At the end of the coronavirus pandemic, climatologists were startled. Atmospheric concentration of methane — the second largest greenhouse gas (GHG), after CO2 — jumped suddenly. The effect of confinement on the oil and gas industry? A temporary climatic phenomenon? Scientists set to work to solve the riddle. A few years later, there is growing consensus on a hypothesis to explain this sudden surge in methane concentration: the flooding of tropical wetlands.
Since the start of the industrial revolution, the quantity of methane (CH4) in the atmosphere has tripled. This gas alone warmed the Earth by 0.6°C. The majority (60%) of methane comes from human activities. It unearths coal, oil and gas operations, but also rice fields, livestock farms and dumps. Methane of natural origin (40%) mainly emanates from wetlands. It forms where organic matter decomposes in the absence of oxygen, such as in swamps, mangroves, peat bogs and salt marshes.
Determining the origin of a gas diluted to 0.0002% in the atmosphere is, however, difficult. To achieve this, an international team looked at the data collected since 2010 by the Japanese GOSAT satellite. Using a digital model, the researchers rewound the instrument’s readings to determine the origin of the methane emissions. Their results, published last week in the journal Proceedings of the National Academy of Sciences (PNAS), show that tropical wetlands fueled the rapid increase in methane in 2020-2022, but also the more moderate increase observed during the previous decade (2010-2019).
These scientists associate the surge in natural methane emissions to two regions, equatorial Asia and Africa, which suffered significant flooding during the pandemic years, when a La Niña climatic episode increased rain. Their conclusions are “entirely plausible,” observes Michelle Garneau, professor of geography at the University of Quebec in Montreal and specialist in carbon in northern wetlands. “When we have water, heat and organic matter, we have the ideal cocktail to promote the activity of the bacteria which produce methane,” she summarizes.
A possible vicious circle
You should know that climate change dries out arid areas, but can also soak rainy regions. By altering the distribution of precipitation, they have the potential to amplify flooding in tropical wetlands, and therefore increase their methane emissions: a vicious circle that scientists call the “methane-wetlands feedback loop”.
For years, there was talk of the “methane bomb” that could explode in the Arctic regions. By softening, the permafrost must have given free rein to the microbes in its sludge. And these microbes would generate millions of tons of methane.
However, specialists are beginning to doubt that this will happen. “We are no longer certain of everything, of all, of all,” explains M.me Garneau, who studies northern ecosystems. The reason? The Arctic is greening. Heat and rain accelerate plant growth in peatlands, which promotes carbon storage. The new study published in PNAS also shows that methane emissions from high northern latitudes decreased in 2020-2022.
The authors of the scientific publication also conclude that the 2020 pandemic lockdowns had a “relatively insignificant” effect on the concentration of methane in the atmosphere. However, this effect is not zero. The phenomena at work are indirect: because we burned fewer fossil fuels, we emitted less nitrogen oxide (NOx). However, a lesser presence of this pollutant slows down the rate of degradation of methane in the atmosphere, which therefore persists there longer.
An open debate
Jason McKeever, the scientific director of GHGSat, a Montreal company that specializes in CO measurement2 and methane from space, believes that the data published in PNAS do not put an end to the riddle of the pandemic methane outbreak. “In atmospheric sciences, it is difficult to have precise data because we are interested in very delicate balances,” he explains. Tropical wetlands certainly play a role, but other contributions should not be overlooked. “The debate is perhaps still open,” thinks the physicist.
One thing is certain: more precise measurements of methane emissions in equatorial wetlands are needed. Climate projections – those of the IPCC, for example – do not take them directly into account. In recent years, planes have flown over large wetlands in Brazil, Bolivia and Zambia to measure methane flows. Surprises awaited the researchers. In Africa, they observed emissions several times higher than expected.
There is therefore a significant “understanding” challenge, according to Mr. McKeever. But to slow down the runaway climate, it is not the wetlands that we must look at. “We must target what is anthropogenic, which comes primarily from the petrochemical industry,” says the specialist. “We have ways to limit it, by modernizing and repairing infrastructure,” he says. The efforts are economic and political. In 2021, around a hundred countries, including Canada, have committed to reducing human-caused methane emissions by 30% by 2030.