Permafrost is not so permanent. Across the Arctic, swathes of once-frozen-solid ground have begun to thaw. If the records preserved in Siberian caves are accurate, much more of the region could melt if temperatures continue to warm.

Geoscientist Anton Vaks of the University of Oxford led an international team of experts—including the Arabica Caving Club in Irkutsk—in sampling the spindly cave growths known as stalagmites and stalactites across Siberia and down into the Gobi Desert of China. Taking samples of such speleothems from six caves, the researchers then reconstructed the last roughly 500,000 years of climate via the decay of radioactive particles in the stone. When the ground is frozen above a cave no water seeps into it, making such formations "relicts from warmer periods before permafrost formed," the researchers wrote in a study published online in Science on 21 February.

The details of the study reveal that conditions were warm enough even in Siberia for these mineral deposits to form roughly 400,000 years ago, when the global average temperature was 1.5 degrees Celsius higher than present. It also suggests that there was no permafrost in the Lena River region at that time, because enough water seeped into the northernmost cave to enable roughly eight centimeters of growth in the formations.

That was, in fact, the last time the formations in the Ledyanaya Lenskaya Cave grew, although other caves further south showed multiple periods of growth coinciding with other warmer periods. "That boundary area of continuous permafrost starts to degrade when the mean global temperature is 1.5 degrees C higher than present," Vaks explains. "Such a warming is a threshold after which continuous permafrost zone starts to be vulnerable to global warming."

Since Vaks's present is the "preindustrial late Holocene," that means the planet is already more than halfway there, having experienced 0.8 degree C warming to date. Such a thaw is no small matter, given that permafrost covers nearly a quarter of the land in the Northern Hemisphere and holds roughly 1,700 gigatonnes of carbon—or roughly twice as much carbon as is currently trapping heat in the atmosphere. Much of that carbon would end up in the atmosphere if the permafrost was to thaw further.

That may not have occurred during the warm period 400,000 years ago, known as Marine Isotope Stage 11 to scientists, which featured elements such as boreal forest on Greenland and higher sea levels. "The thawing was probably very brief because the layer deposited in the northernmost cave stalactite was relatively thin," Vaks says—too thin in fact to determine how long the warm period lasted. "We don't see any extraordinary increase in atmospheric CO2 or methane during MIS-11." And the Gobi Desert might benefit, enjoying wetter conditions in the future if the record in these caves is accurate.

It's not clear how far north such thawing might extend if global average temperatures continue to warm until they match those from long ago. "Now we are looking for caves with speleothems in northern Siberia to answer this question," Vaks notes, adding that the northernmost cave is already much warmer than in the late 18th century based on historical reports. Further research could be done by taking sediment cores from Arctic river deltas or lakes, though this remains an epic task given the vastness and remoteness of the region. But, already, it is clear that global climates not much warmer than present are enough to thaw even more permafrost—as far north as 60 degrees latitude.

"The potential impact of these results extends to global policy: these results indicate the potential release of large amounts of carbon from thawed permafrost even if we attain the 2 degree [C] warming target under negotiation," says Kevin Schaefer, a scientist at the U.S. National Snow and Ice Data Center, who has also studied permafrost but was not involved in this, in his words, "great science" effort. "Permafrost thaws slowly and the carbon will be released into the atmosphere over two to three centuries."

Already, such thawing Arctic ice—whether underground or at sea—has further opened up the territory to exploration for resources, particularly oil. At the same time, the big thaw will make getting the oil out more expensive—billions of dollars in infrastructure investments in pipelines, roads and the like will be damaged as the ground shifts beneath them.