Jump in Tropospheric Methane Concentrations in 2020–2021 and Slowdown in 2022–2024: New Hypotheses on Causation

Earth’s atmospheric methane (CH4) concentration has risen more than 162% since pre-industrial levels in the mid-18th century, and about 30% of the rise in global temperatures since the pre-industrial era is due to CH4 The build-up of methane in the atmosphere in 2020–2022 was the largest since systematic measurements started in 1983, more than double the average yearly growth rate measured over the previous 17 years (15.2 ppb yr−1 vs. 5.71 ppb yr−1, respectively). During 2020, with a growth rate of 14.81 ppb yr−1, the level of atmospheric CH4 broke the previous record (which was set in 1991), and it was broken again immediately the following year, with an increase of 17.64 ppb yr−1 in 2021. For 2022, the final estimate is 13.25 ppb yr−1, the fourth largest annual growth rate. The most recent explanations for this surge in tropospheric CH4 include increased emissions from tropical wetlands, more floods, and increased temperatures. For 2020 and part of 2021, a reduction in the oxidative capacity of the atmosphere due to COVID-19 lockdowns was also proposed. Our main hypothesis is that this CH4 surge in 2020–2021 may also be caused by reduced sulfate emissions, which have been shown to decrease methanotrophy and increase methanogenesis rates in wetlands. Then, for the CH4 slowdown in 2022–2024, our hypotheses are that the emissions from wetlands remained high, but that there was an even higher increase in the oxidative capacity of the atmosphere due to multiple other parameters that are detailed in this article. This perspective review paper is mainly qualitative; it demonstrates that coupled climate–chemistry models will also need to integrate biochemistry, as the evolution of the atmospheric composition is multifactorial and non-linear.