What Are Greenhouse Gases?

Gases that trap heat in the atmosphere are called greenhouse gases. CO₂ is the principal greenhouse gas, but other gases can have the same heat-trapping effect. Some of these other greenhouse gases, however, have a much stronger greenhouse, or heat-trapping, effect than CO₂. For example, methane is 21 times more potent a greenhouse gas than CO₂. Different GHGs have different atmospheric life times, and therefore actions to reduce emissions will take time to effect reductions of gases in the atmosphere. The principal, human-generated greenhouse gases that enter the atmosphere are

• Carbon Dioxide (CO₂): Carbon dioxide enters the atmosphere through the burning of fossil fuels (oil, natural gas and coal).



• Methane (CH₄): Methane is emitted during the production and transport of coal, natural gas and oil. Methane emissions also result from livestock and other agricultural practices and by the decay of organic waste in municipal solid waste landfills and anaerobic wastewater treatment plants. CH⁴ is a greenhouse gas approximately 21 times more potent than CO₂ and has an atmospheric lifespan of roughly 12 years (EPA, 2009c).



• Nitrous Oxide (N₂O): Nitrous oxide is emitted during agricultural and industrial activities, as well as during the combustion of fossil fuels and solid waste. Nitrous oxide is also emitted from wastewater treatment plants during nitrification and denitrification processes. N₂O is 310 times more potent as a greenhouse gas than CO₂ and has an atmospheric lifespan of 120 years (EPA, 2009b).



• Fluorinated Gases: Hydrofluorocarbons (HFCs), perfluorocarbons (PFCs) and sulfur hexafluoride (SF₆) are synthetic, powerful greenhouse gases that are emitted from a variety of industrial processes. Fluorinated gases are sometimes used as substitutes for ozone-depleting substances (i.e., CFCs, HCFCs and halons). These gases are typically emitted in smaller quantities, but because they are potent greenhouse gases, they are sometimes referred to as High Global Warming Potential gases (High GWP gases). HFCs are 140 to 11,700 times more potent than CO₂ and have atmospheric lifespans of 1–260 years. Most commercially used HFCs remain in the atmosphere less than 15 years. PFCs are 6,500 to 9,200 times more potent than CO₂ and have an atmospheric lifespan of several thousand years. Sulfur hexafluoride is 23,900 times a more potent greenhouse gas than CO₂ and is extremely long lived with very few sinks (EPA, 2009c).

References:
EPA. 2009b. High Global Warming Potential (GWP) Gases Web site. High Global Warming Potential (GWP) Gases – Science. accessed August 31, 2009.

EPA. 2009c. Methane Web site. Methane – Science. www.epa.gov/methane/scientific.html, accessed August 31, 2009.

Graphic Reference:
EPA. 2009e. Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007 (April 2009) USEPA #430-R-09-004. accessed August 31, 2009.

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