About 400,000 years ago, the climate was just a little bit warmer than it is now. A new study out in the journal Science shows that small difference means a lot, though, and even a relatively small amount of warming is likely to lead to melting permafrost.
The study, released yesterday, used stalactites, stalagmites and other calcium carbonate cave growths, collectively known as speleothems, to reconstruct the ancient history of permafrost freezing and thawing.
The study was headed by Anton Vaks, a University of Oxford geologist and paleoclimate researcher, and funded by the British Natural Environment Research Council (NERC), the British Royal Society and the Russian Foundation for Basic Research. Vaks and an international team of researchers from Britain, Russia, Switzerland and Mongolia sampled a number of Siberian caves, as well as caves in the Gobi desert.
The northernmost of these, a cave called Lenskaya Ledyanaya, currently sits in an area of continuous permafrost. No stalactites grow there now, but they did 400,000 years ago.
This period, known as Marine Isotopic Stage 11, was 1.5 degrees Celsius warmer than temperatures in the preindustrial age, so Vaks and his colleagues concluded that a temperature increase of that amount will likely lead to thawing of the permafrost.
A puzzle solved through caves
As Vaks explained: "Stalagmites can grow only when you have rain or snowmelt water penetrating into the cave through fissures in the ceiling." When the cave is covered by permafrost, Vaks said, water can't seep through and speleothems do not form.
Based on this knowledge and newer techniques that allow researchers to accurately date stalactites and their kin back 500,000 years, Vaks and fellow researchers uncovered exactly when speleothems were growing in Siberian and Gobi desert caves.
In Siberia, the times of growth correspond to when the permafrost thawed, since water seeped through into the caves, and in the Gobi, speleothem growth only occurred in periods of increased precipitation.
The study is important because it clearly links a specific temperature -- 1.5 degrees warmer than preindustrial conditions -- with permafrost thawing, said Ted Schuur, a University of Florida professor who works on the impacts of current climate change on permafrost.
"As clear as something can be that happened 400,000 years ago" anyway, Schuur said.
Significant loss in permafrost by 2100
Permafrost melt will bring its own climate impacts, as well as affecting roads and infrastructure built atop it. As it thaws, the formerly frozen tundra will release significant quantities of carbon dioxide, contributing to even more warming (ClimateWire, Feb. 12).
Under a business-as-usual scenario, scientists estimate 48 to 63 percent of the surface permafrost will degrade by 2100, Schuur said.
The results from the desert caves sampled of Vaks' work show that parts of Asia experienced significantly more precipitation when the temperature was higher.
"It means the climate of Asia starts to change when we reach the 1.5-degree global warming tipping point," Vaks said.
In the future, Vaks plans to look for even more northern caves, using the same technique to see when the permafrost above them thawed. This could help pinpoint the extent to which scientists can expect permafrost to thaw under different increases in temperature.
Finding the caves won't be an easy task, he noted. Vaks credited the Arabica Caving Club in Irkutsk, Russia, for helping him with this study, but as the researchers search for ever-northern caves, they'll be in increasingly remote, difficult-to-access areas.
Reprinted from ClimateWire with permission from Environment & Energy Publishing, LLC. 202-628-6500. E&E Publishing is the leading source for comprehensive, daily coverage of environmental and energy issues. Click here to start a free trial to E&E's information services.
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