Understanding CO2-Equivalent and GWP100

Last updated: 2026-05-20 · PlainEmissions Editorial

What CO2-equivalent actually means

Greenhouse gases don't all warm the planet the same way. A tonne of methane traps far more heat in the lower atmosphere than a tonne of carbon dioxide does — but methane breaks down within roughly a decade, while CO2 lingers for centuries. To compare gases in a single number, climate scientists use the global-warming potential (GWP) multiplier, which expresses each gas as some number of tonnes of CO2 that would produce equivalent warming over a chosen time horizon.

By far the most commonly used multiplier is GWP100 — the 100-year global-warming potential. Methane's GWP100 is 27.9 in the IPCC AR6 assessment (revised upward from earlier estimates as atmospheric-chemistry models improved). Nitrous oxide's GWP100 is 273. Sulphur hexafluoride (used in electrical-grid equipment) is a staggering 25,200. These multipliers are applied to native emissions figures to produce CO2-equivalent (CO2e) totals.

Why 100 years (and not 20 or 500)?

The time horizon is a policy choice, not a scientific one. GWP100 is the convention adopted by UNFCCC reporting since the 1990s and remains the default in IPCC Assessment Reports. Choosing GWP20 instead would dramatically increase methane's weight — methane's GWP20 is roughly 80, meaning the same tonne of methane suddenly looks three times worse over the 20-year horizon than under GWP100.

For policy contexts that prioritize the next two decades (such as Arctic ice-loss prevention or short-term peak warming), GWP20 is more relevant. For long-horizon multi-century targets (the Paris Agreement's 2100 trajectory framing), GWP100 better reflects cumulative impact. This site standardizes on GWP100 because all four upstream sources use it and switching would break apples-to-apples comparison.

How the conversion looks in practice

A country emits 50 megatonnes of CO2 from coal-fired power, 3 megatonnes of methane from agriculture, and 0.2 megatonnes of nitrous oxide from fertilizer application. Converting each to CO2-equivalent: 50 + (3 × 27.9) + (0.2 × 273) = 50 + 83.7 + 54.6 = 188.3 MtCO2e total. Notice that methane's contribution (84 MtCO2e) is larger than the headline CO2 number, despite being a smaller mass — that's the whole point of the GWP framing.

PlainEmissions always retains the native unit value alongside the CO2e figure in our fact table. When you compare two countries, you can dig in and see whether one looks worse because of CO2 from energy or because of methane from agriculture. The CO2e number alone hides that information; the native gas breakdown surfaces it.

Why GWP values keep being revised

Each IPCC Assessment Report tightens the GWP estimates as atmospheric-chemistry models improve. AR4 (2007) put methane at GWP100 = 25; AR5 (2014) revised to 28; AR6 (2021) put it at 27.9 (with the inclusion of feedback effects pushing it as high as 30 in some interpretations). For consistency, PlainEmissions uses AR6 values across the dataset — but be aware that older studies using AR4 or AR5 multipliers will show methane and N2O slightly lower than current values.

The upstream sources are not perfectly synchronized: UNFCCC inventories have historically used AR4 values for legal-reporting consistency, while EDGAR and Climate TRACE moved to AR6. Where this matters (typically < 5% of the CO2e total for most countries), our methodology page documents the harmonization step.

F-gases: small mass, large impact

Hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), sulphur hexafluoride (SF6), and nitrogen trifluoride (NF3) are tiny in mass terms — millions of tonnes globally rather than gigatonnes — but their GWP100 values range from 1,430 (HFC-134a) to 25,200 (SF6) and 17,400 (NF3). In CO2-equivalent terms F-gases account for roughly 2-3% of global emissions and are growing because they are used in air conditioning, refrigeration, and semiconductor manufacturing.

The Kigali Amendment to the Montreal Protocol commits parties to a phasedown of HFC consumption. Tracking F-gas trends over time is one place where PlainEmissions sector-level slicing comes into its own — the country-level CO2e total often hides the F-gas signal.

Practical implications for readers

When you read an emissions figure anywhere — news article, ESG report, climate-policy document — check whether the number is (a) CO2 only or CO2e, (b) which GWP horizon was used (almost always 100), (c) which IPCC assessment vintage. A 10% difference in the headline number can come from a methodology choice rather than a real change in emissions. PlainEmissions surfaces both the native gas breakdown and the CO2e total precisely so you can audit the conversion when it matters.

Key takeaways

The number is downstream of the methodology — which gas, which source, which framing.
Multi-source disagreement is informative, not embarrassing — it surfaces uncertainty.
Production-based versus consumption-based; LULUCF-included versus excluded; per-capita versus absolute — each framing answers a different question.
Satellite-derived measurements (Climate TRACE) are progressively rebalancing the historical reliance on self-reported inventories.
GWP100 is convention; GWP20 dramatically reweights short-lived gases like methane.

Definitions used on this site

CO2-equivalent (CO2e): any greenhouse gas expressed as the mass of CO2 that would produce equivalent warming over a chosen time horizon, typically 100 years.
GWP100 / GWP20: global-warming potential over 100 (or 20) years; the multiplier used to convert from native gas units to CO2-equivalent.
LULUCF: Land Use, Land-Use Change, and Forestry — the IPCC sector covering carbon stocks in vegetation and soils.
Production-based emissions: emissions attributed to the country where they physically occur.
Consumption-based emissions: emissions attributed to the country where the final goods or services are consumed.
Annex I: the group of historically-developed countries under the UNFCCC with deeper reporting obligations.