Thread by @ciais_philippe

Why did #methane surge in 2020 ?

Our article just published @Nature shows that the high growth rate of CH4 in 2020 is explained half by decreased OH removal & half by more wetland emissions @ShushiPeng @CEA_Officiel @gcarbonproject @IPSL_outreach … 🧶

nature.com/articles/s4158…

We expected in 2020 less anthropogenic emissions from #COVID but in fact the #CH4 growth rate was record high at 15.1 ppt y-1 (@NOAA data )

The #CH4 growth rate results from an imbalance between emissions ( natural and anthropogenic ) and the OH sink in the atmosphere @gcarbonproject

So what happened in 2020 ?

More natural emissions and / or less OH removal ?

Anthropogenic emissions show a small decrease in 2020 compared to 2019 from inventories of agriculture, waste and fossil fuels extraction and use ( @IEA data )

Wetland emissions from wetland models increased ( 2 models LPJ @carBenPoulter and ORCHIDEE @ShushiPeng driven by different climate datasets )

Wetland areas experienced wetter and warmer conditions

But large uncertainties from models / climate datasets +2.5 to +9.4 Mt CH4 y-1

Fire emissions in 2020 were « normal » 6 MtCH4 less than in 2019, that was an extreme years from #amazon and #australia

But all in all, the sum of bottom-up estimates did not explain the abnormal #CH4 growth rate in 2020

Still an a gap of 13 MtCH4 y-1 growth to explain !

Here we come to the hard part : how did OH change ?

We used a 3D chemistry model with realistic drop of NOx & CO anthropogenic emissions @Carbon_Monitor and found a 1.6% drop of OH

Less NOX pollution -> less OH -> more CH4

See below drop of NOx emissions & OH change map

The OH decrease explains half of the growth rate anomaly in 2020 compared to 2019.

OH decrease is consistent with an independent estimate based on F-gases concentrations @rona_thomp = 2% drop

To reconcile atmospheric signals with bottom up emissions, we used a 3D atmospheric inversion model using surface stations data or #GOSAT satellite data

The solution of the inversion gives more sources in the northern hemisphere and confirms the smaller OH sink

See red areas in the North where emissions increased ( northern wetlands, Africa, South and South East Asia )

The methane high growth rate enigma is explained 50% by decreased OH from less pollution (NOx) and 50% bu enhanced natural (#wetlands) sources

Colors : where the emission change ( + or - ) is dominated by a specific source type

Two implications from the « large scale natural experiment «  of the year 2020 ( note high growth rates continued in 2021 )

1) in a wetter and warmer year, #wetland emissions increase strongly, suggesting that in the future, #wetlands could emit more CH4 and amplify warming …

2) reducing NOx pollution increased CH4 in 2020

All future @IPCC scenarios predict NOX reductions, especially when reducing CO2 emissions, and will cause an extra growth of #CH4Th

This NOX CH4 feedback is included in simple climate models like MAGICC used by IPCC @JoeriRogelj but based on simple equations. Where and when NOx reductions happen will matter a lot for OH and #CH4 and simple models may underestimate the CH4 increase from less pollution

Take home below

Links to

Paper doi.org/10.1038/s41586…

Presentation sharebox.lsce.ipsl.fr/index.php/s/bY…

🙏 Huge thanks to an incredible team of co authors 🙏 working over last year to put together the jigsaw puzzle @MethaneData @CLAND_saclay

@ShushiPeng @rona_thomp @carBenPoulter @BoZheng18 @mic_ramonet @BiogeoYi @NOAA M Saunois, X Lin, D Hauglustaine, G Liu, X Lan, Z Zhang