Satellite Shows Arctic Sea Ice Now More Thin Than Thick

WASHINGTON, DC, July 8, 2009 (ENS) - Arctic sea ice has thinned 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, according to the first basin-wide estimate of the thickness and volume of the Arctic Ocean's ice cover.

The total area covered by the thicker, older "multi-year" ice that has survived one or more summers shrank by 42 percent over the four year period.

Based on data from a NASA Earth-orbiting spacecraft, the study provides 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.

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

In the past, scientists had 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.

Arctic sea ice thickness as measured by NASA's ICESat satellite (Photo courtesy Norbert Untersteiner, University of Washington)

The results give scientists a better understanding of the regional distribution of ice and provide better insight into what is happening in the Arctic Ocean.

"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," he said.

ICESat measures the distances to the top of the ice 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.

"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," Kwok said.

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; the thicker, older ice is more likely to survive. Seasonal sea ice usually reaches about six feet in thickness, while multi-year ice averages nine feet.

Using ICESat measurements, scientists found that overall Arctic sea ice thinned about seven inches a year, for a total of 2.2 feet over four winters.

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.

Areas of thick old Arctic sea ice have shrunk since 2004. (Image courtesy NASA)

Between 2004 and 2008, multi-year ice cover shrank 595,000 square miles - nearly the size of Alaska's land area.

During the 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 multi-year 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 multi-year.

"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, Maryland.

"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," said Zwally.

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.

Kwok said, "The near-zero replenishment of the multi-year ice cover, combined with unusual exports of ice out of the Arctic after the summers of 2005 and 2007, have both played significant roles in the loss of Arctic sea ice volume over the ICESat record."

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