'It's telling us there's something big going on': Unprecedented spike in atmospheric methane during the COVID-19 pandemic has a troubling explanation

Methane is a greenhouse gas around 30 times more potent than carbon dioxide and has been increasing in concentration in the atmosphere since measurements began. However, in 2020 scientists were bemused by a sudden unexplained spike in atmospheric levels. With so many possible sources and sinks of this gas, untangling the origins of this anomaly has proven a complex task but researchers think they may now have solved the mystery.

The unprecedented spike in atmospheric methane in 2020 was actually caused principally by reduced human emissions during the pandemic, which temporarily stopped the atmosphere from breaking down the gas, according to a new study.

Lower levels of nitrous oxides — which are released by combustion engines in cars, among other sources — weakened the atmosphere's natural cleanup capability. This, in turn, prompted a dramatic surge in methane as travel ground to a halt in early 2020, and returned to pre-pandemic levels in 2023 as society went back to normal.

The study, reported in the journal Science Feb. 5, combined satellite data, ground station measurements and complex models to untangle the possible sources of the extra gas. It also identified a substantial increase in natural emissions as a secondary contributor to the methane spike.

Atmospheric methane has been increasing steadily since records began, but measurements taken in 2019-2020 revealed an alarming acceleration in this trend. The annual increase almost doubled, reaching 16.2 parts per billion, compared with a more moderate rise of 8.6 parts per billion over the previous 10 years. In the years since, various hypotheses have been put forward to explain this unexpected spike — including rising fossil fuel use, wetland and agricultural emissions, wildfires, and changes in atmospheric chemistry — but untangling which factors were actually responsible is an immensely complex task.

Taking a comprehensive approach, the researchers combined physical data from across the globe with modeling studies and simulations to evaluate the potential contribution of each source.

Their analysis revealed that a staggering 83% of 2020's methane peak likely resulted from a reduction in the atmosphere's ability to remove methane — a phenomenon directly tied to the disruption of human activities caused by the pandemic.

Oxidizing 'all the nasties'

Specifically, the sudden drop in industrial emissions — most notably, toxic nitrous oxides — dramatically decreased the production of hydroxy (OH) radicals in the atmosphere.

"OH is the cleanup molecule of the atmosphere," Euan Nisbet, a professor of Earth sciences at Royal Holloway University of London who was not involved in the research but wrote an accompanying perspectives article on the findings, told Live Science. "It oxidizes all the nasties — it turns carbon monoxide to CO₂, and by grabbing hydrogens, it turns methane into CO₂."

The team fed satellite data about the precursor molecules to OH into a model to map the concentration and distribution of these cleansing radicals between 2019 and 2023. This revealed a sharp decrease in 2020, which is consistent with the observed rise in methane levels. Then, they compared this result with a second model, generated from measured emissions and wind patterns, further confirming the hypothesis that reduced human emissions were the main contributor to elevated methane.

However, cautioned Nisbet, this doesn’t mean that fossil fuel use is the answer to rising methane levels. Although a less potent greenhouse gas, CO₂ persists much longer in the atmosphere so a move to cleaner fuels is still an urgent priority.

The remaining 20% of the spike was therefore the result of direct methane emissions. Working backward from satellite measurements, climate data and isotope ratios, the team created a series of additional "inversion" models to pinpoint the precise source of these emissions.

The relative levels of carbon-12 and carbon-13 isotopes — both versions of carbon with different chemical masses — were particularly crucial to this process. "The sources affect the isotopes, so you've got to fit the isotope data as well," Nisbet said.

Biology prefers to use lighter carbon-12, meaning biological sources of methane, such as cattle or wetlands, have a different effect on the proportions of carbon-12 and carbon-13 in the atmosphere than geological sources like fossil fuels do, Nisbet explained.

"So one of the conclusions to come out of this is that the fossil fuel methane emissions are relatively static," he said. "On the other hand, the biological emissions have grown quite strongly, and that's most probably in wet Africa."

The observed increase in methane emissions between 2020 and 2023 coincided with extremely wet conditions across tropical Africa that resulted from an unusually extended La Niña period and the Indian Ocean Dipole climate oscillation.

"Over recent years, it's been causing huge amounts of rainfall in East Africa, particularly the Nile basin that then floods the Sud, which is one of the most productive wetlands in the world," Nisbet said. "Very wet and very warm means big swamps — cows, antelope and buffalo, and a lot of papyrus growing, dying, rotting and turning into methane."

In 2023, the end of both the pandemic and the wet La Niña conditions across the tropics saw methane increases stabilize back to pre-2020 levels. But while the world appears to have recovered from this temporary blip, that it happened at all is an urgent call to action, Nisbet said.

"It's a first indicator of the state of the global climate," Nisbet said. "Methane has a period of 10 years, so it's turning over all the time and telling us there's something big going on. This is a climate feedback and the big biological sources are turning on, so we've got to work twice as hard."