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What scientific arguments support using a GWP of 100 for methane and which policymakers or studies adopt it?
Executive summary
GWP100 is the dominant, policy‑facing metric for methane because it converts a pulse of methane into a CO2‑equivalent over a 100‑year horizon, producing values around 25–30× CO2 and enabling consistent accounting across gases and agreements (e.g., IPCC and national inventories use GWP100) [1][2][3]. Scientific critiques centre on methane’s ~12‑year lifetime and argue GWP100 can misrepresent short‑term warming and policy incentives; alternatives (GWP20, GWP*, modified GWP100 values) are proposed or used by parts of the research and farming community but are not universally adopted in international reporting [4][5][6].
1. Why 100 years? A pragmatic standard for comparability
The 100‑year time horizon for Global Warming Potential (GWP100) exists primarily as a common standard to compare different greenhouse gases on one consistent basis; it has been widely adopted in international assessments and reporting so that one metric can be used for “basket of gases” targets and national inventories [1][3]. Using a century‑scale window flattens differences in atmospheric lifetimes — giving a single, policy‑usable conversion factor (for methane typically ~27–30 on the 100‑year basis in recent IPCC assessments) — and this consistency is why many policymakers continue to default to GWP100 [2][7].
2. The scientific logic behind GWP100’s continued use
Scientists and agencies that continue to use GWP100 argue it is underpinned by radiative‑forcing calculations that integrate warming over a long period and therefore aligns with long‑term climate goals and cumulative emission frameworks; major assessment reports (IPCC AR6) and reports cited by agencies apply GWP100 to aggregate anthropogenic emissions [3][1]. The U.S. EPA explicitly states methane’s GWP over 100 years is about 27–30 and that the agency “primarily uses the 100‑year GWP” for reporting while noting alternative metrics exist [2].
3. Scientific arguments that support GWP100 for methane
Proponents say GWP100 is appropriate because many policy targets are framed over multi‑decadal to centennial timescales, and because GWP100 ties directly into cumulative CO2‑equivalent accounting used for national commitments and inventories; changing the horizon would reweight targets and could produce unintended shifts in where reductions are prioritized [3][1]. Analyses caution that switching to shorter horizons (e.g., GWP20) would increase the relative weight of short‑lived gases and alter mitigation priorities, an effect stress‑tested by Climate Analytics and others [5].
4. Why scientists criticise GWP100 — the short‑lived gas problem
Opponents focus on the physics: methane’s atmospheric lifetime is about 11–12 years, so its near‑term warming per tonne is much larger than when averaged over 100 years; GWP100 therefore can understate near‑term warming and give misleading impressions about temperature trajectory if methane and CO2 are simply summed into CO2e [1][4]. That critique underlies development of GWP and calls for reporting both short‑ and long‑horizon metrics to convey both instantaneous and cumulative impacts [5][4].
**5. Alternatives in the literature and who is adopting them**
Researchers developed GWP to better represent the warming contribution of short‑lived climate pollutants by mapping changes in methane flow onto warming‑equivalent CO2 emissions; peer‑reviewed work (npj Climate and Atmospheric Science) demonstrates how GWP compares to GWP100 across scenarios [5]. Farming groups, industry bodies and some analysts advocate dual accounting or warmed‑based metrics: agriculture organisations (e.g., NFU and others in a joint statement) and tools like Cool Farm explicitly discuss or pilot GWP or dual reporting while maintaining GWP100 for formal inventories [6][8][9].
6. Policy adoption and limits — where GWP100 still rules
Despite active debate and selective adoption of alternatives for internal or sectoral reporting, international reporting frameworks and many national inventories continue to use GWP100 (as reflected in IPCC risk and emissions summaries and agency guidance) [3][1]. Several commentators warn that using alternative metrics for official targets or trading without consensus could distort mitigation incentives; Climate Analytics argues shifting to a short horizon for targets could force very different reduction pathways [5].
7. Competing viewpoints and implications for policy
One camp stresses clarity and comparability: keep GWP100 for formal reporting to preserve consistent baselines and long‑term goals [1][3]. The other camp stresses physical realism in temperature response: use GWP20 or GWP, or report multiple metrics side‑by‑side, so policymakers grasp near‑term warming risk and tailor measures accordingly [4][5][6]. These are not purely scientific debates — they have distributional and political consequences (e.g., agriculture vs. fossil fuels) that shape which groups press for change [6][8].
**8. What reporting gaps and next steps remain**
Available sources do not mention a single, global shift away from GWP100 for official inventories; rather, they document experimentation with GWP and dual reporting by researchers, industry groups and some tools, while IPCC‑based reporting and many governments still rely on GWP100 [5][9][3]. Policymakers must decide whether to preserve comparability (stick with GWP100), adopt multiple metrics for transparency, or define new rules — a choice with clear winners and losers across sectors [5][6].