Methane's GWP over a 20 year period , is it 30 or 86 or 100 ?

1. What GWP actually measures — short, sharp punch vs long tail

GWP compares the integrated radiative forcing from a pulse emission of a greenhouse gas to that from the same mass of CO2 over a chosen time horizon; because methane lasts only about a decade in the atmosphere but is a strong infrared absorber, its GWP is much larger on a 20‑year horizon than on a 100‑year horizon ^{[1] [4] [5]}.

2. Where the familiar numbers come from — 86, ~30, ~100?

The figure “≈86” for GWP20 appears in IPCC summaries and reference compilations (often rounded 83–86 depending on the exact treatment), and “≈27–30” for GWP100 is the commonly quoted AR6/agency range; therefore “86” corresponds to a 20‑year GWP and “30” (or ~27–28) to a 100‑year GWP — “100” by itself is not a GWP value but the 100‑year time horizon used to produce values like ~30 ^{[1] [2] [3]}.

3. Why different sources quote slightly different numbers

Different documents use different calculation inputs (assessment report version, inclusion of climate‑carbon feedbacks, whether rounding or source attribution corrections are applied) so GWP20 is reported as 83, 84, or 86 in different places and GWP100 as 27, 28, or 30; agencies such as the US EPA report methane GWP100 as 27–30 and some summaries cite IPCC’s 83 for GWP20 while other overviews state 84–86 ^{[1] [2] [3]}.

4. Policy implications — picking 20 vs 100 years is a choice, not a pure science answer

The choice of horizon is a policy decision with trade‑offs: using GWP20 emphasizes near‑term potency of short‑lived gases like methane and can push faster methane reductions, whereas GWP100 gives more weight to long‑lived CO2 and is the conventional basis for many inventories and regulations; technical experts note there is no single “technically objective” correct horizon — it depends on policy goals and intergenerational tradeoffs ^{[6] [7]}.

5. Stakes and debate — winners and losers in metric selection

Switching to GWP20 increases the relative weight of methane in emission “baskets,” which can substantially change targets and incentives; advocates for shorter horizons argue it better addresses near‑term warming, while critics warn it could let long‑lived CO2 reductions slip and distort long‑term climate goals ^{[7] [8] [6]}.

6. Alternative metrics and complication: GWP*, GTP and distinctions by source

Scientists and policymakers have proposed alternatives — GWP* attempts to represent short‑lived gases’ temperature effects differently, and the IPCC/AR6 discussion even distinguishes methane by source (fossil vs non‑fossil) in some updates; these alternatives aim to capture how constant vs changing methane flows affect temperature, and they change how one interprets “equivalent” emissions ^{[9] [10] [4]}.

7. What to take away if you must pick a number for calculations

If you need a single number for comparison: use GWP20 ≈ 83–86 when prioritizing near‑term warming, and use GWP100 ≈ 27–30 for the conventional, longer‑term accounting that most inventories and many policy frameworks currently use; always state which horizon and source you used because the choice materially affects reported CO2e values ^{[1] [2] [3]}.

8. Limitations and how reporting can obscure choices

Reporting often omits the horizon or the assessment version, which creates confusion — the three numbers you asked about ^{[11] [12] [13]} are not competing claims of fact about one horizon but stem from different horizons, rounding and different references; available sources do not treat “100” as a standalone GWP value but as the 100‑year time horizon used to generate values like ~30 ^{[1] [2]}.

If you want, I can: a) pull the exact AR6 numeric table entries and show the different rounded values and their sources, or b) produce a one‑page cheat sheet you can use when comparing GWP20 vs GWP100 in reporting.