Arctic sea ice is dramatically thinning with thin, seasonal ice replacing thick, older ice as the dominant type for the first time on record, according to scientists from NASA and the University of Washington. The researchers used data and observations from NASA's earth-orbiting ICESat (Ice, Cloud and land Elevation Satellite) to provide further evidence of the rapid, ongoing transformation of the Arctic's ice cover.
Previously, scientists relied only on measurements of area to determine how much of the Arctic Ocean is covered in ice, but ICESat makes it possible for the first time to monitor ice thickness and volume changes over the entire Arctic Ocean. Using ICESat measurements, the scientists discovered that overall Arctic sea ice thinned about seven inches a year, for a total of 2.2 feet over four winters.
The total area covered by the thicker, older "multiyear" ice that has survived one or more summers shrank by 42 percent. The research team said the changes in the overall thickness and volume of Arctic Ocean sea ice is due to recent warming and anomalies in patterns of sea ice circulation.
"Ice volume allows us to calculate annual ice production and gives us an inventory of the freshwater and total ice mass stored in Arctic sea ice," Ron Kwok of NASA's Jet Propulsion Laboratory, who led the research team, said in a statement. "Even in years when the overall extent of sea ice remains stable or grows slightly, the thickness and volume of the ice cover is continuing to decline, making the ice more vulnerable to continued shrinkage."
Kwon and his fellow researchers published their findings July 7 in the Journal of Geophysical Research-Oceans.
"Our data will help scientists better understand how fast the volume of Arctic ice is decreasing and how soon we might see a nearly ice-free Arctic in the summer," Kwon said.
The Arctic ice cap grows each winter as the sun sets for several months and intense cold follows. In the summer, wind and ocean currents cause some of the ice naturally to flow out of the Arctic, while much of it melts in place. The thicker, older ice is more likely to survive. Seasonal sea ice usually reaches about six feet in thickness, while multiyear ice averages nine feet.
In recent years, the amount of ice replaced in the winter has not been sufficient to offset summer ice losses. The result is more open water in summer, which then absorbs more heat, warming the ocean and further melting the ice. Between 2004 and 2008, multiyear ice cover shrank 595,000 square miles -- nearly the size of Alaska's land area.
"One of the main things that has been missing from information about what is happening with sea ice is comprehensive data about ice thickness," said Jay Zwally, study co-author and ICESat project scientist at NASA's Goddard Space Flight Center in Greenbelt, Md. "U.S. Navy submarines provide a long-term, high-resolution record of ice thickness over only parts of the Arctic. The submarine data agree with the ICESat measurements, giving us great confidence in satellites as a way of monitoring thickness across the whole Arctic Basin."
During the four-year study period, the relative contributions of the two ice types to the total volume of the Arctic's ice cover were reversed. In 2003, 62 percent of the Arctic's total ice volume was stored in multiyear ice, with 38 percent stored in first-year seasonal ice. By 2008, 68 percent of the total ice volume was first-year ice, with 32 percent multiyear ice.