Global Warming Potential (GWP)

An index used to compare the relative radiative forcing (e.g., the capacity to cause warming) of different greenhouse gases. The capacity of a particular greenhouse gas to cause warming depends upon its radiative properties, its molecular weight, and its lifetime in the atmosphere.

The GWP of a greenhouse gas is calculated as the ratio of the radiative forcing that would occur from the emission of one kilogram of that gas to the forcing that would occur from the emission of one kilogram of carbon dioxide over a set time period (100 years is used in the Kyoto Protocol). Carbon dioxide is assigned a reference value of one. Therefore, if methane has a GWP of 21, it means that one kilogram of methane has the same effect on the atmosphere as 21 kilograms of carbon dioxide.

source: http://www.mondaq.com/unitedstates/article.asp?articleid=87596