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).
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.