The concept of global warming potential (GWP) was invented to allow comparisons of the total cumulative warming effects of different
GHGs over a specified time period. A GWP is a measure of relative contribution to radiative forcing (see The Science of Climate Change).
The warming effect of CO2 is assigned a value of 1, and the warming effects of other gases are calculated as multiples of this value. (see
The GWPs shown in Table 1 are for a 100-year time horizon2 and are from the Intergovernmental Panel on Climate Change (IPCC)
Second Assessment Report (SAR). The Third Assessment Report (TAR) includes revised 100-year GWP estimates, increasing the GWP of
CH4 to 23 and decreasing the GWP of N2O to 296 (IPCC, 2001). Virtually all GWPs used in the literature to date are from the SAR.
Emissions calculations relevant to the United Nations Framework Convention on Climate Change (UNFCCC) and the Kyoto Protocol are
also based on SAR GWPs.
GWP and Carbon Dioxide Equivalents (CO2E)
GWPs are used to convert emissions of non-CO2 gases into their CO2 warming equivalents (CO2Es). The CO2E of a non-CO2 gas is
calculated by multiplying the mass of the emissions of the non-CO2 gas by its GWP.
A 100-year GWP of 21 for CH4 (see Table 1) means that each gram of CH4 emitted is considered to have cumulative warming effects
over the next 100 years equivalent to emitting 21 grams of CO2. Using the SAR 100-year GWP of 21 for CH4, the CO2E of 310 tons of
CH4 is 310 tons x 21 = 6,510 tons CO2E. Emitting 310 tons of CH4 would thus be considered to result in the same cumulative warming
over the next 100 years as emitting 6,510 tons of CO2.
While the 100-year time horizon GWPs shown in Table 1 are the most commonly used, GWPs based on cumulative warming over 20- and
500-year periods were also developed by the IPCC and are used in some analyses. For the HFCs and PFCs, it is common to use the GWP
of the dominant gas (shown in parentheses) to calculate the CO2E for emissions of these gases.
While most international data sources provide emissions data in terms of CO2 or CO2E, many U.S. sources provide emissions data in
units of carbon (C) or carbon equivalents (CE). Emissions reported in CO2 or CO2E units are 3.67 times emissions reported in C or CE
units. To convert from CO2 to C or from CO2E to CE, multiply by 12/44 (0.27), the ratio of the mass of C to the mass of CO2. To convert
from C to CO2 or from CE to CO2E, multiply by 44/12 (3.67).
Gas SAR 20-Year GWPs SAR 500-Year GWPs
CO2 1 1
CH4 56 6
N2O 280 170
HFCs 490-9,100 (3,400) 42-9,800 (420)
PFCs 4,900-6,200 (4,400) 10,000-14,000 (10,000)
SF6 16,300 34,900