02 Sep 2022
02 Sep 2022
Status: a revised version of this preprint is currently under review for the journal ACP.

Atmospheric data support a multi-decadal shift in the global methane budget towards natural tropical emissions

Alice Drinkwater1,2, Paul Palmer1,3, Liang Feng1,3, Tim Arnold1,2, Xin Lan4,5, Sylvia Michel6, Robert Parker7,8, and Hartmut Boesch7,8 Alice Drinkwater et al.
  • 1School of GeoSciences, University of Edinburgh, Edinburgh, UK
  • 2National Physical Laboratory, Teddington, UK
  • 3National Centre for Earth Observation, University of Edinburgh, Edinburgh, UK
  • 4Cooperative Institute for Research in Environmental Sciences, University of Colorado Boulder, Boulder, CO, USA
  • 5Global Monitoring Laboratory, National Oceanic and Atmospheric Administration, Boulder, CO, USA
  • 6Institute of Arctic and Alpine Research, University of Colorado Boulder, Boulder, CO, USA
  • 7National Centre for Earth Observation, Space Park Leicester, University of Leicester, UK
  • 8Earth Observation Science, School of Physics and Astronomy, University of Leicester, UK

Abstract. We use the GEOS-Chem global 3-D model and a Maximum A Posteriori inverse method to infer regional methane emissions and the corresponding carbon stable isotope source signatures, 2004–2020, across the globe using in situ and satellite remote sensing data. Over our study period, we find consistent evidence from both atmospheric CH4 datasets of a progressive increase of methane emissions at tropical (30° N to 30° S) latitudes (+3.80 Tg/yr/yr), accompanied by a progressively lighter atmospheric δ13C signature, consistent with increasing natural emissions. The satellite remote sensing data provide evidence of higher spatially resolved hotspots of methane that are consistent with the location and seasonal timing of wetland emissions, limiting the hypothesis about the hydroxyl radical (OH) sink for methane playing a significant role in observed global growth in atmospheric methane. We find that since 2004, the largest growing regional contributions (2004–2020) are from North Africa (+19.9 Tg/yr), China (+21.6 Tg/yr), and Tropical South America (+14.2 Tg/yr). To quantify the influence of our results to 10 changes in OH, we also report regional emission estimates using an alternative scenario of a 0.5 %/yr decrease in OH since 2004, followed by a 5 % drop in 2020 during the first COVID-19 lockdown. We find that our main findings are robust against those year-to-year changes in OH.

Alice Drinkwater et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2022-561', Anonymous Referee #1, 10 Oct 2022
  • RC2: 'Comment on acp-2022-561', Anonymous Referee #2, 19 Oct 2022
  • AC1: 'Comment on acp-2022-561', Alice Drinkwater, 09 Dec 2022

Alice Drinkwater et al.

Alice Drinkwater et al.


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Short summary
This work concerns global inversions of CH4 and δ13C over the period 2004–2020. We use both ground-based and satellite data. The results show increasing tropical emissions, particularly over North Africa, Tropical Asia, and Tropical South America, at the same time as mid-latitudinal emission proportion decreases. The isotope data suggest natural sources (wetlands) are driving increases, which presents a problem for emissions mitigation as these natural emissions are challenging to reduce.