Ominous sign for global warming, feedback loop could accelerate methane emissions | Science

If carbon dioxide is a grill that regularly grills our planet, methane is a grill explosion: a more potent but shorter-lived greenhouse gas that is responsible for about a third 1.2°C warming since pre-industrial times. Atmospheric methane levels have increased nearly 7% since 2006, and the past 2 years have seen the biggest jumps to date, although the pandemic has slowed oil and gas production, presumably reducing methane leaks. Now researchers are focusing on the source of the mysterious surge. Two new preprints trace it back to microbes in tropical wetlands. Worryingly, climate change itself could fuel the trend by causing increased rainfall in the regions.

If so, emissions from wetlands could end up being an uncontrollable process beyond human control, although the magnitude of the feedback loop is uncertain. “We will have left a little more control over the 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 Chemistry and physics of the atmosphere.

Most climatologists have already agreed that the post-2006 methane spike largely did not come from fossil fuel production. That’s because atmospheric methane became increasingly enriched in carbon-12, the lightest isotope of carbon, reversing what had been a centuries-old trend, says Xin Lan, a carbon cycle scientist at the Research Laboratories. Earth System Survey (ESRL) from the National Oceanic and Atmospheric Administration. “It’s a very important signal,” she says. This identifies microbes as the source as they promote reactions using 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 increase after 2006, as they have grown significantly alongside the populations in the region.

But the strong acceleration of the last two years seemed to require another source. Studies now implicate the Sudd in South Sudan, the largest swamp on the continent and an area that researchers have been unable to survey in the field due to long-term conflict in the area. Using Japan’s greenhouse gas observation satellite, which measures the amount of light absorbed by methane at infrared wavelengths, Palmer and his colleagues were able to show that the Sudd had become a hotspot of methane since 2019, adding some 13 million additional tonnes per year to the air — more than 2% of annual global emissions. A second study, published at the end of June by researchers at Harvard University and submitted to Environmental Research Lettersfind Almost the same history, especially the rise to power in East Africa. When combined with smaller increases in the Amazon and northern forests, it largely explains the observed rise in the atmosphere.

Climate change could determine the rate of emissions. In a book published earlier this year in Nature CommunicationPalmer and his colleagues shown how methane emissions in East Africa from 2010 to 2019, measured by satellite, synchronized with a temperature model in the Indian Ocean that periodically warms the waters off the Horn of Africa, leading to increased precipitation over land. Climate projections predict that this positive phase of the Indian Ocean Dipole, as it is called, will grow in strength and duration with continued global warming. If so, Palmer says, the warming will drive more methane emissions from the Sudd, which in turn could fuel more warming and rainfall — a positive feedback loop.

Ed Dlugokencky, atmospheric chemist at ESRL, agrees that East African wetlands may well play a significant role in methane emissions over the past 2 years. “But whether this is a climate feedback is still very difficult to answer,” simply because of limited records and large annual variations in rainfall and wetland emissions. Nisbet notes, however, that the same dynamic can play out in other tropical wetlands. “A warming world is a wetter world in the humid tropics,” Nisbet says. “We have good reason to expect, if we have an increase in humidity and temperature, then biological productivity follows.” Research flights over wetlands in Zambia found methane levels 10 times higher that the models suggested, Nisbet and his colleagues reported in May.

Methane in motion

After a brief plateau in the early 2000s, atmospheric methane levels have risen rapidly, causing about a third of modern global warming. Tropical wetlands could fuel an acceleration observed after 2019.

GRAPHIC: K. FRANKLIN/SCIENCE; DATA: ED DLUGOKENCKY/NOAA/GLOBAL MONITORING LABORATORY

The researchers who identified the link to East Africa also worked to rule out another possible factor in the 2-year surge: a slowdown in atmospheric methane destruction. Unlike carbon dioxide, which persists for centuries, methane only lasts about a dozen years before being washed out of the air, mostly by an atmospheric cleaner called a hydroxyl (OH) radical. Nitrogen oxides, common pollutants from burning fossil fuels, contribute to the formation of OH – and nitrogen oxides declined as traffic and industry dwindled early in the pandemic, which which should have reduced the 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 of the second study. Matching the estimated pandemic OH reduction in their models led to negligible change in methane levels.

In 2021, more than 100 countries signed on to the Global Methane Pledge, which would reduce emissions by 30% from 2020 levels, primarily by plugging oil and gas infrastructure leaks. Some scientists have even discussed removing methane from the air. But those efforts might not offset increased emissions from wetlands, says Benjamin Poulter, a biogeochemical modeler at NASA’s Goddard Space Flight Center. “I can see a scenario where we’re mitigating methane, but we’re still seeing an increase in atmospheric methane.”

Some might point to methane emissions as a reason to drain or develop wetlands, which are already under threat around the world, says Eoghan Darbyshire, a researcher at the Conflict and Environment Observatory, a UK charity. Last year, following earlier work by Palmer that first pinpointed the Sudd as a source of methane, South Sudan proposed to achieve its climate goals by completing the Jonglei Canal, abandoned in the 1980s, which would divert water from the Sudd to Egypt. But the Sudd’s drainage could simply replace its methane emissions with carbon dioxide generated as the newly exposed peat decomposes, while causing immeasurable damage to its ecosystem, says Darbyshire. “On the surface, these seem like reasonable arguments,” he says. “But if you start to think about it a bit, they start to unravel and you’re left with an overwhelming sense of uncertainty.”

Teresa H. Sadler