The temperature measurements revealed the water just above the ocean bottom warms by more than 12 degrees Fahrenheit (7 degrees Celsius) in some spots during the summer, the researchers found. And the drill core revealed that the surface sediment layers were unfrozen at the drill site, near the Lena River delta.
"We have now proved that the current state of subsea permafrost is incomparably closer to the thaw point than that of terrestrial permafrost," Shakova said.
Shakova and her colleagues attribute the warming of the permafrost to long-term changes initiated when sea levels rose starting at the end of the last glacial period. The seawater is several degrees warmer than the frozen ground, and is slowly melting the ice over thousands of years, they think.
Massive burst But other researchers think the permafrost warming started only recently. "This is the first time in 12,000 years the Arctic Ocean has warmed up 7 degrees in the summer, and that's entirely new because the sea ice hasn't been there to hold the temperatures down," said Peter Wadhams, head of the Polar Ocean Physics Group at the University of Cambridge in the U.K., who was not involved in the study. The summer ice melt season has lasted longer since 2005, giving the sun more time to warm the ocean. [10 Things You Need to Know About Arctic Sea Ice]