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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 converting energy sources to energy—by energy source and by energy-consuming sector by month and year.2

Carbon dioxide

According to EPA, in 2021, 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 GHG emissions and for 92% of total U.S. anthropogenic CO2 emissions in the United States. 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 GHGs 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 (N2O), which comes from using nitrogen fertilizers and burning fossil fuels and certain industrial and waste management processes
  • 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 2021.

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In 2021, fossil fuel combustion was 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 is mostly methane (CH4), it has a high hydrogen content, which means 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 the CO2 burning coal produces.

About half of U.S. energy-related CO2 emissions were from petroleum use in 2022

In 2022, 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 accounted for about 32% of U.S. energy consumption and for about 34% of total annual energy-related CO2 emissions. Coal accounted for about 11% of U.S. energy consumption and for about 21% of total annual energy-related CO2 emissions.2

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

Consumption of fossil fuels accounts for most of the energy-related CO2 emissions of the major energy-consuming sectors: commercial, industrial, residential, transportation, and electric power. Although the industrial sector was the highest energy end-use sector in 2022 (includes direct primary energy use and electricity purchases from the electric power sector) sector, 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 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 2022.

U.S. CO2 emissions by source and sector in 2022 graphic.

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Natural gas was the highest source of industrial sector CO2 emissions in 2022, 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 primary CO2 emissions source related to electricity generation

In 2022, the electric power sector accounted for about 38% of total U.S. primary energy consumption and for about 31% of total U.S. energy-related CO2 emissions. Coal accounted for 55% and natural gas for 43% 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: 1990-2021, April 2023. Includes U.S. Territories.

2 U.S. Energy Information Administration, Monthly Energy Review, Energy consumption by sector and Environment, April 2023, preliminary data for 2022.

Last updated: August 22, 2023, with data from sources as indicated; data for 2022 are preliminary.