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.
Highlights
Methane is a powerful greenhouse gas and important precursor of tropospheric ozone; both are key air pollutants and shortlived climate forcers (SLCFs)
Compared to 2019, the global growth rate of atmospheric methane rose by about 50% in 2020, reaching 15 ppb/yr. 10 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
Summary
Methane is a powerful greenhouse gas and important precursor of tropospheric ozone; both are key air pollutants and shortlived climate forcers (SLCFs). Advances in our understanding of how GWP metrics relate to climate change indicate that it is the magnitude of emissions, and the rates of change of SLCFs like methane that strongly influence near-term global temperature changes (Allen et al, 2018; Cain et al, 2019). These post-2013 updates increase the importance of methane and its evolution in the context of the Paris Climate Agreement target that seeks to limit warming to 1.5°C above pre-industrial levels. Measurements of nitrogen dioxide (NO2) from satellite instruments and nitrogen monoxide (NO) and NO2 from surface sites 45 show that levels of atmospheric NOx (NO + NO2) dramatically fell globally during 2020 (Bauwens et al, 2020) as COVID19 lockdowns around the world reduced emissions, mainly from transportation (Venter et al, 2020; Lamboll et al, 2021)
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