Thread by @piersforster

On @COP26 rest day time to reflect on where we are with the science. First up, #methanepledge. @BBCMoreOrLess fact check needed: Would it save up 0.2C of warming, has methane caused half of global warming to date, is it 84x worse than CO2? bbc.co.uk/news/world-591…

1. Half of global warming? Half right. It's (0.5C) roughly half of NET warming, but misleading as really its 1/3 of GHG warming, and the total GHG warming contribution is countered by aerosol cooling. @IPCC_CH AR6 Fig SPM2

Methane emission causes ozone pollution, so the 0.5C, is not all due to methane change, but 0.35C is. Emissions also increase ozone and CO2, adding an extra 0.15C. Figure 6.12 AR6 @SophieSzopa, Vaishali Naik, @chrisroadmap

2. Will the #methanepledge of 30% by 2030 save 0.2C?: It's more like 0.1C on top of current targets if achieved globally, but pledges only covers 50% of global emissions, ex. Russia and China. However, 0.05C cooling benefit is substantial and immediate carbonbrief.org/guest-post-the…

It's a little more complex as measures this decade will reduce SO2 emissions, leading to warming, so in fact methane cuts are needed to ameliorate this short-term warming constrain-eu.org @CONSTRAIN_EU @JoeriRogelj

Additional methane measures are important though to keep 1.5C in sight. Also huge benefits for surface ozone, air quality and plants @IPCC_CH WGI SPM @SophieSzopa,Vaishali Naik

Both @IPCC_CH WGI AR6 and the SR1.5 report are clear though that BOTH CO2 and CH4 emission reductions needed this decade for 1.5C, AR6 Fig SPM.4

3. Is CH4 84x worse than CO2? No!!! The 84 quoted is a 20 year global warming potential (GWP). But under no circumstances could this ever be a good measure to use, it crazily overestimates methane's impact by at least x10 AR6 fig 7.22

The UNFCCC use a 100 year GWP for accounting approx. 29x CO2, but even this overestimates effect of CH4 in net zero pathways... @civiltalker @ClimateFramo theconversation.com/cop26-a-global…

Issue is that CH4 short lived gas and CO2 long lived so no easy way to compare --> if we don't get to net zero CO2 soon, CO2 warming will wipe out any methane benefit.

Good news is that methane emission reductions start to reverse methane's contribution to warming, whereas CO2s emission reductions only slow warming, only reversing warming when global net negative emissions achieved.

4. All this being said, lots of win-win options for methane abatement - so let's get on with it. a) we know how to stop leaks, food waste and improve agriculture @mikehemsley energy-transitions.org/publications/k…

Check out the #methanepavilion @COP26 for more more more methanemoment.org @sarahuhl