Scientists have long believed melting permafrost emits large amounts of carbon-rich greenhouse gases like methane and carbon dioxide to the atmosphere resulting in a warming climate. But a new study published online by the journal Nature today indicates ancient lakes that formed after permafrost in the Arctic first melted roughly ten thousand years ago may in fact have a net climate cooling effect over long time scales. The study also increases the total amount of carbon estimated in the frozen soils of the Far North by more than 50 percent.
Katey Walter Anthony is an Associate Professor at the University of Alaska Fairbanks Institute of Northern Engineering She studies methane emissions from Arctic thermokarst lakes.
“Until now, we have understood these thermokarst lakes, or lakes where permafrost thaws, to be a really important source of methane, a greenhouse gas that causes the climate to warm,” she says.
A few years ago, Anthony was in a boat on a river in Siberia, when she noticed something in the sediments along the riverbank.
“We could see where ancient lakes had been eroded by the river, so we could see the lakes in cross section,” she says. “It looked like we were looking at a layered cake. Those layers were the layers of sediment in the lake and we saw really thick beds of moss.”
Some time after the last glacial maximum – roughly ten thousand years ago – permafrost began to thaw. Depressions formed, filled with water and eventually millions of small lakes started to dot the Arctic landscape. They were all emitting methane and carbon dioxide, greenhouse gases that warm the climate. Anthony says that process probably lasted for about a thousand years.
“But those waterbodies sit around as lakes for several thousand years,” she explains, “and at some point, they burn up all of the permafrost carbon and so their methane emissions decline and as they slow down in their emissions, they speed up in their ability to soak up carbon out of the atmosphere.”
Over time, Anthony believes thick, carbon rich beds of peat moss grew as microbial decomposition declined. She and colleagues studied more than 50 ancient lakes in parts of Siberia and Alaska. In some places, she says they found beds of peat moss up to four meters, or 12 fee, thick.
“We would walk up to these permafrost exposures and we could pull on those mosses and it was like pulling long tendrils of spaghetti,” she laughs. “They were very well preserved and poorly decomposed, and the reason is that when the mosses grow and then senesce in these lakes, they have anaerobic bottoms. there’s no oxygen down there and so site mosses don’t decompose and eventually, lakes drain and the sediments really quickly refreeze. it’s like flash freezing of those mosses.”
Anthony believes lakes across the landscape have accumulated 1.6 times the amount of carbon they emitted before the lakes refroze. She says that increases the total estimated amount of carbon scientists believe is currently stored in the circumpolar permafrost region by 50 percent. The results also show these ancient lakes actually have a net cooling effect on climate over thousands of years.
“It’s cooling the climate,” she says. “It’s soaking up more climate than its emitting. It’s is offsetting human emissions. It’s not a avery large offset to human emissions and I think there bigger concern is that all of this very large reservoir of lake moss peat, this lake carbon, is stored in permafrost since the sediments refreeze when they drain.”
Anthony doesn’t believe this net cooling effect till offset current predications for a warmer climate in the future. “So, in the future, the projected warming of permafrost across the Arctic, will thaw all of that carbon again and make it vulnerable to decomposition by microbes and return that carbon to the atmosphere as CO2 and methane.”
The study is published in the most recent issue of Nature. Funding comes from the National Science Foundation, the Alfred Wegener Institute, the University of Alaska Fairbanks and the US Geological Survey.
