Carbon dioxide equivalent: the amount of CO2 that would cause the same warming as a given mix of greenhouse gases, worked out by multiplying each gas by its global warming potential.

What is the global warming potential of methane?

Over 100 years, 27.0 for non-fossil methane and 29.8 for fossil methane in the IPCC's sixth assessment; 28 in the fifth, which UK and US factor sets still use.

Why do UK and US factors use older GWP values?

Because UN climate reporting guidelines require the fifth assessment's values. The GHG Protocol recommends the newer AR6 values for corporate inventories.

Term

CO2e (carbon dioxide equivalent)

CO2e, or carbon dioxide equivalent, expresses a mix of greenhouse gases as the amount of CO2 that would cause the same warming. Each gas is multiplied by its global warming potential (GWP): over 100 years, the IPCC's sixth assessment puts methane at 27 to 29.8 and nitrous oxide at 273 times CO2.

Also called CO2 equivalent, carbon dioxide equivalent, CO2eq, global warming potential, GWP100Updated 13 October 2026

GWP values

Gas	IPCC AR6 (2021)	IPCC AR5 — used in UK and US factor sets
Carbon dioxide	1	1
Methane, non-fossil	27.0	28
Methane, fossil	29.8	28
Nitrous oxide	273	265

100-year global warming potentials

The UK government calls CO2e the universal unit for the global warming potential of greenhouse gases, expressed in terms of one unit of carbon dioxide; the US EPA converts gases to CO2e by multiplying by their GWP. Both national factor sets still use the IPCC's fifth assessment values, because UN reporting guidelines require them. The GHG Protocol recommends the latest values, AR6, and the EU's reporting standard asks for the most recent IPCC GWPs. AR6 also gives 20-year values — methane 79.7 non-fossil and 82.5 fossil, nitrous oxide 273 — but the factor sets here use the 100-year figures.

Worked example: a truck's ton-mile factor

EPA 2025 heavy truck, per short ton-mile: 0.186 kg CO2 + 0.0016 g CH4 × 28 ÷ 1,000 + 0.0054 g N2O × 265 ÷ 1,000 = 0.18748 kg CO2e (AR5) · with AR6's 27 and 273: 0.18752

For diesel, methane and nitrous oxide add about 1%; the CO2 dominates. The UK's 2026 diesel factor shows the same split: 2.55035 CO2 + 0.00029 CH4 + 0.0329 N2O = 2.58354 kg CO2e a litre.

The mistake people make

Using the fossil methane value for fuel burned in vehicles. The GHG Protocol's 2024 GWP list says the non-fossil value should be used for combustion emissions, because applying the higher fossil value would double count. The other trap is mixing AR5 and AR6 values in one inventory — pick one edition and say which.

FAQ