Data from a NASA Earth-orbiting spacecraft has revealed a dramatic thinning of Arctic sea ice between the winters of 2004 and 2008, with thin seasonal ice replacing thick older ice as the dominant type for the first time on record.
The new results provide further evidence for the rapid, ongoing transformation of the Arctic's ice cover.
In the research, scientists from NASA and the University of Washington in Seattle conducted the most comprehensive survey to date using observations from NASA's Ice, Cloud and land Elevation Satellite, known as ICESat, to make the first basin-wide estimate of the thickness and volume of the Arctic Ocean's ice cover.
The Arctic ice cap grows each winter as the sun sets for several months and intense cold ensues.
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.
But not all of the Arctic ice melts each summer, as the thicker, older ice is more likely to survive.
Seasonal sea ice usually reaches about 6 feet in thickness, while multi-year ice averages 9 feet.
Using ICESat measurements, scientists found that overall Arctic sea ice thinned about 7 inches a year, for a total of 2.2 feet over four winters.
The total area covered by the thicker, older "multi-year" ice that has survived one or more summers shrank by 42 percent.
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 to monitor ice thickness and volume changes over the entire Arctic Ocean for the first time.
The results give scientists a better understanding of the regional distribution of ice and provide better insight into what is happening in the Arctic.
"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," said Kwok.
"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.
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," he added.
The research team attributes the changes in the overall thickness and volume of Arctic Ocean sea ice to the recent warming and anomalies in patterns of sea ice circulation.