CO2 vs CO2e: global warming potentials (AR4, AR5, AR6) explained

Greenhouse gases differ in how strongly they warm the climate. To add them up, emissions are converted to a common unit: carbon dioxide equivalent (CO2e). See Scope 1, 2 and 3 emissions explained for which gases an inventory covers.

Global warming potential

The conversion factor is the gas's global warming potential (GWP): how much warming a kilogram of the gas causes over a given period compared with a kilogram of CO2. Corporate reporting uses the 100-year time horizon; the GHG Protocol's table is headed "GWP values for 100-year time horizon".

CO2e = mass of gas × GWP

The main gases

Values from the GHG Protocol's table of IPCC assessment reports (August 2024):

So 1 kg of SF6 released counts as 24,300 kg CO2e using AR6. Every gas in the table has its own converter page with the source page cited.

Which assessment report to use

Each IPCC assessment report updated the science, so the same gas has different values. The GHG Protocol's table says "The use of the latest values (AR6) is recommended", while noting AR4 and AR5 values "are sometimes used for inventory and reporting purposes".

Factor publishers make their own choice. The UK government's 2026 conversion factors use AR5 values, with AR6 where no AR5 value exists. If you take factors from one publisher and GWPs from another, the result mixes bases; check what your factor source used and report it.

The rule that matters most is consistency: use the same GWP set across years, and if you change it, recalculate your base year (see base year and recalculation).

CO2 and CO2e are not interchangeable

Some sources publish CO2 only. Italy's national electricity factors from ISPRA are an example: they include carbon dioxide but not methane or nitrous oxide. A result calculated with them is CO2, not CO2e, and should be labelled that way. See Italian electricity factors.

Refrigerant blends

Refrigerants such as R410A are mixtures. Their GWP is the mass-weighted average of their components, and publishers such as DESNZ give a ready-made factor per kilogram of blend: 1,924 kg CO2e per kg for R410A in 2026. Use the blend factor, not the GWP of one component.