Methane Surge: Atmosphere Briefly Halted Breakdown

A temporary weakening of the atmosphere's chemical capacity to break down methane, combined with elevated emissions from tropical wetlands, drove the sharp increase in atmospheric methane observed in 2020 to 2021, according to a new study. Methane (CH₄) is a significant contributor to atmospheric warming. In the early 2020s, the amount of atmospheric CH₄ grew faster than ever before observed, peaking at 16.2 parts per billion per year (ppb yr^-1), before declining to 8.6 ppb yr^-1 in 2023. It's hypothesized that this surge was driven by a combination of increased natural emissions and a coincident decrease in the atmosphere's oxidizing capacity, namely, fewer OH radicals available to chemically break down CH₄ in the atmosphere. However, the main drivers behind this event remain poorly understood, largely due to the difficulty in separating the impacts of changes in CH₄ emissions from changes in oxidizing capacity from OH radicals, particularly during the COVID period when OH precursor emissions were globally reduced. To resolve these uncertainties, Philippe Ciais and colleagues combined atmospheric models and a large suite of bottom-up inventories for various CH₄ sources to update the global and regional CH₄ budget for the period 2019 to 2023. Ciais et al. found that the rapid surge and subsequent decline in CH₄ growth between 2019 and 2023 were primarily driven by changes in the atmosphere's ability to destroy CH₄, rather than by methane emissions alone. According to the findings, a global drop in OH radicals in 2020 to 2021 – followed by their recovery in 2022 to 2023 – explains about 80% of the year-to-year variation in CH₄ growth rate. The remaining share was largely due to increased CH₄ emissions from wetlands and inland waters, particularly in tropical wetlands in Africa, Asia, and the Arctic. In an accompanying Perspective, Euan Nisbet and Martin Manning discuss the study's findings in greater detail.