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Energy and the environment explained Where greenhouse gases come from

In the United States, most of the emissions of human-caused (anthropogenic) greenhouse gases (GHG) come primarily from burning fossil fuels—coal, natural gas, and petroleum—for energy use. Economic growth (with short-term fluctuations in growth rate) and weather patterns that affect heating and cooling needs are the main factors that drive the amount of energy consumed. Energy prices and government policies can also affect the sources or types of energy consumed.

Sources of GHG emissions estimates

The U.S. Environmental Protection Agency (EPA) publishes estimates for total U.S. greenhouse gas emissions to meet annual U.S. commitments under the United Nations Framework Convention on Climate Change (UNFCCC).1 The U.S. Energy Information Administration (EIA) publishes estimates for energy-related carbon-dioxide (CO2) emissions (emissions resulting from conversion of energy sources/fuels to energy) by fuel source and by fuel consuming sector by month and year.2

Carbon dioxide

In 2020, CO2 emissions accounted for about 79% of total U.S. anthropogenic GHG emissions (based on 100-year global warming potential). Fossil fuel combustion (burning) for energy accounted for 73% of total U.S. GHG emissions and for 92% of total U.S. anthropogenic CO2 emissions. CO2 emissions from other anthropogenic sources and activities were about 6% of total GHG emissions and 8% of total CO2 emissions.

Other greenhouse gases

The U.S. and international GHG emissions estimates include several other GHG that are emitted as a result of human activity:

  • Methane (CH4), which comes from landfills, coal mines, agriculture, and oil and natural gas operations
  • Nitrous oxide (N22O), which comes from using nitrogen fertilizers and certain industrial and waste management processes and burning fossil fuels
  • High global warming potential (GWP) gases, which are human-made industrial gases
    • Hydrofluorocarbons (HFCs)
    • Perfluorocarbons (PFCs)
    • Sulfur hexafluoride (SF6)
    • Nitrogen trifluoride (NF3)

The combined emissions of these other greenhouse gases accounted for about 21% of total U.S. anthropogenic GHG emissions in 2020.

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In 2020, fossil fuels were the source of about 73% of total U.S. human-caused (anthropogenic) greenhouse gas emissions.

The energy connection

Fossil fuels consist mainly of carbon and hydrogen. When fossil fuels are combusted (burned), oxygen combines with carbon to form CO2 and with hydrogen to form water (H2O). These reactions release heat, which we use for energy. The amount of CO2 produced (emitted) depends on the carbon content of the fuel, and the amount of heat produced depends on the carbon and hydrogen content. Because natural gas, which is mostly CH4, has a high hydrogen content, combustion of natural gas produces less CO2 for the same amount of heat produced from burning other fossil fuels. For example, for the same amount of energy produced, burning natural gas produces about half of the amount of CO2 produced by burning coal.

About half of U.S. energy-related CO2 emissions were from petroleum use in 2021

In 2021, petroleum accounted for about 36% of U.S. energy consumption but petroleum was the source of 46% of total annual U.S. energy-related CO2 emissions. Natural gas also provided about 32% U.S. energy and accounted for 34% of total annual energy-related CO2 emissions. Coal was the source of about 12% of U.S. energy use and of about 21% of total annual energy-related CO2 emissions.

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The transportation sector accounts for largest share of U.S. energy-related CO2 emissions

Consumption of fossil fuels accounts for most of the CO2 emissions of the major energy consuming sectors: commercial, industrial, residential, transportation, and electric power. Although the industrial sector was the largest end-use (includes direct primary energy use and electricity purchases from the electric power sector) energy consuming sector in 2021, the transportation sector emitted more CO2 because of its near complete dependence on petroleum fuels.

Emissions by the electric power sector can be allocated to each energy end-use sector according to each end-use sector’s share of total annual electric power retail electricity sales. Even when these electric emissions are apportioned to each sector, the transportation sector accounted for the largest share of U.S. energy end-use CO2 emissions in 2021.

U.S. CO2 emissions by source and sector in 2021 graphic.

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Natural gas was the largest source of industrial sector CO2 emissions in 2021, followed by the sector's associated electric emissions, and then by petroleum and coal consumption. Most of the CO2 emissions associated with energy use by the residential and commercial sectors can be attributed to their associated electric emissions.

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Coal is the dominant CO2 emissions source related to electricity generation

In 2021, the electric power sector accounted for about 38% of total U.S. primary energy consumption and for about 32% of total U.S. energy-related CO2 emissions. Coal accounted for 59% and natural gas for 40% of electric power sector CO2 emissions. Emissions from burning petroleum fuels and non-biomass waste (mainly plastics) in waste-to-energy power plants and emissions from some types of geothermal power plants accounted for about 2% of power sector CO2 emissions.

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1 U.S. Environmental Protection Agency, Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2020, April 2022. Includes U.S. Territories.

2 U.S. Energy Information Administration, Monthly Energy Review, Environment, May 2022, preliminary data.

Last updated: June 24, 2022, with data from sources as indicated.