Increase in atmospheric methane during pandemic mainly due to flooding of wetlands, satellite data analysis finds

A new analysis of satellite data reveals that the record increase in atmospheric methane emissions between 2020 and 2022 was driven by increased flooding and water storage in wetlands, combined with a slight decrease in atmospheric hydroxides (OH). The findings have implications for efforts to reduce atmospheric methane and mitigate its impact on climate change.

The research is published in the journal Proceedings of the National Academy of Sciences.

“Between 2010 and 2019, we observed steady increases—with slight accelerations—in atmospheric methane concentrations, but the increases that occurred between 2020 and 2022 and coincided with the COVID-19 shutdown were significantly higher,” said Zhen Qu, assistant professor of marine, earth, and atmospheric sciences at North Carolina State University and lead author of the study. “Global methane emissions increased from about 499 teragrams (Tg) to 550 Tg during 2010–2019, followed by a sharp increase to 570–590 Tg between 2020 and 2022.”

Atmospheric methane emissions are expressed by their mass in teragrams. One teragram is equivalent to approximately 1.1 million US tons.

One of the leading theories regarding the sudden increase in atmospheric methane was the decrease in human-caused air pollution from automobiles and industry during the pandemic shutdowns of 2020 and 2021. Air pollution contributes to the formation of hydroxyl radicals (OH) in the lower atmosphere. In turn, atmospheric OH interacts with other gases, such as methane, to break them down.

“The prevailing idea was that the pandemic had reduced the concentration of OH, and therefore there was less OH available in the atmosphere to react and remove methane,” Qu says.

To test the theory, Qu and a team of researchers from the United States, the United Kingdom and Germany looked at global satellite emissions data and atmospheric simulations for methane and OH over the period 2010 to 2019 and compared them to the same data from 2020 to 2022 to determine the source of the surge.

Using data from satellite surveys of atmospheric composition and chemical transport models, the researchers created a model that allowed them to determine both the amounts and sources of methane and OH for both periods.

They found that most of the increase in methane concentrations between 2020 and 2022 was due to flooding in equatorial Asia and Africa, which accounted for 43% and 30% of the increase in atmospheric methane concentrations, respectively. Although OH concentrations declined during this period, this decline accounted for only 28% of the increase.

“The heavy rainfall in these wetlands and rice-growing regions is likely related to La Niña conditions from 2020 to early 2023,” Qu said. “Wetland microbes produce methane when they metabolize and decompose organic matter anaerobically, meaning without oxygen. Higher water storage in wetlands means greater anaerobic microbial activity and greater release of methane into the atmosphere.”

Researchers say a better understanding of wetland emissions is important for developing mitigation plans.

“Our results indicate that the wet tropics are the main cause of the increase in methane concentrations since 2010,” says Qu. “Better observation of methane emissions from wetlands and how methane production responds to changes in precipitation is essential to understanding the role of precipitation patterns on tropical wetland ecosystems.”