Equivalent carbon dioxide[edit]
Equivalent CO2 (CO2e) is the concentration of CO2 that would cause the same level of radiative forcing as a given type and concentration of greenhouse gas. Examples of such greenhouse gases are methane, perfluorocarbons, and nitrous oxide. CO2e is expressed as parts per million by volume, ppmv.
CO2e calculation examples:
The radiative forcing for pure CO2 is approximated by RF = alpha ln(C/C_0) where C is the present concentration, alpha is a constant, 5.35 and C_0 the pre-industrial concentration, 278 ppm. Hence the value of CO2e for an arbitrary gas mixture with a known radiative forcing is given by C_0 exp(RF/alpha) in ppmv.
To calculate the radiative forcing for a 1998 gas mixture, IPCC 2001 gives the radiative forcing (relative to 1750) of various gases as: CO2=1.46 (corresponding to a concentration of 365 ppmv), CH4=0.48, N2O=0.15 and other minor gases =0.01 W/m2. The sum of these is 2.10 W/m2. Inserting this to the above formula, we obtain CO2e = 412 ppmv.
To calculate the CO2e of the additional radiative forcing calculated from the 2012 averaged data:[2] ∑ RF(GHGs) = 3.234, thus CO2e = 278 e3.234/5.35 ppmv = 508.8 ppmv