harvested species are being managed, impacts of climate change on their population sizes and mortality rates must also be considered.

Ecosystem changes under climate change are most observed in the Arctic, where climate change is proceeding at a greater pace, with woody species advancing into the tundra, tundra melting into thermokarst lakes, and boreal forest both expanding north and collapsing under more intensive fire regimes. However, the most at-risk major ecosystem under climate change is clearly warm-water (shallow-water) coral reefs. We will study these more carefully in the next chapter.

One class of species has an out-sized impact on ecosystems and deserves attention under climate change. Ecosystem engineers are species that have unusually large impacts on other species and on ecosystem processes. Beavers cut down trees and dam rivers, creating ponds and wetlands where forests and rivers once occurred. Prairie dogs create extensive burrow systems that support an entire suite of other species and rework soils over large areas. Tropical termites support their own suite of species and affect soils and nutrient levels throughout their biomes. Reef-building coral organisms create complex structures that provide homes for a immense diversity of species. Whereas, in most ecosystems, the loss or arrival of individual species does not affect the ecosystem in major ways, the loss or arrival of ecosystem engineers may change ecosystems significantly.

Ecosystem and species impacts on climate change 

Carbon sequestration

Climate regulation is an increasingly important ecosystem service, provided especially by wetlands, peatland forests, tropical forests, and tundra, which sequester significant carbon (remove it from atmospheric circulation, into biomass or soil, for meaningful amounts of time) . So-called nature-based solutions to climate change include the protection of these ecosystems and efforts to increase the amount of carbon they hold and the length of time for which they hold it. However, humans continue to drain wetlands, and clear and burn peatland forests and tropical forests. Climate change, itself, is melting tundra, releasing methane and carbon dioxide, and increasing wildfires that darken the land surface and further hasten warming. These anthropogenic factors can turn ecosystems into major sources of GHG emissions, as we have seen. If we can control these factors, ecosystems can continue their natural roles in regulating climate change through carbon uptake and sequestration. However, as ecosystem services are reduced in climate-stressed ecosystems, the amount of climate regulation that can be performed by those ecosystems may decrease.

At present, researchers calculate that natural systems and agricultural soils may be able to absorb approximately one third of greenhouse-gas emissions. However, most of the science behind our understanding of the details of carbon cycling in ecosystems in very young, and researchers stress that we need a much more solid understanding of all phases of carbon cycling in the natural world in order to understand and benefit from the potential for natural systems to mitigate climate change.

Unlike livestock, single species in natural systems mostly do not have a major impact on carbon cycling and climate change. However, termites, by virtue of their role in breaking down large volumes of dead woody material in the tropics, and their production of methane, are often mentioned. Climate change may expand their range and their impacts. Wild ruminants burp methane, just as cattle, sheep and goats do, however their populations are very small relative to livestock populations, and they usually receive at most a passing note.

Other impacts on climate

In addition to sequestering substantial carbon, tropical rainforests also affect climate change through teleconnections. Because of the very large amount of water vapor these forests “breathe out” through evapotranspiration, they affect precipitation patterns around the world. Degradation of these forests reduces rainfall at great distances. And all forest types are now seen as important for supporting local and regional rainfall.

Knowledge Check

Take a moment to complete the short quiz below to assess your understanding of this section. Read each question carefully and refer to the section content as needed. This quiz is not graded – it’s simply an opportunity for you to reflect on what you’ve learned and reinforce key concepts.

Cataloging and monitoring biodiversity

Biodiversity is a lovely word because it embraces so many things. But at the same time, it can make the multiplicity of species and ecosystems less visible. We have many approaches to conserving biodiversity without identifying every bacterium, fungus and flower. But to truly understand biodiversity, at some point, that level of knowledge is needed. Biodiversity surveys continue to find new species, especially as genetic tools become less expensive and more widely known. Such surveys are often undertaken by individual researchers and nonprofits, using grants from governments or funding organizations, rarely by agencies, unless on public lands. These can be one-off missions because their goal is discovery.

In contrast to missions of discovery, monitoring is a forever process that must be ongoing if we are to detect changes in biodiversity that signal a need for increased efforts at protection. Monitoring of all biodiversity is perhaps impossible. Researchers estimate the planet houses millions of species, many of which (the smaller and better hidden ones, to be sure) are yet to be discovered. Most national reports on biodiversity begin with a disclaimer about information gaps in even the most urgent kinds of information, such as the status of at-risk species. Depending on location and level of jurisdiction (national, regional, local), protected areas may have sufficient resources to monitor within their boundaries. But broader-scale monitoring, and especially monitoring on private lands, occurs only irregularly.

At present, approximately 2.1 million species have been formally described, most of which are invertebrates (Fig 1). Of these, the International Union for the Conservation of Nature has assessed status for 172,620, including all terrestrial vertebrates, and found 26% of mammals, 11% of birds, 41% of amphibians, and 21% of reptiles are threatened with extinction.

Figure 1. Numbers of described species on Earth, by taxonomic group. OurWorldinData CC BY.

Clearly, biodiversity is not secure. How, then, is it being protected?

Laws, treaties, and other agreements to conserve biodiversity 

International protection

Species status assessments and recommendations

The International Union for the Conservation of Nature (IUCN) is an international organization important in determining species status and in recommending approaches for conservation. The IUCN compiles the Red List of Threatened Species, covering, in theory, all species on the planet. They report that they have comprehensive assessments for terrestrial vertebrates, freshwater fishes, reef-building corals, and trees (see above). Assessments are updated periodically, but as might be expected, many groups are incompletely unassessed, and updates are not as frequent as the organization would like.

The IUCN Species Survival Commissions oversees a number of international, species-oriented conservation efforts. Specialist groups are convened for individual species (e.g., Asian elephants) and species groups (e.g., antelope, chameleons, butterflies and moths) to bring together experts from many nations and many jurisdictional levels to share knowledge and training to understand the status of species and the best approaches for their conservation and recovery.

Species protection

As we know, international environmental work is undertaken through treaties. Of these, the Convention on International Trade in Endangered Species of Wild Fauna and Flora, CITES, is the most far-reaching. Almost all nations are signatories to the treaty. CITES operates through a series of species lists that indicate level of threat. Depending on level of threat, CITES regulations limit or ban international trade in the species or in its parts. Countries that have signed CITES agree to work to oversee importation of regulated species and ban importation of protected species. Political issues arise involving which species are listed and at what level of protection. Countries can request to have their species listed, but other nations can impose listings, as well, if they believe a species is not well protected by the nations in which it lives, called the species’ range states. The CITES administration provides training in recognition of protected species, helps nations to cooperate in controlling trade, and generally supports world efforts to ensure that international trade does not harm species or render them extinct. CITES has no enforcement powers. Nations undertake their responsibilities under CITES by incorporating CITES requirements into their national system of laws. In the US, CITES protections are enforced under the Endangered Species Act. CITES does not deal with in-country trade, but only with international trade.

A variety of other, less far-reaching international treaties also protect species. The Convention on Migratory Species of Wild Animals helps protect species ranging from bats to whales that migrate across international boundaries. It provides a framework within which nations can set up coordinated efforts at conservation including setting harvest limits that acknowledge all the potential sites of harvest. Like CITES, it provides protection at more than one level, to ensure appropriate conservation measures. A suite of treaties binds the US, Canada, Mexico, Russia, and Japan to protect migratory birds and sets up coordination of hunting regulations to ensure sustainable harvest of ducks and geese.

Ecosystem protection

The oldest international conservation treaty, and the only one aimed at a single ecosystem type, is Ramsar, the Ramsar Convention on Wetlands, signed in Ramsar, Iran, in 1971, as the world was beginning to understand that perhaps wetlands served some useful functions and should not be eliminated by drainage, plowing, and construction, as if they were a blight upon the landscape. Nations that signed on to Ramsar agreed to nominate one wetland site to the list of Wetlands of International Importance and to support the conservation and “wise use” of wetlands within their borders. The Ramsar administration coordinates training opportunities to improve wetland-protection capacity among nations. The vast majority of nations have signed and ratified the treatment. For many developing nations, their Ramsar wetland was their first nationally protected conservation land.

The Convention on Biodiversity (CBD) was opened for signing in Rio de Janeiro, Brazil, in 1992. The US is the only UN member state that has not ratified the treaty, but it does follow the treaty protocols. Treaty nations agree to create National Biodiversity Strategies to safeguard their biodiversity and to develop Action Plans to implement those strategies. These provide a means for nations to collect and coordinate their biodiversity information so that the national biodiversity picture is detailed and clear, and to coordinate conservation actions across jurisdictions within the nation to ensure effective, efficient conservation. Although the CBD is not specifically an ecosystem conservation measure, by seeking to safeguard biodiversity, generally, it provides an umbrella that includes ecosystem protection.

National protection

Nations differ hugely in the level of protection they afford biodiversity, but as most have signed the Convention on Biodiversity, they have at least created national biodiversity strategies that describe national biodiversity and the risks to it. The existence of such a document gives individuals and organizations outside the government a means of holding their nation accountable for biodiversity conservation, and may open doors for collaboration between government and nongovernmental organizations.

The US Endangered Species Act is widely regarded as the most protective species-level statute in the world. However, as of 2025, efforts are underway to severely undermine its strengths. Other countries have similar laws, but they afford less protection to listed species. Some countries will allow populations of a species to be listed, even if the entire species is not yet at risk; others will not. To date, in the US, habitat of species listed under the Endangered Species Act can be protected, as a means of protecting the species. Because habitat loss is such a pervasive threat to biodiversity, this protection is key to recovering many at-risk species, but is weak or absent from many at-risk species statutes throughout the world. A 2018 review of US state at-risk species laws found that 46 of 50 states had legislation protecting imperiled species, although 2 states only protected the species already listed by the national Endangered Species Act. But the remaining 44 extended state protection to additional species. Only 5 protected habitat.

Even under the Endangered Species Act, at-risk species are not guaranteed to recover. Funds are not available to support and recover all at-risk species. Some are essentially put on hold due to lack of resources. The law requires that their status be monitored and that, if they show signs of sudden decline, they are to be protected. But as we have seen earlier, such monitoring is itself expensive and imperfect. Even when species are listed, they may still become extinct if managers and researchers are unable to reverse population declines. Genetic problems, novel diseases, irreversible habitat loss, climate change, and other stresses may not be amenable to correction, despite the best efforts of experts.

Efforts to protect listed species are often met with demands that conservation groups and agencies should be willing to compromise in their requirements. However, most populations and species only become listed when their status has become dire and there is little room left for compromise if they are to be kept from extinction. Listing these species earlier, when they are declining but not yet approaching risk of extinction, would increase the number of listed species dramatically, requiring more conservation planning, imposing more work on municipalities and industries that might harm the species in question, and perhaps reducing public support for biodiversity conservation. However, avoiding the transparency of honest reporting on the status of so many species is often not working well, either.

Protecting biodiversity by protecting land – protected areas

Protected areas occur at many levels of jurisdiction and in all countries. The degree of protection also varies. The IUCN describes 7 levels of protection for protected areas, ranging from areas that can only be accessed for scientific research to areas in which sustainable use of natural resources is permitted. In the US, national parks typically permit only recreational and scientific use, and conservation of cultural resources – these lands are designed to preserve and protect their resources for future generations. In other nations, villages may exist within parks and may use the resources of the park. Elsewhere, inhabitants of villages outside the park may have permission to enter to secure fodder for livestock, building materials, etc. In the UK, sustainable plantation forestry may occur in national parks, and private land is contained within many, where grazing of livestock is a regular use of the land. Conservation grazing is often used to manage habitats.

Measures that exclude local and indigenous human populations from their lands in the name of creating protected areas have sometimes given national parks and other protected areas a bad name, in the same way that dam construction that requires the relocation of local and indigenous residents has a bad name. In addition to being unjust, disenfranchising people who are knowledgeable about species and ecosystems of an area risks losing valuable historical and current information that may be needed for informed management of the land and its species.

The ability of protected areas to conserve biodiversity is related to the characteristics of the protected areas. Experts have developed recommendations for designing protected areas and systems of protected areas and additional recommendations are designed to maximize protection against impacts of climate change.

Big blocks are best. Large areas have large core areas where the impacts of surrounding land uses are muted. For many species, large areas can protect one or more viable populations that can persist in the face of population reductions due to climate change and other disturbances. Disturbances such as fire or flood or storms are less likely to affect all of a large area, leaving intact areas to shelter plants and animals and to provide colonizers to reestablish populations in affected areas, if necessary.

Topographic variation provides diverse microclimates and habitats. Protected areas that include mountains or whatever topographic variation is available will have lower warmer areas and higher cooler areas, with attendant variation in soil moisture and precipitation. The variety of conditions supports a variety of species, and the elevational gradient provides some opportunity for mobile species to find preferred temperatures and moisture levels in a relatively short distance compared to shifting poleward. Poleward facing slopes (north-facing in the Northern Hemisphere, south-facing in the Southern Hemisphere) receive less sunlight during the day than equator-facing slopes, and different soil types provide different levels of drainage, suiting a variety of plants and the food webs they support.

Connectivity and permeability provide passage as species seek appropriate climates and resources. Modern landscapes present a series of obstacles and barriers to organisms moving through them – highways, cities, large agricultural regions, etc. For many species, such barriers render landscapes impermeable – the probability of surviving passage through them is very low, and the landscapes become killing fields. Corridors of natural habitat – for example, streams with intact streamside habitat – can improve connectivity among natural areas. Protected areas that include such corridors can facilitate dispersal of young animals, seasonal migrations, and movement to adjust to climate change or other disturbance. For more mobile species, stepping stones may suffice – islands of habitat close enough together to provide resting spots, foraging spots, etc. But for less mobile species and species at higher risk in highly modified habitats, intact corridors may be necessary.

Certain settings create local refuges from climate change. Within any landscape, some locations will be cooler and moister than others – these are climate-change refugia. Poleward-facing ravines and canyons are naturally cooler than equator-facing depressions. Spring-fed rivers may have cooler waters than rivers that are fed only by rainfall and runoff. Mountain areas downslope of glaciers and snowpack are cooler than areas without such natural refrigeration. Areas shaded by forest and cooled by evapotranspiration from the vegetation are cooler than more exposed ecosystems. Land along large bodies of water will have more moderate climates because water warms slowly and cools slowly. These areas will not maintain constant conditions under climate change – everything is getting warmer. But they will remain cooler than nearby areas, and may give species longer to adjust to changing conditions.

Species-specific protection

The guidelines above are helpful for identifying areas that can protect biodiversity broadly. But areas that are broadly protective may still fail to meet the needs of some species, particularly endemic species that inhabit only small areas or only certain very specific habitat types. Endemic species may not need large areas, only areas that preserve their particular needs. However, it is important to keep in mind that, under climate change, these areas may no longer meet the needs of the species for which they were created, and flexibility in protected area system design will be needed to accommodate change needs under changing conditions.

Protected areas may be hard to situate in high-quality agricultural areas or in dense urban areas, simply because the land is not available. But these areas have (or once had) their own biodiversity, which may be at particularly high risk. In the US, schools in urban areas have begun to provide pollinator gardens to support breeding and provide habitat connectivity for pollinator species, including monarch butterflies. In Europe and elsewhere, climate-smart urban planning is increasingly restoring wetlands and building parks in areas that flood easily, to collect flood waters in areas that will not be harmed by being inundated and reduce flood risk in other, more built environments.

In some instances, protected areas are created with a strong emphasis on one or a few species – usually these are smaller areas for endemic species. Local communities and indigenous people may protect culturally significant species and ensure sustainable harvest of species important to subsistence within lands they control. Climate change is complicating such efforts, as species’ ranges shift. Typically, it’s not feasible to have a movable protected area. Similarly, local communities and indigenous people likely cannot track across the landscape the species with which they have special relationships.

Captive propagation

As a means of protecting biodiversity, captive propagation is the practice of taking individuals of an at-risk species into captivity in order to safeguard genetic diversity, to breed individuals to supplement faltering wild populations, or to take entire species into protective custody if their populations are extremely low. Captive propagation is expensive, time-consuming, and intensive. It requires deep knowledge of species biology and ecology which is often lacking, with the result that efforts may not initially be particularly successful. Some species need breeding or other conditions that cannot be met in captivity. When it works, captive propagation can save species. But the capacity of existing facilities can support only a handful of species in this way. Triage to identify the most needy species and the species for which efforts are most likely to be successful are constantly ongoing, and failures are heartbreaking.

One limitation of captive propagation is that it cannot fix whatever was broken in the species’ environment that led to its extreme decline. One aspect of triage is often whether there will be a place to release captive-bred and reared individuals – a place to reestablish the species. If conflict, habitat loss, or changing conditions such as climate change have eliminated appropriate release areas, then captive propagation can only maintain a semblance of the species in protective custody. Prolonged captivity leads to changes in genetics and behavior that can rapidly reduce the chances for successful release of individuals and that can permanently change the nature of the species. Such changes will occur quickly for some species and much slower for others, but unless donors come forward to fund permanent custody, facilities will not be able to afford to support species in captivity without a clear path to eventual release.

With the advent of biotechnology, some groups are seeking to resurrect extinct species using recovered DNA and by intensively breeding existing related species for the characteristics of the lost species. Such efforts can attract considerable attention, but they can detract from efforts to conserve existing biodiversity. If (or when) such efforts are successful, the vanished species will compete with existing biodiversity for space and resources which are already in short supply.

Knowledge Check

Take a moment to complete the short quiz below to assess your understanding of this section. Read each question carefully and refer to the section content as needed. This quiz is not graded – it’s simply an opportunity for you to reflect on what you’ve learned and reinforce key concepts.

The case study text for this chapter is broader than the chapter, but much of it is focused on ecosystem health and biodiversity. It is explicitly urgent and it makes alarming reading. It is also the most recent compilation of the range of climate-change impacts on the planet. The text here is an article, in contrast to the thousands of pages of the IPCC reports that assess climate change exhaustively every 5-7 years.

Ripple WJ et al. The 2025 state of the climate report: a planet on the brink. BioScience Special Report. https://doi.org/10.1093/biosci/biaf149 .

Salinity and temperature

Earth’s major oceans are a series of linked basins interrupted in places by continents that can only manage to amount to about one-quarter of the planet’s surface. When Earth was young, the oceans were much less salty, but a few billion years of rain washing dissolved minerals out of the rocks have brought it to its present salinity of approximately 35 parts per thousand, about 6 times saltier than our blood.

Salinity, like many marine water characteristics, varies more than one might expect in a well-mixed system. But some parts of the oceans are less well mixed. In very shallow areas, evaporation may increase salinity. Where ice is melting and running into the oceans, and where rivers meet the oceans, salinity will be lower.

Figure 1. Sea-surface temperatures around the world. Note that cooler temperatures come closer to the equator along the west coasts of North America, South America and Africa than they do farther out into the oceans, due to cold-water upwellings. US National Oceanographic and Space Administration. Public domain.

Ocean water is colder at depths than near the surface. In part, this is due to solar warming, which obviously occurs only at the surface.  As well, cold water is dense, and sinks, ensuring that deeper waters are always colder. Mixing by wind in shallow waters and by currents throughout much of the oceans modifies the temperature profile. In areas where cold water is brought to the surface by currents meeting obstacles or by wind moving surface waters out of the way and providing an opening for rising water, surface waters can be quite cold (Fig 1). These cold-water upwellings are places of high productivity and are often biodiversity hotspots as well as fishing hotspots. Except for cold-water upwellings, sea-surface temperatures largely vary by latitude, but the depths are uniformly cold, except for a few hot-water vents.

Salinity and water temperature, together, create the global circulation pattern among ocean waters that is called the Meridional Overturning Circulation (Fig 2), which we saw briefly in one of the Chapter 1 videos. Warm water in the tropics moves polewards (north in the Northern Hemisphere, south in the Southern Hemisphere) because of the Coriolis effect, carrying warm, salty water away from the equator, where evaporation slightly increases salinity. In the Atlantic, the Gulf Stream carries the warm water from the Gulf of Mexico clockwise along the North American East Coast and across the Atlantic towards the United Kingdom. The water cools as it travels, and as it nears Greenland, that cooling, dense, salty water sinks towards the ocean bottom. The cold water travels along the ocean floor, south through the Atlantic Ocean, until it meets the circumpolar current of the South Ocean, and joins it. Because of wind patterns in the South Ocean that blow towards the equator, water is lifted up from the deep Southern Ocean at several points, warming as it reaches the surface, and returning towards the Atlantic.

Figure 2. Global ocean circulation called the Meridional Overturning Circulation. Cold, dense, salty water sinks in the North Atlantic (red lines becoming blue lines) and rises in the Southern Ocean in response to wind pattern (blue lines becoming red). UK Met Office. Contains public sector information licensed under the UK Open Government License v3.0.

Because the driving part of the circulation pattern, the sinking cold, salty water, is in the North Atlantic, the pattern is often called the Atlantic Meridional Overturning Circulation (the AMOC),  although it affects all the main oceans of the world. The branch of the AMOC that comes from the Gulf of Mexico towards Europe is responsible for milder temperatures in Europe than would otherwise occur at that latitude.

Dissolved gases

As always, gases dissolve best in cold water. Although plants take carbon dioxide from the air, phytoplankton use dissolved CO₂, which dissolves readily in water and is not limiting. Under climate change, levels of dissolved CO₂ are increasing as CO₂ in the atmosphere increases.

Fish and other marine organisms rely on dissolved oxygen, which is usually not limiting. However, the ocean has dead zones, often as the result of anthropogenic eutrophication, as we saw in Chapter 3. Some eutrophic areas occur naturally, in upwelling areas where nutrients come to the surface in abundance. Where eutrophic areas create large algal blooms, and dead algae feed decomposer bacteria that use up oxygen in the water, oxygen can be limiting, as we saw in the discussion of dead zones in Chapter 3.

Minerals

Ocean water contains many dissolved minerals, most in very small quantities. Gold, for example, is present in parts per trillion – levels that are currently not useful for recovery.

Over long periods of time, some minerals precipitate out into mineral deposits on the ocean floor as a result of chemical reactions that create insoluble forms. Mineral deposits may occur in crusts, nodules, and large ore bodies. Metals and critical minerals are of particular interest to industry.

Biodiversity and ecology

Owing to their vastness and the difficulty researchers face in studying oceans, more than 90% of ocean species may still be uncatalogued, and some 80% of the oceans are unmapped.

As in lakes, productivity is related most to light availability and nutrient availability. As a result, coastal areas are often productive because shallower water and nutrients from runoff supply nutrients that surface-water organisms such as phytoplankton (the base of the food web) can access. In contrast, in deeper parts of the ocean, the surface waters have sunlight, but nutrients are much less available, except where upwellings bring them to the surface from the depths.

Phytoplankton – algae and blue-green algae (actually bacteria) produce approximately 50% of the world’s oxygen. The tiniest species of these produces some 20% of world oxygen, outstripping all the world’s tropical rainforests.

Carbon and nutrient cycles in the ocean are somewhat complex. Gravity wins a lot of battles, and many minerals and a lot of carbon end up on the sea floor. But in the water column, organisms capture other living and dying organisms, eat fecal pellets or dead material, and keep some carbon and nutrients in constant circulation in the upper parts of the water column (Fig 3). We are still learning about oceanic circulation in important ways, particularly as concerns about climate change lead us to understand carbon cycling better.

Figure 3. Ocean nutrient cycling and food web. US Department of Energy Office of Science. Public domain.

Only recently, for example, researchers realized that, before humans hunted whales to near extinction, whale urine and feces and whale bodies were an important source of carbon to the sea floor and to the organisms that inhabit those depths. The whales that eat plankton feed in cold, productive waters and then travel to warm, nutrient-poor waters to give birth, transporting nutrients in their urine, feces, and bodies, and improving productivity in these waters, in much the same way that large herbivores move resources around grasslands. And just by existing, whales can, over their lifetimes, store more carbon than long-lived trees. As some whale populations are recovering, it is becoming easier to understand these mechanism. The overall movement of carbon from the atmosphere into the oceans is called the carbon pump. It is an important mechanism of climate regulation. The whale portion of it has been referred to as the whale pump.

Ecosystem services of oceans

Provisioning services of oceans provide food, raw materials for building and manufacturing, medicines, and minerals. Although aquaculture production is increasing, capture fisheries – fish and other marine organisms taken wild from the waters of the world –  still comprise about half of aquatic food production, and about 90% of capture fisheries are oceanic. Coral and sand are used for construction, with sand (necessary for cement and concrete production) increasingly taken not only from beaches but also sucked off the ocean floor along with any living things inhabiting it. Marine organisms contribute to cosmetics, dyes, food additives, fertilizers, paints, abrasives, and many other kinds of products. Marine products are used for medicines including anti-cancer drugs and painkiller, among others. Minerals are not yet widely produced from ocean sources, but as technological demand for them increases, the availability of ocean sources for metals and other critical minerals is expected to result in mining of the sea floor. Oceans also provide a medium for transportation. An estimated 80% of international trade moves by sea.

Among regulatory services, climate regulation is perhaps the ocean ecosystem service most under study. Approximately 90% of anthropogenic heat generated through climate change has been absorbed by the oceans, so far, and about 30% of anthropogenic CO₂. Without these services, the planet would have warmed much faster than it has to date. Additional regulatory services include protection of coasts, which is afforded by coral reefs, mangrove forests, and salt marshes, which break the force of storm surges, and dilution and breakdown of water pollutants.

Supporting services provided by oceans include oxygen production which supports most life forms, primary production via photosynthesis, which provides the basis for the food webs of the ocean, and the provision of habitat for marine organisms. These are quick to describe, but enormous in their impact.

Life on Earth began in oceans and although the land now supports the majority of biomass and biodiversity, oceans are crucial to providing a sustainable environment for all life.

Governance of the oceans

The overarching statute governing human interactions with oceans is the UN Convention on the Law of the Sea, sometimes abbreviated UNCLOS. Most nations of the world, but not all, have ratified the treaty. The US has not ratified it, but abides by its provisions, to date. UNCLOS sets up a variety of frameworks for dealing with ocean issues. It defines national territorial waters as extending outward from national seashores for 12 nautical miles (13.8 statute miles, 22.2 km). The waters within that distance are part of the sovereign territories of the respective nations and they may make and enforce laws governing any aspects of these waters without interference from other nations. Further, nations have sovereign rights over natural resources such as fish and minerals within 200 nautical miles (230 statute miles, 370 km), regions known as exclusive economic zones or EEZs. Beyond territorial waters (for matters not related to natural resources) or the EEZs (for matters related to natural resources), issues of navigation, resources management and use, environmental protection, research, and settlement of disputes about these are governed by UNCLOS, for those nations that have ratified the treaty.

UNCLOS also sets up a means for additional agreements to be created to deal with specific marine issues. For example, the International Seabed Authority was created to address seabed mining. The UN Fish Stocks Agreement, which manages stocks of fish in international waters and stocks that cross international boundaries, also arose out of the UNCLOS framework.

Early overharvest of marine resources such as whaling and harvesting of seabirds illustrated clearly the potential for tragedies of the marine commons. UNCLOS and the treaties and agreements that grew out of it are the major means of converting high seas resources from common-pool resources that can be freely (and excessively) exploited by all into resources subject to sustainable management.

Knowledge Check

Take a moment to complete the short quiz below to assess your understanding of this section. Read each question carefully and refer to the section content as needed. This quiz is not graded – it’s simply an opportunity for you to reflect on what you’ve learned and reinforce key concepts.

Humans often describe only a single marine biome, which obscures lots of environmentally useful variability. It’s true that the open ocean is a huge expanse of fairly similar conditions – the kind of situation that makes a good biome. However, the places of greatest interest for biodiversity and other natural resources are often in much smaller bits that punctuate that huge expanse – on underwater ridges and sea mounts and islands and atolls. These smaller bits don’t make good biomes, but they are certainly good ecosystems.

Read the accompanying PowerPoint on marine ecosystems.

Knowledge Check

Take a moment to complete the short quiz below to assess your understanding of this section. Read each question carefully and refer to the section content as needed. This quiz is not graded – it’s simply an opportunity for you to reflect on what you’ve learned and reinforce key concepts.

Anthropogenic threats to ecosystem health and biodiversity of oceans can modify the abiotic aquatic environment, for example, through water temperature and chemistry, by adding noise, and by changing atmospheric weather patterns. Threats can target specific biotic and abiotic natural resources for subsistence use or for economic gain, possibly affecting the aquatic environment at the same time. For much of human history, we lacked the capacity to cause harm to the oceans as a whole, however, as ships long ago became available that could cross oceans, and technology more recently developed that can seek out individual whales and schools of fish, human capacity for harm has advanced as well. And climate change has taken the concept of harm to oceans to new levels.

Among the UN Sustainable Development Goals, life in water is obviously relevant here, but so many human actions and benefits intersect with marine ecosystems and marine life that all of them are involved to some extent. Among the planetary boundaries, ocean acidification is directly tied to marine ecosystems, but it is caused by climate change and both of these threaten biosphere integrity. Modification of biogeochemical flows, especially of N and P causes the dead zones we first met in Chapter 2.

Pollution

Land-based pollution

In Chapter 8 we discussed threats to land-based ecosystems and biodiversity, including threats to rivers. Because most rivers discharge into the oceans, those threats are also marine threats. River pollution from industrial and municipal waste and runoff including plastic waste and the microplastics that derive from it, and run-off from agricultural lands all contribute to pollution of oceans. Coastal waters are most affected, due to their proximity to the land source. However, ocean currents carry pollution throughout the oceans.

Nutrient pollution from land activity is associated with coastal dead zones around the world (Fig 1). These are sometimes associated with agricultural lands but are often associated with high nutrient loads from municipal water treatment.

Figure 1. Highly productive agricultural areas on land, eutrophic and hypoxic coastal zones and areas of high particulate carbon in the oceans. US National Aeronautic and Space Administration Science Visualization Studio. Public domain.

Land-based air pollution can also affect oceans. We saw, in chapter 2, that Saharan dust – particulate pollution – fertilizes the Atlantic, providing iron and other minerals that are in short supply in the surface waters of deep oceans. In the early 2000s, acid rain impacts on coastal waters were noted, but these have been reduced by reductions in sulfate air pollution. Now, ocean acidification occurs primarily as a result of climate change, as we will see shortly.

Pollution from sources on the oceans

Ocean-going ships contribute antifouling chemicals, sewage, leaking oil and gas, and air pollution to marine pollution. Antifouling paints are used on ships’ hulls to repel barnacles and other hitchhiking organisms that would slow the ship as it moves through the water. They are toxic and leach in small quantities. They also save fuel which reduces GHG emissions, and reduce chances of ships introducing hitchhikers into new waters where they could be invasive. Oil and gas leaks are generally in small quantities and are not considered major sources of water pollution for oceans.

Figure 2. Container ships offloading and onloading cargo in the Port of Oakland, California, USA. Container ships presently carry about 90% of packaged and manufactured goods. The largest ships can carry over 24,000 of the shipping containers, which double as trucking containers to reach their final destinations. US National Oceanic and Atmospheric Administration. Public domain.

GHG emissions and air pollution from ocean shipping (Fig 2) are a sufficient cause for concern that the International Maritime Organization, a rule-making body formed under UNCLOS, passed regulations in 2020 sharply reducing limits for sulfur in ship fuel. The resulting decrease in sulfate emissions reduced albedo and may have contributed slightly to warming over the Atlantic, but did improve air quality and reduce associated acid rain. The IMO is also working to achieve zero emissions from the world shipping fleet by 2050; presently, the fleet is responsible for 2-3% of GHG emissions, globally.

Spills from ships carrying fossil fuels (Fig 3) and toxic chemicals cause local mortality of sea life and foul waters, in some cases for long periods. When the Exxon Valdez ran aground in 1989 in Prince William Sound, off the coast of Alaska, it dumped approximately 11 million gallons (41,640 cu m) of crude oil into coastal waters, much of which escaped initial clean-up efforts.

Figure 3. Ultra large crude carrier AbQaiq, a commercial oil tanker of the largest class, over 1000 feet (333 m) long. Fully loaded, she would sit near the line of the color change of the hull. Kevin H. Tierney, US Navy. Public domain.

In the cold waters of the Alaskan coast, the oil that came onshore congealed into a mousse-like consistency that was largely undiminished on affected beaches in 2015. Wildlife deaths from acute effects included an estimated 250,000 seabirds, 302 harbor seals, and up to 2800 sea otters. Chronic effects included higher than usual mortality of salmon eggs in exposed streams at least 4 years later, prolonged recovery in sea otters (by 2014) and seabirds, some of which had not recovered as of 2014.  In contrast, a 2007 spill of about one third the size of the Exxon Valdez spill occurred as a result of an accident involving the Hebei Spirit in the port of Daesan in South Korea was dealt with quickly, with extensive clean-up efforts. By 2013, intertidal species were considered “fairly recovered” and fish and plankton were fully recovered. Impacts to marine mammals and birds were not reported in Marron et al. (2020). Such spills are uncommon and localized, but coastal habitats are biodiverse and spills can affect many species.

Ocean-going fishing fleets contribute substantial amounts of discarded fishing gear to the oceans each year. These are now the largest component of the various “garbage patches” that occupy large areas of the major ocean basins. The circulating currents in these basins – gyres – collect floating debris into a large, very loose masses. The material includes microplastics and much of it floats slightly below the surface, making it less visible. A research sampling mission into the North Pacific Garbage Patch in 2019 attributed 47% of the mass of plastic collected to fishing gear, with another 13% from plastic bottles from land. They traced most of the plastics in the gyre to the major industrial fishing nations of the region: Japan, China, Korea, USA, Taiwan and Russia. The materials in garbage patches leach toxins into the ocean and can entangle wildlife (see the discussion of ghost nets in Chapter 8). They degrade into microplastics, whose impacts we saw in section 3.4. The garbage patches become homes for marine organisms, and being novel habitats, they can support species that are out of place – such as crabs, which are not common in the surface waters of the deep oceans – and likely to become invasive. They can then transport these species around the oceans, increasing the odds that these potential invasive species will become truly invasive.

Fossil-fuel production at sea contributes GHG emissions and leaking oil and gas, as do ships. But failures at drilling rigs can be more substantial. In 2010, an explosion on the Deepwater Horizon resulted in continuous release of oil from the wellhead for more than 4 months. The US government estimated that 210 million gallons of oil were released (780,000 cu m). Clean-up efforts in these warmer waters (warmer at the surface – at the wellhead on the ocean floor, water temperatures were approximately 4°C (40°F)) included the use of chemicals to keep the oil from clumping, which were themselves potentially harmful to sea life, but reduced harm from the oil itself. Unlike the Exxon Valdez and Hebei Spirit spills, which occurred close to shore, the Deepwater Horizon wellhead was 41 miles (66 km) from shore; oil was released into the pelagic zone and oiled deep-sea corals in addition to fish, shellfish, marine mammals, and seabirds. The government agency tasked with the damage assessment estimated, among other losses, 4-8.3 billion harvestable oysters lost, 51-84,000 birds killed, 56,000 to 166,000 juvenile sea turtles killed, the local dolphin population decreased by 51%, and the local recreation industry lost over half a million dollars. Recovery times for longer-lived species such as sea turtles are estimated in decades. Recovery of the local dolphin population was estimated to need 39 years unless active restoration was undertaken.

Seabed mining is a proposed activity that has not yet commenced, owing to disagreements about how to proceed. Seabed mining is anticipated to cause substantial turbulence and turbidity and has the potential to be associated with the same kind of leakage and spills as accompanies shipping. In 2025, in the US, an executive order directed fast-tracking of extraction of critical minerals from the ocean floor, potentially in contravention of UNCLOS, which the US has not ratified, and by which it is therefore not bound.

Noise pollution from ocean-going ships, from sonar “thumping” used in oil exploration, and from in-ocean installations such as drilling rigs and wind turbines, disrupts navigation by and communication among cetaceans – whales and porpoises. The interference reduces feeding success and increases vulnerability to ship strikes, which are an important source of mortality.

Overfishing, including destructive fishing

Food and Agriculture Organization estimates show that the proportion of overfished stocks increased rather steadily from about 10% of stocks to about 30% of stocks from 1975 to 2021, while the proportion of stocks fished at the limit of sustainability increased from about 50% to about 60% (Fig 4).

Figure 4. Global trends in the state of the world’s marine fishery stocks, 1974-2021. Food and Agriculture Organization. CC0.

In Section 8.4, we saw that many kinds of fishing gear catch many different species, including many non-target species, and may continue catching things even after they are discarded. Drift nets that hang in the surface waters are limited by the UN to 2.5 km (1.55 mi) in length, but illegal nets may be much longer – nets as long as 50 km (31 mi) have been reported.

Baited longlines can be set at any depth, depending on the main species targeted. Longlines in US waters average 28 miles (45 km long); in international waters they may extend over 100 miles (160 km). Longlines contain 25-50 hooks per mile.

Trawl nets can be large enough to enclose commercial aircraft, but the front-end mesh is usually quite large, to allow escape of some species such as smaller whales and dolphins. Slower swimming species are less likely to escape this way, as they must swim faster than the trawl is being dragged. Trawl nets can be dragged at any depth; they are particularly destructive when dragged along the bottom.

In addition to fishing gear with great potential to catch target and nontarget organisms, fishing ships also often employ a variety of sonar and other fish-finding technologies. The intensity of effort is often underwritten by governments supporting fishing fleets that have become too large to be efficient but that remain important to coastal economies. It’s not surprising, then, that overfishing results, even in managed fishing. But as we saw earlier, illegal take amounts to perhaps 20% of fishing harvest, overall.

One practice supporting illegal fishing practices is reflagging, in which ship owners register their ships under a nation other than their nation of citizenship or the nation of the port at which they do most of their business. Reflagging is (more or less) legal, and a few nations attract most of the ship owners who reflag. Often, regulations in the attractive nations allow owners to remain anonymous, set low taxes on ship profits, have few requirements associated with pay and treatment of ship crews, and have low enforcement of fishing regulations. The UN estimates that 73% of ships operate under so-called flags of convenience. The 4 largest registries are Liberia, Panama, the Marshall Islands, and Hong Kong.

Figure 5. The fish in the image are all bycatch in a shrimp harvest. US National Oceanographic and Atmospheric Administration. Public domain.

As we have seen, legal fishing can have considerable bycatch of unwanted but still dead organisms. Illegal fishing is no different. Bycatch mortality is the leading threat listed for sea turtles, the tiny vaquita porpoise, most species of albatross and many other endangered marine species (Fig 5).

Destructive fishing destroys habitat in search of fish. Use of dynamite and poisons in reefs destroy reef structure or kill coral. Small doses of cyanide can be used to stun fish for the aquarium trade; the practice is considered a major threat to reefs where it is practiced. Dynamite is less target-specific and kills fish – it is used for subsistence fishing.

Invasive species

Estimates of economic harm from invasive species typically run into billions of dollars. As with terrestrial invasive species, marine invaders can disrupt food webs; destroy habitat; cause damage, disease and mortality; and modify entire ecosystems. They include algae that smother coral reefs; diseases that can extirpate native species; and toxic and poisonous species.

Marine invaders may travel on debris carried by wind and currents. Plastics and other anthropogenic debris are much more common than naturally occurring debris, and may last longer in the water, increasing their ability to disperse species around the world, thereby supporting invasions.

Ocean-going ships spread invasives in two major ways. Like pieces of debris, they can acquire hitchhikers that travel with them, attaching and dropping off as the ship travels. In addition, ocean-going ships take on ballast water to stabilize the ship and strengthen the hull. The less cargo the ship carries, and the more rambunctious the seas in which it travels, the more weight of water it must take on in order to sit properly in the water. With the water come larval fish and other marine life, along with plankton of all kinds. When a ship carrying ballast water receives more load, it needs less ballast water and discharges it, along with at least a sample of the organisms it scooped up with the water when it took on the ballast water. An American comb jellyfish introduced to the Black Sea, probably in ballast water, consumes such volumes of plankton that they severely reduce populations of native fish, including commercially important species.

The International Maritime Organization of UNCLOS has a Ballast Water Management Convention that provides guidance on ballast water practices that limit invasive species – ships take up coastal water in port, then exchange it, in the open ocean, where coastal organisms won’t likely survive, then dump the deep-ocean water when they get to port, into the coastal waters where the deep-ocean organisms are less likely to survive. Many nations have adopted the guidance into national law.

The ocean-bridging canals – Suez and Panama – are also potential avenues for invasion. Invasions of the Mediterranean from the Red Sea are fairly well documented and include 137 species of mollusks. Red Sea fish species have become so common in the Mediterranean that there are markets for some. Others, such as lionfish, are pest species that disrupt tourism and are harmful to human health.

Climate-change impacts on oceans

If climate-change impacts are increasingly bad on land, they are worse in the oceans. Because water is slow to change temperature, the marine environment is much less variable, thermally, than the terrestrial environment and marine species are less likely to tolerate temperature changes.

Impacts on ocean circulation

In Section 2.2, when we discussed tipping points, we briefly visited the possibility of a tipping point involving the major ocean circulatory pattern called the Atlantic Meridional Overturning Circulation (AMOC). This ocean conveyor belt is driven by sinking of cold salty water in the North Atlantic, together with the forces of a spinning planet, which turn the waters of the ocean in clockwise loops in the Northern Hemisphere and counterclockwise loops in the Southern Hemisphere. The Gulf Stream of the North Atlantic is an important part of AMOC that sends warm water from the Gulf of Mexico north and east across the Atlantic towards the United Kingdom and Europe, where it provides a warming effect. A collapse in the AMOC is predicted to have greatest impact on weather in this region.

Our understanding of the physics of the AMOC is still evolving. Very early climate-change literature suggested the AMOC might collapse under climate change, as melting freshwater from the Greenland icesheets diluted and floated over the Gulf Stream, reducing the rates of sinking salty water. Later but still early work suggested this was less likely. In 2020, a review of the earth system models used to study climate change showed that most of the models predicted a 34-45% weakening of the current. A more recent study concurs, but with a wider range of uncertainty: 18-43%, underscoring that we still have much to learn about this important aspect of planetary function. Note that, the article estimating a 34-45% decline uses the phrase “significant 21st century decline” while the other, indicating an 18-43% decline, describes it as “limited future … weakening!”

Impacts on marine ecosystems and species of warming waters

The oceans of the world have absorbed approximately 90% of the warming created by anthropogenic GHGs, saving the terrestrial world from warming far more than it has. The top 700 m (2300 ft) has already warmed approximately 1.5°F (0.8°C) since 1901. Because water takes so much energy to warm, the waters have warmed much less than the terrestrial world would have. But that energy is now bound in the ocean waters, and to return to pre-climate-change temperatures, that heat will need to dissipate.

Even more concerning is that ocean temperatures seem to have passed a tipping point with respect to extreme temperatures – marine heat waves – which have become the new normal. In 2019, 57% of the global ocean surface recorded extreme heat, which was rare before 1870-1919. Significant increases in the extent of extreme marine events over the past century resulted in many local climates shifting out of their historical sea-surface temperature bounds.

For the global ocean, 2014 was the first year to exceed the 50% threshold of extreme heat, with the South Atlantic (1998) and Indian (2007) basins crossing this barrier earlier. “Extreme” heat thus became the norm, rather than an extreme, in 2014.

Because of water’s thermal inertia – because it takes considerable energy to change the temperature of water – aquatic environments are thermally stable, relative to terrestrial environments. As a result, aquatic organisms tend to have narrower thermal tolerances than terrestrial organisms. Warming that might stress a terrestrial organism slightly is likely to stress an aquatic organism more. In addition to temperature stress, decreasing oxygen levels due to warming waters are also provoking range shifts. More mobile aquatic species can move poleward to cooler and more oxygen-rich waters, and they are moving, as we saw in Section 8.3.

Corals are not mobile. Under heat stress, reef-building corals expel their partner algae, called zooxanthellae, creating a condition called bleaching. Zooxanthellae are responsible for the colors in corals and also provide most of the food for coral, through photosynthesis. When they are gone, only the white skeleton remains. If heat stress continues for more than a few days, corals begin to starve. Some zooxanthellae and some corals are more resilient to heat than others, but most are already badly stressed by marine heat.

During 2014-2017, during a three-year heat event, more than 75% of all coral reefs bleached. Reefs can recover from bleaching over time, but repeated heat stress prevents recovery. In addition, coral diseases are increasing in frequency and severity with warming waters. In the 6th Assessment Report in 2023, the IPCC warned that a tipping point for coral extirpation was nearly reached. In 2025, researchers reported that it had passed. The world-wide 2023-2025 coral bleaching event affected 84% of world corals, and the warming point associated with the tipping point – 1.2 C above historical average temperature, was passed. Researchers predict that 95% of coral reefs will succumb to climate change by 2100. Even if all GHG emissions were to cease immediately, anticipated loss of coral reefs would still proceed, due to the momentum of existing GHG levels and because the ocean will be very slow to cool, once GHG emissions return to pre-industrial levels.

As we saw in Chapter 8, marine heat waves harm more than coral ecosystems. Extended heat waves can collapse ocean productivity and cause the collapse of entire food webs. As heat waves become the new normal, these impacts become more common.

Because species do not respond to climate change in the same way and at the same pace, phenological mismatches are increasingly being observed. Phenology is the study of life-history events – breeding, hatching, flowering, migration, leaf fall, seed set, etc. Phenological mismatches occur when coordinated life history events – like breeding by birds and hatching of insects in spring – become uncoordinated. For example, the timing of juvenile salmon migration from freshwater to the oceans is becoming misaligned to the availability of prey in the nearshore ocean waters.

Impacts of sea-level rise

As it warms above 4°C (40°F), water expands. More warmth, more volume of water. The IPCC Sixth Assessment Report indicated that thermal expansion of the global ocean accounted for 38% of sea-level rise from 1901 to 2018. For 1993–2003, thermal expansion was estimated to cause about half of the observed rate of global sea-level rise.

Melting of floating ice does not contribute to sea-level rise. When the ice in your soft drink melts, your glass does not overflow. But melting of glaciers and ice sheets on land does contribute to sea-level rise and it is increasing. Nevertheless, warmer, plumper water is still causing over one-third of sea-level rise. As ice sheets melt faster and provide a competing source of sea-level rise, that proportion will drop.

Sea-level rise is not constant around the planet, owing to wind activity that piles water up in some areas and blows it away from other areas. But, on average, experts predict that by the end of the century, sea level will have increased by 30 cm (1 ft) over levels in 2000, even for fairly mild increases in GHG.

Sea-level rise affects coastlines by increasing erosion, flooding coastal ecosystems such as salt marshes and mangroves forests, and overwashing into freshwater coastal wetlands during storms. Storm damage to ecosystems and property is greater because both property and ecosystems were established during times of lower sea levels. Carbon sequestered in coastal and marine ecosystems is at risk from erosion and ecosystem loss due to rising seas.

Rising seas also flood beaches of nesting sea turtles. In many places, inland migration of ecosystems and sea turtles is not possible due to coastal development.

Sea-level rise deepens waters over seagrass beds, kelp forests, and warm-water coral reefs. Deeper waters are colder, which can reduce heat damage, but they also limit light, reducing productivity. Increasing depth will exceed tolerances of shallow-water species.

Ocean acidification

As the amount of CO₂ in the atmosphere increases, the amount of CO₂ dissolved in the ocean increases. CO₂ dissolved in water creates carbonic acid, which lowers the pH of the ocean, increasing acidity, just as it makes rainwater acid. Ocean acidity has already increased nearly 30% compared to pre-industrial times and a doubling is predicted by 2100.

Carbonate chemistry is slightly complicated, because of buffering reactions but the simple version is that acid dissolves calcium carbonate. Calcium carbonate makes up coral reefs, mollusk and other marine shells, and shells of crabs and lobsters. Calcium carbonate occurs in three different crystalline forms, and they are differently susceptible to dissolution by acids. The form in coral and mollusk shells is more vulnerable than the form in crab and lobster shells.

Because gases dissolve better in cold water than in warm water, ocean acidification is proceeding more quickly in polar and deep waters. Thus, while warm-water corals are most vulnerable to warming oceans, cold-water corals are more vulnerable to ocean acidification. But both kinds of corals are exposed to both stressors; the deep oceans are warming significantly, and acidification is underway in warm waters, just slower than in cold waters.

Corals can still grow under acid conditions, but they grow longer and thinner and are more susceptible to damage. But the reef structure, which is purely mineral, is increasingly dissolved by the acid water, weakening the whole reef and potentially, over time, reducing it to rubble.

Figure 6. A pteropod or sea butterfly. US National Oceanographic and Atmospheric Administration. Public domain.

Shelled sea species are having increasing trouble building their shells against the dissolving forces of the acid ocean. Pteropods (also called sea butterflies) are shelled planktonic organisms important in ocean food webs (Fig 6). Because their shells sink to the seabed after death, pteropods also contribute to the ocean carbon pump. As early as 2012, researchers observed severely eroded shells in pteropods in the South Ocean.

In Washington State in the US, Puget Sound has some of the most acidic waters in the world, and oyster hatcheries in the region are unable to raise oysters in their earliest stages; they raise juveniles elsewhere and bring them back to Washington when they are big enough. Native shellfish do not have the option of being reared elsewhere and will disappear from the region when increasing acidity causes similar problems with their larvae and young.

A 2025 report indicated that, like the coral warming threshold – a threshold for a group of species, rather than for the planet – the planetary boundary associated with ocean acidification is also in the process of being crossed. Although acidification, like warming and sea-level rise, is variable from place to place, all three are increasing everywhere. Range shifts poleward can alleviate warming impacts, but not acidification or sea-level rise.

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Protecting resources that exist, in large part, as a common-pool resource would be challenging under any circumstances. Simultaneous pressures from increasing pollution, increasing human population, and accelerating climate change increase the need for a sustainable approach to these important resources.

Regulatory approaches to conservation and management of marine natural resources

Within their exclusive economic zones – in the waters within 200 nautical miles of their coasts – nations manage and conserve natural resources as they choose. They can exclude harvest of sea organisms by other nations, set their own harvest levels, control gear use for harvest, establish protected areas, and so forth. Other nations are typically excluded from mining minerals and fossil fuels.

Certification programs for sustainable seafood, mentioned in Chapter 7, provide additional support for efforts to use marine resources sustainably. These include international NGOs such as the Marine Stewardship Council and the Global Seafood Alliance’s Best Seafood Practices, as well as national-level efforts such as Fair Trade USA. An umbrella organization, the Certification and Ratings Collaboration, makes available a data tool that compiles information both on environmental sustainability and human rights/social responsibility aspects of fisheries.

Mining and drilling under the high seas

Mining for minerals and petroleum products under the high seas is subject to control of the International Seabed Authority, under UNCLOS. Although the ISA has issued permits for exploration for mineral deposits, no mining has yet been allowed.  Existing rules require that mining companies wishing to work in the high seas must partner with a UN member state, but no further regulatory structure exists. The most recent round of talks aimed at determining how mining might proceed broke off in July 2025, without any resolution.

Because the US has not signed UNCLOS, it is free to ignore the standstill in regulatory work at the International Seabed Authority and proceed to mine as it pleases. In the past, the US has followed UNCLOS processes, but under the present administration, it has expressed strong interest in developing deep sea mining. It is considering mining within the US EEZ around American Samoa, despite Samoan resistance, and is exploring two requests from companies seeking to mine in international waters with US partnership.

Harvest of living resources on the high seas

On the high seas, the UN Convention on the Law of the Sea (UNCLOS) is the overarching authority on management of living resources, but specific treaties provide a patchwork approach to sustainable management. Harvest in the Southern Ocean, the waters surrounding Antarctica, is managed separately, by the Convention for the Conservation of Antarctic Marine Living Resources, as part of international agreements in the Antarctic Treaty System.

Under UNCLOS, the management of fish stocks of species that breed in freshwater but live mostly in salt water, including the high seas, is the responsibility of the nations in whose freshwater the species breed. Thus, the US and Canada are responsible for management of the salmon that breed in rivers on the east and west coasts of North America.

Fully marine species that migrate within and across the high seas (highly migratory species) and whose ranges include the high seas as well as the waters of multiple nations (straddling stocks) are managed by the nations in whose freshwater the species breeds. Thus, Canada and the US manage the salmon stocks that breed in the coastal rivers of North America.

A variety of specific fish management systems are enabled under UNCLOS. For example, the International Commission for the Conservation of Atlantic Tuna (ICCAT) protects tuna, species related to tuna, and highly migratory sharks and rays in the Atlantic Ocean and adjacent seas. It also has measures to reduce bycatch of sea turtles and seabirds in the fisheries it oversees.

ICCAT has had some successes, but it has also been criticized for failing to act quickly and strongly enough to protect the stocks for which it is responsible. A single, high-value tuna, which can weigh as much as 600 lbs (270 kg) can bring over USD 1 million at auction, for sushi. As a result, the economic pressure is high to allow fishing on these stocks and illegal fishing pressure is also high. Nevertheless, some recovery has occurred.

The most recent effort at protecting living marine resources under UNCLOS is the ponderously named Agreement under the United Nations Convention on the Law of the Sea on the Conservation and Sustainable Use of Marine Biological Diversity of Areas beyond National Jurisdiction, abbreviated as BBNJ, which required almost 20 years of negotiation to create. It was ratified by the 60th member state in September 2025 and came into effect at that point, for those nations that have ratified it. The agreement, which is informally called the High Seas Treaty “establishes legally binding rules to conserve and sustainably use marine biodiversity, share benefits from marine genetic resources more fairly, create protected areas, and strengthen scientific cooperation and capacity building.”

Marine Protected Areas (MPAs)

Under UNCLOS, nations are empowered to create Marine Protected Areas (MPAs) in their territorial waters and within their EEZs. In addition to protecting biodiversity, protection of coastal ecosystems and the seabed in MPAs helps to ensure that sequestered carbon remains undisturbed and can continue to accrete, helping to mitigate climate change.

The International Union for the Conservation of Nature (IUCN) reports that slightly more than 6% of world oceans is protected in MPAs, but less than 2% is protected at a level that prohibits taking of resources (living and mineral). The IUCN also reports that MPAs are typically understaffed and underfunded, making it more difficult to protect resources from threats including climate change.

No marine protected areas have been created in the high seas under UNCLOS, but an area three times the size of California has been protected in the Ross Sea, on the coast of Antarctica, under a separate treaty, the Convention for the Conservation of Antarctic Marine Living Resources, that governs the Southern Ocean. The Ross Sea Marine Protected Area is quite large, but its creation was contentious, and it is more than 40% smaller than its originally proposed size. All biodiversity is protected in 72% of the area, which is managed as a no-take zone.

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As mentioned in section 9.4, the High Seas Treaty is a new (September 2025) treaty under UNCLOS that seeks to protect living marine resources.

Read an explanation of main goals of the agreement by the Pew Charitable Trusts, here.

If you wish to see it, the text of the treaty (technically, an agreement) is available from the UN website here.

Usually when we think of protecting the environment, we think about wilderness, places ‘out in nature’ where animals live without direct human influence. When we think of cities, we think of buildings and streets and cars, the opposite of nature. So why are cities an essential part of environmental sustainability? Here are three big reasons.

Life in cities

The way we set up our cities determines how healthy and safe the urban environment is for humans and other species to live in. More than half of the global population lives in cities, and all those people deserve good, healthy lives. Clean air, clean water, and access to natural spaces are essential to our health. Urban development and climate change threaten those necessities, causing more direct harm to certain groups of people than others. The people whose lives are affected by those threats deserve a say in how their cities develop and how the world works to address climate change. In addition to humans, thousands of plant and animal species are found in urban and suburban areas – some cities are even considered biodiversity hotspots and critical conservation areas. A lot of people who care about protecting the rest of nature live in cities, and they have the opportunity to make a huge difference right where they are.

Cities’ local impact

Urbanization has a huge effect on the land where it happens. Replacing fields and forests with pavement and human structures destroys habitat, and when humans in a city use resources and produce waste, it can pollute the surrounding air and water. However, not all urbanization has the same level of impact on its surroundings. There are more and less responsible ways to put together cities, and because increased urbanization is inevitable (the human population is still growing, and many people are moving to cities), it’s critical that we do it responsibly.

Cities’ global impact

People’s lifestyles, like what they eat, what they buy, how they get around, and how much energy they use, determine the environmental impact they have. As of 2020, the 55% of the world’s population that lived in cities consumed two thirds of the world’s energy and caused 70% of CO₂ emissions, meaning the average city-dweller had a larger environmental impact than the average person in the world. Cities can be set up to facilitate low-impact lifestyles, and many of the choices that decrease energy use and emissions also improve the health of city residents.

Figure 1. People relaxing in Central Park, New York City, New York, USA. Ingfbruno, CC BY-SA 3.0

Figure 1. Urban sprawl: a suburb of London, Ontario, Canada. Adam Colvin, adapted by Haljackey. CC BY-SA.

Over the past century, the proportion of all humans who live in cities has dramatically increased, reaching 56% in 2021 and expected to reach 68% by 2050. As urban populations grow, cities need to physically expand: outward, upward, or both (Fig 1). Without planning and intentional strategies for how to use land, urban development spreads uncontrollably into surrounding natural areas, creating urban sprawl. The UN reports that “between 2000 and 2020, cities sprawled up to 3.7 times faster than they densified, resulting in negative impacts on the natural environment.” Because sprawl increases the distances people have to travel across a city, it leads to dependence on personal vehicles and an increase in transportation-related GHG emissions. However, compared to rural areas, cities allow for more efficient use of resources, because having housing, schools, and businesses together in one location allows for economies of scale in infrastructure and shorter travel distances. If cities are designed to maximize this efficiency, they can be really sustainable places.

Zoning laws determine what each parcel of land in a city can be used for, such as single-family housing, apartments, mixed-use buildings (businesses on the ground floor and apartments above), parks, or factories. The placement and density of these different options shapes the urban environment. Zoning decisions can reflect bias and perpetuate injustice. For example, in the United States in the 1930s, each neighborhood in many large cities was rated at one of four levels of desirability for homeowners’ loans, and areas with non-White and/or immigrant populations were often assigned the lowest rating. This practice, called redlining, made it harder for people in those communities to buy homes and advance economically, and it also influenced infrastructure and land use in those areas, such as parks that provide recreational opportunities or industrial facilities that produce pollution. Redlined areas were associated with increased health risks, less greenspace, increased crime, and persistent poverty. These effects persist to the present day, decades after the practice was ended. In the southeastern United States, minority communities have significantly higher levels of soil contamination than white communities.

City planners can promote approaches to development that decrease the environmental impacts of city layout. The first policy tool is mixed-use zoning, which allows housing and businesses to be interspersed in the same area (Fig 2). People might live in apartments above a bank or grocery store and be within walking distance of a school, a park, and a doctor’s office. This decreases pollution from transportation and increases people’s access to essential services and resources. Another city planning solution is to fill in low-density areas by replacing things like parking lots and single-story businesses with apartments or a mixed-use zone. Finally, when cities do expand outward, they can do so in a way that allows for efficient, low-emission living and movement around the city. One example of this is Copenhagen, Denmark’s 1947 Finger Plan, which set up new development in five lines radiating from the city center, each served by public transportation and separated by preserved natural areas so that people living along each line had easy access both to the city center and to greenspace near their homes. Another example comes from Freiburg, Germany, whose government set environmental, transportation, and energy guidelines for new developments, involving dense, mixed-use neighborhoods, tram lines connecting to the city center and train station, limited parking (in communal, solar-powered garages), preserved greenspace, and rainwater retention systems.

Figure 2. Mixed-use development on a street in Brussels, Belgium. Adisa. Adobe Education license.

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Figure 1. Construction of a suburb. Robert Myers. CC BY-SA.

Heat

Creating a city involves a change in land use (Fig 1): areas of vegetation are replaced with roads, sidewalks, parking lots, and buildings. Impervious surfaces (those that can’t absorb water) such as roofs and pavement emit and reflect solar energy differently than natural surfaces, often absorbing heat during the day and then releasing it at night. Additionally, buildings can trap air between them and alter the movement of wind currents, and human activities like cooking and driving produce heat. All these factors combine to create the phenomenon of urban heat islands (UHIs; Fig 2), in which the air temperature in a city is higher than in the surrounding area. Usually, the difference is on the scale of 2-4ºC, but the air in a big city can be up to 5-10ºC hotter than in nearby rural areas.

Figure 2. Urban heat island temperature profile. TheNewPhobia and US NOAA. Public domain.

Areas with more impervious surfaces and fewer plants have higher surface temperatures, and this pattern holds during both the day and night. Heat threatens human health by magnifying the effects of other health conditions, and heat waves can cause thousands of deaths, especially when high temperatures continue overnight. As climate change progresses, many regions are dealing with hotter temperatures, which exacerbate the heat-island effect.

Just like the locations of polluting industries, built-up areas of cities where the heat-island effect is worst are often home to disadvantaged communities, reflecting historic prejudice and perpetuating inequality in these communities’ physical environments. In addition to health impacts, heat leads to increased electricity use for air conditioning. Each degree of temperature increase leads to a 0.5-8.5% increase in electricity demand, raising costs for residents or causing blackouts. For buildings whose electricity comes from fossil fuels, this leads to increased GHG emissions as well. Heat-mitigating infrastructure and energy conservation will be discussed in later sections.

Water

Figure 3. Rainwater absorption and runoff in natural and urban areas. US EPA. Public domain.

When rain falls in a field or forest, most of it either infiltrates into the ground or collects on plants and then evaporates. On a road or sidewalk, however, most of it runs off (Fig 3). This can cause flooding in cities, and the extra water increases the runoff’s flow rate, causing erosion on the way to nearby streams and rivers. The water is often heated unnaturally by the city’s impervious surfaces, and warm water holds less oxygen than cold water. Many aquatic species are very sensitive to sediment, temperature, and oxygen levels, so this runoff can cause serious disruption to nearby stream and river ecosystems.

When just 10-20% of a watershed is covered by impervious surfaces, the amount of runoff into streams doubles. Densely built-up parts of cities tend to have a higher percentage of impervious surface cover than low-density areas and therefore lead to more stormwater runoff; however, a dense city that takes up less land area overall creates less total runoff than a low-density, sprawling city with the same population.

Impervious surfaces collect contaminants (gasoline, road salt, industrial waste products, litter and household waste, fertilizers and pesticides, and many more), which rainwater runoff or floodwater can then carry into waterways. Replacing just 10% of a watershed with impervious surfaces is enough to damage the water quality in that watershed.

Water pollution is one of the top threats to human health, connected to 80% of diseases and 50% of child deaths around the world. Impervious surfaces can also contribute to the spread of disease by holding pools of water where mosquitoes (whose natural predators may not live in urban areas) can lay their eggs.

Because impervious surfaces cause so many problems, an obvious solution is to replace them with permeable surfaces where possible. There are many alternatives to pavement, such as flagstones and gravel, as well as various types of permeable pavement. There are also ways to store rainwater so it doesn’t overwhelm watersheds, including gray infrastructure, such as rain barrels and large storage tanks, and green infrastructure, which makes use of plants’ ability to absorb and filter water. For an in-depth look at Copenhagen’s flood mitigation efforts, including both green and gray infrastructure, you can watch this 11-minute video .

Figure 4. A channelized river. Nathalie. Adobe Education license.

Rivers and streams naturally meander, or follow a path that curves back and forth. This gives the water enough space that when it rains, the flow rate doesn’t increase drastically, and when a stream floods, there’s plenty of land along the banks to absorb it. In many cases when a city was built around a river, people constructed straight concrete banks for the river to flow through, a technique called channelization (Fig 4). This shortens the river’s path and prevents water from being absorbed into the ground. After a large rain, this type of channel can easily flood the surrounding areas of the city, and the fast-flowing water can have significant erosive force. You can watch a 6-minute video on the history of humans’ modifications to rivers and a few places in Europe that are experimenting with restoring rivers to their original meandering paths. Many rivers were not only channelized, but buried in pipes underground. You can watch a 7-minute video about a city in Canada that is working to daylight a buried river, bringing it back to the surface. Daylighting often involves restoring the riverbanks to a more natural form, improving the river’s ability to absorb rainwater and creating aquatic and riparian habitat.

Air

Figure 5. Urban smog caused by cars. Martin Vorel. Public domain.

Most air pollution comes from burning things, including fuel for heat, cooking, transportation, and industrial processes. A lot of this happens in and around urban areas. In developing countries, cooking and industry produce the majority of air pollution, while in developed countries, most air pollution comes from vehicles (Fig 5).

Not all city residents are at the same risk for air pollution – disadvantaged communities deal with higher levels. For example, in the United States, Black people are 1.54 times as likely as white people to live near factories that emit PM_2.5.

As we saw in Chapter 2.3, the world death rate from air pollution dropped by nearly half from 1990 to 2021 (Fig 6). That change is due to a decrease in deaths from indoor air pollution, while deaths from outdoor air pollution have stayed fairly constant. Reducing outdoor air pollution depends on transportation and power generation solutions and on decreasing industrial pollution. Governments can raise air quality standards, carefully monitor industries, and set strong financial incentives to decrease pollution. Research and development of new, green technologies can also decrease the environmental impacts of industrial processes.

Figure 6. Death rates from indoor and outdoor air pollution. Our World in Data. CC BY..

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One of the defining features of cities is traffic. All the people working, shopping, and spending time downtown need to get around, and many people who live in surrounding suburbs commute to and from the city center. That means a lot of cars, often with only one person in them, emitting greenhouse gases and air pollution as they idle in traffic jams. Globally, transportation is the second-largest source of GHG emissions, not counting land use change. Some city planners have tried to deal with congestion by building bigger roads with more lanes, but this creates induced demand – because there’s more space, more people drive on those roads, and before long, they’re as congested as they were before the expansion (Fig 1).

Figure 1. Traffic jam on ten-lane road in Moscow, Russia. sablinstanislav. Adobe education license.

Vehicles are a major source of both water and air pollution, and vehicle infrastructure (impervious roads and parking lots) increases stormwater runoff and urban heat islands. Driving damages our health in other ways as well, including physical inactivity, social isolation, stress, noise pollution, and injuries from vehicle accidents. Physical inactivity contributes to obesity and other diseases, and people who drive tend to have a higher body mass index than people who use more active modes of transportation.

For the many people who can’t drive (too young or too old, have a disability, can’t afford a car), living in a place where driving is the only feasible transportation option means having limited access to jobs, activities, grocery stores, and social connections. Roads that are built through existing neighborhoods can similarly limit access and isolate people. Car commuters also have to deal with sitting in traffic, searching for parking, and sometimes conflict with other drivers, all of which cause stress. People living and working near loud roads have to deal with noise pollution, which causes so much harm to physical and mental health that its disease burden is comparable to that of second-hand smoke. Finally, road travel crashes cause millions of deaths and tens of millions of injuries around the world every year (see footnote #2 for more on all these health impacts).

Public transit can solve many of these problems if it is designed well. Routes have to connect housing to destinations and destinations to each other. Different modes of transport have to be well integrated in space, time, and payment (like having one access card that works on all modes of transportation) so that passengers can easily switch between them. Transit has to come often and on time, and it has to be straightforward to use (app, tickets, etc.). It has to be at least as convenient as driving, which typically means, among other things, getting people to their destinations faster. Metros and many tram systems have this advantage since they are separate from traffic, but tunnels and tracks are very expensive to build.

Even in cities growing large enough to need extensive transit systems, governments may be reluctant to invest in those travel modes. Buses do not require nearly as much new infrastructure, but they often get stuck in traffic. Bus rapid transit (BRT) systems solve this problem by giving buses their own lanes and sometimes priority at intersections. You can click here for an in-depth look at one of the longest-running BRT systems, in Curitiba, Brazil, which includes specially designed metro-like bus stations that allow for prepaid, same-level boarding. Figure 2 shows the share of the urban population in each country with convenient access to public transit, as of 2022.

Figure 2. Share of urban populations with convenient access to public transit, as of 2022. This is the percentage of city residents who live within a 500 meter walk of low-capacity public transit, like a small bus, or within a 1 kilometer walk of high-capacity public transit, like a metro. Our World in Data. CC BY.

Active transportation (walking, biking, etc.) provides healthy physical activity, produces zero emissions, and takes up less space than cars. As any pedestrian or cyclist knows, some parts of cities are much more walkable and bikeable than others. Pedestrians and cyclists need designated space (sidewalks and bike lanes) so they don’t have to dodge cars as they go. Cyclists need bike racks to park at, located conveniently and protected from weather (Fig 3). Pedestrians appreciate having benches to rest on, as well as shade (trees, awnings, or constructed shelters) for hot days. People with physical disabilities need accessibility infrastructure like sidewalk curb cuts (which also benefit cyclists) and ramps.

Figure 3. Public bicycle parking in Tokyo, Japan. toptop28. Adobe education license.

However, physical infrastructure is not enough to create a large-scale shift toward active transportation. Walking and biking need to be accepted culturally, and people need to feel safe while doing them. Many people avoid walking, at least in certain areas, because they fear harassment or crime. Cultural shifts and improvements to public safety can thus also facilitate shifts to sustainable lifestyles. In many parts of the world, cycling is associated with poverty, and in others, people see it as a form of exercise but not daily transportation. In some cultures, however, it’s seen as a normal and respectable way of getting around; Denmark, for example, is known for having government officials who commute by bike. You can click here for an interesting blog post on the contrast between cultural attitudes toward biking in Tehran, Iran, and Amsterdam, the Netherlands. For ideas on how to nurture a bike-positive culture, you can read this article.

Once a city has good alternatives to cars available, it can start making driving a less appealing option. Since giving cars more space increases the number of people driving, logically, giving them less space will make more people choose other modes of transportation. Many European cities are doing this, in a variety of ways. Paris, France has banned motorized through-traffic in the city center. Barcelona, Spain has created ‘superblocks:’ only a wide grid of streets is open to cars, and the smaller streets within each large square are reserved for pedestrians and cyclists.

Delft, the Netherlands created the ‘woonerf,’ or ‘living street,’ method of managing transportation, which is actually the reverse of the previous approaches because it removes the divide between car and pedestrian space. Woonerf streets don’t have sidewalks, and instead, pedestrians, bikes, and cars all share the same space, which forces drivers to go slowly and give priority to others. Cities in South America are also shifting from car-centric to people-centric streets. You can watch this interview with the Minister of Public Space and Urban Hygiene of Buenos Aires, Argentina about improving walkability. Most recently, New York City has implemented congestion pricing, charging drivers to enter the city center, and the first few months have shown decreased traffic, accidents, noise complaints, and school bus delays and increased bus use and speeds.

Figure 4. Electric car charging at a station in Europe. Artūrs Laucis photo. Adobe education license.

When people drive less, cities don’t need as much space for parking, and that space can be used to increase density or natural areas. For the vehicles that remain, cities can encourage people to choose low- or zero-emissions vehicles by providing convenient charging infrastructure (Fig 4) and establishing low-emissions zones (Fig 5). Electrifying bus fleets is also a key step, as Santiago, Chile is modelling. Cities can also set up car and bike share programs, which reduce people’s need to own their own vehicles and are useful for tourists.

Figure 5. London, England’s congestion pricing and low emissions zone. Travers. Adobe education license.

You can click here for an 11-minute video on entrenched car culture, essential features of usable public transit systems, and ways to disincentivize driving, with examples from Jakarta, Indonesia and several European cities.

Knowledge Check

Take a moment to complete the short quiz below to assess your understanding of this section. Read each question carefully and refer to the section content as needed. This quiz is not graded – it’s simply an opportunity for you to reflect on what you’ve learned and reinforce key concepts.

Figure 1. Forest remnants in Echizen, Japan. Japan Ministry of Land, Infrastructure and Transport. CC BY-SA.

Most of the ecosystem services we focus on in urban areas are regulating services. Trees, grass, and other plants can mitigate heat islands, air pollution, water pollution, and flooding. Urban greenspaces can also provide wildlife habitat and support biodiversity, which is essential to the continued health of urban ecosystems and the services they provide. Each location where a city now exists originally had a certain landscape based on the biome it’s part of, like a forest, prairie, or desert. Some cities still have patches of that original landscape within them, and these areas are called remnants (Fig 1).

Remnants can be extremely valuable habitat for plants and animals. For example, a study done in Italy found that landscape remnants made up 5% of one city’s land area but hosted over 85% of all the plant species in the city, including some that were not found anywhere outside the remnants.

A study in Australia surveyed bee species in bushland remnants and in residential gardens and found that the remnants were home to more rare and unique bee species than the gardens, as well as hosting higher overall bee diversity. This section addresses many ecosystem services that newly-planted areas of vegetation can provide. However, if there’s a choice between preserving original habitat or creating new habitat, preserving the original is almost certainly the more environmentally responsible option (and will likely require less time and money).

Soil, the foundation of terrestrial ecosystems, is dramatically different in urban areas than in undisturbed natural areas. Soil in a forest, for example, is made of small pieces of clay, sand, and organic matter, loosely piled together so that there are tiny air pockets between them. That kind of soil has high porosity, which means water can easily infiltrate those air pockets and plant roots can easily work their way in between the soil particles. Constructing buildings and roads involves compressing the soil both intentionally, to ensure it stays stable, and unintentionally, as heavy equipment travels to and from the construction site. In some areas of cities, people and vehicles then move around on the soil and compact it further. This compaction dramatically decreases the soil’s porosity, so that some urban soils can be almost as dense as concrete.

When it rains, most of the water runs off the compacted soil just like it would off of pavement. It’s also difficult for roots of trees and other plants to grow into this dense soil. Even when they manage it, the low-porosity soil can’t hold much water, so those plants are more vulnerable to drought than they would be in more porous soil. Urban soil is polluted by traffic, industry, waste incineration, and more. Contaminants like heavy metals, microplastics, pesticides, and antibiotic resistance genes are found at high levels both within cities and in the surrounding natural areas.

Brownfields are polluted sites, such as former industrial areas, gas stations, or landfills, where soil contamination can make it unsafe to use the land for housing or public space. In some cases, developers simply excavate the soil or put a cap over the top, but plants, fungi, and microorganisms can also be used to break down, neutralize, or absorb pollutants, an approach known as bioremediation. Planting former brownfields can simultaneously mitigate pollution and provide public green spaces for city residents. You can read more about cities turning brownfields into parks here.

Figure 2. Street trees in New York City, New York, USA. Lance Cheung. Public domain.

Trees in cities provide a variety of ecosystem services and economic benefits (Fig 2). They mitigate urban heat islands, first by shading surfaces, then through evapotranspiration. They intercept 10-40% of rainwater on its way to the ground, and their roots help absorb the rest. They filter air pollution and carbon dioxide, remove heavy metals, excess nutrients, and other pollutants from the soil, and shade buildings, which reduces the energy needed for cooling. ‘Green screens,’ lines of trees along highways, not only mitigate air pollution but can reduce traffic noise and improve the view for people living nearby.

Unfortunately, in the United States, another effect of policies like redlining means that urban street trees are inequitably distributed. The temperature difference between white and POC neighborhoods, due to white neighborhoods having higher tree canopy cover, leads to significantly more deaths, more doctors’ visits, and higher electricity consumption in POC neighborhoods.

Overall, urban tree cover in the United States is decreasing by tens of millions of trees every year. This is a problem in cities around the world, as urbanization replaces forested areas with human structures. In Freetown, Sierra Leone, deforestation around the city combined with heavy rains led to huge landslides, prompting a large-scale reforestation effort. The program has planted hundreds of thousands of trees, including many in disadvantaged areas with low canopy cover, creating over 1,000 jobs and decreasing flooding (more details click here). Urban reforestation can give a huge return on investment because trees provide such valuable benefits to people living near them.

However, a high number of trees is not enough, on its own, to ensure long-term ecosystem services; taxonomic and age diversity of trees are also essential. The more diverse an ecosystem is, the healthier and more resilient it can be. Diseases and pests can be specialized to target one species, or they can affect a group of related species. If every tree on a particular street belongs to the same family, they could all get wiped out at the same time, as happened when Dutch elm disease spread across the US in the mid-1900s. eliminating elm trees – very popular street trees – in many cities.

Planting a wide range of species that can each tolerate different conditions and threats maximizes the strength of the ecosystem as a whole. It is also important to have a range of ages so that the trees in an area do not all get old and die at the same time. In addition to planting a variety of trees, it’s essential to have plans and resources in place for maintaining them so they can live as long as possible in the hostile urban environment. You can click here for a 10-minute video on tree equity and efforts to increase and maintain urban canopy cover in the United States.

As mentioned in section 3, plants are a major part of the solution to flooding and water pollution issues in cities. They can be used on roofs (which make up 20-25% of total urban area), on slopes and in ditches, and in rain gardens (Fig 3). You can click here to view a Polish company that creates green bus stops (planters and trellises that eventually cover bus shelters in plants); their website lists the many benefits of this infrastructure. Watch this video click here TED talk to learn about green flood-mitigation infrastructure that doubles as beautiful public space in Bangkok, Thailand.

Figure 3. Green infrastructure at a range of scales. US Environmental Protection Agency. Public domain.

In temperate regions, the most common type of vegetation in cities is short-mown turf grass lawn, which is maintained both in parks and on private land. Lawn makes up 25% of city land area in the UK, 23% in the US, and 22.5% in Sweden. Lawns are deeply entrenched in Western culture and valued for their neat appearance and suitability for recreation. Lawns provide some ecosystem services, including erosion control and carbon storage, but they are time-intensive and expensive to maintain, requiring frequent mowing as well as fertilizers, herbicides, and/or irrigation. Mowing produces GHG emissions, chemical inputs can pollute the soil and water, and irrigation can contribute to the depletion of water resources. In addition, lawns are often maintained as monocultures, which decreases the biodiversity they can support. Even when lawns include multiple plant species, they are low-quality habitat for invertebrates and other animals. Mowing prevents plants from flowering and going to seed, limiting the availability of nesting opportunities for insects and foraging opportunities for many animal species.

An increasingly popular alternative to lawns is urban meadows, or ‘pocket prairies’: infrequently-mowed open areas of native grasses and wildflowers, created in urban areas to mimic original ecosystems like North American prairie or Eurasian steppe. In dry biomes, xeriscape meadows can include succulents and drought-tolerant bushes and grasses. Meadows with a variety of species growing to their full height can support impressive biodiversity, including microbes in the soil, insects that nest in different parts of plants, bees and other pollinators that rely on flowers for food, and birds and mammals that forage on and around plants. Pollinators provide a huge service to human agriculture, so maintaining pollinator habitat is essential to protecting our own food supply.

Native plants have much deeper roots than non-natives and turf grass, so they do much more to absorb rainwater, stabilize soil, capture heavy metals and nutrients, and prevent flooding. They can filter more particulate air pollution than lawns can, and because they require so much less mowing, they also reduce GHG emissions and noise pollution. In some urban areas, homeowners are not allowed to create meadows because of regulations against tall plants that are seen as weeds. However, even in these places, native plants can be used in landscaped gardens and still provide valuable ecosystem services and habitat (Fig 4).

Figure 4. Pollinator garden, Washington, DC, USA. John Boggan, Smithsonian Gardens. CC BY.

A surprising variety of plant and animal life can exist in cities. Over 14,000 plant species and 2,000 bird species live in cities around the world, and most of these are native to the areas they inhabit.

The first step to protecting urban biodiversity is to measure it by conducting a species inventory (like this example, click here done in Mexico City, Mexico). These efforts can engage local residents in citizen science projects, which help get people interested in the nature around their homes and motivate them to protect it. Once city planners know the locations and identities of species (pollinators, endangered species, invasives, etc.) or habitats (wetlands, coastlands, biodiversity hotspots), they can target their efforts at those species and areas. Check out this article click here for an in-depth look at how birds have adapted to urban environments and how to make cities more bird-friendly.

Cities can be nexuses for invasive species, because humans transport plant and animal species both intentionally and by accident. Exotic plants are often used in gardens and then spread to other areas, and people buy pets such as snakes and lizards and then release them into the wild. In the United States, Callery pear trees and Burmese pythons, among many other species, were brought to the country intentionally and became invasive. Seeds and small animals can also travel on ships, in luggage and packaging, and on people’s shoes. Many invasive species are well-adapted to urban environments and so can form established populations in cities. From there, they can move to nearby natural areas and threaten native species. Because many invasives are spread by people who don’t realize the effect they will have, educating the public is key to limiting this issue.

In supporting pollinators and other wildlife, the total amount of habitat in a city matters, but the connectivity of the habitat is also important. Many animal species can only travel short distances, and plant seeds can also only travel so far. Animals are often killed while crossing roads. These limitations on movement can lead to a population dying out when it gets stuck in a small patch of habitat that can’t support it. Habitat corridors address this issue by connecting patches of green space across urban areas, often along rivers, in rights-of-way next to roads, or under power lines.

City governments can protect corridors and include habitat connectivity as a priority in urban planning. In 2016, Medellín, Colombia started a green corridor program that trained citizens to plant trees around the city, and in the following three years, they planted nearly 9,000 trees in 30 urban corridors. The new trees have lowered average city temperatures, improved air quality, and increased biodiversity.

Cities can also acquire land for habitat restoration, create incentives for private developers and landowners to create habitat on their properties, and educate the public on the value of green space, both for nature and people.

Figure 5. People socializing in a park in Pyongyang, North Korea. John Pavelka. CC BY.

Spending time in nature (Fig 5) has both physical and mental health benefits: lowered blood pressure, heart rate, and stress hormones and improved attention, cognitive performance, and mood.

Public greenspaces can give people opportunities to build social connections and learn about the world around them. However, fewer than half of urban residents around the world have convenient access to open public spaces, including greenspaces (Fig 6). Many demographics that are marginalized in other ways (socioeconomics, race, culture, age, physical ability, etc.) also have less access to greenspace. This gives those groups worse health and quality of life and makes them more sensitive to the effects of climate change.

Figure 6. Share of urban populations with convenient access to public transit, as of 2022. This is the percentage of city residents who live within a 500 meter walk of low-capacity public transit, like a small bus, or within a 1 kilometer walk of high-capacity public transit, like a metro. Our World in Data. CC BY.

In 2019, Lima, Peru started an urban regeneration program, in collaboration with local communities and targeted at vulnerable populations, that has increased tree cover, pedestrian streets, and access to greenspace.

Many activities people enjoy in parks and greenspace, such as picnics and sports, require turf grass lawn. However, mixing patches of forest and native plants into lawn areas dramatically increases the quality of the habitat and provides beauty and variety for human visitors. Many city residents may not have opportunities to visit pristine natural areas, but healthy, vibrant ecosystems within the city can provide experiences of plants and animals. This contact with nature can help people understand the importance of conservation, so urban greenspace can have an additional, indirect benefit to the health of the planet.

Additionally, cities are in some cases even better places for conservation than rural areas. Farmers and other people whose livelihoods rely on a particular land use may be reluctant to reserve parts of their land for habitat, whereas in urban and suburban areas, both governments and individuals pay to maintain little-used lawn in public parks and private yards. Replacing lawn with habitat can save taxpayers and urban landowners money by requiring less maintenance, as well as providing beauty, interest, and the satisfaction of having improved the city’s environmental health.

Knowledge Check

Take a moment to complete the short quiz below to assess your understanding of this section. Read each question carefully and refer to the section content as needed. This quiz is not graded – it’s simply an opportunity for you to reflect on what you’ve learned and reinforce key concepts.

Buildings contribute 21% of global GHG emissions, and they do it in two main ways, known as embodied and operational carbon. Embodied carbon refers to the emissions that come from producing building materials, such as steel and concrete. Operational carbon is what’s emitted in the production of the energy used in buildings for heating and cooling, lighting, and other appliances and devices. As the global urban population grows, many more buildings will need to be built, and the way they are designed will determine how much they contribute to greenhouse gas emissions in each way. The process of constructing buildings also produces emissions, but the growing availability of electric construction equipment will make it possible to eliminate on-site emissions, as Oslo, Norway is demonstrating.

Building materials are extracted from the earth, and most are not renewable. Wood, however, is renewable, and as trees grow, they sequester carbon, which is then stored for long periods when their wood is used in buildings. However, because deforestation is another major environmental challenge, many people are reluctant to promote building with wood as a solution. There are also ways to create building materials out of other natural, renewable sources, as well as to design buildings to require less material in the first place. If you’re interested in this topic, see this video of a TED talk from a Canadian architect working to develop these techniques.

Building operations account for 30% of global energy use and contribute 26% of energy-related greenhouse gas emissions, and this energy use is increasing. Decreasing energy use in buildings saves owners money on electricity and reduces emissions. The top priority is insulation: if a building loses heat through the walls in the winter or soaks up the sun’s heat in the summer, the heating and air conditioning have to work harder to keep it comfortable inside. Most buildings can have their insulation improved (Fig 1), and new building design can take advantage of physical phenomena to reduce the need for heating and cooling. If you’re interested in the technical details of these design elements, check out this 6-minute video.

Figure 1. Thermal image of a house showing heat escaping through the walls. Dario Sabljak. Adobe Education license.

The next step in reducing energy use is to heat, cool, and light only as needed and to use energy-efficient appliances. You can watch this 13-minute video to learn about the problems with existing air conditioning technology and the variety of options for improving it.

Heat pumps are a recently developed technology that uses electricity instead of burning anything to produce heat. Electrification is important in every part of the house; for example, electric stoves use less energy than gas stoves and do not pollute indoor air. In addition to technological advancement, improving building efficiency requires a cultural shift, because at least in the United States, many people have very wasteful attitudes and habits: they want big houses, heat and cool buildings much more than necessary, and leave lights and appliances on far beyond when they’re using them.

Buildings can provide habitat for some wildlife species, such as birds and bats that nest on ledges and under eaves. However, many birds die from flying into windows, and light pollution from buildings can disrupt the daily and seasonal cycles of many kinds of animals. For example, artificial lighting can disorient sea turtle hatchlings and cause them to crawl inland rather than toward the ocean. Flagstaff, Arizona and other cities around the world have worked to reduce light pollution and become certified as Dark Sky Cities, protecting wildlife and promoting astro tourism.

Extreme weather, increasing due to climate change, threatens buildings in many parts of the world. Buildings must be resilient to protect people from disasters, and because each region has its own landscape and natural disasters, customizing buildings’ design based on their location improves their resilience. For example, in a dry region that is prone to wildfires, buildings would be kept safer by having a xeriscaped yard with plants adapted to a dry climate, whereas in a region with frequent floods, growing tall plants around a house would help protect it. On a larger scale, mangrove trees along a coastline can reduce damage to buildings from hurricanes.

City governments can speed the transition to more efficient housing by setting building codes to encourage energy efficiency and, as mentioned in section 2, zoning for multi-family housing, which is much more efficient than single-family homes. Heidelberg, Germany created a district of all passive-house buildings, which are highly insulated and powered by district heating and electricity. Many of the buildings have green roofs, and there are water retention basins to absorb stormwater. The hi-tech Al Bahar Towers in Abu Dhabi, UAE use a lattice of smart blinds that open and close to shade the interior spaces from the sun, reducing the need for air conditioning by 50%.

The generation of electricity and heat used in buildings makes up 18% of global energy-related GHG emissions. Solving this part of the problem depends on the energy transition, replacing fossil fuels with renewable energy. Buildings can be a big part of that solution, or at least use less energy from electric grids that rely on fossil fuels, by generating their own clean energy. Globally, there are enough roofs available (on residential, public, and commercial buildings) for rooftop solar PV to provide nearly two thirds of the energy the world currently uses. In Cape Town, South Africa, electricity blackouts motivated homeowners to install rooftop solar, and the country’s total rooftop solar capacity quintupled in less than two years. Cities can also create solar farms to provide clean power for people who can’t install their own solar panels.

Figure 2. Green roof on the city hall of Chicago, Illinois, USA. TonyTheTiger. CC BY-SA.

The many benefits of plants are not limited to designated greenspaces; they can also be applied to buildings in the form of green walls and roofs (Fig 2). The plants filter air pollutants, absorb carbon dioxide, and release oxygen, which provides health benefits both indoors and out. When used on sunny exteriors, plants shade buildings, lowering the internal temperature and decreasing the energy required for cooling. Green walls and roofs can provide some of the psychological benefits of exposure to nature and serve as habitat even in a dense urban area without room for a park. For example, the Oasia Hotel in Singapore has plants growing all over its exterior, covering far more area than the building itself takes up on the ground. For almost two decades, Rotterdam, in the Netherlands, has been adding parks, water storage infrastructure, and solar installations to its many, previously empty roofs.

Homelessness and slums are major problems around the world, and cities have the opportunity to provide both affordable and sustainable housing. Quezon City, the Philippines, has an affordable housing program that builds good quality, environmentally friendly homes (with parks and gardens in each community) to house thousands of families who used to live in slums.

Knowledge Check

Take a moment to complete the short quiz below to assess your understanding of this section. Read each question carefully and refer to the section content as needed. This quiz is not graded – it’s simply an opportunity for you to reflect on what you’ve learned and reinforce key concepts.

The 56% of humans who live in cities consume 75% of all the natural resources we use as a species and produce half of all our waste.  We have a long way to go in increasing the efficiency of how we use resources, so that we consume less in the first place, and improving how we manage things we’re done using, so that not so much of it goes to waste.

Food waste and food deserts in cities

A huge part of this body of resources is food. Globally, about one third of all food produced goes to waste (Fig 1). That’s one third of all the resources put into growing plants and raising animals wasted, one third of all the time, energy, and space used to store and transport the food wasted, and one third of food that could’ve been eaten rotting and releasing greenhouse gases in the process. At the same time, hunger is a massive problem around the world. One out of every eleven people on earth were undernourished as of 2023. We produce more than enough food to feed all of them, but waste is great enough to create hunger.

In many cases, food goes to waste because it isn’t packaged or stored properly and so goes bad, but single-use food packaging is also a massive source of waste. For the food that does need to be thrown away, composting is much better than putting it in the trash. Not only can it be turned into valuable fertilizer, but in the composting process, food decomposes aerobically, producing carbon dioxide, whereas in landfills, it decomposes anaerobically and produces methane, which is a much more potent greenhouse gas. In the US, food waste makes up 24% of landfill contents. New York City in the US requires businesses and individuals to compost yard and food waste and is working to educate residents on the process.

Figure 1. Fruits and vegetables in a dumpster in Luxembourg. OpenIDUser2. CC0.

For many people, the problem with food access is not that they can’t get enough calories, it’s that they only have access to unhealthy, processed food. Areas of cities with low access to affordable, healthy food are known as food deserts.

Policy approaches can shrink food deserts, including incentives for grocery stores to open locations in food deserts and improved public transportation between low-income neighborhoods and grocery stores. Cities can also address this issue through urban agriculture, which is growing in popularity around the world. Community gardens can be set up in empty lots and on rooftops and provide fresh produce to people living nearby. As mentioned previously, high diversity of plant species in gardens and other urban greenspaces supports the pollinators that many food crops depend on. The city of Rosario, Argentina has a program that has trained thousands of families and dozens of schools on agricultural skills, and many of those people have gone on to start vegetable gardens. And in Seoul, South Korea, a small company partnered with the metro to create an LED-lit, vertical hydroponic farm in a station, as well as information about urban agriculture and multiple cafés in other stations.

Resources and Waste in Cities

Modern societies also use non-food resources in inefficient and wasteful ways. Consumer culture incentivizes businesses to produce low-quality products that will need to be replaced often and encourages individuals to buy things they don’t really need. In contrast, a circular economy, based on how ecosystems function, would use resources efficiently without producing waste or pollution (more details in a short article click here and 4-minute video click here). The first two steps toward this goal are to reduce consumption in the first place and to reuse as many things as possible. The growing sharing economy allows people to sell or give away items they no longer need and to seek out used items instead of buying everything brand new.

Cities may have limited power to influence business practices, but they can help provide alternatives to continuous consumption. Around the world, people are starting libraries of things, which can include items like tools, games, kitchen equipment, musical instruments, computers, and sports gear so that people who can’t afford or have no reason to own something long-term can use it when they want to and then give it back for others to use (Fig 2). Cities can also facilitate reuse of things like clothes and furniture by hosting swap events and locations. Belo Horizonte, Brazil has a Computer Reconditioning Center where low-income residents are trained to refurbish used IT equipment. The computers and other items then go to ‘digital inclusion sites,’ offering free computer use and internet access to people around the city (details and impressive stats click here). For more ideas of how cities can promote reuse and repair, you can check out C40’s implementation guide.

Figure 2. Tool library at the Berkeley Public Library, California, USA. Dreamyshade. CC-BY SA.

As a last resort if an item must be thrown away, recycling (and compost for food) are the best option. However, at least in the United States, people have attitudes and beliefs about recycling that do not lead to useful behavior. For decades, the plastics industry promoted recycling as the solution to plastic waste, despite knowing that producing new plastic is cheaper than recycling it and so recycling would not solve anything. Instead of taking responsibility for all the trash they were producing, companies used this misinformation to put the responsibility on consumers. To this day, many people believe that it doesn’t matter how much plastic they consume as long as they recycle it afterwards. What’s more, people don’t recycle properly at all. Only about two thirds of paper and paperboard waste in the US gets recycled, and the rest makes up 12% of the contents of landfills. The solution to all of this, instead of asking consumers to be aware of and responsible for disposing of all their waste properly, is to incentivize businesses to reduce waste and create easy-to-use recycling and compost infrastructure. Cape Town, South Africa created a program to match companies based on materials they use and produce: one’s waste products are another’s raw materials, so they exchange instead of throwing them away. A pair of companies in China is working not only to recycle materials from batteries, but improve battery design so they last longer and are easier to disassemble for components.

Figure 3. Percentage of urban population in each world region with convenient access to public open spaces, as of 2020. Our World in Data. CC BY.

Only 19% of the world’s municipal solid waste gets recycled (UN GWMO), and dealing with the remaining 81% is where waste management comes in: making sure the waste goes somewhere where it will stay contained and won’t contaminate the environment. Globally, 700 million people in urban areas don’t have waste collection services (Fig 3), and only 20% of waste workers have formal jobs, like city-paid trash collectors. The rest work independently, doing things like salvaging usable materials from landfills.

Accra, Ghana created a program to train informal waste workers and formalize the waste management industry. This not only improved their working conditions, but also increased waste collection from 53% to 90% and recycling rates from 5% to 18%. Curitiba, Brazil has neighborhoods that municipal waste vehicles can’t access. They ask residents in these areas to bring their waste (sorted into compostable and non-compostable) to designated collection points, offering bus tickets and fresh produce in exchange for the waste.

Knowledge Check

Take a moment to complete the short quiz below to assess your understanding of this section. Read each question carefully and refer to the section content as needed. This quiz is not graded – it’s simply an opportunity for you to reflect on what you’ve learned and reinforce key concepts.