Carbon dioxide equivalent
A common unit that expresses different greenhouse gases relative to CO₂ using a selected emissions metric.
Example
Reports often multiply each gas's mass by a global warming potential before adding them.
The catch
A CO₂e figure must identify the metric, time horizon and assessment version.
Related terms
Global warming, Global warming potential, Greenhouse gas, Absolute emissions, Activity data, Base year
In more depth
Carbon dioxide equivalent, explained.
Carbon dioxide equivalent lets a report add methane, nitrous oxide and CO₂ into a single total, but the total depends on the exchange rate chosen. The usual rate is a global warming potential. Mass for mass, methane warms about 80 times as much as CO₂ over 20 years and about 30 times over 100 years, so the same methane leak produces sharply different CO₂e figures under GWP20 and GWP100.
GWP100 is the convention for most official reporting, including national inventories under the Paris Agreement. Values shift between assessments as the science improves, so two reports of identical emissions can disagree slightly. The bigger limitation is timing. A single CO₂e number hides the fact that methane does most of its warming in the first decades after release while CO₂ keeps warming for centuries, so trading one gas for the other at a fixed rate changes when the warming happens.
Use the unit with its label attached, for example CO₂e under GWP100, and where methane makes up a large share, report each gas separately as well. Keep CO₂e apart from plain CO₂ too. A target of net zero CO₂ covers carbon dioxide alone, while a CO₂e target covers whichever gases its scope names.