Which authoritative sources report methane GWP as 30, 86, or 100 and why do they disagree?

1. Why you see “~30” — the standard 100‑year number favored in reporting

Most climate inventories and many regulators rely on a 100‑year global warming potential (GWP100) that places methane’s potency around 27–30 times CO2 over 100 years; the US EPA states CH4 is “estimated to have a GWP of 27 to 30 over 100 years,” and multiple toolmakers and NGOs adopt values near 28–30 for GWP100 because the 100‑year horizon is the convention used in many international reporting rules ^{[1] [3] [6]}. The IPCC’s AR6 tables — and derivative guidance used by practitioners — also provide GWP100 values and, in some implementations, separate values for fossil vs non‑fossil methane, which can slightly change the number used for specific accounting ^{[4] [5]}.

2. Why you see “~86” (or ~80–83) — the short‑term, 20‑year perspective

The much larger figure around 80–86 comes from using a 20‑year time horizon (GWP20) instead of 100 years — methane is short‑lived (roughly a decade), so its initial warming per tonne is large but fades as it oxidizes; classic IPCC and other summaries list GWP20 at roughly 80–86 while GWP100 is ~30 ^{[2] [3]}. Many advocates and analysts who stress near‑term climate benefits of cutting methane therefore prefer GWP20 because it highlights immediate warming avoidance and the “quick win” potential from reducing methane emissions ^[7].

3. Where “100” shows up and why some communications use it

Some sources or stakeholders sometimes state a GWP of “100” in shorthand or in specific accounting choices — available sources do not show a canonical authoritative body publishing methane GWP = 100 as a standard scientific value, but the number appears in policy or rhetoric when people intentionally choose a larger short‑term multiplier or apply different conversion conventions; the supplied materials don’t document an endorsement of exactly 100 as an IPCC or EPA baseline (not found in current reporting). The supplied sources do show that manipulation of assumptions (e.g., whether to add the CO2 from methane oxidation over the 100‑year window) can shift GWP100 values slightly (for instance a calculation of 30.65 that adds full CO2 oxidation effect) ^[8].

4. The technical nitty‑gritty: chemistry, timeframes and oxidation

GWP compares integrated radiative forcing of a pulse of gas to that of CO2 over a chosen time horizon; methane’s short atmospheric lifetime (~12 years) means its relative impact drops sharply between 20 and 100 years, so the metric is time‑sensitive by design ^{[2] [1]}. Some accounting approaches explicitly add the persistent radiative forcing from the CO2 produced when methane oxidizes — that can raise a GWP100 figure marginally (the Greenhouse Gas Management Institute notes a calculated 30.65 when including full 100‑year forcing from oxidized CO2 in fossil methane scenarios) ^[8].

5. Competing viewpoints and why the debate matters for policy

Scientists and institutions generally agree on the physics that time horizon and oxidation choices drive the numeric differences, but they disagree on which metric is appropriate for policy and markets. Proponents of GWP100 point to international reporting norms (Paris Rulebook and common practice), while advocates for GWP20 or alternative metrics (like GWP) argue short‑lived climate pollutants need different treatment to reflect actual warming dynamics and policy goals; critics warn that switching metrics can shift responsibility between CO2 and methane and produce misleading comparisons ^{[3] [9] [10] [7]}.

**6. Practical guidance for readers and practitioners**

Use the metric that fits your objective but be explicit: if you’re reporting to an international inventory or following many national rules, GWP100 (~27–30) is standard ^{[1] [11]}. If your policy focus is near‑term warming avoided, compare using GWP20 (~80–86) or consider newer metrics designed for short‑lived gases (GWP), but note critics and methodological limitations and disclose which convention you used ^{[3] [9] [10]}.

Limitations: this briefing uses only the supplied search results; detailed AR6 tables, the Paris CMA decisions, and some policy documents are referenced in those sources but not quoted in full here — consult the IPCC AR6 tables and specific national guidance documents for exact numeric choices and reporting rules ^{[4] [5]}.