CO2e (CO2 equivalent)
A common unit that expresses emissions of different greenhouse gases as the mass of CO2 that would cause the same warming over a chosen time horizon.
CO2 equivalent lets inventories add methane, nitrous oxide and fluorinated gases to carbon dioxide on a single scale. Each gas is multiplied by its global warming potential, almost always the 100-year value (GWP100).
With IPCC AR6 values, 1 t of fossil methane equals about 29.8 t CO2e (27.0 for biogenic methane), 1 t of N2O equals 273 t CO2e, and 1 t of SF6 equals roughly 25,200 t CO2e. Many regulatory inventories, including UNFCCC reporting under the Paris rulebook, still use AR5 values (methane 28, N2O 265), so the GWP set should always be stated.
The choice of time horizon matters for short-lived gases. On a 20-year basis (GWP20) methane is about 80 times as potent as CO2, which is why methane leakage can erase much of the climate advantage of gas over coal in near-term analyses. Global greenhouse gas emissions were roughly 53–57 Gt CO2e per year in the early 2020s (UNEP Emissions Gap Report), of which fossil CO2 was about 37 Gt.
Related terms
All terms →- Global warming potential (GWP)
The time-integrated radiative forcing of one kilogram of a gas relative to one kilogram of CO2, over a stated period such as 20 or 100 years.
- Carbon intensity of electricity
The mass of CO2 (or CO2e) emitted per unit of electricity generated or consumed, usually expressed in grams per kilowatt-hour (gCO2/kWh).
- Methane leakage
Unintended release of methane from oil, gas and coal supply chains through leaks, venting and incomplete flaring, often expressed as a percentage of gas produced.
- Lifecycle emissions
Total greenhouse gas emissions of a product or power source across its full life, from raw material extraction and manufacturing through operation to decommissioning.