Q&A: What the 'controversial' GWP* methane metric means for farming emissions
Carbon BriefOrla Dwyer03 Oct 2025cc-by-nc-ndRead the original
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Carbon Brief's Q&A sets out what GWP* is and why it is disputed. GWP* weights short-lived gases such as methane by the change in their emission rate over time, rather than by the fixed multiplier used in the conventional 100-year global warming potential. Under GWP*, methane emissions that fall by about 10 percent over 30 years, roughly 0.3 percent a year, are described in research and policy as causing no additional warming. Supporters, including Professor Myles Allen, characterise it as a way of calculating the warming impact of activities. Critics, including Dr Caspar Donnison, argue the framing implies a neutral climate impact when existing warming is in fact being sustained, and that larger reductions in herd numbers would lower warming rather than hold it constant. The choice of metric changes how livestock methane counts against national and corporate climate targets. No country has adopted GWP*, though New Zealand, Ireland, Australia and Paraguay have weighed it.
Contested
Extract
"GWP* calculates the warming contributions of long- and short-lived gases at different rates, accounting for their varying lifetimes in the atmosphere. One of the researchers behind GWP*, Dr Michelle Cain, explained in a 2018 Carbon Brief guest post that a constant rate of methane emissions can maintain stable atmospheric concentrations of the gas, assuming methane sinks remain constant as well." "Under GWP*, methane emissions stop causing new warming once they reduce by 10% over the course of 30 years – around 3% each decade, or 0.3% each year. These emissions are then described in research and policy as causing 'no additional warming'." "However, many critics take issue with the 'no additional warming' concept. The main criticism is that, although a gradually reducing herd of cattle may stabilise methane emissions, it still emits the polluting gas. If animal numbers were instead drastically reduced, this would cut methane emissions and lower warming rather than maintaining current levels."
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The MyGreenSuit briefing
Commentary by MyGreenSuit, not the words of Carbon Brief.
This is the most consequential metric argument in agriculture, and most people in the industry cannot explain it — which is a problem when the metric decides your target.
The physics is not really in dispute. Methane's roughly twelve-year lifetime means a constant emission rate maintains a constant atmospheric concentration rather than adding to it, and GWP100 — designed for one-off pulses of CO2 — genuinely does handle that badly.
The dispute is about what follows. Under GWP*, a herd shrinking about 0.3 percent a year is described as causing "no additional warming." The critics' objection is not to the maths; it is that the phrase implies a neutral climate impact, when what is actually happening is that a large existing warming contribution is being held steady rather than reduced.
Both of those statements are true at the same time, which is why the argument does not resolve.
The honest position for anyone in this industry: GWP* is a better description of methane's physics and a convenient answer to an uncomfortable question. It is possible to say both without being accused of either.