Overview of Greenhouse Gases

U.S. Greenhouse Gas Emissions in 2013

Total Emissions in 2013 = 6,673 Million Metric Tons of CO2equivalent

Gases that trap heat in the atmosphere are called greenhouse gases. This section provides information on emissions and removals of the main greenhouse gases to and from the atmosphere. For more information on the science of climate change and other climate forcers, such as black carbon, please visit the Climate Change Science Home Page.

• Carbon dioxide (CO2) : Carbon dioxide enters the atmosphere through burning fossil fuels (coal, natural gas and oil), solid waste, trees and wood products, and also as a result of certain chemical reactions (e.g., manufacture of cement). Carbon dioxide is removed from the atmosphere (or "sequestered") when it is absorbed by plants as part of the biological carbon cycle.
• Methane (CH4) : Methane is emitted during the production and transport of coal, natural gas, and oil. Methane emissions also result from livestock and other agricultural practices and by the decay of organic waste in municipal solid waste landfills.
• Nitrous oxide (N2O) : Nitrous oxide is emitted during agricultural and industrial activities, as well as during combustion of fossil fuels and solid waste.
• Fluorinated gases : Hydrofluorocarbons, perfluorocarbons, sulfur hexafluoride, and nitrogen trifluoride are synthetic, powerful greenhouse gases 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). These gases are typically emitted in smaller quantities, but because they are potent greenhouse gases, they are sometimes referred to as HighGlobal Warming Potential gases ("High GWP gases").

Each gas's effect on climate change depends on three main factors:

How much of these gases are in the atmosphere?

Concentration, or abundance, is the amount of a particular gas in the air. Larger emissions of greenhouse gases lead to higher concentrations in the atmosphere. Greenhouse gas concentrations are measured in parts per million, parts per billion, and even parts per trillion. One part per million is equivalent to one drop of water diluted into about 13 gallons of liquid (roughly the fuel tank of a compact car). To learn more about the increasing concentrations of greenhouse gases in the atmosphere, visit the Causes of Climate Change and the Climate Change Indicators Atmospheric Concentrations of Greenhouse Gases pages.

How long do they stay in the atmosphere?

Each of these gases can remain in the atmosphere for different amounts of time, ranging from a few years to thousands of years. All of these gases remain in the atmosphere long enough to become well mixed, meaning that the amount that is measured in the atmosphere is roughly the same all over the world, regardless of the source of the emissions.

How strongly do they impact global temperatures?

Some gases are more effective than others at making the planet warmer and "thickening the Earth's blanket."

For each greenhouse gas, a Global Warming Potential (GWP) has been calculated to reflect how long it remains in the atmosphere, on average, and how strongly it absorbs energy. Gases with a higher GWP absorb more energy, per pound, than gases with a lower GWP, and thus contribute more to warming Earth.

Note: All emission estimates are from the Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2013.

Carbon dioxide (CO2) is the primary greenhouse gas emitted through human activities. In 2013, CO2 accounted for about 82% of all U.S. greenhouse gas emissions from human activities. Carbon dioxide is naturally present in the atmosphere as part of the Earth's carbon cycle (the natural circulation of carbon among the atmosphere, oceans, soil, plants, and animals). Human activities are altering the carbon cycle—both by adding more CO2 to the atmosphere and by influencing the ability of natural sinks, like forests, to remove CO2 from the atmosphere. While CO2 emissions come from a variety of natural sources, human-related emissions are responsible for the increase that has occurred in the atmosphere since the industrial revolution. [1]

U.S. Carbon Dioxide Emissions, By Source

Note: All emission estimates from the Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2013.

The main human activity that emits CO2 is the combustion of fossil fuels (coal, natural gas, and oil) for energy and transportation, although certain industrial processes and land-use changes also emit CO2. The main sources of CO2 emissions in the United States are described below.

• Electricity. Electricity is a significant source of energy in the United States and is used to power homes, business, and industry. The combustion of fossil fuels to generate electricity is the largest single source of CO2 emissions in the nation, accounting for about 37% of total U.S. CO2emissions and 31% of total U.S. greenhouse gas emissions in 2013. The type of fossil fuel used to generate electricity will emit different amounts of CO2. To produce a given amount of electricity, burning coal will produce more CO2than oil or natural gas.
• Transportation. The combustion of fossil fuels such as gasoline and diesel to transport people and goods is the second largest source of CO2 emissions, accounting for about 31% of total U.S. CO2 emissions and 26% of total U.S.greenhouse gas emissions in 2013. This category includes transportation sources such as highway vehicles, air travel, marine transportation, and rail.
• Industry. Many industrial processes emit CO2 through fossil fuel combustion. Several processes also produce CO2 emissions through chemical reactions that do not involve combustion, for example, the production and consumption of mineral products such as cement, the production of metals such as iron and steel, and the production of chemicals. Fossil fuel combustion from various industrial processes accounted for about 15% of total U.S. CO2 emissions and 12% of total U.S.greenhouse gas emissions in 2013. Note that many industrial processes also use electricity and therefore indirectly cause the emissions from the electricity production.

Carbon dioxide is constantly being exchanged among the atmosphere, ocean, and land surface as it is both produced and absorbed by many microorganisms, plants, and animals. However, emissions and removal of CO2 by these natural processes tend to balance. Since the Industrial Revolution began around 1750, human activities have contributed substantially to climate change by adding CO2 and other heat-trapping gases to the atmosphere.

In the United States, since 1990, the management of forests and non-agricultural land has acted as a net sink of CO2, which means that more CO2 is removed from the atmosphere, and stored in plants and trees, than is emitted. This sink offset about 13% of total emissions in 2013 and is discussed in more detail in the Land Use, Land-Use Change, and Forestry section.

To find out more about the role of CO2 warming the atmosphere and its sources, visit the Causes of Climate Change page and the Greenhouse Gas Indicators page in the Science section.

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Emissions and Trends

Carbon dioxide (CO2) emissions in the United States increased by about 7% between 1990 and 2013. Since the combustion of fossil fuel is the largest source of greenhouse gas emissions in the United States, changes in emissions from fossil fuel combustion have historically been the dominant factor affecting total U.S. emission trends. Changes in CO2 emissions from fossil fuel combustion are influenced by many long-term and short-term factors, including population growth, economic growth, changing energy prices, new technologies, changing behavior, and seasonal temperatures. Between 1990 and 2013, the increase in CO2 emissions corresponded with increased energy use by an expanding economy and population, and an overall growth in emissions from electricity generation. Transportation emissions also contributed to the 7% increase, largely due to an increase in miles traveled by motor vehicles.

U.S. Carbon Dioxide Gas Emissions, 1990-2013

Note: All emission estimates from the Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2013.

Going forward, CO2 emissions in the United States are projected to grow by about 1.5% between 2005 and 2020. [2]

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Reducing Carbon Dioxide Emissions

The most effective way to reduce carbon dioxide (CO2) emissions is to reduce fossil fuel consumption. Many strategies for reducing CO2 emissions from energy are cross-cutting and apply to homes, businesses, industry, and transportation.

EPA is taking common sense regulatory actions to reduce greenhouse gas emissions from our nation's largest sources, including power plants and motor vehicles.

• Learn about what EPA is doing to reduce carbon pollution from power plants.
• Learn about EPA’s motor vehicle standards.
• Learn more about EPA’s regulatory initiatives to reduce greenhouse gas emissions.

Examples of Reduction Opportunities for Carbon Dioxide

Strategy	Examples of How Emissions Can be Reduced
Energy Efficiency	Improving the insulation of buildings, traveling in more fuel-efficient vehicles, and using more efficient electrical appliances are all ways to reduce energy consumption, and thus CO2 emissions. See EPA's ENERGY STAR® program for more information on energy-efficient appliances. See EPA's and DOE's fueleconomy.gov site for more information on fuel-efficient vehicles. Learn about EPA's motor vehicle standards that improve vehicle efficiency and save drivers money.
Energy Conservation	Reducing personal energy use by turning off lights and electronics when not in use reduces electricity demand. Reducing distance traveled in vehicles reduces petroleum consumption. Both are ways to reduce energy CO2 emissions through conservation. Learn more about What You Can Do at Home, at School, in the Office, and on the Road to save energy and reduce your carbon footprint.
Fuel Switching	Producing more energy from renewable sources and using fuels with lower carbon contents are ways to reduce carbon emissions.
Carbon Capture and Sequestration	Carbon dioxide capture and sequestration is a set of technologies that can potentially greatly reduce CO2 emissions from new and existing coal- and gas-fired power plants, industrial processes, and other stationary sources of CO2. Learn more.

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*Carbon dioxide's lifetime is poorly defined because the gas is not destroyed over time, but instead moves among different parts of the ocean–atmosphere–land system. Some of the excess carbon dioxide will be absorbed quickly (for example, by the ocean surface), but some will remain in the atmosphere for thousands of years, due in part to the very slow process by which carbon is transferred to ocean sediments.

References

1. NRC (2010). Advancing the Science of Climate Change .  National Research Council. The National Academies Press, Washington, DC, USA.

2. U.S. Department of State (2007). Fourth Climate Action Report to the UN Framework Convention on Climate Change:Projected Greenhouse Gas Emissions. U.S. Department of State, Washington, DC, USA.

Methane (CH4) is the second most prevalent greenhouse gas emitted in the United States from human activities. In 2013, CH4 accounted for about 10% of all U.S. greenhouse gas emissions from human activities. Methane is emitted by natural sources such as wetlands, as well as human activities such as leakage from natural gas systems and the raising of livestock. Natural processes in soil and chemical reactions in the atmosphere help remove CH4 from the atmosphere. Methane's lifetime in the atmosphere is much shorter than carbon dioxide (CO2), but CH4 is more efficient at trapping radiation than CO2. Pound for pound, the comparative impact of CH4 on climate change is 25 times greater than CO2 over a 100-year period.

Globally, over 60% of total CH4 emissions come from human activities. [1] Methane is emitted from industry, agriculture, and waste management activities, described below.

• Industry. Natural gas and petroleum systems are the largest source of CH4 emissions from industry in the United States. Methane is the primary component of natural gas. Some CH4 is emitted to the atmosphere during the production, processing, storage, transmission, and distribution of natural gas. Because gas is often found alongside petroleum, the production, refinement, transportation, and storage of crude oil is also a source of CH4 emissions. For more information, see the Inventory of U.S. Greenhouse Gas Emissions and Sinks sections on Natural Gas Systems and Petroleum Systems.
• Agriculture. Domestic livestock such as cattle, buffalo, sheep, goats, and camels produce large amounts of CH4 as part of their normal digestive process. Also, when animals' manure is stored or managed in lagoons or holding tanks, CH4 is produced. Because humans raise these animals for food, the emissions are considered human-related. Globally, the Agriculture sector is the primary source of CH4 emissions. For more information, see the Inventory of U.S. Greenhouse Gas Emissions and Sinks Agriculture chapter.
• Waste from Homes and Businesses. Methane is generated in landfills as waste decomposes and in the treatment of wastewater. Landfills are the third largest source of CH4 emissions in the United States. For more information see the U.S. Inventory's Waste chapter.

Methane is also emitted from a number of natural sources. Wetlands are the largest source, emitting CH4 from bacteria that decompose organic materials in the absence of oxygen. Smaller sources include termites, oceans, sediments, volcanoes, and wildfires.

To find out more about the role of CH4 in warming the atmosphere, and its sources, visit the Causes of Climate Changepage and the Greenhouse Gas Indicators page in the Science section.

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Emissions and Trends

Methane (CH4) emissions in the United States decreased by almost 15% between 1990 and 2013. During this time period, emissions increased from sources associated with agricultural activities, while emissions decreased from sources associated with the exploration and production of natural gas and petroleum products.

U.S. Methane Emissions, 1990-2013

Note: All emission estimates from the Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2013. These estimates use aglobal warming potential for methane of 25, based on reporting requirements under the United Nations Framework Convention on Climate Change.

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Reducing Methane Emissions

There are a number of ways to reduce methane (CH4) emissions. Some examples are discussed below. EPA has a series ofvoluntary programs for reducing CH4 emissions, and is supporting the President’s Strategy to Reduce Methane Emissions (PDF) (15 pp, 1.88MB).

Examples of Reduction Opportunities for Methane

Emissions Source	How Emissions Can be Reduced
Industry	Upgrading the equipment used to produce, store, and transport oil and gas can reduce many of the leaks that contribute to CH4 emissions. Methane from coal mines can also be captured and used for energy. Learn more about the EPA'sNatural Gas STAR Program and Coalbed Methane Outreach Program.
Agriculture	Methane can be reduced and captured by altering manure management strategies at livestock operations or animal feeding practices. Learn more about these strategies and EPA's AgSTAR Program.
Waste from Homes and Businesses	Because CH4 emissions from landfill gas are a major source of CH4 emissions in the United States, emission controls that capture landfill CH4 are an effective reduction strategy. Learn more about these opportunities and the EPA's Landfill Methane Outreach Program.

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References

1. EPA (2010). Methane and Nitrous Oxide Emissions from Natural Sources . U.S. Environmental Protection Agency, Washington, DC, USA.

2. U.S. Department of State (2007). Projected Greenhouse Gas Emissions. In: Fourth Climate Action Report to the UN Framework Convention on Climate Change . U.S. Department of State, Washington, DC, USA.

In 2013, nitrous oxide (N2O) accounted for about 5% of all U.S. greenhouse gas emissions from human activities. Nitrous oxide is naturally present in the atmosphere as part of the Earth's nitrogen cycle, and has a variety of natural sources. However, human activities such as agriculture, fossil fuel combustion, wastewater management, and industrial processes are increasing the amount of N2O in the atmosphere. Nitrous oxide molecules stay in the atmosphere for an average of 114 years before being removed by a sink or destroyed through chemical reactions. The impact of 1 pound of N2O on warming the atmosphere is almost 300 times that of 1 pound of carbon dioxide.

Globally, about 40% of total N2O emissions come from human activities. [1] Nitrous oxide is emitted from agriculture, transportation, and industry activities, described below.

• Agriculture. Nitrous oxide is emitted when people add nitrogen to the soil through the use of synthetic fertilizers. Agricultural soil management is the largest source of N2O emissions in the United States, accounting for about 74% of total U.S. N2O emissions in 2013. Nitrous oxide is also emitted during the breakdown of nitrogen in livestock manure and urine, which contributed to 5% of N2O emissions in 2013.
• Transportation. Nitrous oxide is emitted when transportation fuels are burned. Motor vehicles, including passenger cars and trucks, are the primary source of N2O emissions from transportation. The amount of N2O emitted from transportation depends on the type of fuel and vehicle technology, maintenance, and operating practices.
• Industry. Nitrous oxide is generated as a byproduct during the production of nitric acid, which is used to make synthetic commercial fertilizer, and in the production of adipic acid, which is used to make fibers, like nylon, and other synthetic products.

Nitrous oxide emissions occur naturally through many sources associated with the nitrogen cycle, which is 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 N2O. Natural emissions of N2O are mainly from bacteria breaking down nitrogen in soils and the oceans. Nitrous oxide is removed from the atmosphere when it is absorbed by certain types of bacteria or destroyed by ultraviolet radiation or chemical reactions.

To find out more about the role of N2O in warming the atmosphere and its sources, visit the Causes of Climate Changepage and the Greenhouse Gas Indicators page in the Science section.

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Emissions and Trends

Nitrous oxide (N2O) emissions in the United States have increased by about 8% between 1990 and 2013. This increase in emissions is due in part to annual variation in agricultural soil emissions, and an increase in emissions from the electric power sector. Nitrous oxide emissions from agricultural soils have varied during this period and were about 18% higher in 2013 than in 1990.

Going forward, N2O emissions are projected to increase by 5% between 2005 and 2020, driven largely by increases in emissions from agricultural activities. [2]

U.S. Nitrous Oxide Emissions, 1990-2013

Note: All emission estimates from the Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2013.

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Reducing Nitrous Oxide Emissions

There are a number of ways to reduce emissions of nitrous oxide (N2O), discussed below.

Examples of Reduction Opportunities for Nitrous Oxide Emissions

Emissions Source	Examples of How Emissions Can be Reduced
Agriculture	The application of fertilizers accounts for the majority of N2O emissions. Emissions can be reduced by reducing nitrogen-based fertilizer applications and applying fertilizers more efficiently, [3] as well as following better manure management practices.
Transportation	Nitrous oxide is a byproduct of fuel combustion, so reducing mobile fuel consumption in motor vehicles can reduce transportation emissions. Additionally, the introduction of pollution control technologies, such as catalytic converters to reduce exhaust pollutants from passenger cars, can also reduce emissions of N2O.
Industry	Nitrous oxide is generally emitted from industry through fossil fuel combustion so technological upgrades and fuel switching are effective ways to reduce industry emissions of N2O. Production of adipic acid results in N2O emissions that can be reduced through technological upgrades.

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References

1. EPA (2010). Methane and Nitrous Oxide Emissions from Natural Sources (PDF). U.S. Environmental Protection Agency, Washington, DC, USA.

2. U.S. Department of State (2007). Projected Greenhouse Gas Emissions. In: Fourth Climate Action Report to the UN Framework Convention on Climate Change . U.S. Department of State, Washington, DC, USA.

3. EPA (2005). Greenhouse Gas Mitigation Potential in U.S. Forestry and Agriculture . U.S. Environmental Protection Agency, Washington, DC, USA.

Unlike many other greenhouse gases, fluorinated gases have no natural sources and only come from human-related activities. They are emitted through a variety of industrial processes such as aluminum and semiconductor manufacturing. Many fluorinated gases have very high global warming potentials (GWPs) relative to other greenhouse gases, so small atmospheric concentrations can have large effects on global temperatures. They can also have long atmospheric lifetimes--in some cases, lasting thousands of years. Like other long-lived greenhouse gases, fluorinated gases are well-mixed in the atmosphere, spreading around the world after they're emitted. Fluorinated gases are removed from the atmosphere only when they are destroyed by sunlight in the far upper atmosphere. In general, fluorinated gases are the most potent and longest lasting type of greenhouse gases emitted by human activities.

There are four main categories of fluorinated gases--hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), sulfur hexafluoride (SF6), and nitrogen trifluoride (NF3). The largest sources of fluorinated gas emissions are described below.

• Substitution for Ozone-Depleting Substances.Hydrofluorocarbons are used as refrigerants, aerosol propellants, solvents, and fire retardants. The major emissions source of these compounds is their use as refrigerants--for example, in air conditioning systems in both vehicles and buildings. These chemicals were developed as a replacement for chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) because they do not deplete the stratospheric ozone layer. Chlorofluorocarbons and HCFCs are being phased out under an international agreement, called the Montreal Protocol. Unfortunately, HFCs are potent greenhouse gases with long atmospheric lifetimes and high GWPs, and they are released into the atmosphere through leaks, servicing, and disposal of equipment in which they are used.
• Industry. Perfluorocarbons are compounds produced as a by-product of various industrial processes associated with aluminum production and the manufacturing of semiconductors. Like HFCs, PFCs generally have long atmospheric lifetimes and high GWPs. Sulfur hexafluoride is used in magnesium processing and semiconductor manufacturing, as well as a tracer gas for leak detection. HFC-23 is produced as a by-product of HCFC-22 production.
• Transmission and Distribution of Electricity. Sulfur hexafluoride is used in electrical transmission equipment, including circuit breakers. The GWP of SF6 is 22,800, making it the most potent greenhouse gases that the Intergovernmental Panel on Climate Change has evaluated.

To find out more about the role of fluorinated gases in warming the atmosphere, and their sources, visit the Causes of Climate Change page and the Greenhouse Gas Indicators page in the Science section.

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Emissions and Trends

Overall, fluorinated gas emissions in the United States have increased by about 73% between 1990 and 2013. This increase has been driven by a 250% increase in emissions of hydrofluorocarbons (HFCs) since 1990 as they have been widely used as a substitute for ozone-depleting substances. Emissions of perfluorocarbons (PFCs) and sulfur hexafluoride (SF6) have actually declined during this time due to emission reduction efforts in the aluminum production industry (PFCs) and the electricity transmission and distribution industry (SF6).

Going forward, HFC emissions are projected to grow by nearly 140% between 2005 and 2020 as demands for refrigeration continue to grow and as more ozone-depleting substances are replaced. During this same period, emissions of SF6 are expected to decline by over 25%, while emissions of PFCs are projected to remain flat. Since the emissions from HFCs are far greater than the emissions of the other gases on a carbon dioxide-equivalence basis, substantial growth in emissions of F-gases is expected to continue. [1]

U.S. Fluorinated Gas Emissions, 1990-2013

Note: All emission estimates from the Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2013.

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Reducing Fluorinated Gas Emissions

Because most fluorinated gases have a very long atmospheric lifetime, it will take many years to see a noticeable decline in current concentrations. However, there are a number of ways to reduce emissions of fluorinated gases, described below.

Examples of Reduction Opportunities for Fluorinated Gases

Emissions Source	Examples of How Emissions Can be Reduced
Substitution of Ozone-Depleting Substances in Homes and Businesses	Hydrofluorocarbons are used as refrigerants, aerosol propellants, solvents, and fire retardants. Emissions can be reduced by use of substitutes with lower global warming potentials and other technological improvements. Visit EPA's Ozone Layer Protection site to learn more about reduction opportunities in this sector, and EPA's Significant New Alternatives Policy (SNAP) page to learn about available substitutes that pose less overall risk to human health and the environment.
Industry	Industrial users of fluorinated gases can reduce emissions by adopting fluorinated gas recycling and destruction processes, optimizing production to minimize emissions, and replacing these gases with alternatives. The following are EPA programs working to reduce these gases in the Industry sector: Voluntary Aluminum Industrial Partnership (VAIP) The SF6 Emission Reduction Partnership for the Magnesium Industry The PFC Reduction/Climate Partnership for the Semiconductor Industry
Electricity Transmission and Distribution	Sulfur hexafluoride is an extremely potent greenhouse gas that is used for several purposes when transmitting electricity through the power grid. EPA is working with industry to reduce emissions through the SF6Emission Reduction Partnership for Electric Power Systems which promotes leak detection and repair, use of recycling equipment, and employee training.
Transportation	Hydrofluorocarbons (HFCs) are released through the leakage of refrigerants used in vehicle air-conditioning systems. Leakage can be reduced through better system components, and through the use of alternative refrigerants with lower global warming potentials than those presently used. One important way the EPA is working to reduce HFC emissions is through its light-duty and heavy-duty vehicle standards.

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References

1. U.S. Department of State (2007). Projected Greenhouse Gas Emissions In: Fourth Climate Action Report to the UN Framework Convention on Climate Change . U.S. Department of State, Washington, DC, USA.