Why atmospheric methane surged in 2020?

<p><strong>Methane (CH₄) levels in the atmosphere increased by 15.1 &#177; 0.4 ppb in 2020, the highest annual increase from 1984 to 2020, despite a likely decrease in anthropogenic CH₄ emissions during COVID-19 confinements. Here, we used bottom-up and top-down methods to quantify the changes in different sources of CH₄, and in its atmospheric sink due to the hydroxyl radical (OH) in 2020 compared to 2019. Bottom-up methods showed that, globally, total anthropogenic emissions slightly decreased by ~1.2 Tg CH₄ yr^-1, fire emissions were lower than in 2019 by ~6.5 Tg CH₄ yr^-1, and wetland emissions increased by 6.0 &#177; 2.3 Tg CH₄ yr^-1. In addition to higher wetland emissions in 2020 than 2019 from bottom-up, we found a decrease of 1.6&#8211;1.8% in tropospheric OH concentration relative to 2019, mainly due to lower anthropogenic NO_x emissions and associated lower free tropospheric ozone during the confinements. Based on atmospheric CH₄ observations from the surface network, and considering the decrease in OH, using top-down inversions, we infer that global net emissions increased by 6.9 &#177; 2.1 Tg CH₄ yr^-1 in 2020 relative to 2019, while the global CH₄ removal from reaction with OH in the atmosphere decreased in 2020 by 7.5 &#177; 0.8 Tg CH₄ yr^-1. Therefore, we attribute the positive growth rate anomaly of atmospheric CH₄ in 2020 relative to 2019 to lower OH</strong><strong> sink (53 &#177; 10%) and higher natural emissions (47 &#177; 16%), mostly from wetlands. Warmer and wetter climate conditions in the Northern Hemisphere promoted wetland emissions, but fires decreased in the Southern Hemisphere, compared to the previous year. Our study highlights&#160;that northern microbial emissions of CH₄ are highly sensitive to a warmer and wetter climate and could act as a positive feedback in the future. Our study also hints that the global CH₄ pledge must be implemented by taking into account NO_x emissions trend, whose reduction lengthens the lifetime of atmospheric CH₄. </strong></p>