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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.

Carbon dioxide

In 2017, carbon dioxide (CO2) emissions from burning fossil fuels for energy were equal to about 76% of total U.S. anthropogenic GHG emissions (based on global warming potential) and about 93% of total U.S. anthropogenic CO2 emissions. Carbon dioxide emissions from other anthropogenic sources and activities were about 5% of total GHG emissions and about 6% of total CO2 emissions.1

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 18% of total U.S. anthropogenic GHG emissions in 2017.

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In 2017, fossil fuels were the source of about 76% 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 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 2018, about 45% of U.S. energy-related CO2 emissions came from burning petroleum fuels, 31% came from burning natural gas, and 24% came from burning coal.2 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 sector and the commercial sector 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 the residential and commercial sectors.

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Coal is the dominant CO2 emissions source related to electricity generation

In 2018, the electric power sector accounted for about 38% of U.S. primary energy consumption and produced 33% of total U.S. energy-related CO2 emissions. Coal accounted for 65% and natural gas for 33% 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: 1990-2017, Executive Summary, April 2019. Includes U.S. Territories.

2 U.S. Energy Information Administration, Monthly Energy Review, Environment, May 2019, preliminary data.

Last updated: June 19, 2019