www.epa.gov /ghgemissions/overview-greenhouse-gases

Overview of Greenhouse Gases | US EPA

28-35 minutes 12/24/2015

On this page:

• Overview
• Carbon Dioxide
• Methane
• Nitrous Oxide
• Fluorinated Gases

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 other climate forcers, such as black carbon, please visit the Climate Change Indicators: Climate Forcing page.

• Carbon dioxide (CO₂): Carbon dioxide enters the atmosphere through burning fossil fuels (coal, natural gas, and oil), solid waste, trees and other biological materials, and also as a result of certain chemical reactions (e.g., cement production). 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, land use, and by the decay of organic waste in municipal solid waste landfills.
• Nitrous oxide (N₂O): Nitrous oxide is emitted during agricultural, land use, and industrial activities; combustion of fossil fuels and solid waste; as well as during treatment of wastewater.
• Fluorinated gases: Hydrofluorocarbons, perfluorocarbons, sulfur hexafluoride, and nitrogen trifluoride are synthetic, powerful greenhouse gases that are emitted from a variety of household, commercial, and industrial applications and processes. Fluorinated gases (especially hydrofluorocarbons) are sometimes used as substitutes for stratospheric ozone-depleting substances (e.g., chlorofluorocarbons, hydrochlorofluorocarbons, and halons). Fluorinated gases are typically emitted in smaller quantities than other greenhouse gases, but they are potent greenhouse gases. With global warming potentials (GWPs) that typically range from thousands to tens of thousands, they are sometimes referred to as high-GWP gases because, for a given amount of mass, they trap substantially more heat than CO₂.

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

How abundant are greenhouse gases 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 Climate Change Indicators: Atmospheric Concentrations of Greenhouse Gases page.

How long do greenhouse gases 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 greenhouse gases impact the atmosphere?

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

For each greenhouse gas, a Global Warming Potential (GWP) was developed to allow comparisons of the global warming impacts of different gases. Specifically, it is a measure of how much energy the emissions of 1 ton of a gas will absorb over a given period of time, typically a 100-year time horizon, relative to the emissions of 1 ton of carbon dioxide (CO₂). Gases with a higher GWP absorb more energy, per ton emitted, 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: 1990–2021. The Inventory uses 100-year GWPs from IPCC’s Fifth Assessment Report (AR5).

Carbon Dioxide Emissions

Carbon dioxide (CO₂) is the primary greenhouse gas emitted through human activities. In 2021, CO₂ accounted for 79% 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 and soils, to remove and store 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.²

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

• Transportation. The combustion of fossil fuels such as gasoline and diesel to transport people and goods was the largest source of CO₂ emissions in 2021, accounting for 35% of total U.S. CO₂ emissions and 28% of total U.S. greenhouse gas emissions. This category includes domestic transportation sources such as highway and passenger vehicles, air travel, marine transportation, and rail.
• Electricity. Electricity is a key source of energy in the United States and is used to power homes, business, and industry. In 2021, the combustion of fossil fuels to generate electricity was the second largest source of CO₂ emissions in the nation, accounting for 31% of total U.S. CO₂ emissions and 24% of total U.S. greenhouse gas emissions. The types of fossil fuel used to generate electricity emit different amounts of CO₂. To produce a given amount of electricity, burning coal will produce more CO₂ than natural gas or oil.
• Industry. Many industrial processes emit CO₂ through fossil fuel consumption. Several processes also produce CO₂ emissions through chemical reactions that do not involve combustion, and examples include the production of mineral products such as cement, the production of metals such as iron and steel, and the production of chemicals. The fossil fuel combustion component of various industrial processes accounted for 15% of total U.S. CO₂ emissions and 12% of total U.S. greenhouse gas emissions in 2021. Many industrial processes also use electricity and therefore indirectly result in CO₂ emissions from electricity generation.

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. Emissions and removals of CO₂ by these natural processes, however, tend to balance over time, absent anthropogenic impacts. 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, the management of forests and other land (e.g., cropland, grasslands, etc.) 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 carbon sink offset about 13% of total emissions in 2021. For more details, see the discussion in the Land Use, Land-Use Change, and Forestry section.

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

Trends

Carbon dioxide emissions in the United States decreased by 2% between 1990 and 2021. 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. In 2021, the increase in CO₂ emissions from fossil fuel combustion corresponded with an increase in energy use as a result of economic activity rebounding after the height of the COVID-19 pandemic, in addition to an increase in coal use in the electric power sector.

Reducing Carbon Dioxide Emissions

The most effective way to reduce 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.

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 use, and thus CO2 emissions. See EPA's ENERGY STAR® program for more information on energy-efficient appliances and ways to save at home and work. 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 (CCS)	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. For example, a CCS project might capture CO2 from the stacks of a coal-fired power plant before it enters the atmosphere, transport the CO2 via pipeline, and inject the CO2 deep underground at a carefully selected and suitable subsurface geologic formation, such as a nearby abandoned oil field, where it is securely stored. Learn more about CCS.
Changes in Uses of Land and Land Management Practices	Learn more about Land Use, Land Use Change and Forestry Sector.

¹ Atmospheric CO₂ is part of the global carbon cycle, and therefore its fate is a complex function of geochemical and biological processes. 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.