July 7, 2009 / 9:11 PM / 11 years ago

RPT-Arctic ice thinned dramatically since 2004 - NASA

(Repeating for wider distribution)

* Tough old ice melts, replaced by frail young ice

* Arctic ice influences global weather patterns

By Deborah Zabarenko, Environment Correspondent

WASHINGTON, July 7 (Reuters) - Arctic sea ice has thinned dramatically since 2004, with the older, thicker ice giving way to a younger, thinner kind that melts in the northern summer, NASA scientists reported on Tuesday.

Researchers have known for years that ice covering in the Arctic Sea has been shrinking in area, but new satellite data that measure the thickness of ice show that the volume of sea ice is declining as well.

That is important because thicker ice is more resilient and can last from summer to summer. Without ice cover, the Arctic Sea’s dark waters absorb the sun’s heat more readily instead of reflecting it as the light-colored ice does, accelerating the heating effect.

Using NASA’s ICESat spacecraft, scientists figured that overall Arctic sea ice thinned about 7 inches (17.78 cm) a year since 2004, for a total of 2.2 feet (0.67 metres) over four winters. Their findings were reported in the Journal of Geophysical Research-Oceans.

The total area covered by thicker, older ice that has survived at least one summer shrank by 42 percent.

Beyond that, the new satellite data showed that the proportion of tough old ice is decreasing at the same time as the amount of young fragile ice is increasing, information that was hard to discern from earlier data.

LOSING THE OLD ICE

In 2003, 62 percent of the Arctic’s total ice volume was stored in multi-year ice and 38 percent in first-year seasonal ice. By last year, 68 percent was first-year ice and 32 percent the tougher multi-year ice.

The research team blamed these changes on recent warming and anomalies in sea ice circulation.

“We’re losing a lot more of the old ice, and that’s significant,” said Ron Kwok of the Jet Propulsion Laboratory in Pasadena, California. “Basically we knew how much the area (of ice) was shrinking, but we didn’t know how thick it was.”

To find the volume of ice, NASA’s ICESat spacecraft measured how high the ice rose above sea level in the Arctic, Kwok said in a telephone interview.

“If we know how much is floating on top, we can use that to compute the rest of the ice thickness,” Kwok said in a telephone interview. About nine-tenths of the ice is beneath the water, he said.

The ICESat measurements cover virtually the entire Arctic, and they tally with ice volume measurements made by submarines, which cover only a few passes across the area.

Arctic sea ice melted to its second-lowest level last year, rising slightly from its all-time low in 2007, according to the U.S. National Snow and Ice Data Center.

Arctic ice is a factor in global climate and weather patterns, because the difference between the cool air at the poles and the warm air around the Equator drives air and water currents, including the jet stream.

More information and images are available online here

Editing by Cynthia Osterman

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