If carbon dioxide is an oven steadily roasting our planet, methane is a blast from the broiler: a more potent but shorter lived greenhouse gas that’s responsible for roughly one-third of the 1.2°C of warming since preindustrial times. Atmospheric methane levels have risen nearly 7% since 2006, and the past 2 years saw the biggest jumps yet, even though the pandemic slowed oil and gas production, presumably reducing methane leaks. Now, researchers are homing in on the source of the mysterious surge. Two new preprints trace it to microbes in tropical wetlands. Ominously, climate change itself might be fueling the trend by driving increased rain over the regions.
If so, the wetlands emissions could end up being a runaway process beyond human control, although the magnitude of the feedback loop is uncertain. “We will have handed over a bit more control of Earth’s climate to microorganisms,” says Paul Palmer, an atmospheric chemist at the University of Edinburgh and co-author of one of the studies, posted late last month for review at Atmospheric Chemistry and Physics.
Most climate scientists already agreed that the post-2006 methane spike has largely not come from fossil fuel production. That’s because atmospheric methane has become ever more enriched in carbon-12, the lighter isotope of carbon, reversing what had been a multicentury trend, says Xin Lan, a carbon cycle scientist at the Earth System Research Laboratories (ESRL) of the National Oceanic and Atmospheric Administration. “This is a very significant signal,” she says. It points to microbes as the source because they favor reactions that use light carbon, giving the methane they produce a distinctive light signature.
Yet the isotopic signal cannot distinguish between microbes in a swamp, a landfill, or a cow’s gut. “A cow is a walking wetland,” says Euan Nisbet, an atmospheric chemist at Royal Holloway, University of London. Most researchers think a mix of cattle ranching and landfills in the tropics are the main driver of the post-2006 increase, because they have expanded dramatically alongside populations in the region.
But the sharp acceleration in the past couple of years seemed to require some other source. Studies are now implicating the Sudd in South Sudan, the continent’s largest swamp and a region researchers have been unable to study on the ground because of the long-term conflict in the region. Using Japan’s Greenhouse Gases Observing Satellite, which measures the amount of light absorbed by methane at infrared wavelengths, Palmer and his colleagues were able to show the Sudd had grown as a methane hot spot since 2019, adding some 13 million extra tons per year to the air—more than 2% of annual global emissions. A second study, posted in late June by Harvard University researchers and submitted to Environmental Research Letters, finds nearly the same story, especially the surge in East Africa. When combined with smaller increases from the Amazon and the northern forests, it largely explains the observed rise in the atmosphere.
Climate change may be setting the pace of the emissions. In work published earlier this year in Nature Communications, Palmer and colleagues showed how East African methane emissions from 2010 to 2019, measured by satellite, synced up with a temperature pattern in the Indian Ocean that periodically warms the waters off the Horn of Africa, causing increased rainfall on land. Climate projections call for this positive phase of the Indian Ocean dipole, as it’s known, to grow in strength and duration with continued global warming. If it does, Palmer says, warming will beget more methane emissions from the Sudd, which in turn could fuel more warming and rains—a positive feedback loop.
Ed Dlugokencky, an atmospheric chemist at ESRL, agrees East African wetlands may well play a big role in the methane emissions of the past 2 years. “But the question of whether it’s a climate feedback yet is very difficult to answer,” simply because of limited records and large yearly variations in rainfall and wetland emissions. Nisbet notes, though, that the same dynamic may be playing out across other tropical wetlands. “A warming world is a wetter world in the moist tropics,” Nisbet says. “We have good reason to expect, if we have a moisture and temperature increase, then biological productivity follows.” Research flights over wetlands in Zambia found methane levels 10 times higher than models suggested, Nisbet and his colleagues reported in May.
The researchers who identified the East Africa link also worked to rule out another possible driver of the 2-year surge: a slowdown in the destruction of atmospheric methane. Unlike carbon dioxide, which lingers for centuries, methane only lasts a dozen years or so before it is washed out of the air, primarily by an atmospheric cleanser called the hydroxyl radical (OH). Nitrogen oxides, common pollutants from fossil fuel burning, help form OH—and nitrogen oxides declined as traffic and industry subsided during the early part of the pandemic, which should have reduced OH and allowed more methane to survive. “But we find that’s not the case at all,” says Daniel Jacob, an atmospheric chemist at Harvard and co-author on the second study. Matching the pandemic’s estimated OH reduction in their models led to a negligible change in methane levels.
In 2021, more than 100 countries signed the Global Methane Pledge, which would cut emissions 30% from 2020 levels, primarily by plugging leaks from oil and gas infrastructure. Some scientists have even discussed removing methane from the air. But those efforts might not offset rising wetland emissions, says Benjamin Poulter, a biogeochemical modeler at NASA’s Goddard Space Flight Center. “I can see a scenario where we mitigate methane—but we still see increases in atmospheric methane.”
Some might seize on methane emissions as a reason for draining or developing wetlands, which are already under threat worldwide, says Eoghan Darbyshire, a researcher at the Conflict and Environment Observatory, a U.K. charity. Last year, following earlier work from Palmer that first highlighted the Sudd as a methane source, South Sudan proposed achieving its climate goals by finishing the Jonglei Canal, abandoned in the 1980s, which would divert water from the Sudd to Egypt. But draining the Sudd might just replace its methane emissions with carbon dioxide generated as newly exposed peat decomposes, while doing immeasurable damage to its ecosystem, Darbyshire says. “On the surface these seem like reasonable arguments,” he says. “But if you start to think about them a little bit, they start to unravel and you’re left with an overwhelming sense of uncertainty.”
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