A drop in air pollution during COVID-19 lockdowns altered atmospheric chemistry, leading to a sharp rise in methane concentrations from 2020 to 2022. Researchers attribute most of this surge to fewer hydroxyl radicals that normally break down the potent greenhouse gas. The findings highlight potential risks as countries reduce emissions further.
The COVID-19 lockdowns in 2020 drastically cut emissions from transportation, aviation, and shipping, including nitrogen oxides (NOx). These compounds help produce hydroxyl radicals (OH), which degrade methane in the atmosphere. With less NOx, OH levels fell, allowing methane to accumulate faster.
Methane, which traps heat more effectively than carbon dioxide but persists for only about a decade, had been rising since the 1980s, initially from fossil fuel leaks and later from microbial activity in wetlands, agriculture, and landfills. The growth rate jumped from around 20 million tonnes per year to 40 million tonnes between 2020 and 2022, before dropping back to 20 million in 2023.
A study led by Shushi Peng at Peking University modeled these changes and found that the decline in OH radicals from 2020 to 2021, followed by a recovery in 2022-2023, explained 83 percent of the variation in methane growth. Aviation emissions stayed low into 2021, and other sectors rebounded slowly. The remaining increase came from wetlands, fueled by La Niña-driven rains expanding areas like the Sudd and Cuvette Centrale in Africa, wetter conditions in Asian rice paddies, and warming in Arctic regions.
"It’s like having a hangover or something from our addiction to fossil fuels," says Matthew Johnson at the University of Copenhagen, who was not involved. "We’re emitting [methane] pollution and the catalyst at the same time, so if we reduce emissions of the catalyst, the pollution takes over."
Peng warns that as China and India shift to electrification, cutting NOx further could weaken the methane sink. "The air will become more and more clean, so it means that we have less and less methane sink in the atmosphere," he says. "So we need to reduce more and more anthropogenic emissions."
However, estimates of OH are uncertain, with some models predicting a decrease and others an increase. Paul Palmer at the University of Edinburgh notes surprise at OH's dominance over emission changes and calls for re-examining tropospheric controls. Regardless, rising wetland emissions from climate feedbacks mean human sources, like coal mine vents and oil leaks, must be curbed urgently.
In a related commentary, Euan Nisbet and Martin Manning highlight opportunities in China and India to capture methane from landfills and sewage. "We have to do something, because the system is starting to spin out of control," Johnson adds.
