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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, hydrocarbon gas liquids, 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 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 2018, carbon dioxide (CO2) emissions from fossil fuel combustion (burning) for energy were equal to about 75% of total U.S. anthropogenic GHG emissions (based on 100-year global warming potential) and about 93% of total U.S. anthropogenic CO2 emissions. Carbon dioxide emissions from other anthropogenic sources and activities were about 7% of total CO2 emissions and about 6% of total GHG 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 (N2O), 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 19% of total U.S. anthropogenic GHG emissions in 2018.

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In 2018, fossil fuels were the source of about 75% 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 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.

Nearly half of U.S. energy-related CO2 emissions are from petroleum use

In 2019, about 46% of U.S. energy-related CO2 emissions came from burning petroleum fuels, 33% came from burning natural gas, and 21% came from burning coal. Although the industrial sector is the largest consumer of energy (including direct fuel use and electricity purchases from the electric power sector), the transportation sector emits more CO2 because of its near complete dependence on petroleum fuels.

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The residential and commercial sectors have lower CO2 emissions levels than the transportation sector and the industrial sector. Most of the CO2 emissions associated with energy use by the residential and commercial sectors can be attributed to fossil fuel combustion by the electric power sector to produce the electricity that it sells to end users in those sectors. Emissions from combustion of the fossil fuel components of primary energy consumption by the electric power sector are allocated to each end-use sector's share of total electricity sales to all sectors.

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Coal is the dominant CO2 emissions source related to electricity generation

In 2019, the electric power sector accounted for about 37% of total U.S. primary energy consumption and produced 32% of total U.S. energy-related CO2 emissions. Coal accounted for 60% and natural gas for 38% 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: 1990-2018, April 2020. Includes U.S. Territories.

2 U.S. Energy Information Administration, Monthly Energy Review, Environment, July 2020, preliminary data.

Last updated: August 11, 2020