03 Aug 2021
03 Aug 2021
Status: this preprint is currently under review for the journal ACP.

COVID-19 lockdown NOx emission reductions can explain most of the coincident increase in global atmospheric methane

David Stevenson1, Richard Derwent2, Oliver Wild3, and William Collins4 David Stevenson et al.
  • 1School of GeoSciences, The University of Edinburgh, Edinburgh EH9 3FF, UK
  • 2rdscientific, Newbury, UK
  • 3Lancaster Environment Centre, Lancaster University, Lancaster, UK
  • 4Department of Meteorology, University of Reading, Reading, UK

Abstract. Compared to 2019, the global growth rate of atmospheric methane rose by about 50 % in 2020, reaching 15 ppb/yr. Models of global atmospheric chemistry show that reductions in nitrogen oxide (NOx) emissions reduce levels of the hydroxyl radical, and lengthen the methane lifetime. Using estimates of NOx emission reductions associated with COVID-19 lockdowns around the world in 2020, together with model-derived regional and sectoral sensitivities of methane to NOx emissions, we find that NOx emissions reductions can fully explain the observed surge in the global methane growth rate. Whilst changes in NOx emissions are probably not the only important factor that has influenced methane since the beginning of 2020, it is clear that they are a key factor that will need to be included within any attribution study, and that they may well be the dominant driver of these recent methane changes. The major global scale changes in composition of the Earth’s atmosphere measured during lockdown provide unprecedented constraints on the sensitivity of the atmospheric chemical system to changes in emissions, and are of great utility for evaluating policy-relevant models.

David Stevenson 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-2021-604', Anonymous Referee #1, 31 Aug 2021
  • RC2: 'Comment on acp-2021-604', Anonymous Referee #2, 21 Sep 2021
  • RC3: 'Comment on acp-2021-604', Anonymous Referee #3, 30 Sep 2021
  • AC1: 'Authors' response to 3 referees on acp-2021-604', David Stevenson, 17 Nov 2021

David Stevenson et al.

David Stevenson et al.


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Short summary
Atmospheric methane’s growth rate rose by 50 % in 2020 relative to 2019. Lower nitrogen oxide (NOx) emissions tend to increase methane’s atmospheric residence time. We find that COVID-19 lockdown reductions in NOx emissions can explain the observed changes in methane. Changes in atmospheric composition measured during lockdown provide unprecedented constraints on the sensitivity of the atmospheric chemical system to emissions changes, and are of great use in evaluating policy-relevant models.