"The ocean has gained so much heat it takes a while to release it," said lead study author Julienne Stroeve, a senior scientist at the National Snow and Ice Data Center in Boulder, Colo. "That's delaying the autumn freeze-up."
In the past decade, the additional heat stored in the upper ocean has increased Arctic sea surface temperatures by 0.9 degrees to 2.7 degrees Fahrenheit (0.5 to 1.5 degrees Celsius), Stroeve and her colleagues report. These warmer ocean temperatures prolong the summer melt season because the ocean must fall below about 29 F (minus 1.9 C) before new sea ice forms.
In the Kara, Laptev, East Siberian, Chukchi and Beaufort seas, the fall freeze now comes between six and 11 days later each decade since 1979. The researchers found a similar trend in the East Greenland and Barents seas, where the fall freeze may now be delayed by as much as 40 days per decade.
Oil and gas companies are already exploiting this delay by pushing for drilling leases that allow extraction and exploration well into autumn, Stroeve said. But year-to-year ice conditions can still vary dramatically.