The 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 equivalent (CDE) and equivalent carbon dioxide (CO2
e and CO2
eq) are 2 related but distinct measures for estimating 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.
CO2e thus reflects the time-integrated radiative forcing of a quantity of emissions or rate of greenhouse gas emission - a flow into the atmosphere - rather than the instantaneous value of the radiative forcing of the stock (concentration) of greenhouse gases in the atmosphere described by CO2e.
CO2e for a gas is obtained by multiplying the mass and the global warming potential (GWP) of the gas.
The following units are commonly used:
by the UN climate change panel (IPCC): n×109 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 over 100 years is 34, and for nitrous oxide, 298.
This means that emissions of 1 million tonnes of methane or nitrous oxide are equivalent to emissions of 34 or 298 million tonnes of carbon dioxide, respectively.