Abatement. Refers to reducing the degree or intensity of greenhouse-gas emissions.
Adaptation. Actions taken to help communities and ecosystems cope with such changing climate conditions as the construction of flood walls to protect property from stronger storms and heavier precipitation, or planting agricultural crops and trees more suited to warmer temperatures and drier soil conditions.
Afforestation. Planting new forests on lands that have not produced forests.
Anthropogenic emissions. Greenhouse-gas emissions resulting from human activies.
Biomass fuels. Energy sources that are renewable as long as the vegetation producing them is maintained or replanted, such as firewood, alcohol fermented from sugar, and combustible oils extracted from soy beans. Their use in place of fossil fuels cuts greenhouse gas emissions because the plants that are their sources recapture carbon dioxide from the atmosphere.
Carbon market. A popular, but misleading term for a trading system through which countries may buy or sell units of greenhouse-gas emissions in an effort to meet their national limits on emissions, either under the Kyoto Protocol or under other such agreements as arrived at by members of the European Union. The term comes from the fact that carbon dioxide is the predominant greenhouse gas and other gases are measured in units called "carbon-dioxide equivalents."
Deforestation. The direct human-induced conversion of forested land to non-forested land.
Emission-reduction unit (ERU). A unit equal to one metric ton of carbon dioxide equivalent, applicable to binding emissions-reductions targets under the Kyoto Protocol, and generated through joint implementation projects.
Emissions trading. Mechanism under the Kyoto Protocol through which parties with emissions commitments may trade units of their emissions allowances with other parties. The aim is to improve the overall flexibility and economic efficiency of making emissions cuts.
Fugitive fuel emissions. Greenhouse-gas emissions as byproducts or waste or loss in the process of fuel production, storage, or transport, such as methane given off during oil and gas drilling and refining, or leakage of natural gas from pipelines.
Greenhouse gases (GHGs). The atmospheric gases responsible for causing global warming and climate change. Less prevalent — but very powerful — greenhouse gases are hydrofluorocarbons (HFCs), perfluorocarbons (PFCs) and sulphur hexafluoride (SF6).
"Hot air." Refers to the concern that some governments will be able to meet their commitment targets for greenhouse-gas emissions under the Kyoto Protocol with minimal effort and could then flood the market for emissions credits, reducing the incentive for other countries to cut their own domestic emissions.
Kyoto Protocol. An international agreement standing on its own, and requiring separate ratification by governments, but linked to the UNFCCC. The protocol is yet to go into effect.
Land use, land-use change, and forestry (LULUCF). Refers to the impact of land use by humans — and changes in such land use — on greenhouse-gas emissions: Expanding forests reduces atmospheric carbon dioxide; deforestation releases additional carbon dioxide; various agricultural activities may add to atmospheric levels of methane and nitrous oxide.
Leakage. That portion of cuts in greenhouse-gas emissions by developed countries — countries trying to meet mandatory limits under the Kyoto Protocol — that may reappear in other countries not bound by such limits. For example, multinational corporations may shift factories from developed countries to developing countries to escape restrictions on emissions.
No-regrets options. Technology for reducing greenhouse-gas emissions whose other benefits (in terms of efficiency or reduced energy costs) are so extensive that the investment is worth it for those reasons alone. For example, combined-cycle gas turbines — in which the heat from the burning fuel drives steam turbines while the thermal expansion of the exhaust gases drives gas turbines — may boost the efficiency of electricity generating plants by 70 percent.
Sinks. Any process that removes a greenhouse gas from the atmosphere. However, calculating the effects of sinks is methodologically complex and the standards for doing so still need to be clarified.
Spill-over effects. Reverberations in developing countries caused by actions taken by developed countries to cut greenhouse-gas emissions. Current estimates are that full-scale implementation of the Kyoto Protocol may cause five to 20 percent of emissions reductions in industrialized countries to "leak" into developing countries.
Sustainable development. Development that meets the needs of the present without compromising the ability of future generations to meet their own needs.
Third Assessment Report (TAR). The third extensive review of global scientific research on climate change, published by the Intergovernmental Panel on Climate Change (IPCC) in 2001. Among other things, the report stated that "The Earth's climate system has demonstrably changed on both global and regional scales since the pre-industrial era, with some of these changes attributable to human activities. There is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities."
Vulnerability. The degree to which a community, population, species, ecosystem, region, agricultural system, or some other quantity is susceptible to, or unable to cope with, adverse effects of climate change.
Global Warming 101
A: Since the Industrial Revolution, the global annual temperature has increased in total by a little more than 1 degree Celsius, or about 2 degrees Fahrenheit. Between 1880—the year that accurate recordkeeping began—and 1980, it rose on average by 0.07 degrees Celsius (0.13 degrees Fahrenheit) every 10 years. Since 1981, however, the rate of increase has more than doubled: For the last 40 years, we’ve seen the global annual temperature rise by 0.18 degrees Celsius, or 0.32 degrees Fahrenheit, per decade.
The result? A planet that has never been hotter. Nine of the 10 warmest years since 1880 have occurred since 2005—and the 5 warmest years on record have all occurred since 2015. Climate change deniers have argued that there has been a “pause” or a “slowdown” in rising global temperatures, but numerous studies, including a 2018 paper published in the journal Environmental Research Letters, have disproved this claim. The impacts of global warming are already harming people around the world.
Now climate scientists have concluded that we must limit global warming to 1.5 degrees Celsius by 2040 if we are to avoid a future in which everyday life around the world is marked by its worst, most devastating effects: the extreme droughts, wildfires, floods, tropical storms, and other disasters that we refer to collectively as climate change. These effects are felt by all people in one way or another but are experienced most acutely by the underprivileged, the economically marginalized, and people of color, for whom climate change is often a key driver of poverty, displacement, hunger, and social unrest.
Q: What causes global warming?
A: Global warming occurs when carbon dioxide (CO2) and other air pollutants collect in the atmosphere and absorb sunlight and solar radiation that have bounced off the earth’s surface. Normally this radiation would escape into space, but these pollutants, which can last for years to centuries in the atmosphere, trap the heat and cause the planet to get hotter. These heat-trapping pollutants—specifically carbon dioxide, methane, nitrous oxide, water vapor, and synthetic fluorinated gases—are known as greenhouse gases, and their impact is called the greenhouse effect.
Though natural cycles and fluctuations have caused the earth’s climate to change several times over the last 800,000 years, our current era of global warming is directly attributable to human activity—specifically to our burning of fossil fuels such as coal, oil, gasoline, and natural gas, which results in the greenhouse effect. In the United States, the largest source of greenhouse gases is transportation (29 percent), followed closely by electricity production (28 percent) and industrial activity (22 percent).
Curbing dangerous climate change requires very deep cuts in emissions, as well as the use of alternatives to fossil fuels worldwide. The good news is that countries around the globe have formally committed—as part of the 2015 Paris Climate Agreement—to lower their emissions by setting new standards and crafting new policies to meet or even exceed those standards. The not-so-good news is that we’re not working fast enough. To avoid the worst impacts of climate change, scientists tell us that we need to reduce global carbon emissions by as much as 40 percent by 2030. For that to happen, the global community must take immediate, concrete steps: to decarbonize electricity generation by equitably transitioning from fossil fuel–based production to renewable energy sources like wind and solar to electrify our cars and trucks and to maximize energy efficiency in our buildings, appliances, and industries.
Q: How is global warming linked to extreme weather?
A: Scientists agree that the earth’s rising temperatures are fueling longer and hotter heat waves, more frequent droughts, heavier rainfall, and more powerful hurricanes.
In 2015, for example, scientists concluded that a lengthy drought in California—the state’s worst water shortage in 1,200 years—had been intensified by 15 to 20 percent by global warming. They also said the odds of similar droughts happening in the future had roughly doubled over the past century. And in 2016, the National Academies of Science, Engineering, and Medicine announced that we can now confidently attribute some extreme weather events, like heat waves, droughts, and heavy precipitation, directly to climate change.
The earth’s ocean temperatures are getting warmer, too—which means that tropical storms can pick up more energy. In other words, global warming has the ability to turn a category 3 storm into a more dangerous category 4 storm. In fact, scientists have found that the frequency of North Atlantic hurricanes has increased since the early 1980s, as has the number of storms that reach categories 4 and 5. The 2020 Atlantic hurricane season included a record-breaking 30 tropical storms, 6 major hurricanes, and 13 hurricanes altogether. With increased intensity come increased damage and death. The United States saw an unprecedented 22 weather and climate disasters that caused at least a billion dollars’ worth of damage in 2020, but 2017 was the costliest on record and among the deadliest as well: Taken together, that year's tropical storms (including Hurricanes Harvey, Irma, and Maria) caused nearly $300 billion in damage and led to more than 3,300 fatalities.
The impacts of global warming are being felt everywhere. Extreme heat waves have caused tens of thousands of deaths around the world in recent years. And in an alarming sign of events to come, Antarctica has lost nearly four trillion metric tons of ice since the 1990s. The rate of loss could speed up if we keep burning fossil fuels at our current pace, some experts say, causing sea levels to rise several meters in the next 50 to 150 years and wreaking havoc on coastal communities worldwide.
Q: What are the other effects of global warming?
A: Each year scientists learn more about the consequences of global warming, and each year we also gain new evidence of its devastating impact on people and the planet. As the heat waves, droughts, and floods associated with climate change become more frequent and more intense, communities suffer and death tolls rise. If we’re unable to reduce our emissions, scientists believe that climate change could lead to the deaths of more than 250,000 people around the globe every year and force 100 million people into poverty by 2030.
Global warming is already taking a toll on the United States. And if we aren’t able to get a handle on our emissions, here’s just a smattering of what we can look forward to:
- , early snowmelt, and severe droughts will cause more dramatic water shortages and continue to increase the risk of wildfires in the American West. will lead to even more coastal flooding on the Eastern Seaboard, especially in Florida, and in other areas such as the Gulf of Mexico.
- Forests, farms, and cities will face troublesome new pests, heat waves, heavy downpours, and increased flooding. All of these can damage or destroy agriculture and fisheries.
- Disruption of habitats such as coral reefs and alpine meadows could drive many plant and animal species to extinction.
- Allergies, asthma, and infectious disease outbreaks will become more common due to increased growth of pollen-producing ragweed, higher levels of air pollution, and the spread of conditions favorable to pathogens and mosquitoes.
Though everyone is affected by climate change, not everyone is affected equally. Indigenous people, people of color, and the economically marginalized are typically hit the hardest. Inequities built into our housing, health care, and labor systems make these communities more vulnerable to the worst impacts of climate change—even though these same communities have done the least to contribute to it.
Q: Where does the United States stand in terms of global-warming contributors?
A: In recent years, China has taken the lead in global-warming pollution, producing about 26 percent of all CO2 emissions. The United States comes in second. Despite making up just 4 percent of the world’s population, our nation produces a sobering 13 percent of all global CO2 emissions—nearly as much as the European Union and India (third and fourth place) combined. And America is still number one, by far, in cumulative emissions over the past 150 years. As a top contributor to global warming, the United States has an obligation to help propel the world to a cleaner, safer, and more equitable future. Our responsibility matters to other countries, and it should matter to us, too.
Q: Is the United States doing anything to prevent global warming?
A: We’ve started. But in order to avoid the worsening effects of climate change, we need to do a lot more—together with other countries—to reduce our dependence on fossil fuels and transition to clean energy sources.
Under the administration of President Donald Trump (a man who falsely referred to global warming as a “hoax”), the United States withdrew from the Paris Climate Agreement, rolled back or eliminated dozens of clean-air protections, and opened up federally managed lands, including culturally sacred national monuments, to fossil fuel development. Although President Biden has pledged to get the country back on track, years of inaction during and before the Trump administration—and our increased understanding of global warming’s serious impacts—mean we must accelerate our efforts to reduce greenhouse gas emissions.
Despite the lack of cooperation from the Trump administration, local and state governments made great strides during this period through efforts like the American Cities Climate Challenge and ongoing collaborations like the Regional Greenhouse Gas Initiative. Meanwhile, industry and business leaders have been working with the public sector, creating and adopting new clean-energy technologies and increasing energy efficiency in buildings, appliances, and industrial processes. Today the American automotive industry is finding new ways to produce cars and trucks that are more fuel efficient and is committing itself to putting more and more zero-emission electric vehicles on the road. Developers, cities, and community advocates are coming together to make sure that new affordable housing is built with efficiency in mind, reducing energy consumption and lowering electric and heating bills for residents. And renewable energy continues to surge as the costs associated with its production and distribution keep falling. In 2020 renewable energy sources such as wind and solar provided more electricity than coal for the very first time in U.S. history.
President Biden has made action on global warming a high priority. On his first day in office, he recommitted the United States to the Paris Climate Agreement, sending the world community a strong signal that we were determined to join other nations in cutting our carbon pollution to support the shared goal of preventing the average global temperature from rising more than 1.5 degrees Celsius above preindustrial levels. (Scientists say we must stay below a 2-degree increase to avoid catastrophic climate impacts.) And significantly, the president has assembled a climate team of experts and advocates who have been tasked with pursuing action both abroad and at home while furthering the cause of environmental justice and investing in nature-based solutions.
Q: Is global warming too big a problem for me to help tackle?
A: No! While we can’t win the fight without large-scale government action at the national level, we also can’t do it without the help of individuals who are willing to use their voices, hold government and industry leaders to account, and make changes in their daily habits.
Wondering how you can be a part of the fight against global warming? Reduce your own carbon footprint by taking a few easy steps: Make conserving energy a part of your daily routine and your decisions as a consumer. When you shop for new appliances like refrigerators, washers, and dryers, look for products with the government’s ENERGY STAR ® label they meet a higher standard for energy efficiency than the minimum federal requirements. When you buy a car, look for one with the highest gas mileage and lowest emissions. You can also reduce your emissions by taking public transportation or carpooling when possible.
And while new federal and state standards are a step in the right direction, much more needs to be done. Voice your support of climate-friendly and climate change preparedness policies, and tell your representatives that equitably transitioning from dirty fossil fuels to clean power should be a top priority—because it’s vital to building healthy, more secure communities.
You don’t have to go it alone, either. Movements across the country are showing how climate action can build community, be led by those on the front lines of its impacts, and create a future that’s equitable and just for all.
Glossary of Climate Change Terms
Abrupt Climate Change
Sudden (on the order of decades), large changes in some major component of the climate system, with rapid, widespread effects.
Adjustment or preparation of natural or human systems to a new or changing environment which moderates harm or exploits beneficial opportunities.
The ability of a system to adjust to climate change (including climate variability and extremes) to moderate potential damages, to take advantage of opportunities, or to cope with the consequences.
Small particles or liquid droplets in the atmosphere that can absorb or reflect sunlight depending on their composition.
Planting of new forests on lands that historically have not contained forests. 
The amount of solar radiation reflected from an object or surface, often expressed as a percentage.
Energy derived from nontraditional sources (e.g., compressed natural gas, solar, hydroelectric, wind). 
Annex I Countries/Parties
Group of countries included in Annex I (as amended in 1998) to the United Nations Framework Convention on Climate Change, including all the developed countries in the Organization of Economic Co-operation and Development, and economies in transition. By default, the other countries are referred to as Non-Annex I countries. Under Articles 4.2 (a) and 4.2 (b) of the Convention, Annex I countries commit themselves specifically to the aim of returning individually or jointly to their 1990 levels of greenhouse gas emissions by the year 2000. 
Made by people or resulting from human activities. Usually used in the context of emissions that are produced as a result of human activities. 
The gaseous envelope surrounding the Earth. The dry atmosphere consists almost entirely of nitrogen (78.1% volume mixing ratio) and oxygen (20.9% volume mixing ratio), together with a number of trace gases, such as argon (0.93% volume mixing ratio), helium, radiatively active greenhouse gases such as carbon dioxide (0.035% volume mixing ratio), and ozone. In addition the atmosphere contains water vapor, whose amount is highly variable but typically 1% volume mixing ratio. The atmosphere also contains clouds and aerosols. 
Atmospheric lifetime is the average time that a molecule resides in the atmosphere before it is removed by chemical reaction or deposition. In general, if a quantity of a compound is emitted into the atmosphere at a particular time, about 35 percent of that quantity will remain in the atmosphere at the end of the compound's atmospheric lifetime. This fraction will continue to decrease in an exponential way, so that about 15 percent of the quantity will remain at the end of two times the atmospheric lifetime, etc. (Some compounds, most notably carbon dioxide, have more complex lifecycles, and their atmospheric lifetimes are not defined by a simple exponential equation.) Greenhouse gas lifetimes can range from a few years to a few thousand years.
A - B - C - D - E - F - G - H - I - J - K - L - M - N - O - P - Q - R - S - T - U - V - W - X - Y - Z - #
Gas or liquid fuel made from plant material. Includes wood, wood waste, wood liquors, peat, railroad ties, wood sludge, spent sulfite liquors, agricultural waste, straw, tires, fish oils, tall oil, sludge waste, waste alcohol, municipal solid waste, landfill gases, other waste, and ethanol blended into motor gasoline. 
Movements through the Earth system of key chemical constituents essential to life, such as carbon, nitrogen, oxygen, and phosphorus. 
Materials that are biological in origin, including organic material (both living and dead) from above and below ground, for example, trees, crops, grasses, tree litter, roots, and animals and animal waste. 
The part of the Earth system comprising all ecosystems and living organisms, in the atmosphere, on land (terrestrial biosphere) or in the oceans (marine biosphere), including derived dead organic matter, such as litter, soil organic matter and oceanic detritus. 
Black Carbon Aerosol
Black carbon (BC) is the most strongly light-absorbing component of particulate matter (PM), and is formed by the incomplete combustion of fossil fuels, biofuels, and biomass. It is emitted directly into the atmosphere in the form of fine particles (PM2.5).
Any exploratory hole drilled into the Earth or ice to gather geophysical data. Climate researchers often take ice core samples, a type of borehole, to predict atmospheric composition in earlier years. See ice core.
A - B - C - D - E - F - G - H - I - J - K - L - M - N - O - P - Q - R - S - T - U - V - W - X - Y - Z - #
All parts (reservoirs) and fluxes of carbon. The cycle is usually thought of as four main reservoirs of carbon interconnected by pathways of exchange. The reservoirs are the atmosphere, terrestrial biosphere (usually includes freshwater systems), oceans, and sediments (includes fossil fuels). The annual movements of carbon, the carbon exchanges between reservoirs, occur because of various chemical, physical, geological, and biological processes. The ocean contains the largest pool of carbon near the surface of the Earth, but most of that pool is not involved with rapid exchange with the atmosphere. 
A naturally occurring gas, and also a by-product of burning fossil fuels and biomass, as well as land-use changes and other industrial processes. It is the principal human caused greenhouse gas that affects the Earth's radiative balance. It is the reference gas against which other greenhouse gases are measured and therefore has a Global Warming Potential of 1. See climate change and global warming. 
Carbon Dioxide Equivalent
A metric measure used to compare the emissions from various greenhouse gases based upon their global warming potential (GWP). Carbon dioxide equivalents are commonly expressed as "million metric tons of carbon dioxide equivalents (MMTCO2Eq)." The carbon dioxide equivalent for a gas is derived by multiplying the tons of the gas by the associated GWP.
MMTCO2Eq = (million metric tons of a gas) * (GWP of the gas)
Carbon Dioxide Fertilization
The enhancement of the growth of plants as a result of increased atmospheric CO2 concentration. Depending on their mechanism of photosynthesis, certain types of plants are more sensitive to changes in atmospheric CO2 concentration. 
The total amount of greenhouse gases that are emitted into the atmosphere each year by a person, family, building, organization, or company. A persons carbon footprint includes greenhouse gas emissions from fuel that an individual burns directly, such as by heating a home or riding in a car. It also includes greenhouse gases that come from producing the goods or services that the individual uses, including emissions from power plants that make electricity, factories that make products, and landfills where trash gets sent.
Terrestrial, or biologic, carbon sequestration is the process by which trees and plants absorb carbon dioxide, release the oxygen, and store the carbon. Geologic sequestration is one step in the process of carbon capture and sequestration (CCS), and involves injecting carbon dioxide deep underground where it stays permanently.
Carbon Capture and Sequestration
Carbon capture and sequestration (CCS) is a set of technologies that can greatly reduce carbon dioxide emissions from new and existing coal- and gas-fired power plants, industrial processes, and other stationary sources of carbon dioxide. It is a three-step process that includes capture of carbon dioxide from power plants or industrial sources transport of the captured and compressed carbon dioxide (usually in pipelines) and underground injection and geologic sequestration, or permanent storage, of that carbon dioxide in rock formations that contain tiny openings or pores that trap and hold the carbon dioxide.
Gases covered under the 1987 Montreal Protocol and used for refrigeration, air conditioning, packaging, insulation, solvents, or aerosol propellants. Since they are not destroyed in the lower atmosphere, CFCs drift into the upper atmosphere where, given suitable conditions, they break down ozone. These gases are being replaced by other compounds: hydrochlorofluorocarbons, an interim replacement for CFCs that are also covered under the Montreal Protocol, and hydrofluorocarbons, which are covered under the Kyoto Protocol. All these substances are also greenhouse gases. See hydrochlorofluorocarbons, hydrofluorocarbons, perfluorocarbons, ozone depleting substance. 
Climate in a narrow sense is usually defined as the "average weather," or more rigorously, as the statistical description in terms of the mean and variability of relevant quantities over a period of time ranging from months to thousands of years. The classical period is 3 decades, as defined by the World Meteorological Organization (WMO). These quantities are most often surface variables such as temperature, precipitation, and wind. Climate in a wider sense is the state, including a statistical description, of the climate system. See weather. 
Climate change refers to any significant change in the measures of climate lasting for an extended period of time. In other words, climate change includes major changes in temperature, precipitation, or wind patterns, among others, that occur over several decades or longer.
A process that acts to amplify or reduce direct warming or cooling effects.
The delay that occurs in climate change as a result of some factor that changes only very slowly. For example, the effects of releasing more carbon dioxide into the atmosphere occur gradually over time because the ocean takes a long time to warm up in response to a change in radiation. See climate, climate change.
A quantitative way of representing the interactions of the atmosphere, oceans, land surface, and ice. Models can range from relatively simple to quite comprehensive. See General Circulation Model. 
In Intergovernmental Panel on Climate Change (IPCC) reports, equilibrium climate sensitivity refers to the equilibrium change in global mean surface temperature following a doubling of the atmospheric (equivalent) CO2 concentration. More generally, equilibrium climate sensitivity refers to the equilibrium change in surface air temperature following a unit change in radiative forcing (degrees Celsius, per watts per square meter, (C/Wm-2). One method of evaluating the equilibrium climate sensitivity requires very long simulations with Coupled General Circulation Models (Climate model). The effective climate sensitivity is a related measure that circumvents this requirement. It is evaluated from model output for evolving non-equilibrium conditions. It is a measure of the strengths of the feedbacks at a particular time and may vary with forcing history and climate state. See climate, radiative forcing. 
Climate System (or Earth System)
The five physical components (atmosphere, hydrosphere, cryosphere, lithosphere, and biosphere) that are responsible for the climate and its variations. 
Coal Mine Methane
Coal mine methane is the subset of coalbed methane that is released from the coal seams during the process of coal mining. For more information, visit the Coalbed Methane Outreach program site.
Coalbed methane is methane contained in coal seams, and is often referred to as virgin coalbed methane, or coal seam gas. For more information, visit the Coalbed Methane Outreach program site.
The benefits of policies that are implemented for various reasons at the same time including climate change mitigation acknowledging that most policies designed to address greenhouse gas mitigation also have other, often at least equally important, rationales (e.g., related to objectives of development, sustainability, and equity).
Amount of a chemical in a particular volume or weight of air, water, soil, or other medium. See parts per billion, parts per million. 
Conference of the Parties
The supreme body of the United Nations Framework Convention on Climate Change (UNFCCC). It comprises more than 180 nations that have ratified the Convention. Its first session was held in Berlin, Germany, in 1995 and it is expected to continue meeting on a yearly basis. The COP's role is to promote and review the implementation of the Convention. It will periodically review existing commitments in light of the Convention's objective, new scientific findings, and the effectiveness of national climate change programs. See United Nations Framework Convention on Climate Change.
The process in which a coral colony, under environmental stress expels the microscopic algae (zooxanthellae) that live in symbiosis with their host organisms (polyps). The affected coral colony appears whitened.
One of the interrelated components of the Earth's system, the cryosphere is frozen water in the form of snow, permanently frozen ground (permafrost), floating ice, and glaciers. Fluctuations in the volume of the cryosphere cause changes in ocean sea level, which directly impact the atmosphere and biosphere. 
A - B - C - D - E - F - G - H - I - J - K - L - M - N - O - P - Q - R - S - T - U - V - W - X - Y - Z - #
Those practices or processes that result in the conversion of forested lands for non-forest uses. Deforestation contributes to increasing carbon dioxide concentrations for two reasons: 1) the burning or decomposition of the wood releases carbon dioxide and 2) trees that once removed carbon dioxide from the atmosphere in the process of photosynthesis are no longer present. 
Land degradation in arid, semi-arid, and dry sub-humid areas resulting from various factors, including climatic variations and human activities. Further, the UNCCD (The United Nations Convention to Combat Desertification) defines land degradation as a reduction or loss, in arid, semi-arid, and dry sub-humid areas, of the biological or economic productivity and complexity of rain-fed cropland, irrigated cropland, or range, pasture, forest, and woodlands resulting from land uses or from a process or combination of processes, including processes arising from human activities and habitation patterns, such as: (i) soil erosion caused by wind and/or water (ii) deterioration of the physical, chemical and biological or economic properties of soil and (iii) long-term loss of natural vegetation. Conversion of forest to non-forest.
A technique that uses soil moisture conservation and seed selection to optimize production under dry conditions.
The extent to which the Earth's orbit around the Sun departs from a perfect circle.
Any natural unit or entity including living and non-living parts that interact to produce a stable system through cyclic exchange of materials. 
El Niño - Southern Oscillation (ENSO)
El Niño in its original sense, is a warm water current that periodically flows along the coast of Ecuador and Peru, disrupting the local fishery. This oceanic event is associated with a fluctuation of the intertropical surface pressure pattern and circulation in the Indian and Pacific Oceans, called the Southern Oscillation. This coupled atmosphere-ocean phenomenon is collectively known as El Niño-Southern Oscillation. During an El Niño event, the prevailing trade winds weaken and the equatorial countercurrent strengthens, causing warm surface waters in the Indonesian area to flow eastward to overlie the cold waters of the Peru current. This event has great impact on the wind, sea surface temperature, and precipitation patterns in the tropical Pacific. It has climatic effects throughout the Pacific region and in many other parts of the world. The opposite of an El Niño event is called La Niña. 
The release of a substance (usually a gas when referring to the subject of climate change) into the atmosphere.
A unique value for scaling emissions to activity data in terms of a standard rate of emissions per unit of activity (e.g., grams of carbon dioxide emitted per barrel of fossil fuel consumed, or per pound of product produced). 
Using less energy to provide the same service. 
A U.S. Environmental Protection Agency voluntary program that helps businesses and individuals save money and protect our climate through superior energy efficiency. Learn more about ENERGY STAR.
Enhanced Greenhouse Effect
The concept that the natural greenhouse effect has been enhanced by increased atmospheric concentrations of greenhouse gases (such as CO2 and methane) emitted as a result of human activities. These added greenhouse gases cause the earth to warm. See greenhouse effect.
Livestock, especially cattle, produce methane as part of their digestion. This process is called enteric fermentation, and it represents one third of the emissions from the agriculture sector.
The process by which water changes from a liquid to a gas or vapor. 
The combined process of evaporation from the Earth's surface and transpiration from vegetation. 
Factors which increase or amplify (positive feedback) or decrease (negative feedback) the rate of a process. An example of positive climatic feedback is the ice-albedo feedback. See climate feedback. 
Powerful synthetic greenhouse gases such as hydrofluorocarbons, perfluorocarbons, and sulfur hexafluoride that are emitted from a variety of industrial processes. Fluorinated gases are sometimes used as substitutes for stratospheric ozone-depleting substances (e.g., chlorofluorocarbons, hydrochlorofluorocarbons, and halons) and are often used in coolants, foaming agents, fire extinguishers, solvents, pesticides, and aerosol propellants. These gases are emitted in small quantities compared to carbon dioxide (CO2), methane (CH4), or nitrous oxide (N2O), but because they are potent greenhouse gases, they are sometimes referred to as High Global Warming Potential gases (High GWP gases).
Carbon-fluorine compounds that often contain other elements such as hydrogen, chlorine, or bromine. Common fluorocarbons include chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), hydrofluorocarbons (HFCs), and perfluorocarbons (PFCs). See chlorofluorocarbons, hydrochlorofluorocarbons, hydrofluorocarbons, perfluorocarbons, ozone depleting substance. 
A process that alters the energy balance of the climate system, i.e. changes the relative balance between incoming solar radiation and outgoing infrared radiation from Earth. Such mechanisms include changes in solar irradiance, volcanic eruptions, and enhancement of the natural greenhouse effect by emissions of greenhouse gases. See radiation, infrared radiation, radiative forcing.
A general term for organic materials formed from decayed plants and animals that have been converted to crude oil, coal, natural gas, or heavy oils by exposure to heat and pressure in the earth's crust over hundreds of millions of years. 
In general, this is substituting one type of fuel for another. In the climate-change discussion it is implicit that the substituted fuel produces lower carbon emissions per unit energy produced than the original fuel, e.g., natural gas for coal.
General Circulation Model (GCM)
A global, three-dimensional computer model of the climate system which can be used to simulate human-induced climate change. GCMs are highly complex and they represent the effects of such factors as reflective and absorptive properties of atmospheric water vapor, greenhouse gas concentrations, clouds, annual and daily solar heating, ocean temperatures and ice boundaries. The most recent GCMs include global representations of the atmosphere, oceans, and land surface. See climate modeling. 
The soils, sediments, and rock layers of the Earth's crust, both continental and beneath the ocean floors.
A multi-year surplus accumulation of snowfall in excess of snowmelt on land and resulting in a mass of ice at least 0.1 km2 in area that shows some evidence of movement in response to gravity. A glacier may terminate on land or in water. Glacier ice is the largest reservoir of fresh water on Earth, and second only to the oceans as the largest reservoir of total water. Glaciers are found on every continent except Australia. 
Global Average Temperature
An estimate of Earths mean surface air temperature averaged over the entire planet.
The recent and ongoing global average increase in temperature near the Earths surface.
Global Warming Potential
A measure of the total energy that a gas absorbs over a particular period of time (usually 100 years), compared to carbon dioxide.
Trapping and build-up of heat in the atmosphere (troposphere) near the Earths surface. Some of the heat flowing back toward space from the Earth's surface is absorbed by water vapor, carbon dioxide, ozone, and several other gases in the atmosphere and then reradiated back toward the Earths surface. If the atmospheric concentrations of these greenhouse gases rise, the average temperature of the lower atmosphere will gradually increase. See greenhouse gas, anthropogenic, climate, global warming. 
A process during which larger areas of habitat are broken into a number of smaller patches of smaller total area, isolated from each other by a matrix of habitats unlike the original habitat. (Fahrig 2003)
Compounds containing either chlorine, bromine or fluorine and carbon. Such compounds can act as powerful greenhouse gases in the atmosphere. The chlorine and bromine containing halocarbons are also involved in the depletion of the ozone layer. 
An urban area characterized by temperatures higher than those of the surrounding non-urban area. As urban areas develop, buildings, roads, and other infrastructure replace open land and vegetation. These surfaces absorb more solar energy, which can create higher temperatures in urban areas. 
A prolonged period of excessive heat, often combined with excessive humidity. 
Substances containing only hydrogen and carbon. Fossil fuels are made up of hydrocarbons.
Compounds containing hydrogen, fluorine, chlorine, and carbon atoms. Although ozone depleting substances, they are less potent at destroying stratospheric ozone than chlorofluorocarbons (CFCs). They have been introduced as temporary replacements for CFCs and are also greenhouse gases. See ozone depleting substance.
Compounds containing only hydrogen, fluorine, and carbon atoms. They were introduced as alternatives to ozone depleting substances in serving many industrial, commercial, and personal needs. HFCs are emitted as by-products of industrial processes and are also used in manufacturing. They do not significantly deplete the stratospheric ozone layer, but they are powerful greenhouse gases with global warming potentials ranging from 140 (HFC-152a) to 11,700 (HFC-23).
The process of evaporation, vertical and horizontal transport of vapor, condensation, precipitation, and the flow of water from continents to oceans. It is a major factor in determining climate through its influence on surface vegetation, the clouds, snow and ice, and soil moisture. The hydrologic cycle is responsible for 25 to 30 percent of the mid-latitudes' heat transport from the equatorial to polar regions. 
The component of the climate system comprising liquid surface and subterranean water, such as: oceans, seas, rivers, fresh water lakes, underground water etc. 
A cylindrical section of ice removed from a glacier or an ice sheet in order to study climate patterns of the past. By performing chemical analyses on the air trapped in the ice, scientists can estimate the percentage of carbon dioxide and other trace gases in the atmosphere at a given time. Analysis of the ice itself can give some indication of historic temperatures.
Indirect emissions from a building, home or business are those emissions of greenhouse gases that occur as a result of the generation of electricity used in that building. These emissions are called "indirect" because the actual emissions occur at the power plant which generates the electricity, not at the building using the electricity.
A period of rapid industrial growth with far-reaching social and economic consequences, beginning in England during the second half of the 18th century and spreading to Europe and later to other countries including the United States. The industrial revolution marks the beginning of a strong increase in combustion of fossil fuels and related emissions of carbon dioxide. 
Infrared radiation consists of light whose wavelength is longer than the red color in the visible part of the spectrum, but shorter than microwave radiation. Infrared radiation can be perceived as heat. The Earths surface, the atmosphere, and clouds all emit infrared radiation, which is also known as terrestrial or long-wave radiation. In contrast, solar radiation is mainly short-wave radiation because of the temperature of the Sun. See radiation, greenhouse effect, enhanced greenhouse effect, global warming. 
Intergovernmental Panel on climate Change (IPCC)
The IPCC was established jointly by the United Nations Environment Programme and the World Meteorological Organization in 1988. The purpose of the IPCC is to assess information in the scientific and technical literature related to all significant components of the issue of climate change. The IPCC draws upon hundreds of the world's expert scientists as authors and thousands as expert reviewers. Leading experts on climate change and environmental, social, and economic sciences from some 60 nations have helped the IPCC to prepare periodic assessments of the scientific underpinnings for understanding global climate change and its consequences. With its capacity for reporting on climate change, its consequences, and the viability of adaptation and mitigation measures, the IPCC is also looked to as the official advisory body to the world's governments on the state of the science of the climate change issue. For example, the IPCC organized the development of internationally accepted methods for conducting national greenhouse gas emission inventories.
The submergence of land by water, particularly in a coastal setting. 
Land waste disposal site in which waste is generally spread in thin layers, compacted, and covered with a fresh layer of soil each day. 
The location north or south in reference to the equator, which is designated at zero (0) degrees. Lines of latitude are parallel to the equator and circle the globe. The North and South poles are at 90 degrees North and South latitude. 
Least Developed Country
A country with low indicators of socioeconomic development and human resources, as well as economic vulnerability, as determined by the United Nations. 
Radiation emitted in the spectral wavelength greater than about 4 micrometers, corresponding to the radiation emitted from the Earth and atmosphere. It is sometimes referred to as 'terrestrial radiation' or 'infrared radiation,' although somewhat imprecisely. See infrared radiation. 
Cities with populations over 10 million.
A hydrocarbon that is a greenhouse gas with a global warming potential most recently estimated at 25 times that of carbon dioxide (CO2). Methane is produced through anaerobic (without oxygen) decomposition of waste in landfills, animal digestion, decomposition of animal wastes, production and distribution of natural gas and petroleum, coal production, and incomplete fossil fuel combustion. The GWP is from the IPCC's Fourth Assessment Report (AR4). For more information visit EPA's Methane page.
Common international measurement for the quantity of greenhouse gas emissions. A metric ton is equal to 2205 lbs or 1.1 short tons. See short ton. 
A human intervention to reduce the human impact on the climate system it includes strategies to reduce greenhouse gas sources and emissions and enhancing greenhouse gas sinks. 
A volcano in the Philippine Islands that erupted in 1991. The eruption of Mount Pinatubo ejected enough particulate and sulfate aerosol matter into the atmosphere to block some of the incoming solar radiation from reaching Earth's atmosphere. This effectively cooled the planet from 1992 to 1994, masking the warming that had been occurring for most of the 1980s and 1990s. 
Municipal Solid Waste (MSW)
Residential solid waste and some non-hazardous commercial, institutional, and industrial wastes. This material is generally sent to municipal landfills for disposal. See landfill.
Underground deposits of gases consisting of 50 to 90 percent methane (CH4) and small amounts of heavier gaseous hydrocarbon compounds such as propane (C3H8) and butane (C4H10).
Variations in the mean state and other statistics (such as standard deviations or statistics of extremes) of the climate on all time and space scales beyond that of individual weather events. Natural variations in climate over time are caused by internal processes of the climate system, such as El Niño as well as changes in external influences, such as volcanic activity and variations in the output of the sun.  
The natural circulation of nitrogen among the atmosphere, plants, animals, and microorganisms that live in soil and water. Nitrogen takes on a variety of chemical forms throughout the nitrogen cycle, including nitrous oxide (N2O) and nitrogen oxides (NOx).
Nitrogen Oxides (NOx)
Gases consisting of one molecule of nitrogen and varying numbers of oxygen molecules. Nitrogen oxides are produced in the emissions of vehicle exhausts and from power stations. In the atmosphere, nitrogen oxides can contribute to formation of photochemical ozone (smog), can impair visibility, and have health consequences they are thus considered pollutants. 
Nitrous Oxide (N2O)
A powerful greenhouse gas with a global warming potential of 298 times that of carbon dioxide (CO2). Major sources of nitrous oxide include soil cultivation practices, especially the use of commercial and organic fertilizers, fossil fuel combustion, nitric acid production, and biomass burning. The GWP is from the IPCC's Fourth Assessment Report (AR4). 
Natural emissions of N2O are mainly from bacteria breaking down nitrogen in soils and the oceans. Nitrous oxide is mainly removed from the atmosphere through destruction in the stratosphere by ultraviolet radiation and associated chemical reactions, but it can also be consumed by certain types of bacteria in soils.
Non-Methane Volatile Organic Compounds (NMVOCs)
Organic compounds, other than methane, that participate in atmospheric photochemical reactions.
Increased concentrations of carbon dioxide in sea water causing a measurable increase in acidity (i.e., a reduction in ocean pH). This may lead to reduced calcification rates of calcifying organisms such as corals, mollusks, algae and crustaceans. 
To chemically transform a substance by combining it with oxygen. 
Ozone, the triatomic form of oxygen (O3), is a gaseous atmospheric constituent. In the troposphere, it is created by photochemical reactions involving gases resulting both from natural sources and from human activities (photochemical smog). In high concentrations, tropospheric ozone can be harmful to a wide range of living organisms. Tropospheric ozone acts as a greenhouse gas. In the stratosphere, ozone is created by the interaction between solar ultraviolet radiation and molecular oxygen (O2). Stratospheric ozone plays a decisive role in the stratospheric radiative balance. Depletion of stratospheric ozone, due to chemical reactions that may be enhanced by climate change, results in an increased ground-level flux of ultraviolet (UV-) B radiation. See atmosphere, ultraviolet radiation. 
Ozone Depleting Substance (ODS)
A family of man-made compounds that includes, but are not limited to, chlorofluorocarbons (CFCs), bromofluorocarbons (halons), methyl chloroform, carbon tetrachloride, methyl bromide, and hydrochlorofluorocarbons (HCFCs). These compounds have been shown to deplete stratospheric ozone, and therefore are typically referred to as ODSs. See ozone. 
The layer of ozone that begins approximately 15 km above Earth and thins to an almost negligible amount at about 50 km, shields the Earth from harmful ultraviolet radiation from the sun. The highest natural concentration of ozone (approximately 10 parts per million by volume) occurs in the stratosphere at approximately 25 km above Earth. The stratospheric ozone concentration changes throughout the year as stratospheric circulation changes with the seasons. Natural events such as volcanoes and solar flares can produce changes in ozone concentration, but man-made changes are of the greatest concern. See stratosphere, ultraviolet radiation. 
Chemical compounds, such as carbon monoxide, methane, non-methane hydrocarbons, and nitrogen oxides, which in the presence of solar radiation react with other chemical compounds to form ozone, mainly in the troposphere. See troposphere. 
Particulate matter (PM)
Very small pieces of solid or liquid matter such as particles of soot, dust, fumes, mists or aerosols. The physical characteristics of particles, and how they combine with other particles, are part of the feedback mechanisms of the atmosphere. See aerosol, sulfate aerosols. 
Parts Per Billion (ppb)
Number of parts of a chemical found in one billion parts of a particular gas, liquid, or solid mixture. See concentration.
Parts Per Million by Volume (ppmv)
Number of parts of a chemical found in one million parts of a particular gas, liquid, or solid. See concentration.
Parts Per Trillion (ppt)
Number of parts of a chemical found in one trillion parts of a particular gas, liquid or solid. See concentration.
A group of chemicals composed of carbon and fluorine only. These chemicals (predominantly CF4 and C2F6) were introduced as alternatives, along with hydrofluorocarbons, to the ozone depleting substances. In addition, PFCs are emitted as by-products of industrial processes and are also used in manufacturing. PFCs do not harm the stratospheric ozone layer, but they are powerful greenhouse gases: CF4 has a global warming potential (GWP) of 7,390 and C2F6 has a GWP of 12,200. The GWP is from the IPCC's Fourth Assessment Report (AR4). These chemicals are predominantly human-made, though there is a small natural source of CF4. See ozone depleting substance.
Perennially (continually) frozen ground that occurs where the temperature remains below 0ºC for several years. 
The timing of natural events, such as flower blooms and animal migration, which is influenced by changes in climate. Phenology is the study of such important seasonal events. Phenological events are influenced by a combination of climate factors, including light, temperature, rainfall, and humidity.
The process by which plants take CO2 from the air (or bicarbonate in water) to build carbohydrates, releasing O2 in the process. There are several pathways of photosynthesis with different responses to atmospheric CO2 concentrations. See carbon sequestration, carbon dioxide fertilization. 
The wobble over thousands of years of the tilt of the Earths axis with respect to the plane of the solar system. 
Energy transfer in the form of electromagnetic waves or particles that release energy when absorbed by an object. See ultraviolet radiation, infrared radiation, solar radiation, longwave radiation. 
A measure of the influence of a particular factor (e.g. greenhouse gas (GHG), aerosol, or land use change) on the net change in the Earths energy balance.
Collecting and reprocessing a resource so it can be used again. An example is collecting aluminum cans, melting them down, and using the aluminum to make new cans or other aluminum products. 
The ability of a surface material to reflect sunlight including the visible, infrared, and ultraviolet wavelengths. 
Planting of forests on lands that have previously contained forests but that have been converted to some other use. 
Relative Sea Level Rise
The increase in ocean water levels at a specific location, taking into account both global sea level rise and local factors, such as local subsidence and uplift. Relative sea level rise is measured with respect to a specified vertical datum relative to the land, which may also be changing elevation over time. 
Energy resources that are naturally replenishing such as biomass, hydro, geothermal, solar, wind, ocean thermal, wave action, and tidal action. 
The average time spent in a reservoir by an individual atom or molecule. With respect to greenhouse gases, residence time refers to how long on average a particular molecule remains in the atmosphere. For most gases other than methane and carbon dioxide, the residence time is approximately equal to the atmospheric lifetime. 
A capability to anticipate, prepare for, respond to, and recover from significant multi-hazard threats with minimum damage to social well-being, the economy, and the environment.
The process whereby living organisms convert organic matter to CO2, releasing energy and consuming O2. 
Salt Water Intrusion
Displacement of fresh or ground water by the advance of salt water due to its greater density, usually in coastal and estuarine areas. 
A plausible and often simplified description of how the future may develop based on a coherent and internally consistent set of assumptions about driving forces and key relationships.
Sea Surface Temperature
The temperature in the top several feet of the ocean, measured by ships, buoys and drifters. 
The degree to which a system is affected, either adversely or beneficially, by climate variability or change. The effect may be direct (e.g., a change in crop yield in response to a change in the mean, range or variability of temperature) or indirect (e.g., damages caused by an increase in the frequency of coastal flooding due to sea level rise). 
Common measurement for a ton in the United States. A short ton is equal to 2,000 lbs or 0.907 metric tons. See metric ton.
Any process, activity or mechanism which removes a greenhouse gas, an aerosol or a precursor of a greenhouse gas or aerosol from the atmosphere. 
A seasonal accumulation of slow-melting snow. 
A major component of the terrestrial biosphere pool in the carbon cycle. The amount of carbon in the soil is a function of the historical vegetative cover and productivity, which in turn is dependent in part upon climatic variables. 
Radiation emitted by the Sun. It is also referred to as short-wave radiation. Solar radiation has a distinctive range of wavelengths (spectrum) determined by the temperature of the Sun. See ultraviolet radiation, infrared radiation, radiation. 
An abnormal rise in sea level accompanying a hurricane or other intense storm, whose height is the difference between the observed level of the sea surface and the level that would have occurred in the absence of the cyclone. 
Region of the atmosphere between the troposphere and mesosphere, having a lower boundary of approximately 8 km at the poles to 15 km at the equator and an upper boundary of approximately 50 km. Depending upon latitude and season, the temperature in the lower stratosphere can increase, be isothermal, or even decrease with altitude, but the temperature in the upper stratosphere generally increases with height due to absorption of solar radiation by ozone. 
See ozone layer.
The volume of water that moves over a designated point over a fixed period of time. It is often expressed as cubic feet per second (ft3/sec). 
The downward settling of the Earth's crust relative to its surroundings. 
Particulate matter that consists of compounds of sulfur formed by the interaction of sulfur dioxide and sulfur trioxide with other compounds in the atmosphere. Sulfate aerosols are injected into the atmosphere from the combustion of fossil fuels and the eruption of volcanoes like Mt. Pinatubo. Sulfate aerosols can lower the Earth's temperature by reflecting away solar radiation (negative radiative forcing). General Circulation Models which incorporate the effects of sulfate aerosols more accurately predict global temperature variations. See particulate matter, aerosol, General Circulation Models. 
Sulfur Hexafluoride (SF6)
A colorless gas soluble in alcohol and ether, slightly soluble in water. A very powerful greenhouse gas used primarily in electrical transmission and distribution systems and as a dielectric in electronics. The global warming potential of SF6 is 22,800. This GWP is from the IPCC's Fourth Assessment Report (AR4). See Global Warming Potential. 
1 trillion (1012) grams = 1 million (106) metric tons.
The increase in volume (and decrease in density) that results from warming water. A warming of the ocean leads to an expansion of the ocean volume, which leads to an increase in sea level. 
Large-scale density-driven circulation in the ocean, caused by differences in temperature and salinity. In the North Atlantic the thermohaline circulation consists of warm surface water flowing northward and cold deep water flowing southward, resulting in a net poleward transport of heat. The surface water sinks in highly restricted sinking regions located in high latitudes. 
Any one of the less common gases found in the Earth's atmosphere. Nitrogen, oxygen, and argon make up more than 99 percent of the Earth's atmosphere. Other gases, such as carbon dioxide, water vapor, methane, oxides of nitrogen, ozone, and ammonia, are considered trace gases. Although relatively unimportant in terms of their absolute volume, they have significant effects on the Earth's weather and climate. 
The lowest part of the atmosphere from the surface to about 10 km in altitude in mid-latitudes (ranging from 9 km in high latitudes to 16 km in the tropics on average) where clouds and "weather" phenomena occur. In the troposphere temperatures generally decrease with height. See ozone precursors, stratosphere, atmosphere. 
Tropospheric Ozone (O3)
Tropospheric Ozone Precursors
See ozone precursors.
A treeless, level, or gently undulating plain characteristic of the Arctic and sub-Arctic regions characterized by low temperatures and short growing seasons. 
Ultraviolet Radiation (UV)
The energy range just beyond the violet end of the visible spectrum. Although ultraviolet radiation constitutes only about 5 percent of the total energy emitted from the sun, it is the major energy source for the stratosphere and mesosphere, playing a dominant role in both energy balance and chemical composition.
Most ultraviolet radiation is blocked by Earth's atmosphere, but some solar ultraviolet penetrates and aids in plant photosynthesis and helps produce vitamin D in humans. Too much ultraviolet radiation can burn the skin, cause skin cancer and cataracts, and damage vegetation. 
United Nations Framework Convention on Climate Change (UNFCCC)
The Convention on Climate Change sets an overall framework for intergovernmental efforts to tackle the challenge posed by climate change. It recognizes that the climate system is a shared resource whose stability can be affected by industrial and other emissions of carbon dioxide and other greenhouse gases. The Convention enjoys near universal membership, with 189 countries having ratified.
Under the Convention, governments:
- gather and share information on greenhouse gas emissions, national policies and best practices
- launch national strategies for addressing greenhouse gas emissions and adapting to expected impacts, including the provision of financial and technological support to developing countries
- cooperate in preparing for adaptation to the impacts of climate change
The Convention entered into force on 21 March 1994. 
The degree to which a system is susceptible to, or unable to cope with, adverse effects of climate change, including climate variability and extremes. Vulnerability is a function of the character, magnitude, and rate of climate variation to which a system is exposed its sensitivity and its adaptive capacity. 
Water that has been used and contains dissolved or suspended waste materials. 
The most abundant greenhouse gas, it is the water present in the atmosphere in gaseous form. Water vapor is an important part of the natural greenhouse effect. While humans are not significantly increasing its concentration through direct emissions, it contributes to the enhanced greenhouse effect because the warming influence of greenhouse gases leads to a positive water vapor feedback. In addition to its role as a natural greenhouse gas, water vapor also affects the temperature of the planet because clouds form when excess water vapor in the atmosphere condenses to form ice and water droplets and precipitation. See greenhouse gas. 
Atmospheric condition at any given time or place. It is measured in terms of such things as wind, temperature, humidity, atmospheric pressure, cloudiness, and precipitation. In most places, weather can change from hour-to-hour, day-to-day, and season-to-season. Climate in a narrow sense is usually defined as the "average weather", or more rigorously, as the statistical description in terms of the mean and variability of relevant quantities over a period of time ranging from months to thousands or millions of years. The classical period is 30 years, as defined by the World Meteorological Organization (WMO). These quantities are most often surface variables such as temperature, precipitation, and wind. Climate in a wider sense is the state, including a statistical description, of the climate system. A simple way of remembering the difference is that climate is what you expect (e.g. cold winters) and 'weather' is what you get (e.g. a blizzard). See climate.
100-Year Flood Levels
Severe flood levels with a one-in-100 likelihood of occurring in any given year.
3. Attaining cultural literacy on a world scale
World history contributes to our cultural literacy. Human beings, unlike other species, have the gift of language, that is, symbolic thinking and communication. That means that humans also have what World History for Us All calls collective learning, the ability to learn from one another and to transmit knowledge from one generation to the next.
Communicating intelligently in any language, whether English, Spanish, or Vietnamese, requires that we share a common fund of knowledge, information, vocabulary, and conceptual tools. We need shared knowledge and understandings partly because we live in a world where people in specialized occupations and professions tend to use special words, terms, and concepts that "outsiders" do not understand.
Making world history a core subject in schools broadens the fund of knowledge that we all share. It helps us speak and write to one another in clearer and more intricate ways. This does not mean that world history courses should be exactly the same in every school district. But societies should aim for general agreement regarding the common stock of both world-scale knowledge and historical thinking skills that children ought to possess when they graduate from high school.
All past societies that we know of have had an endowment of collective knowledge. World history is shared knowledge that citizens, whatever their country of allegiance, need to function on our planet in the twenty-first century. The complexity of human interrelations today means that cultural literacy must be global in range and depth.
Taking a Global Perspective on Earth's Climate
NASA maintains a fleet of Earth science spacecraft and instruments in orbit studying all aspects of the Earth system (oceans, land, atmosphere, biosphere, cryosphere), with more planned for launch in the next few years.
NASA conducts a program of breakthrough research on climate science, enhancing the ability of the international scientific community to advance global integrated Earth system science using space-based observations.
The agency's research encompasses solar activity, sea level rise, the temperature of the atmosphere and the oceans, the state of the ozone layer, air pollution, and changes in sea ice and land ice. NASA scientists regularly appear in the mainstream press as climate experts. So how did the space agency end up taking such a big role in climate science?
When NASA was first created by the National Aeronautics and Space Act of 1958, it was given the role of developing technology for “space observations,” but it wasn’t given a role in Earth science. The agency’s leaders embedded the technology effort in an Earth Observations program centered at the new Goddard Space Flight Center in Greenbelt, Maryland, in the U.S.. It was an “Applications” program, in NASA-speak. Other agencies of the federal government were responsible for carrying out Earth science research: the Weather Bureau (now the National Oceanic and Atmospheric Administration or NOAA) and the U.S. Geological Survey (USGS). The Applications program signed cooperative agreements with these other agencies that obligated NASA to develop observational technology while NOAA and the USGS carried out the scientific research. The Nimbus series of experimental weather satellites and the Landsat series of land resources satellites were the result of the Applications program.
This Applications model of cross-agency research failed during the 1970s, though, due to the bad economy and an extended period of high inflation. Congress responded by cutting the budgets of all three agencies, leaving NOAA and the USGS unable to fund their part of the arrangement and putting pressure on NASA, too. At the same time, congressional leaders wanted to see NASA doing more research toward “national needs.” These needs included things like energy efficiency, pollution, ozone depletion and climate change. In 1976, Congress revised the Space Act to give NASA authority to carry out stratospheric ozone research, formalizing the agency’s movement into the Earth sciences.
NASA’s planetary program had a lot to do with scientific and congressional interest in expanding the agency’s role in Earth science. The Jet Propulsion Laboratory, NASA's lead center for planetary science, sent Mariner series probes to Venus and Mars. Astronomers considered these to be the "Earth-like" planets in the solar system, most likely to have surface conditions able to support life.
But that's not what they found. Venus had been roasted by a super-charged greenhouse effect. In contrast to Earth, Venus had about 300 times more carbon dioxide in its atmosphere, no significant water vapor and a surface temperature hotter than molten lead. Mars, on the other hand, had an atmospheric pressure about 1 percent of that of planet Earth and temperatures far below freezing. Pictures showed no surface water - it would have been frozen anyway - but they also seemed to show that it once had liquid water.
These discoveries left planetary scientists with unanswered questions. How did Earth, Venus and Mars wind up so radically different from similar origins? How could Mars have once been warm enough to be wet, but be frozen solid now? These questions revolve around climate and the intersection of climate, atmospheric chemistry and, on Earth, life.
Moving Back to Earth
But just as planetary scientists began confronting these questions, Congress lost interest in planetary exploration. NASA's planetary exploration budget sank dramatically starting in 1977, and the Reagan administration threatened to terminate planetary exploration entirely. This was partly due to high inflation in the U.S., and partly due to the agency's focus on the space shuttle, which could only reach low Earth orbit. The shuttle focused agency leaders’ attention on studying the Earth from orbit, not on the other planets.
The Space Shuttle Atlantis is back-dropped against Earth.
The same decade had witnessed a revolution in scientists' understanding of Earth's climate. Prior to the mid 1960s, geoscientists believed that our climate could only change relatively slowly, on timescales of thousands of years or longer. But evidence from ice and sediment cores showed that belief was wrong. Earth's climate had changed rapidly in the past—in some cases, within mere decades. Recognition that climate could change on human timescales made climate processes much more interesting research topics. It also spurred political interest.
It had been known since 1960 that humans were increasing the amount of heat-trapping greenhouse gases in the atmosphere. Would this warm the climate noticeably? Scientists also knew that human emissions of aerosols could cool the Earth. Which effect would dominate? A 1975 study by the U.S. National Academy of Science said, in effect, "We don't know. Give us money for research." A 1979 study of carbon dioxide's role in the climate put it slightly differently. They had found "no reason to doubt that climate changes will result and no reason to believe that these changes will be negligible."
Declining planetary funding and growing scientific interest in the Earth's climate caused planetary scientists to start studying the Earth. It was closer, and much less expensive, to do research on. And NASA followed suit, starting to plan for an Earth observing system aimed at questions of "global change." This phrase included climate change as well as changes in land use, ocean productivity and pollution. But the Earth science program that it established was modeled on NASA’s space and planetary science programs, not the old Applications program. NASA developed the technology and funded the science. In 1984, Congress again revised the Space Act, broadening NASA’s Earth science authority from the stratosphere to “the expansion of human knowledge of the Earth.”
In the early 1980s, NASA began working on an expansive Earth science program plan called Global Habitability, and that eventually became the Mission to Planet Earth. At the same time, a multi-agency effort called the Global Change Research Program was also taking form. NASA's role in that larger U.S. program was the provision of global data from space. Approved in the fiscal year 1991 budget, the resulting Earth Observing System would be the agency's primary contribution to American climate science.
The Earth Observing System era
Grace, one of NASA's more recent Earth-observing missions, has revealed unexpectedly rapid changes in the Earth's great ice sheets.
Fast forward to 2007, and NASA had 17 space missions collecting climate data. Today, it runs programs to obtain and convert data from Defense Department and NOAA satellites as well as from certain European, Japanese and Russian satellites. NASA also sponsors field experiments to provide "ground truth" data to check space instrument performance and to develop new measurement techniques.
Instruments on NASA’s Terra and Aqua satellites have provided the first global measurements of aerosols in our atmosphere, which come from natural sources such as volcanoes, dust storms and man-made sources such as the burning of fossil fuels. Other instruments onboard the Aura satellite study the processes that regulate the abundance of ozone in the atmosphere. Data from the GRACE and ICESat missions and from spaceborne radar show unexpectedly rapid changes in the Earth's great ice sheets, while the Jason-3, OSTM/Jason-2 and Jason-1 missions have recorded a sea level rise of an average of 3 inches since 1992. NASA’s Earth Observing System’s weather instruments have demonstrated significant improvements in global forecast skill.
These capabilities -- nearly 30 years of satellite-based solar and atmospheric temperature data -- helped the Intergovernmental Panel on Climate Change come to the conclusion in 2007 that "Most of the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations." But there's still a lot to learn about what the consequences will be. How much warmer will it get? How will sea level rise progress? NASA scientists and engineers will help answer these and other critical questions in the future.
GLOSSARY OF TERMS
Biodiversity &mdash The variability among living organisms on the Earth, including the variability within and between species and within and between ecosystems. We might think of it as the number of different species in a given place, or the world, plus the degree of difference among them. Learn more about biodiversity.
Climate change (often called global warming) &mdash A process occurring because the planet's atmosphere is becoming increasingly full of carbon dioxide and other greenhouse gases, which become trapped close to the Earth and interfere with our weather &mdash often making it hotter or causing drought (and melting the Arctic), but also sometimes just messing with the weather in general, making it colder than usual, and definitely contributing to the commonness of big storms, or "superstorms" like Hurricane Sandy.
Climate change has happened naturally in the planet's history, but right now we call it "anthropogenic" climate change, which means it's caused by humans. Human activities (like driving cars and burning coal in power plants) are emitting the greenhouse gases causing this transformation. Learn more about climate change.
Ecosystem &mdash A community of living organisms (plants, animals and microbes) in conjunction with the nonliving components of their environment (things like air, water and mineral soil), interacting as a system.
Endangered species &mdash In general, any animal or plant in danger of extinction in the relatively near future. In formal or technical use, this refers to an animal or plant protected under a federal law called the Endangered Species Act.
Endangered Species Act &mdash Generally refers to the U.S. Endangered Species Act, the federal law enacted in 1973 to protect any species that the U.S. Fish and Wildlife Service officially declares "endangered" or "threatened" (with the "threatened" designation meaning that the species in question is less at risk of extinction than a species designated as "endangered"). States also have their own Endangered Species Acts, under which a species may be protected at a state level &mdash lesser protections than at the federal level (but still valuable).
Greenhouse gases &mdash Sometimes shortened to "GHGs," these are the gases causing the greenhouse effect that's heating up Earth's atmosphere. The most common ones are water vapor, carbon dioxide, methane, nitrous oxide and ozone.
Habitat &mdash The area or environment where species normally live or occur. The ocean, for example, is a marine habitat a coral reef is a specific type of marine habitat.
Imperiled &mdash This is a loose term that can be applied to most animals and plants that are in danger of extinction, whether or not they're protected under the Endangered Species Act.
Native species &mdash An animal or plant that evolved in the location where it currently lives (as opposed to invasive species, which take over land and habitat from plants and animals that had already been living there for centuries).
Natural resources &mdash Things humans use that come from nature. For example, we get fossil fuels like oil and coal from the Earth, and we get water from waterways and the ground (and we can also harvest rainwater). Land is a natural resource, used to build on and raise crops and livestock to eat, and species like medicinal plants are in this category, too, since we use them to make medicine for people. Even wind in a natural resource, since we can use it to spin turbines and make energy into electricity.
Species &mdash In biology, a species is one of the basic units of biological classification of living things. A species is often defined as the largest group of organisms capable of interbreeding and producing fertile offspring. Find out more about some of the species the Center works to protect.
Threat &mdash Any factor that hurts an animal or its habitat, such as climate change, pesticides, oil development or mining. You can find out about many more threats on our campaign pages.
U.S. Fish and Wildlife Service &mdash This is the federal agency that manages wildlife and plants across the country and has the authority to designate a species as "endangered" or "threatened" under the U.S. Endangered Species Act. Almost always, this agency only protects an animal or plant once an individual or group (like the Center for Biological Diversity) sends it a petition that the agency believes shows the species in need of designation, though the agency may also decide to protect a species of its own accord, through its own biologists.
Wildlands &mdash A general term referring to any wild place that we don't want to see ruined by humans.
A B C
Acceptability of policy or system change
The extent to which a policy or system change is evaluated unfavourably or favourably, or rejected or supported, by members of the general public (public acceptability) or politicians or governments (political acceptability). Acceptability may vary from totally unacceptable/fully rejected to totally acceptable/fully supported individuals may differ in how acceptable policies or system changes are believed to be.
In human systems, the process of adjustment to actual or expected climate and its effects, in order to moderate harm or exploit beneficial opportunities. In natural systems, the process of adjustment to actual climate and its effects human intervention may facilitate adjustment to expected climate and its effects.
Adaptation that maintains the essence and integrity of a system or process at a given scale. In some cases, incremental adaptation can accrue to result in transformational adaptation (Termeer et al., 2017 Tàbara et al., 2018) 2 .
Adaptation that changes the fundamental attributes of a socio-ecological system in anticipation of climate change and its impacts.
The point at which an actor’s objectives (or system needs) cannot be secured from intolerable risks through adaptive actions.
- Hard adaptation limit: No adaptive actions are possible to avoid intolerable risks.
- Soft adaptation limit: Options are currently not available to avoid intolerable risks through adaptive action.
See also Adaptation options, Adaptive capacity and Maladaptive actions (Maladaptation).
The array of strategies and measures that are available and appropriate for addressing adaptation. They include a wide range of actions that can be categorized as structural, institutional, ecological or behavioural. See also Adaptation, Adaptive capacity and Maladaptive actions (Maladaptation).
The ability of systems, institutions, humans and other organisms to adjust to potential damage, to take advantage of opportunities, or to respond to consequences. This glossary entry builds from definitions used in previous IPCC reports and the Millennium Ecosystem Assessment (MEA, 2005) 3 . See also Adaptation, Adaptation options and Maladaptive actions (Maladaptation).
A suspension of airborne solid or liquid particles, with a typical size between a few nanometres and 10 μm that reside in the atmosphere for at least several hours. The term aerosol, which includes both the particles and the suspending gas, is often used in this report in its plural form to mean aerosol particles. Aerosols may be of either natural or anthropogenic origin. Aerosols may influence climate in several ways: through both interactions that scatter and/or absorb radiation and through interactions with cloud microphysics and other cloud properties, or upon deposition on snow- or ice-covered surfaces thereby altering their albedo and contributing to climate feedback. Atmospheric aerosols, whether natural or anthropogenic, originate from two different pathways: emissions of primary particulate matter (PM), and formation of secondary PM from gaseous precursors. The bulk of aerosols are of natural origin. Some scientists use group labels that refer to the chemical composition, namely: sea salt, organic carbon, black carbon (BC), mineral species (mainly desert dust), sulphate, nitrate, and ammonium. These labels are, however, imperfect as aerosols combine particles to create complex mixtures. See also Short-lived climate forcers (SLCF) and Black carbon (BC).
Planting of new forests on lands that historically have not contained forests. For a discussion of the term forest and related terms such as afforestation, reforestation and deforestation, see the IPCC Special Report on Land Use, Land-Use Change, and Forestry (IPCC, 2000) 4 , information provided by the United Nations Framework Convention on Climate Change (UNFCCC, 2013) 5 and the report on Definitions and Methodological Options to Inventory Emissions from Direct Human-induced Degradation of Forests and Devegetation of Other Vegetation Types (IPCC, 2003) 6 . See also Reforestation, Deforestation, and Reducing Emissions from Deforestation and Forest Degradation (REDD+).
In this report, the degree of agreement within the scientific body of knowledge on a particular finding is assessed based on multiple lines of evidence (e.g., mechanistic understanding, theory, data, models, expert judgement) and expressed qualitatively (Mastrandrea et al., 2010) 7 . See also Evidence, Confidence, Likelihood and Uncertainty.
Degradation of air quality with negative effects on human health or the natural or built environment due to the introduction, by natural processes or human activity, into the atmosphere of substances (gases, aerosols) which have a direct (primary pollutants) or indirect (secondary pollutants) harmful effect. See also Aerosol and Short-lived climate forcers (SLCF).
The fraction of solar radiation reflected by a surface or object, often expressed as a percentage. Snow-covered surfaces have a high albedo, the surface albedo of soils ranges from high to low, and vegetation-covered surfaces and the oceans have a low albedo. The Earth’s planetary albedo changes mainly through varying cloudiness and changes in snow, ice, leaf area and land cover.
Ambient persuasive technology
Technological systems and environments that are designed to change human cognitive processing, attitudes and behaviours without the need for the user’s conscious attention.
The deviation of a variable from its value averaged over a reference period.
The ‘Anthropocene’ is a proposed new geological epoch resulting from significant human-driven changes to the structure and functioning of the Earth System, including the climate system. Originally proposed in the Earth System science community in 2000, the proposed new epoch is undergoing a formalization process within the geological community based on the stratigraphic evidence that human activities have changed the Earth System to the extent of forming geological deposits with a signature that is distinct from those of the Holocene, and which will remain in the geological record. Both the stratigraphic and Earth System approaches to defining the Anthropocene consider the mid-20th Century to be the most appropriate starting date, although others have been proposed and continue to be discussed. The Anthropocene concept has been taken up by a diversity of disciplines and the public to denote the substantive influence humans have had on the state, dynamics and future of the Earth System. See also Holocene.
Resulting from or produced by human activities. See also Anthropogenic emissions and Anthropogenic removals.
Emissions of greenhouse gases (GHGs), precursors of GHGs and aerosols caused by human activities. These activities include the burning of fossil fuels, deforestation, land use and land-use changes (LULUC), livestock production, fertilisation, waste management and industrial processes. See also Anthropogenic and Anthropogenic removals.
Anthropogenic removals refer to the withdrawal of GHGs from the atmosphere as a result of deliberate human activities. These include enhancing biological sinks of CO2 and using chemical engineering to achieve long-term removal and storage. Carbon capture and storage (CCS) from industrial and energy-related sources, which alone does not remove CO2 in the atmosphere, can reduce atmospheric CO2 if it is combined with bioenergy production (BECCS). See also Anthropogenic emissions, Bioenergy with carbon dioxide capture and storage (BECCS) and Carbon dioxide capture and storage (CCS).
Artificial intelligence (AI)
Computer systems able to perform tasks normally requiring human intelligence, such as visual perception and speech recognition.
The gaseous envelope surrounding the earth, divided into five layers – the troposphere which contains half of the Earth’s atmosphere, the stratosphere, the mesosphere, the thermosphere, and the exosphere, which is the outer limit of the atmosphere. The dry atmosphere consists almost entirely of nitrogen (78.1% volume mixing ratio) and oxygen (20.9% volume mixing ratio), together with a number of trace gases, such as argon (0.93 % volume mixing ratio), helium and radiatively active greenhouse gases (GHGs) such as carbon dioxide (CO2) (0.04% volume mixing ratio) and ozone (O3). In addition, the atmosphere contains the GHG water vapour (H2O), whose amounts are highly variable but typically around 1% volume mixing ratio. The atmosphere also contains clouds and aerosols. See also Troposphere, Stratosphere, Greenhouse gas (GHG) and Hydrological cycle.
Atmosphere–ocean general circulation model (AOGCM)
See Detection and attribution.
In much of the literature the term is also synonymous with the term business-as-usual (BAU) scenario, although the term BAU has fallen out of favour because the idea of business as usual in century-long socio-economic projections is hard to fathom. In the context of transformation pathways, the term baseline scenarios refers to scenarios that are based on the assumption that no mitigation policies or measures will be implemented beyond those that are already in force and/or are legislated or planned to be adopted. Baseline scenarios are not intended to be predictions of the future, but rather counterfactual constructions that can serve to highlight the level of emissions that would occur without further policy effort. Typically, baseline scenarios are then compared to mitigation scenarios that are constructed to meet different goals for greenhouse gas (GHG) emissions, atmospheric concentrations or temperature change. The term baseline scenario is often used interchangeably with reference scenario and no policy scenario. See also Emission scenario and Mitigation scenario.
Battery electric vehicle (BEV)
Stable, carbon-rich material produced by heating biomass in an oxygen-limited environment. Biochar may be added to soils to improve soil functions and to reduce greenhouse gas emissions from biomass and soils, and for carbon sequestration. This definition builds from IBI (2018) 8 .
Biological diversity means the variability among living organisms from all sources, including, inter alia, terrestrial, marine and other aquatic ecosystems and the ecological complexes of which they are part this includes diversity within species, between species and of ecosystems (UN, 1992) 9 .
Energy derived from any form of biomass or its metabolic by-products. See also Biomass and Biofuel.
Bioenergy with carbon dioxide capture and storage (BECCS)
Carbon dioxide capture and storage (CCS) technology applied to a bioenergy facility. Note that depending on the total emissions of the BECCS supply chain, carbon dioxide (CO2) can be removed from the atmosphere. See also Bioenergy and Carbon dioxide capture and storage (CCS).
A fuel, generally in liquid form, produced from biomass. Biofuels currently include bioethanol from sugarcane or maize, biodiesel from canola or soybeans, and black liquor from the paper-manufacturing process. See also Biomass and Bioenergy.
Living or recently dead organic material. See also Bioenergy and Biofuel.
Designing cities with green roofs, green walls and green balconies to bring nature into the densest parts of cities in order to provide green infrastructure and human health benefits. See also Green infrastructure.
Black carbon (BC)
Operationally defined aerosol species based on measurement of light absorption and chemical reactivity and/or thermal stability. It is sometimes referred to as soot. BC is mostly formed by the incomplete combustion of fossil fuels, biofuels and biomass but it also occurs naturally. It stays in the atmosphere only for days or weeks. It is the most strongly light-absorbing component of particulate matter (PM) and has a warming effect by absorbing heat into the atmosphere and reducing the albedo when deposited on snow or ice. See also Aerosol.
Blue carbon is the carbon captured by living organisms in coastal (e.g., mangroves, salt marshes, seagrasses) and marine ecosystems, and stored in biomass and sediments.
Burden sharing (also referred to as Effort sharing)
In the context of mitigation, burden sharing refers to sharing the effort of reducing the sources or enhancing the sinks of greenhouse gases (GHGs) from historical or projected levels, usually allocated by some criteria, as well as sharing the cost burden across countries.
Business as usual (BAU)
This term refers to three concepts in the literature: (1) an assessment of carbon cycle sources and sinks on a global level, through the synthesis of evidence for fossil fuel and cement emissions, land-use change emissions, ocean and land CO2 sinks, and the resulting atmospheric CO2 growth rate. This is referred to as the global carbon budget (2) the estimated cumulative amount of global carbon dioxide emissions that that is estimated to limit global surface temperature to a given level above a reference period, taking into account global surface temperature contributions of other GHGs and climate forcers (3) the distribution of the carbon budget defined under (2) to the regional, national, or sub-national level based on considerations of equity, costs or efficiency. See also Remaining carbon budget.
The term used to describe the flow of carbon (in various forms, e.g., as carbon dioxide (CO2), carbon in biomass, and carbon dissolved in the ocean as carbonate and bicarbonate) through the atmosphere, hydrosphere, terrestrial and marine biosphere and lithosphere. In this report, the reference unit for the global carbon cycle is GtCO2 or GtC (Gigatonne of carbon = 1 GtC = 10 15 grams of carbon. This corresponds to 3.667 GtCO2).
Carbon dioxide (CO2)
A naturally occurring gas, CO2 is also a by-product of burning fossil fuels (such as oil, gas and coal), of burning biomass, of land-use changes (LUC) and of industrial processes (e.g., cement production). It is the principal anthropogenic greenhouse gas (GHG) that affects the Earth’s radiative balance. It is the reference gas against which other GHGs are measured and therefore has a global warming potential (GWP) of 1. See also Greenhouse gas (GHG).
Carbon dioxide capture and storage (CCS)
A process in which a relatively pure stream of carbon dioxide (CO2) from industrial and energy-related sources is separated (captured), conditioned, compressed and transported to a storage location for long-term isolation from the atmosphere. Sometimes referred to as Carbon capture and storage. See also Carbon dioxide capture and utilisation (CCU), Bioenergy with carbon dioxide capture and storage (BECCS) and Uptake.
Carbon dioxide capture and utilisation (CCU)
A process in which CO2 is captured and then used to produce a new product. If the CO2 is stored in a product for a climate-relevant time horizon, this is referred to as carbon dioxide capture, utilisation and storage (CCUS). Only then, and only combined with CO2 recently removed from the atmosphere, can CCUS lead to carbon dioxide removal. CCU is sometimes referred to as carbon dioxide capture and use. See also Carbon dioxide capture and storage (CCS).
Carbon dioxide capture, utilisation and storage (CCUS)
See Carbon dioxide capture and utilisation (CCU).
Carbon dioxide removal (CDR)
Anthropogenic activities removing CO2 from the atmosphere and durably storing it in geological, terrestrial, or ocean reservoirs, or in products. It includes existing and potential anthropogenic enhancement of biological or geochemical sinks and direct air capture and storage, but excludes natural CO2 uptake not directly caused by human activities. See also Mitigation (of climate change), Greenhouse gas removal (GGR), Negative emissions, Direct air carbon dioxide capture and storage (DACCS) and Sink.
The amount of emissions of carbon dioxide (CO2) released per unit of another variable such as gross domestic product (GDP), output energy use or transport.
See Net zero CO2 emissions.
The price for avoided or released carbon dioxide (CO2) or CO2-equivalent emissions. This may refer to the rate of a carbon tax, or the price of emission permits. In many models that are used to assess the economic costs of mitigation, carbon prices are used as a proxy to represent the level of effort in mitigation policies.
The process of storing carbon in a carbon pool. See also Blue carbon, Carbon dioxide capture and storage (CCS), Uptake and Sink.
Clean Development Mechanism (CDM)
A mechanism defined under Article 12 of the Kyoto Protocol through which investors (governments or companies) from developed (Annex B) countries may finance greenhouse gas (GHG) emission reduction or removal projects in developing countries (Non-Annex B), and receive Certified Emission Reduction Units (CERs) for doing so. The CERs can be credited towards the commitments of the respective developed countries. The CDM is intended to facilitate the two objectives of promoting sustainable development (SD) in developing countries and of helping industrialised countries to reach their emissions commitments in a cost-effective way.
Climate in a narrow sense is usually defined as the average weather, or more rigorously, as the statistical description in terms of the mean and variability of relevant quantities over a period of time ranging from months to thousands or millions of years. The classical period for averaging these variables is 30 years, as defined by the World Meteorological Organization. The relevant quantities are most often surface variables such as temperature, precipitation and wind. Climate in a wider sense is the state, including a statistical description, of the climate system.
Climate change refers to a change in the state of the climate that can be identified (e.g., by using statistical tests) by changes in the mean and/or the variability of its properties and that persists for an extended period, typically decades or longer. Climate change may be due to natural internal processes or external forcings such as modulations of the solar cycles, volcanic eruptions and persistent anthropogenic changes in the composition of the atmosphere or in land use. Note that the Framework Convention on Climate Change (UNFCCC), in its Article 1, defines climate change as: ‘a change of climate which is attributed directly or indirectly to human activity that alters the composition of the global atmosphere and which is in addition to natural climate variability observed over comparable time periods.’ The UNFCCC thus makes a distinction between climate change attributable to human activities altering the atmospheric composition and climate variability attributable to natural causes. See also Climate variability, Global warming, Ocean acidification (OA) and Detection and attribution.
Climate change commitment
Climate change commitment is defined as the unavoidable future climate change resulting from inertia in the geophysical and socio-economic systems. Different types of climate change commitment are discussed in the literature (see subterms). Climate change commitment is usually quantified in terms of the further change in temperature, but it includes other future changes, for example in the hydrological cycle, in extreme weather events, in extreme climate events, and in sea level.
Constant composition commitment
The constant composition commitment is the remaining climate change that would result if atmospheric composition, and hence radiative forcing, were held fixed at a given value. It results from the thermal inertia of the ocean and slow processes in the cryosphere and land surface.
Constant emissions commitment
The constant emissions commitment is the committed climate change that would result from keeping anthropogenic emissions constant.
Zero emissions commitment
The zero emissions commitment is the climate change commitment that would result from setting anthropogenic emissions to zero. It is determined by both inertia in physical climate system components (ocean, cryosphere, land surface) and carbon cycle inertia.
Feasible scenario commitment
The feasible scenario commitment is the climate change that corresponds to the lowest emission scenario judged feasible.
The infrastructure commitment is the climate change that would result if existing greenhouse gas and aerosol emitting infrastructure were used until the end of its expected lifetime.
Climate-compatible development (CCD)
A form of development building on climate strategies that embrace development goals and development strategies that integrate climate risk management, adaptation and mitigation. This definition builds from Mitchell and Maxwell (2010) 10 .
Climate extreme (extreme weather or climate event)
The occurrence of a value of a weather or climate variable above (or below) a threshold value near the upper (or lower) ends of the range of observed values of the variable. For simplicity, both extreme weather events and extreme climate events are referred to collectively as ‘climate extremes’. See also Extreme weather event.
An interaction in which a perturbation in one climate quantity causes a change in a second and the change in the second quantity ultimately leads to an additional change in the first. A negative feedback is one in which the initial perturbation is weakened by the changes it causes a positive feedback is one in which the initial perturbation is enhanced. The initial perturbation can either be externally forced or arise as part of internal variability.
A numerical representation of the climate system based on the physical, chemical and biological properties of its components, their interactions and feedback processes, and accounting for some of its known properties. The climate system can be represented by models of varying complexity that is, for any one component or combination of components a spectrum or hierarchy of models can be identified, differing in such aspects as the number of spatial dimensions, the extent to which physical, chemical or biological processes are explicitly represented, or the level at which empirical parametrizations are involved. There is an evolution towards more complex models with interactive chemistry and biology. Climate models are applied as a research tool to study and simulate the climate and for operational purposes, including monthly, seasonal and interannual climate predictions. See also Earth system model (ESM).
Concept of a state in which human activities result in no net effect on the climate system. Achieving such a state would require balancing of residual emissions with emission (carbon dioxide) removal as well as accounting for regional or local biogeophysical effects of human activities that, for example, affect surface albedo or local climate. See also Net zero CO2 emissions.
A climate projection is the simulated response of the climate system to a scenario of future emission or concentration of greenhouse gases (GHGs) and aerosols, generally derived using climate models. Climate projections are distinguished from climate predictions by their dependence on the emission/concentration/radiative forcing scenario used, which is in turn based on assumptions concerning, for example, future socioeconomic and technological developments that may or may not be realized.
Climate-resilient development pathways (CRDPs)
Trajectories that strengthen sustainable development and efforts to eradicate poverty and reduce inequalities while promoting fair and cross-scalar adaptation to and resilience in a changing climate. They raise the ethics, equity and feasibility aspects of the deep societal transformation needed to drastically reduce emissions to limit global warming (e.g., to 1.5°C) and achieve desirable and liveable futures and well-being for all.
Iterative processes for managing change within complex systems in order to reduce disruptions and enhance opportunities associated with climate change. See also Development pathways (under Pathways), Transformation pathways (under Pathways), and Climate-resilient development pathways (CRDPs).
Climate sensitivity refers to the change in the annual global mean surface temperature in response to a change in the atmospheric CO2 concentration or other radiative forcing.
Equilibrium climate sensitivity
Refers to the equilibrium (steady state) change in the annual global mean surface temperature following a doubling of the atmospheric carbon dioxide (CO2) concentration. As a true equilibrium is challenging to define in climate models with dynamic oceans, the equilibrium climate sensitivity is often estimated through experiments in AOGCMs where CO2 levels are either quadrupled or doubled from pre-industrial levels and which are integrated for 100-200 years. The climate sensitivity parameter (units: °C (W m –2 ) –1 ) refers to the equilibrium change in the annual global mean surface temperature following a unit change in radiative forcing.
Effective climate sensitivity
An estimate of the global mean surface temperature response to a doubling of the atmospheric carbon dioxide (CO2) concentration that is evaluated from model output or observations for evolving non-equilibrium conditions. It is a measure of the strengths of the climate feedbacks at a particular time and may vary with forcing history and climate state, and therefore may differ from equilibrium climate sensitivity.
Transient climate response
The change in the global mean surface temperature, averaged over a 20-year period, centered at the time of atmospheric CO2 doubling, in a climate model simulation in which CO2 increases at 1% yr -1 from pre-industrial. It is a measure of the strength of climate feedbacks and the timescale of ocean heat uptake.
Climate services refers to information and products that enhance users’ knowledge and understanding about the impacts of climate change and/or climate variability so as to aid decision-making of individuals and organizations and enable preparedness and early climate change action. Products can include climate data products.
Climate-smart agriculture (CSA)
Climate-smart agriculture (CSA) is an approach that helps to guide actions needed to transform and reorient agricultural systems to effectively support development and ensure food security in a changing climate. CSA aims to tackle three main objectives: sustainably increasing agricultural productivity and incomes, adapting and building resilience to climate change, and reducing and/or removing greenhouse gas emissions, where possible (FAO, 2018) 11 .
The climate system is the highly complex system consisting of five major components: the atmosphere, the hydrosphere, the cryosphere, the lithosphere and the biosphere and the interactions between them. The climate system evolves in time under the influence of its own internal dynamics and because of external forcings such as volcanic eruptions, solar variations and anthropogenic forcings such as the changing composition of the atmosphere and land-use change.
Climate target refers to a temperature limit, concentration level, or emissions reduction goal used towards the aim of avoiding dangerous anthropogenic interference with the climate system. For example, national climate targets may aim to reduce greenhouse gas emissions by a certain amount over a given time horizon, for example those under the Kyoto Protocol.
Climate variability refers to variations in the mean state and other statistics (such as standard deviations, the occurrence of extremes, etc.) of the climate on all spatial and temporal scales beyond that of individual weather events. Variability may be due to natural internal processes within the climate system (internal variability), or to variations in natural or anthropogenic external forcing (external variability). See also Climate change.
CO2 equivalent (CO2-eq) emission
The amount of carbon dioxide (CO2) emission that would cause the same integrated radiative forcing or temperature change, over a given time horizon, as an emitted amount of a greenhouse gas (GHG) or a mixture of GHGs. There are a number of ways to compute such equivalent emissions and choose appropriate time horizons. Most typically, the CO2-equivalent emission is obtained by multiplying the emission of a GHG by its global warming potential (GWP) for a 100-year time horizon. For a mix of GHGs it is obtained by summing the CO2-equivalent emissions of each gas. CO2-equivalent emission is a common scale for comparing emissions of different GHGs but does not imply equivalence of the corresponding climate change responses. There is generally no connection between CO2-equivalent emissions and resulting CO2-equivalent concentrations.
The positive effects that a policy or measure aimed at one objective might have on other objectives, thereby increasing the total benefits for society or the environment. Co-benefits are often subject to uncertainty and depend on local circumstances and implementation practices, among other factors. Co-benefits are also referred to as ancillary benefits.
Common but Differentiated Responsibilities and Respective Capabilities (CBDR-RC)
Common but Differentiated Responsibilities and Respective Capabilities (CBDR–RC) is a key principle in the United Nations Framework Convention on Climate Change (UNFCCC) that recognises the different capabilities and differing responsibilities of individual countries in tacking climate change. The principle of CBDR–RC is embedded in the 1992 UNFCCC treaty. The convention states: “… the global nature of climate change calls for the widest possible cooperation by all countries and their participation in an effective and appropriate international response, in accordance with their common but differentiated responsibilities and respective capabilities and their social and economic conditions.” Since then the CBDR-RC principle has guided the UN climate negotiations.
Conference of the Parties (COP)
The supreme body of UN conventions, such as the United Nations Framework Convention on Climate Change (UNFCCC), comprising parties with a right to vote that have ratified or acceded to the convention. See also United Nations Framework Convention on Climate Change (UNFCCC).
The robustness of a finding based on the type, amount, quality and consistency of evidence (e.g., mechanistic understanding, theory, data, models, expert judgment) and on the degree of agreement across multiple lines of evidence. In this report, confidence is expressed qualitatively (Mastrandrea et al., 2010) 12 . See Section 1.6 for the list of confidence levels used. See also Agreement, Evidence, Likelihood and Uncertainty.
A coherent group of agronomic and soil management practices that reduce the disruption of soil structure and biota.
Constant composition commitment
See Climate change commitment.
Constant emissions commitment
See Climate change commitment.
The ability of people, institutions, organizations, and systems, using available skills, values, beliefs, resources, and opportunities, to address, manage, and overcome adverse conditions in the short to medium term. This glossary entry builds from the definition used in UNISDR (2009) 13 and IPCC (2012a) 14 . See also Resilience.
Monetary assessment of all negative and positive impacts associated with a given action. Cost–benefit analysis enables comparison of different interventions, investments or strategies and reveals how a given investment or policy effort pays off for a particular person, company or country. Cost–benefit analyses representing society’s point of view are important for climate change decision-making, but there are difficulties in aggregating costs and benefits across different actors and across timescales. See also Discounting.
A measure of the cost at which policy goal or outcome is achieved. The lower the cost the greater the cost-effectiveness.
Coupled Model Intercomparison Project (CMIP)
The Coupled Model Intercomparison Project (CMIP) is a climate modelling activity from the World Climate Research Programme (WCRP) which coordinates and archives climate model simulations based on shared model inputs by modelling groups from around the world. The CMIP3 multimodel data set includes projections using SRES scenarios. The CMIP5 data set includes projections using the Representative Concentration Pathways (RCPs). The CMIP6 phase involves a suite of common model experiments as well as an ensemble of CMIP-endorsed model intercomparison projects (MIPs).
The total amount of emissions released over a specified period of time. See also Carbon budget, and Transient climate response to cumulative CO2 emissions (TCRE).
Global Warming Glossary - History
Warming to evolution
July 2006, updated July 2008
Global warming is, quite literally, a hot topic. Though the mechanism of global warming temperature rise due to humans' production of heat-trapping greenhouse gases may not be big news, the projected impact of global warming often makes headlines. Al Gore's recent documentary on the topic has focused even more attention on the potentially disastrous effects of even a few degree temperature rise. Whole island countries could disappear into the ocean as polar ice melts and sea level rises. Hurricanes and tropical storms may intensify. And ecological interactions could change in unpredictable ways. For example, a recent news story reports that melting sea ice may be forcing some polar bears into cannibalism now that fewer seal hunting opportunities are available. Increasingly, it seems, global warming shows up on the front page of the newspaper but the evolutionary implications of global warming often remain hidden.
|Global warming is changing the world in startling ways. On the left is a photo of Boulder Glacier in Glacier National Park, Montana, taken in July of 1932. On the right is a photo taken at the same spot in July of 1988. The glacier is gone.|
Where's the evolution?
Global warming is certainly a climatic and environmental issue but it is also an evolutionary one. Over the past 20 years, biologists have uncovered several cases of evolution right under our noses evolution caused by global warming.
In this interview, Susumu Tomiya discusses how today's high extinction rates may indicate that Earth is experiencing a sixth mass extinction. This video is produced by the National Evolutionary Synthesis Center (NESCent) and UCMP.
Over the past 25 years, global surface temperatures have increased about ½°F. That might not sound like much, but it turns out to be more than enough to change the ecology and evolution of life on Earth. In many cases, these changes are simply non-evolutionary examples of phenotypic plasticity, where an organism expresses different traits depending on environmental conditions. For example, many organisms respond to warmer weather by reproducing earlier and taking advantage of an earlier spring but this early reproduction is not caused by genetic changes in the population and so is not an example of evolutionary change. Similarly, many species have shifted their ranges in response to this tiny temperature difference, spreading towards the poles, as those habitats warm but this change in range cannot be traced to a genetic shift in the population and so is not an example of evolution. And still other species simply seem to be on the path to endangerment or extinction as their habitats (like coral reefs) are degraded and their population sizes drop.
However, in a few cases, we know that species have actually evolved experienced a change in gene frequency in the population in response to global warming. Interestingly, in those cases, the species are not necessarily becoming more heat tolerant, but are adapting to changes in seasonal timing:
|Canadian squirrels are evolving to take advantage of an earlier spring and are breeding sooner, which allows them to hoard more pinecones for winter survival and next year's reproduction. Squirrels with genes for earlier breeding are more successful than squirrels with genes for later breeding.|
|European great tits (a type of bird) are also evolving different breeding times. Birds that are able to adjust egg-laying to earlier in the spring can time hatching so that it coincides with greater food (caterpillar) abundance and with recent climatic changes, the caterpillars have been maturing earlier in the spring. Birds with genes for more flexible egg-laying times are more successful than birds with less flexibility in their egg-laying.|
|Another European bird, the blackcap, has been evolving due to changes in its migration patterns. Some blackcaps have begun to overwinter in the now slightly warmer Britain instead of in Spain, Portugal, and North Africa, as they historically did. The British sub-population has evolved genetic differences from the other birds and is more successful at reproducing since its members arrive at the nesting grounds earlier and have first choice of territories and mates.|
|One North American mosquito species has evolved to take advantage of longer summers to gather resources while the weather is good. Mosquitoes with genes that allow them to wait longer before going dormant for the winter are more successful than mosquitoes that go dormant earlier.|
In a sense, these populations are the lucky ones. Small animals (like the birds, squirrels and mosquitoes described above) tend to have large population sizes and short generation times and that bodes well for their ability to evolve along with a changing environment. Large population size means that the species is more likely to have the genetic variation necessary for evolution, and having a short generation time means that their rate of evolutionary change may be able to keep pace with environmental change. However, other species may not be so lucky: larger animals tend to have longer generation times and so evolve more slowly and larger animals also tend to have smaller population sizes, which means that their populations are simply less likely to contain the gene versions that would allow the population to adapt to warmer climates. If global warming continues, such species may come face to face with extinction, as the environments to which they have been adapted over the course of thousands or millions of years change right out from underneath them in the course of a few decades.
Since we published this report in July 2006, we've been monitoring the news for other examples of evolution in response to global warming and have identified two to add to the list:
- Field mustard plants have evolved in response to an extreme, four-year-long drought in southern California, which some sources have linked to global warming. These plants flower and produce seeds near the end of the rainy season, but when the rainy season is cut short by a drought, late blooming plants may wither and die before they can produce seeds. This form of natural selection favors early bloomers. Is just four years enough time to see the results of this evolutionary shift? Researchers compared plants grown from wild seeds collected before and after the drought and found that post-drought plants had evolved to flower much earlier sometimes by as much as 10 days!
- Scientists have been studying fruit fly genetics for a century. When they began to examine the genes found in whole populations of wild flies, they noticed a curious pattern. Certain chromosomal markers (inversions) were common in populations living in warmer climates near the equator, and others were common in more polar, cool-weather populations. It wasn't clear what the genes associated with these different markers did exactly, but they seemed to help the flies cope with their divergent climates. Now, scientists have gone back to many of the fly populations first studied and have found that as the global climate has warmed, the warm-weather genetic markers are becoming more and more common. Of the 22 fly populations on three continents that experienced warming trends, 21 seem to have already evolved in response to the climactic shift.
With rising temperatures and further climate fluctuations, we expect more examples of evolution in response to global warming to come to light. Such rapid evolutionary shifts are disturbing and suggest the gravity of this global threat, but even more unsettling is the likely fate of many species with long generation times and low levels of genetic variation: extinction. For these organisms, climate change may simply outpace their ability to evolve.
- For an easy-to-understand summary of global warming's potential impact on many species, check out this article from Smithsonian.com.
- Balanya, J., Oller, J. M., Huey, R. B., Gilchrist, G. W., and Serra, L. (2006). Global genetic change tracks global climate warming in Drosophila subobscura. Science 313:1773-1775.
- from National Geographic News
from the University of Alberta
Understanding Evolution resources:
Discussion and extension questions
- How could global warming affect the evolutionary paths of different species?
Related lessons and teaching resources
- : In this classroom activity for grades 3-5, students observe and conduct an experiment to see whether differences in salinity (the environment) have an affect on the hatching rate and survival of brine shrimp.
: In this classroom activity for grades 9-12, students experience one mechanism of evolution through a simulation that models the principles of natural selection and helps answer the question: How might biological change have occurred and been reinforced over time?
- Balanya, J., Oller, J. M., Huey, R. B., Gilchrist, G. W., and Serra, L. (2006). Global genetic change tracks global climate warming in Drosophila subobscura. Science 313:1773-1775.
Learn more about Earth's changing temperature on the Understanding Global Change site.
Is Human Activity Primarily Responsible for Global Climate Change?
Average surface temperatures on earth have risen more than 2°F over the past 100 years. During this time period, atmospheric levels of greenhouse gases such as carbon dioxide (CO2) and methane (CH4) have notably increased. This site explores the debate on whether climate change is caused by humans (also known as anthropogenic climate change).
The pro side argues rising levels of atmospheric greenhouse gases are a direct result of human activities such as burning fossil fuels, and that these increases are causing significant and increasingly severe climate changes including global warming, loss of sea ice, sea level rise, stronger storms, and more droughts. They contend that immediate international action to reduce greenhouse gas emissions is necessary to prevent dire climate changes.
The con side argues human-generated greenhouse gas emissions are too small to substantially change the earth’s climate and that the planet is capable of absorbing those increases. They contend that warming over the 20th century resulted primarily from natural processes such as fluctuations in the sun’s heat and ocean currents. They say the theory of human-caused global climate change is based on questionable measurements, faulty climate models, and misleading science. Read more background…
Pro & Con Arguments
Overwhelming scientific consensus finds human activity primarily responsible for climate change.
According to many peer-reviewed studies, over 97% of climate scientists agree that human activity is extremely likely to be the cause of global climate change.  Most scientific organizations also support this view, including the American Medical Association and an international coalition of science academies. 
A prominent review of 11,944 peer-reviewed studies on climate change found that only 78 studies (0.7%) explicitly rejected the idea of anthropogenic (resulting from human activity) global warming.  A separate review of 13,950 peer-reviewed studies on climate change found only 24 that rejected human-caused global warming.  An examination of scientific papers that didn’t agree that humans cause climate change found serious flaws and bias in their research. Read More
Rising levels of human-produced gases released into the atmosphere create a greenhouse effect that traps heat and causes global warming.
Gases released into the atmosphere trap heat and cause the planet to warm through a process called the greenhouse effect.  When we burn fossil fuels to heat our homes, drive our cars, and run factories, we’re releasing emissions that cause the planet to warm. 
Methane, which is increasing in the atmosphere due to agriculture and fossil fuel production, traps 84 times as much heat as CO2 for the first 20 years it is in the atmosphere,  and is responsible for about one-fifth of global warming since 1750.  Nitrous oxide, primarily released through agricultural practices, traps 300 times as much heat as CO2.  Over the 20th century, as the concentrations of CO2, CH4, and NO2 increased in the atmosphere due to human activity,   the earth warmed by approximately 1.4°F. Read More
The rise in atmospheric CO2 over the last century was clearly caused by human activity, as it occurred at a rate much faster than natural climate changes could produce.
Over the past 650,000 years, atmospheric CO2 levels did not rise above 300 ppm until the mid-20th century.  Atmospheric levels of CO2 have risen from about 317 ppm in 1958 to 415 ppm in 2019.   According to the Scripps Institution of Oceanology, the “extreme speed at which carbon dioxide concentrations are increasing is unprecedented. An increase of 10 parts per million might have needed 1,000 years or more to come to pass during ancient climate change events.”  Some climate models predict that by the end of the 21st century an additional 5°F-10°F of warming will occur. Read More
The specific type of CO2 that is increasing in earth’s atmosphere can be directly connected to human activity.
We can tell that CO2 produced by humans burning fossil fuels such as oil and coal  is different than naturally occurring CO2 by looking at the specific isotopic ratio.  According to the Intergovernmental Panel on Climate Change (IPCC), 20th century measurements of CO2 isotope ratios in the atmosphere confirm that rising CO2 levels are the result of human activity as opposed to gas coming off the oceans, volcanic activity, or other natural causes. 
The US Environmental Protection Agency says that “Human activities are responsible for almost all of the increase in greenhouse gases in the atmosphere over the last 150 years.” Read More
Average temperatures on earth have increased at a rate far faster than can be explained by natural climate changes.
Average surface temperatures on earth have risen more than 2°F over the past 100 years.  According to NASA, “The current warming trend is of particular significance because most of it is extremely likely (greater than 95 percent probability) to be the result of human activity since the mid-20th century and proceeding at a rate that is unprecedented over decades to millennia.” 
A 2008 study comparing data from tree rings, ice cores, and corals over the past millennium created the famous “hockey stick” graph showing a steady trend in the earth’s temperature over the last 1,700 years followed by a steep jump in the previous decade (forming a shape like a hockey stick).  Berkeley scientists found that the average temperature of the earth’s land increased 2.5°F over 250 years (1750-2000), 1.5°F of which “appears likely” to be attributable to humans over the past 50 years. Read More
Natural changes in the sun’s activity cannot explain 20th century global warming.
The amount of solar energy received by the earth goes up and down in cycles, but overall there is no net change since the 1950s. There has, however, been a big increase in global temperatures that is too large to attribute to the sun. For this reason, NASA and other scientists say the sun is not responsible for global warming.  The sun has had only a minor effect on the Northern Hemisphere climate over the past 1,000 years, and global warming from human-produced greenhouse gases has been the primary cause of climate change since 1900.  A study found that solar activity could not have contributed to more than 10% of the observed global warming over the 20th century. 
Global warming caused by human-produced greenhouse gases is causing the Arctic ice cap to melt at an increasing rate.
From 1953–2006, Arctic sea ice declined 7.8% per decade. Between 1979 and 2006, the decline was 9.1% each decade.  By 2019, Arctic sea ice was being lost at a rate of 12.9% per decade.  As the Arctic ice cover continues to decrease, the amount of the sun’s heat reflected by the ice back into space also decreases. This positive-feedback loop amplifies global warming at a rate even faster than previous climate models had predicted.  Some studies predicted the Arctic could become nearly ice free sometime between 2020-2060. Read More
Sea levels are rising at an unprecedented rate due to human activities.
Sea levels rise due to thermal expansion of warming ocean waters and melt water from receding glaciers and the polar ice cap.  According to the IPCC, there has been a “substantial” human contribution to the global mean sea-level rise since the 1970s.  As much as 87% of the rise in sea levels since 1970 resulted from human activities such as burning fossil fuels. 
A study found that “significant acceleration” of sea-level rise occurred from 1870 to 2004.  Between 1961 and 2003, global sea levels rose 8 inches a 2019 UN report said they could rise by 3 feet in the next 80 years, displacing hundreds of millions of people.   A study published in the Proceedings of the National Academy of Sciences concluded that the rate of sea level rise over the past century is unprecedented over the last 6,000 years.  Read More
Ocean acidity levels are increasing at an unprecedented rate that can only be explained by human activity.
As excess human-produced CO2 in the atmosphere is absorbed by the oceans, the acidity level of the water increases. Acidity levels in the oceans are 25-30% higher than prior to human fossil fuel use.  The US Government Accountability Office (GAO) said oceans have absorbed about 30% of the CO2 emitted by humans over the past 200 years, and ocean acidity could rise approximately 100-200 percent above preindustrial levels by 2100. 
The World Meteorological Organization said the current acceleration in the rate of ocean acidification “appears unprecedented” over the last 300 million years.  High ocean acidity levels threaten marine species,  and slows the growth of coral reefs.  The Convention on Biological Diversity said “it is now nearly inevitable” that within 50-100 years continued human-produced CO2 emissions will increase ocean acidity to levels that harm marine organisms and ecosystems. Read More
Ocean temperatures are rising at an unprecedented rate due to anthropogenic global warming.
Peter Gleckler, PhD, a climate scientist at Lawrence Livermore National Laboratory, said, “The bottom line is that… most of the observed global ocean warming over the past 50 years is attributable to human activities.”  The IPCC stated in a report that due to human-caused global warming, it is “virtually certain” (99-100% probability) that the upper ocean warmed between 1971 and 2010.  The oceans absorb more than 90% of the heat generated by human-caused global warming.  Since 1970, the upper ocean (above 700 meters) has been warming 24-55% faster than previous studies had predicted. 
Warmer ocean waters can harm coral reefs and impact many species including krill, which are vital to the marine food chain and which reproduce significantly less in warmer water.  Warming oceans also contribute to sea level rise due to thermal expansion, and can add to the intensity of storm systems. 
Glaciers are melting at unprecedented rates due to global warming, causing additional climate changes.
About a quarter of the globe’s glacial loss from 1851-2010, and approximately two thirds of glacial loss between 1991-2010, is attributable directly to global warming caused by human-produced greenhouse gases.  According to the National Snow and Ice Data Center, global warming from human-produced greenhouse gases is a primary cause of the “unprecedented” retreat of glaciers around the world since the early 20th century. 
Since 1980, glaciers worldwide have lost nearly 40 feet (12 meters) in average thickness.  According to an IPCC report, “glaciers have continued to shrink almost worldwide” over the prior two decades, and there is “high confidence” (about an 8 out of 10 chance) that Northern Hemisphere spring snow continues to decrease.  If the glaciers forming the Greenland ice sheet were to melt entirely, global sea levels could increase by up to 20 feet. Read More
Human-caused global warming is changing weather systems and making heat waves and droughts more intense and more frequent.
A National Climate Assessment report said human-caused climate changes, such as increased heat waves and drought, “are visible in every state.”  The American Meteorological Society found that anthropogenic climate change “greatly increased” (up to 10 times) the risk for extreme heat waves.  Globally, 75% of extremely hot days are attributable to warming caused by human activity.  A World Weather Attribution study found that anthropogenic climate change increased the likelihood of wildfires such as the ones that raged across Australia in 2019-2020 by at least 30% since 1900. 
Dramatic changes in precipitation, such as heavier storms and less snow, are another sign that humans are causing global climate change.
As human-produced greenhouse gases heat the planet, increased humidity (water vapor in the atmosphere) results. Water vapor is itself a greenhouse gas.  In a process known as a positive feedback loop, more warming causes more humidity which causes even more warming.  Higher humidity levels also cause changes in precipitation. According to a report published in the Proceedings of the National Academy of Sciences , the recorded changes in precipitation over land and oceans “are unlikely to arise purely due to natural climate variability.” 
According to researchers at the Scripps Institution of Oceanography, up to 60% of the changes in river flow, winter air temperature, and snow pack in the western United States (1950-1999) were human-induced.  Since 1991, heavy precipitation events have been 30% above the 1901-1960 average in the Northeast, Midwest, and upper Great Plains regions.  A study found that global warming caused by human actions has increased extreme precipitation events by 18% across the globe, and that if temperatures continue to rise an increase of 40% can be expected. Read More
Permafrost is melting at unprecedented rates due to global warming, causing further climate changes.
According to the IPCC, there is “high confidence” (about an 8 out of 10 chance) that anthropogenic global warming is causing permafrost, a subsurface layer of frozen soil, to melt in high-latitude regions and in high-elevation regions.  As permafrost melts it releases methane, a greenhouse gas that absorbs 84 times more heat than CO2 for the first 20 years it is in the atmosphere, creating even more global warming in a positive feedback loop.  
By the end of the 21st century, warming temperatures in the Arctic will cause a 30%-70% decline in permafrost.  As human-caused global warming continues, Arctic air temperatures are expected to increase at twice the global rate, increasing the rate of permafrost melt, changing the local hydrology, and impacting critical habitat for native species and migratory birds.  According to the 2014 National Climate Assessment, some climate models suggest that near-surface permafrost will be “lost entirely” from large parts of Alaska by the end of the 21st century. Read More
Many scientists disagree that human activity is primarily responsible for global climate change.
A report found more than 1,000 scientists who disagreed that humans are primarily responsible for global climate change.  The claim that 97% of scientists agree on the cause of global warming is inaccurate. The research on 11,944 studies actually found that only 3,974 even expressed a view on the issue. Of those, just 64 (1.6%) said humans are the main cause. 
A Purdue University survey found that 47% of climatologists challenge the idea that humans are primarily responsible for climate change and instead believe that climate change is caused by an equal combination of humans and the environment (37%), mostly by the environment (5%), or that there’s not enough information to say (5%). Read More
Earth’s climate has always warmed and cooled, and the 20th century rise in global temperature is within the bounds of natural temperature fluctuations over the past 3,000 years.
Although the planet has warmed 1-1.4°F over the 20th century, it is within the +/- 5°F range of the past 3,000 years.  A study by researchers at the Harvard-Smithsonian Center for Astrophysics found that “many records reveal that the 20th century is probably not the warmest nor a uniquely extreme climatic period of the last millennium.” 
A study published in Nature found that “high temperatures – similar to those observed in the twentieth century before 1990 – occurred around AD 1000 to 1100” in the Northern Hemisphere.  A study published in Boreas found that summer temperatures during the Roman Empire and Medieval periods were “consistently higher” than temperatures during the 20th century. Read More
Rising levels of atmospheric CO2 do not necessarily cause global warming.
Earth’s climate record shows that warming has preceded, not followed, a rise in CO2. According to a study published in Science, measurements of ice core samples showed that over the last four climactic cycles (past 240,000 years), periods of natural global warming preceded global increases in CO2.  The Proceedings of the National Academy of Sciences published a study of the earth’s climate 460-445 million years ago which found that an intense period of glaciation, not warming, occurred when CO2 levels were 5 times higher than they are today.  According to ecologist and former Director of Greenpeace International Patrick Moore, PhD, “there is some correlation, but little evidence, to support a direct causal relationship between CO2 and global temperature through the millennia.” Read More
Human-produced CO2 is re-absorbed by oceans, forests, and other “carbon sinks,” negating any climate changes.
A paper published in Asia-Pacific Journal of Atmospheric Sciences found that some climate models overstated how much warming would occur from additional C02 emissions.  About 50% of the CO2 released by the burning of fossil fuels and other human activities has already been re-absorbed by the earth’s carbon sinks.  From 2002-2011, 26% of human-caused CO2 emissions were absorbed specifically by the world’s oceans.  A study published in the Proceedings of the National Academy of Sciences found evidence that forests are increasing their growth rates in response to elevated levels of CO2,  which will in turn, lower atmospheric CO2 levels.
CO2 is so saturated in earth’s atmosphere that more CO2, manmade or natural, will have little impact on the climate.
As CO2 levels in the atmosphere rise, the amount of additional warming caused by the increased concentration becomes less and less pronounced.  According to Senate testimony by William Happer, PhD, Professor of Physics at Princeton University, “[a]dditional increments of CO2 will cause relatively less direct warming because we already have so much CO2 in the atmosphere that it has blocked most of the infrared radiation that it can. The technical jargon for this is that the CO2 absorption band is nearly ‘saturated’ at current CO2 levels.” 
According to the Heartland Institute’s 2013 Nongovernmental International Panel on Climate Change (NIPCC) report, “it is likely rising atmospheric CO2 concentrations will have little impact on future climate.” Read More
Global warming and cooling are primarily caused by fluctuations in the sun’s heat (solar forcing), not by human activity.
According to a study published in the Journal of Atmospheric and Solar-Terrestrial Physics , 50-70% of warming throughout the 20th century could be associated with an increased amount of solar activity.  Between 1900 and 2000 solar irradiance increased 0.19%, and correlated with the rise in US surface temperatures over the 20th century. 
A study published in Energy & Environment wrote, “variations in solar activity and not the burning of fossil fuels are the direct cause of the observed multiyear variations in climatic responses.”  In a study by Willie Soon, PhD, Physicist at the Harvard-Smithsonian Center for Astrophysics, a strong correlation between solar radiation and temperatures in the Arctic over the past 130 years was identified. 
The rate of global warming has slowed over the last decade even though atmospheric CO2 continues to increase.
The Heartland Institute’s 2013 NIPCC report stated that the earth “has not warmed significantly for the past 16 years despite an 8% increase in atmospheric CO2.”  According to Emeritus Professor of Meteorology at the Massachusetts Institute of Technology Richard Lindzen, PhD, the IPCC’s “excuse for the absence of warming over the past 17 years is that the heat is hiding in the deep ocean. However, this is simply an admission that the [climate] models fail to simulate the exchanges of heat between the surface layers and the deeper oceans” Read More
Sea levels have been steadily rising for thousands of years, and the increase has nothing to do with humans.
A report by the Global Warming Policy Foundation found that a slow global sea level rise has been ongoing for the last 10,000 years.  When the earth began coming out of the Pleistocene Ice Age 18,000 years ago, sea levels were about 400 feet lower than they are today and have been steadily rising ever since. 
According to Professor of Earth and Atmospheric Sciences at the Georgia Institute of Technology, Judith Curry, PhD, “it is clear that natural variability has dominated sea level rise during the 20th century, with changes in ocean heat content and changes in precipitation patterns.” Read More
The acidity levels of the oceans are within past natural levels and the current rise in acidity is a natural fluctuation.
 The pH of average ocean surface water is 8.1 and has only decreased 0.1 since the beginning of the industrial revolution (neutral is pH 7, acid is below pH 7).  Science published a study of ocean acidity levels over the past 15 million years, finding that the “samples record surface seawater pH values that are within the range observed in the oceans today.” 
Increased atmospheric CO2 absorbed by the oceans results in higher rates of photosynthesis and faster growth of ocean plants and phytoplankton, which increases pH levels keeping the water alkaline, not acidic.  According to the Science and Public Policy Institute, “our harmless emissions of trifling quantities of carbon dioxide cannot possibly acidify the oceans.” Read More
A lot of climate change fears are based on predictions and inadequate or flawed computer climate models.
Climate models have been unable to simulate major known features of past climate such as the ice ages or the very warm climates of the Miocene, Eocene, and Cretaceous periods. If models cannot replicate past climate changes they should not be trusted to predict future climate changes.  A Asia-Pacific Journal of Atmospheric Science study using observational data rather than computer climate models concluded that “the models are exaggerating climate sensitivity” and overestimate how fast the earth will warm as CO2 levels increase. 
Two other studies using observational data found that IPCC projections of future global warming are too high.   Climatologist and former NASA scientist Roy Spencer, PhD, concluded that 95% of climate models have “over-forecast the warming trend since 1979.”  According to Emeritus Professor of Geography at the University of Winnipeg, Tim Ball, PhD, “IPCC computer climate models are the vehicles of deception… [T]hey create the results they are designed to produce.” 
Glaciers have been growing and receding for thousands of years due to natural causes, not human activity.
The IPCC predicted that Himalayan glaciers would likely melt away by 2035, a prediction they disavowed in 2010.  In 2014 a study of study of 2,181 Himalayan glaciers from 2000-2011 showed that 86.6% of the glaciers were not receding. 
A study of ice cores published in Nature Geoscience said the current melting of glaciers in Western Antarctica was due to “atmospheric circulation changes” that have “caused rapid warming over the West Antarctic Ice Sheet” and cannot be directly attributed to human caused climate change.  According to one of the study authors, “[i]f we could look back at this region of Antarctica in the 1940s and 1830s, we would find that the regional climate would look a lot like it does today, and I think we also would find the glaciers retreating much as they are today.”  According to Christian Schlüchter, Professor of Geology at the University of Bern, the retreat of glaciers in the Alps began in the mid-19th century, before large amounts of human caused CO2 had entered the atmosphere. Read More
Deep ocean currents, not human activity, are a primary driver of natural climate warming and cooling cycles.
Over the 20th century there have been two Arctic warming periods with a cooling period (1940-1970) in between. According to a study in Geophysical Research Letters , natural shifts in the ocean currents are the major cause of these climate changes, not human-generated greenhouse gases.  William Gray, PhD, Emeritus Professor of Atmospheric Science at Colorado State University, said most of the climate changes over the last century are natural and “due to multi-decadal and multi-century changes in deep global ocean currents.” 
Global cooling from 1940 to the 1970s, and warming from the 1970s to 2008, coincided with fluctuations in ocean currents and cloud cover driven by the Pacific Decadal Oscillation (PDO) – a naturally occurring rearrangement in atmospheric and oceanic circulation patterns.  According to Don Easterbrook, PhD, Professor Emeritus of Geology at Western Washington University, the “PDO cool mode has replaced the warm mode in the Pacific Ocean, virtually assuring us of about 30 years of global cooling, perhaps much deeper than the global cooling from about 1945 to 1977.” Read More
Increased hurricane activity and other extreme weather events are a result of natural weather patterns, not human-caused climate change.
According to a report from the Tropical Meteorology Project at Colorado State University, the increase in human-produced CO2 over the past century has had “little or no significant effect” on global tropical cyclone activity. The report stated that specific hurricanes, including Sandy, Ivan, Katrina, Rita, Wilma, and Ike, were not a direct consequence of human-caused global warming.  Between 1995-2015, increased hurricane activity (including Katrina) was recorded however, according to the NOAA, this was the result of cyclical tropical cyclone patterns driven primarily by natural ocean currents. 
Professor of Earth and Atmospheric Sciences at the Georgia Institute of Technology, Judith Curry, PhD, stated that she was “unconvinced by any of the arguments that I have seen that attributes a single extreme weather event, a cluster of extreme weather events, or statistics of extreme weather events” to human-caused climate change.  Experts have noted that many factors beyond climate change are to blame for events such as wildfires, including failed policies on clearing brush, too much population density, and people who set the fires either deliberately or through carelessness. Read More
Climate Change: How Do We Know?
This graph, based on the comparison of atmospheric samples contained in ice cores and more recent direct measurements, provides evidence that atmospheric CO2 has increased since the Industrial Revolution. (Credit: Luthi, D., et al.. 2008 Etheridge, D.M., et al. 2010 Vostok ice core data/J.R. Petit et al. NOAA Mauna Loa CO2 record.) Find out more about ice cores (external site).
Earth's climate has changed throughout history. Just in the last 650,000 years there have been seven cycles of glacial advance and retreat, with the abrupt end of the last ice age about 11,700 years ago marking the beginning of the modern climate era &mdash and of human civilization. Most of these climate changes are attributed to very small variations in Earth&rsquos orbit that change the amount of solar energy our planet receives.
The current warming trend is of particular significance because most of it is extremely likely (greater than 95% probability) to be the result of human activity since the mid-20 th century and proceeding at a rate that is unprecedented over millennia. 1
Earth-orbiting satellites and other technological advances have enabled scientists to see the big picture, collecting many different types of information about our planet and its climate on a global scale. This body of data, collected over many years, reveals the signals of a changing climate.
The heat-trapping nature of carbon dioxide and other gases was demonstrated in the mid-19th century. 2 Their ability to affect the transfer of infrared energy through the atmosphere is the scientific basis of many instruments flown by NASA. There is no question that increased levels of greenhouse gases must cause Earth to warm in response.
Ice cores drawn from Greenland, Antarctica, and tropical mountain glaciers show that Earth&rsquos climate responds to changes in greenhouse gas levels. Ancient evidence can also be found in tree rings, ocean sediments, coral reefs, and layers of sedimentary rocks. This ancient, or paleoclimate, evidence reveals that current warming is occurring roughly ten times faster than the average rate of ice-age-recovery warming. Carbon dioxide from human activity is increasing more than 250 times faster than it did from natural sources after the last Ice Age. 3