Carbon dioxide equivalent

Carbon dioxide equivalent (CDE) and Equivalent carbon dioxide (or CO2e) are two related but distinct measures for describing how much global warming a given type and amount of greenhouse gas may cause, using the functionally equivalent amount or concentration of carbon dioxide (CO2) as the reference.

Equivalent carbon dioxide
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 example:
 * 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.

Carbon dioxide equivalent
Carbon dioxide equivalency is a quantity that describes, for a given mixture and amount of greenhouse gas, the amount of CO2 that would have the same global warming potential (GWP), when measured over a specified timescale (generally, 100 years). Carbon dioxide equivalency thus reflects the time-integrated radiative forcing, rather than the instantaneous value described by CO2e.

The carbon dioxide equivalency for a gas is obtained by multiplying the mass and the GWP of the gas. The following units are commonly used:
 * By the UN climate change panel IPCC: billion metric tonnes of CO2 equivalent (GtCO2eq).
 * In industry: million metric tonnes of carbon dioxide equivalents (MMTCDE).
 * For vehicles: g of carbon dioxide equivalents / km (gCDE/km).

For example, the GWP for methane is 21 and for nitrous oxide 310. This means that emissions of 1 million metric tonnes of methane and nitrous oxide respectively is equivalent to emissions of 21 and 310 million metric tonnes of carbon dioxide.