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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. Ron Kwok of NASA's Jet Propulsion Laboratory in Pasadena, Calif., led the research team, which published its findings July 7 in the Journal of Geophysical Research-Oceans.

Figure 1:ICESat measures the distances to the top of the snow cover and to the sea surface. The difference between the two quantities gives the total “freeboard” measurement; that is, the amount of ice above the water line relative to the local sea level. Credit: Courtesy of Norbert Untersteiner, University of Washington. Larger Image

Figure 2: This schematic shows the geometric relationship between freeboard (the amount of ice above the water line), snow depth, and ice thickness. Buoyancy causes a fraction (about 10 percent) of sea ice to stick out above the sea surface. By knowing the density of the ice and applying “Archimedes’ Principle” -- an object immersed in a fluid is buoyed up by a force equal to the weight of the fluid displaced by the object -- the total thickness of the ice can be calculated. Credit: Ron Kwok, NASA/JPL. Larger Image

Figure 3: ICESat measurements of the distribution of winter sea ice thickness over the Arctic Ocean between 2004 and 2008, along with the corresponding trends in overall, multi-year and first-year winter ice thickness. Credit: Ron Kwok, NASA/JPL. Larger Image

Figure 4: ICESat measurements of winter multi-year ice cover in the Arctic Ocean between 2004 and 2008, along with the corresponding downward trend in overall winter sea ice volume, and switch in dominant ice type from multi-year ice to first-year ice. Credit: Ron Kwok, NASA/JPL. Larger Image

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