Reply on AC1

Abstract

The global atmospheric methane growth rates reported by NOAA for 2020 and 2021 are the largest since systematic measurements began in 1983. To explore the underlying reasons for these anomalous growth rates we use newly available methane data from the Japanese Greenhouse gases Observing SATellite (GOSAT) to estimate methane surface emissions. Relative to baseline values in 2019 we see the largest annual increases in methane emissions during 2020 over Eastern Africa (13 Tg), tropical Asia (4 Tg), tropical South America (3 Tg), and temperate Eurasia (3 Tg), and the largest reductions over China (-6 Tg) and India (-2 Tg). We find comparable emission changes in 2021, relative to 2019, except for tropical and temperate South America where emissions increased to 9 Tg and 5 Tg, respectively, and tropical Asian emissions increased to 8 Tg. The elevated contributions we saw in 2020 over the western half of Africa (-5 Tg) and Europe (-3 Tg) are substantially reduced in 2021, compared to our 2019 baseline. We find statistically significant positive correlations between anomalies of tropical methane emissions and groundwater, consistent with recent studies that have highlighted a growing role for microbial sources over the tropics. Emission reductions over India and China are expected in 2020 due to the Covid-19 shutdown but continued in 2021, which we do not currently understand. Based on a sensitivity study for which we assume a conservative 5 % decrease in hydroxyl concentrations in 2020, due to reduced pollutant emissions during the Covid-19 shutdown, we find that the global increase in our a posteriori emissions in 2020 is ~22 % lower than our control calculation. We conclude therefore that most of the observed increase in atmospheric methane during 2020 and 2021 is due to increased emissions.