carbon dioxide equivalent

Carbon dioxide is not the only gas that traps heat in the atmosphere. National emissions statistics also count methane, nitrous oxide and a group of industrial chemicals known as fluorinated gases, and a tonne of each warms the planet by a different amount. Adding them up in raw tonnes would mix quantities that do not compare.

Carbon dioxide equivalent solves this by weighting each gas by its global warming potential, which measures the gas's warming influence relative to carbon dioxide over a 100 year period. A tonne of a gas with ten times carbon dioxide's warming effect counts as ten tonnes of carbon dioxide equivalent. The weighted amounts can then be added into a single total.

The UK's official statistics use this unit for seven gases: carbon dioxide, methane, nitrous oxide, hydrofluorocarbons, perfluorocarbons, sulphur hexafluoride and nitrogen trifluoride, following international reporting rules and the warming potentials published by the Intergovernmental Panel on Climate Change. Large national totals are usually written in millions of tonnes, shortened to MtCO2e.