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Global Warming Potentials
Gases in the atmosphere can contribute to the greenhouse effect both directly and indirectly. Direct effects occur when the gas itself is a greenhouse gas; indirect radiative forcing occurs when chemical transformations of the original gas produce a gas or gases that are greenhouse gases, or when a gas influences the atmospheric lifetimes of other gases. The concept of a Global Warming Potential (GWP) has been developed to compare the ability of each greenhouse gas to trap heat in the atmosphere relative to another gas. Carbon dioxide was chosen as the reference gas to be consistent with IPCC guidelines.
Global Warming Potentials are not provided for the criteria pollutants CO, NOx, NMVOCs, and SO2 because there is no agreed upon method to estimate the contribution of gases that have only indirect effects on radiative forcing (IPCC 1996).
All gases in this executive summary are presented in units of million metric tons of carbon equivalents (MMTCE). Carbon comprises 12/44ths of carbon dioxide by weight. The relationship between gigagrams (Gg) of a gas and MMTCE can be expressed as follows:
MMTCE=(Tg of gas)x(GWP)x(12/44)
The GWP of a greenhouse gas is the ratio of global warming, or radiative forcing – both direct and indirect – from one unit mass of a greenhouse gas to that of one unit mass of carbon dioxide over a period of time. While any time period can be selected, the 100 year GWPs recommended by the IPCC and employed by the United States for policy making and reporting purposes were used in this report (IPCC 1996). GWP values are listed below in Table ES-6.

Table ES-6: Global Warming Potentials
(100 Year Time Horizon)

Gas GWP
Carbon dioxide (CO2) 1
Methane (CH4)* 21
Nitrous oxide (N2O) 310
HFC-23 11,700
HFC-125 2,800
HFC-134a 1,300
HFC-143a 3,800
HFC-152a 140
HFC-227ea 2,900
HFC-236fa 6,300
HFC-4310mee 1,300
CF4 6,500
C2F6 9,200
C4F10 7,000
C6F14 7,400
SF6 23,900
Source: (IPCC 1996).
* The methane 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.


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Last Updated on April 6, 2001