Why methane surged in the atmosphere during the early 2020s.

The atmospheric methane (CH4) growth rate surged after 2019, peaking at 16.2 parts per billion per year (ppb year−1) in 2020 before declining to 8.6 ppb year−1 in 2023. Using multiple atmospheric inversions constrained by observation- and model-based prescribed hydroxyl radical (OH) fields and CH4 atmospheric data, we show that a drop of OH radicals in 2020–2021, followed by recovery in 2022–2023, accounted for 83% of year-on-year variations in the CH4 growth rate, the rest being explained by wetland and inland water emissions, which increased between 2019 and 2020–2022 [+8.6 ± 2.6 teragrams of CH4 per year (TgCH4 year−1)] and then decreased between 2022 and 2023 (−9.9 ± 3.3 TgCH4 year−1). Most emission changes from 2019 to 2023 occurred in northern tropical wetlands in Africa and Asia, whereas South American wetlands emissions declined and Arctic emissions increased after 2019. Editor's summary: Methane, the second most important trace greenhouse gas after carbon dioxide, experienced an unexpected surge in its growth rate after 2019. Ciais et al. showed that the primary cause for this sudden increase was a decrease in the abundance of atmospheric hydroxyl radicals, the species most responsible for the oxidation of atmospheric methane and thus its major sink. This was compounded by subsequently larger wetland and inland water emissions (see the Perspective by Nisbet and Manning). The change in hydroxyl radical abundance was global, whereas the growing methane source was the result of increased emissions from northern tropical wetlands in Africa and Asia and from the Arctic. —Jesse Smith INTRODUCTION: Atmospheric methane (CH4), the second most important anthropogenic greenhouse gas after carbon dioxide (CO2), experienced an unprecedented surge in the early 2020s. After rising steadily since 2007, the CH4 growth rate accelerated sharply in 2019–2020, peaking at 16.2 parts per billion per year (ppb year−1) in 2020, before declining to 8.6 ppb year−1 by 2023. The drivers of this rise and fall have been unclear, with hypotheses pointing to both enhanced natural emissions and shifts in atmospheric chemistry. RATIONALE: This study integrated multiple lines of evidence—including satellite retrievals of column CH4 mixing ratio (GOSAT), ground-based atmospheric observations from the National Oceanic and Atmospheric Administration (NOAA)'s global network, updated OH (hydroxyl radical) fields, and three atmospheric inversion systems (PYVAR-LMDz, WESTLAKE, and GONGGA)—to disentangle the processes underlying the methane surge. This approach allowed the explicit inclusion of year-to-year variations in OH, the dominant methane sink, using constraints provided by its observed concentrations and recent emissions of its precursors inferred by using a chemistry-climate model. RESULTS: Our analysis shows that reductions in OH during the COVID-19 period (2020–2021), followed by a recovery through 2023, accounted for ~80% of the interannual variations in the CH4 growth rate. These reductions were linked to declines in NOx and CO emissions from human activities during the COVID-19 lockdowns, which temporarily weakened the oxidizing capacity of the atmosphere. When OH rebounded, the methane sink strengthened, slowing the growth rate. Superimposed on this sink variability were substantial changes in surface emissions. Inversions constrained with OH fields revealed a global emission increase of 22 teragrams of CH4 per year (TgCH4 year−1) between 2019 and 2020, driven mainly by wetlands and inland waters. Emissions peaked during the extended La Niña period from June 2020 to June 2023, when wetter conditions expanded inundated areas and raised soil moisture, particularly across tropical Africa (Sudd and Cuvette Centrale) and Southeast Asia. The isotopic composition of atmospheric methane (δ13CCH4) provided an additional constraint. Observed declines in δ13CCH4 were consistent with microbial sources (wetlands, agriculture, inland waters) dominating global emission changes, whereas scenarios relying heavily on fossil or fire emissions were inconsistent. Inversions that over- or underestimated fossil fuel and fire contributions failed to reproduce the observed isotopic shifts, underscoring the importance of microbial processes and their sensitivity to recent climate anomalies. Bottom-up inventories, based on wetland models, inland water emission simulations, livestock statistics, rice models, fossil fuel data, and fire datasets, showed significant mismatches with the top-down inversion results. Although bottom-up estimates captured a general trend of increasing emissions from 2019 to 2023, they underestimated the sharp rise of wetland and inland water emissions in 2020–2022 in the northern tropics, predicted a large decrease of wetland emissions in the southern tropics, and overestimated emissions in 2023. This mismatch highlights persistent weaknesses in present wetland and inland water models and the need for better monitoring of inundation dynamics, small-scale wetlands, and soil water-table depth. CONCLUSION: The methane surge of the early 2020s resulted from a combination of reduced atmospheric sink capacity and climate-driven boosts to microbial emissions. The temporary collapse of OH during COVID-19 explains most of the year-to-year variability, and the wetter La Niña climate amplified wetland and inland water emissions. The subsequent decline in 2023 reflected both OH recovery and drought-induced emission reductions. These findings underscore the importance of integrating atmospheric chemistry, climate variability, and microbial processes to explain methane dynamics. They also reveal critical gaps in bottom-up models, calling for improved wetland monitoring, higher-resolution inversions, and better isotopic measurements. By providing an updated global methane budget through 2023, this study clarifies the drivers of recent methane variability and sets priorities for reducing uncertainties in future methane monitoring and mitigation. Attribution of the early 2020s global methane surge.: Recent changes in methane emissions and OH removal relative to 2019 for (A) different source sectors and (B) different processes. [ABSTRACT FROM AUTHOR]