Abstract/Summary

Compared to 2019, measurements of the global growth rate of background (marine air) atmospheric methane rose by 5.3 ppb/yr in 2020, reaching 15.0 ppb/yr. Global atmospheric chemistry models have previously shown that reductions in nitrogen oxide (NOx) emissions reduce levels of the hydroxyl radical (OH), and lengthen the methane lifetime. Acting in the opposite sense, reductions of carbon monoxide (CO) and non-methane volatile organic compound (NMVOC) emissions increase OH and shorten methane’s lifetime. Using estimates of NOx, CO and NMVOC emission reductions associated with COVID-19 lockdowns around the world in 2020, together with model-derived regional and aviation sensitivities of methane to these emissions, we find that NOx emissions reductions led to a 4.8 (3.8 to 5.8) ppb/yr increase in the global methane growth rate. Reductions in CO and NMVOC emissions partly counteracted this, changing (reducing) the methane growth rate by -1.4 (-1.1 to -1.7) ppb/yr (CO) and -0.5 (-0.1 to -0.9) ppb/yr (NMVOC), yielding a net increase of 2.9 (1.7 to 4.0) ppb/yr. Uncertainties refer to ±1 standard deviation model ranges in sensitivities. Whilst changes in anthropogenic emissions related to COVID-19 lockdowns are probably not the only important factor that influenced methane during 2020, these results indicate that they have had a large impact, and that the net effect of NOx, CO and NMVOC emissions changes can explain over half of the observed 2020 methane changes. Large uncertainties remain in both emissions changes during the lockdowns and methane’s response to them; nevertheless, this analysis suggests that further research into how atmospheric composition changed over the lockdown periods will help us to interpret past methane changes and to constrain future methane projections.