A metric measure used to compare the emissions from various greenhouse gases on the basis of their global-warming potential (GWP), by converting amounts of other gases to the equivalent amount of carbon dioxide with the same global warming potential. Carbon dioxide equivalents are commonly expressed as million metric tonnes of carbon dioxide equivalents, abbreviated as Mt CO2e. The carbon dioxide equivalent for a gas is derived by multiplying the tonnes of the gas by the associated GWP: Mt CO2e = (million metric tonnes of a gas) * (GWP of the gas). For example, the GWP for methane is 21 (minus 1 unit if pertaining to biogenic CH4 as that would alternatively have become 1 CO2) 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.