Certain gases effectively trap heat in the earth's atmosphere. The net effect is much like a greenhouse
where the glass allows light but keeps heat from escaping. The so-called greenhouse gases can have a similar
effect and may actually influence the earth's climate. Our climate appears to be warming and many scientists
believe that human greenhouse gas emissions are contributing. The warming potentially can lead to serious
consequences, including changes in weather patterns, increased droughts, increased extreme weather events
and adverse impacts on various species.
The substances of concern include individual gases as well as groups of gases that have similar
characteristics. The typical list includes six items:
The greatest amounts of emissions, both naturally and man-made, occur from carbon dioxide, followed by
Greenhouse gases are not created equal. Each possesses a different efficiency for trapping heat. For example
methane is about 21 times more potent than CO2 and sulfur hexafluoride is thousands of times more
potent than CO2. Therefore, to estimate the net effect on climate, GHG emissions are usually
converted to CO2 equivalent (CO2e) and then expressed in metric tons.
Global Warming Potentials (100-Year Time Horizon)
Source: IPCC (1996) and listed as Table ES-1 from the Executive Summary of the Inventory of U.S. Greenhouse
Gas Emissions and Sinks: 1990-2009, page 3
*The CH4 GWP includes the direct effects and those indirect effects due to the production of
tropospheric ozone and stratospheric water vapor. The indirect effect due to the production of
CO2 is not included.