Arctic sea ice thinned dramatically 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, based on data from a NASA Earth-orbiting spacecraft, provide further evidence for the rapid, ongoing transformation of the Arctic's ice cover.

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, bringing intense cold. In the summer, wind and ocean currents cause some of the ice to naturally flow out of the Arctic, while much of it melts in place. But not all of the Arctic ice melts each summer - 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. The results give scientists a better understanding of the regional distribution of ice and provide better insight into what is happening in the Arctic.

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