Permafrost and stalagmites found in Siberian caves have answered a question that has long puzzled scientists: why don’t Arctic sea ice and permafrost in the Northern Hemisphere melt at the same time?
New research by British, Israeli and Russian scientists, led by the University of Oxford, reveals that Siberian permafrost (permanently frozen ground) is not affected by warming when Arctic sea ice is present – but is vulnerable when sea ice is absent.
Permafrost covers nearly a quarter of Northern Hemisphere land, and its longevity is important because its frozen state enables it to store large amounts of carbon.
“We were surprised to find that times when permafrost melted in the past did not simply match up with times when the Earth was at its warmest but were much more likely when the Arctic was free of ice in the summer,” says Oxford’s Gideon Henderson.
“This discovery about the past behaviour of permafrost suggests that the expected loss of Arctic sea ice in the future will accelerate melting of the permafrost presently found across much of Siberia.”
This is a hugely significant finding for carbon levels in the Earth’s atmosphere.
If the Arctic has no summer sea ice in the coming decades, the thawing of permafrost will be accelerated. This has the potential to release enough carbon to triple the amount already present.
The study’s results – published in the journal Nature – rely on caves that contain records of periods when permafrost was absent in the past because of its relationship to stalagmites.
Stalagmites, stalactites and flowstones – carbonate deposits known as speleothems – can only form when there is liquid water. During a permafrost there is no liquid water available.
So, a cave with both stalagmites and a permafrost reveals that the permafrost is younger than the carbonate formations.
Stalagmite dating now uses measurements of natural uranium and lead to record the last 1.5 million years.
The study found that the stalagmites commonly grew more when the Arctic Ocean was free of summer sea ice and when the permafrost was absent.
The study team believes this is because the lack of sea ice leads to an increase in heat and moisture transfer from ocean to atmosphere, and therefore to warmer air transported far overland into Siberia and permafrost areas.
Moisture transport also increases snowfall over Siberia during autumn. This blanket of snow insulates the ground from the extreme cold of winters, leading to an increase in average annual ground temperatures, which warms permafrost.
Consequently, in regions with increased snow cover and insulation, permafrost will start to thaw, releasing carbon dioxide that trapped for millennia.
Ian Connellan is a journalist and editor for the Royal Institution of Australia.
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