Frequently Asked Questions

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

The major greenhouse gases that the United States emitted as a result of human activity in 20121 (and their share of total emissions) were2

  • Carbon dioxide (83%)
  • Methane (9%)
  • Nitrous oxide (6%)
  • Hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (collectively 3%)

There are other greenhouse gases that are not counted in U.S. or international greenhouse gas inventories:

  • Water vapor -- 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. Therefore, EIA does not estimate emissions of water vapor. Research by NASAsuggests a stronger impact from the indirect human effects on water vapor concentrations in the atmosphere.
  • Ozone -- technically a greenhouse gas because it has an effect on global temperature. However, at higher elevations in the atmosphere (stratosphere), where it occurs naturally, it is needed to block harmful UV light. At lower elevations of the atmosphere (troposphere) it 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.

1UnitedStates Environmental Protection Agency, National Greenhouse Gas Emissions Data
2Based on global warming potential.

Learn more:

What are greenhouse gases and how much are emitted by the United States?

Energy and the Environment — Greenhouse Gases

Most recent monthly and annual estimates for U.S. Carbon Dioxide Emissions From Energy Consumption.

Emissions of Greenhouse Gases in the United States, 2009, EIA's last annual report on total U.S. greenhouse gas emissions.

Last updated: June 27, 2014

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