Scientists have long worried about what many call the "methane bomb" - the potentially catastrophic release of methane from thawing swamps in Siberia's permafrost.
But now a study by three geologists says the 2020 heatwave has revealed a spike in methane emissions "potentially in much larger quantities" from another source: thawing rock formations in Arctic permafrost.
The difference is that when swamps thaw, "microbial" methane is released, which is formed as a result of decomposition of soil and organic matter, and when limestone - or carbonate rocks - thaws, hydrocarbons and gas hydrates are released from reservoirs located both under the permafrost and inside it., which makes this phenomenon "much more dangerous" than past research has suggested.
Nikolaus Freutzheim, who teaches at the Institute of Geosciences at the University of Bonn, said that he and two colleagues used satellite maps that measured intense methane concentrations over two "prominent elongated regions" of limestone - bands several miles wide and up to 375 miles long - across the Taimyr Peninsula and in the region of northern Siberia.
The study was published in the journal Proceedings of the National Academy of Sciences.
Surface temperatures during the 2020 heatwave rose 10.8 degrees Fahrenheit above the 1979-2000 norm. The long strips have almost no soil and the vegetation is sparse, the study said. Therefore, limestone grows from the surface. As the rock formations heat up, cracks and pockets open, releasing the methane that was trapped inside.
According to Freutzheim, the methane concentration has increased by about 5 percent. Further tests showed that methane concentrations persisted until spring 2021, despite the return of low temperatures and snow in the region.
“We would expect higher methane levels in wetland areas,” said Freutzheim. "But these sites were not over marshes, but on limestone outcrops. There was very little soil in them. It was a really amazing signal from hard rocks, not from wetlands."
According to the US Geological Survey, the carbonates in these outcrops date from the Paleozoic era of 541 million years.
"This is scary. This is not good news," said Robert Max Holmes, senior fellow at the Woods Hole Research Center. "Nobody wants to see more potentially nasty feedbacks, and this is potentially one of them."
“What we know with a high degree of certainty is how much carbon is trapped in permafrost. This is a big figure, and as the Earth warms and the permafrost thaws, this ancient organic matter becomes available to microbes for microbiological processes, resulting in the release of CO2 and methane, "said Holmes. "If something in the Arctic keeps me awake at night, this is it."
The study says gas hydrates in Earth's permafrost are estimated to contain 20 gigatons of carbon. This is a small percentage of all carbon trapped in permafrost, but the continued warming of gas hydrates could lead to a destructive and rapid release of methane from rock outcrops.
"It will be important to continue comparing methane over the coming years to accurately determine how much additional geological methane is released into the atmosphere as the permafrost thaws," said Ted Schur, professor of ecosystem ecology at Northern Arizona University.