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Frequently Asked Questions (FAQs)

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What are greenhouse gases and how do they affect the climate?

The major greenhouse gases emitted by the United States as a result of human activity, and their percentage share of U.S. total gross greenhouse gas emissions of 6,340.2 million metric tons in 2021 were:1

  • Carbon dioxide (CO2)—79.4%
  • Methane (CH4)—11.5%
  • Nitrous oxide (N2O)—6.2%
  • Hydrofluorocarbons (HFCs)—2.8%
  • Perfluorocarbons (PFCs), sulfur hexafluoride (SF6), and nitrogen triflouride (NF3) combined—0.2%

There are other greenhouse gases that are not counted in United States or international greenhouse gas inventories:

  • Water vapor is the most abundant greenhouse gas. Most scientists believe that water vapor produced directly by human activity contributes very little to the amount of water vapor in the atmosphere. The U.S. Energy Information Administration (EIA) does not estimate emissions of water vapor. Research conducted by NASA suggests a stronger impact from the indirect human effects on water vapor concentrations in the atmosphere.
  • Ozone is technically a greenhouse gas because it has an effect on global temperature. However, at higher elevations in the atmosphere (stratosphere), where it occurs naturally, ozone is needed to block harmful ultraviolet light. At lower elevations of the atmosphere (troposphere), ozone is harmful to human health and is a pollutant regulated independently of its warming effects.

Greenhouse gases are transparent to incoming (short-wave) radiation from the sun but block infrared (long-wave) radiation from leaving the earth's atmosphere. This greenhouse effect traps radiation from the sun and warms the planet's surface. As concentrations of these gases increase, more warming occurs than would happen naturally.

1 Based on global warming potential.
Data source: U.S. Environmental Protection Agency (EPA), Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2021, Executive Summary, April 2023

Learn more:
Energy and the Environment—Greenhouse Gases
Most recent monthly and annual estimates for U.S. carbon dioxide emissions from energy consumption

Last updated: June 1, 2023

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