GWP and Carbon Dioxide Equivalents (CO₂E)

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 below)

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.

Box 1

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).