In 2019 and 2020, the annual growth rate of atmospheric methane almost doubled, despite COVID-19-related lockdowns that drastically reduced greenhouse gas emissions overall.
“Why is that? We just didn’t know,” said Euan Nisbet, emeritus professor of Earth sciences at the Royal Holloway University of London.
Although the debate is far from settled, a new paper suggests the methane spike was a result of natural, weather-related wetland emissions and changes in atmospheric chemistry associated with COVID-19-related lockdowns.
Radicals and Emissions
Methane levels have been rising since the Industrial Revolution, but scientists noticed a discrete spike between 2019 and 2023. Prior to this period, methane was increasing at a more moderate pace, for example, 8.7 parts per billion per year in 2018. This annual growth rate rose to more than 9 parts per billion per year in 2019 and surged to 16.2 parts per billion per year in 2020 before falling back to 8.6 parts per billion per year by 2023.
Most analyses attributed the surge to increased weather-related emissions from tropical wetlands and ruminant animals like cows and bison.
But the new study, published last month in Science, offers a more comprehensive explanation, one in which changes in atmospheric chemistry associated with COVID-19 shutdowns also contributed to increased methane levels.
Starting in 2020, governments introduced a series of lockdowns to stop the spread of COVID-19: Cars came off the roads, planes sat on the tarmac, the world was put on pause.
Carbon emissions declined by 2.4 billion tons, and emissions of other greenhouse gases also fell. Among those gases were nitrogen oxides (NOx), which serve as precursor gases that contribute to the formation of hydroxyl radicals (OH) in the atmosphere.
“OH is essentially the laundry of the air.”
Reduced NOx concentrations weakened the photochemical cycling that sustains hydroxyl radicals, leading to a decline in atmospheric OH.
“OH is essentially the laundry of the air,” explained Nisbet, who was not involved in the new research but coauthored an accompanying perspective in the same issue of Science. OH radicals are largely responsible for converting methane to carbon dioxide and water vapor, therefore acting as an atmospheric methane sink.
This “weaker ‘self-cleaning’ capacity of the atmosphere” explains much of the methane spike, “but not all of it,” said Philippe Ciais, a researcher at the Laboratoire des Sciences du Climat et de l’Environnement in Paris and lead author of the new study.
Increased emissions from wetlands also likely contributed to the spike, said Shushi Peng, a professor at Peking University in Beijing and one of the paper’s authors. “If we have a warmer and wetter climate, we expect that the [wetland] methane emissions increase,” he said.
And satellite observations did show elevated methane concentrations over tropical wetlands in East Africa and the Amazon during this period, consistent with La Niña–driven increases in precipitation. These signals weakened in 2023, when drier El Niño conditions coincided with a decline in inferred wetland emissions.
Policy-Relevant Implications
The exact causes of the 2020 methane spike remain disputed among the scientific community. Some scientists attribute it primarily to weather-related wetland emissions, while others now hypothesize that much of the year-to-year variation in methane growth rates can be linked to a temporary decline in atmospheric OH concentrations.
Paul Palmer, a professor at the University of Edinburgh who was not involved in the new research, said that the implications of studies like this are important to how governments approach climate change.
“I am very interested in how much nature is going to change in a warming world,” he said. “But there are serious political and policy-relevant implications for understanding whether it’s emissions or OH.”
—Ryan Green, Science Writer
Citation: Green, R. (2026), COVID-19 shutdowns saw human emissions slow, but atmospheric methane surged, Eos, 107, https://doi.org/10.1029/2026EO260077. Published on 6 March 2026.
Text © 2026. The authors. CC BY-NC-ND 3.0
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