Yahoo News 7 Jul 09;
WASHINGTON (AFP) – Arctic sea ice thinned dramatically between the winters of 2004 and 2008, with thick older ice shrinking by the equivalent of Alaska's land area, a study using data from a NASA satellite showed Tuesday.
Using information from NASA's Ice, Cloud and Land Satellite (ICESat), scientists from the US space agency and the University of Washington in Seattle estimated both the thickness and volume of the Arctic Ocean's ice cover.
ICESat allows scientists to measure changes in the thickness and volume of Arctic ice, whereas previously scientists relied only on measurements of area to determine how much of the Arctic Ocean is covered in ice.
Scientists found that Arctic sea ice thinned some seven inches (17.8 centimeters) a year, or 2.2 feet (67 centimeters) over four winters, according to the study by NASA and the University of Washington, published in the Journal of Geophysical Research-Oceans.
They also found that thicker, older ice, which has survived one or more summers, shrank by 42 percent.
"Between 2004 and 2008, multi-year ice cover shrank 595,000 square miles (1.5 million square kilometers) -- nearly the size of Alaska's land area," a report of the study's findings said.
The Arctic ice cap grows each winter, when the northerly region grows intensely cold as the sun sets for several months.
Then, in the summer, wind and ocean currents cause some of the ice to flow out of the Arctic, while warmer temperatures make much of it melt in place.
Thicker, older ice is less vulnerable than thinner ice to melting in the summer months.
But in recent years, the amount of ice replaced in the winter has not been sufficient to offset summer ice losses, the ICESat study showed.
That makes for more open water in summer, which absorbs more heat, warming the ocean and further melting the ice, the report of the scientists' findings said.
The research team attributed the changes in the overall thickness and volume of Arctic Ocean sea ice to recent warming and anomalies in patterns of sea ice circulation.
"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," said Ron Kwok of NASA's Jet Propulsion Laboratory in California who led the study.
Data from the study will help scientists to better understand how fast the volume of Arctic ice is decreasing and how soon the region might be "nearly ice-free in the summer," said Kwok.
A study published in April by the Colorado-based National Snow and Ice Data Center (NSIDC) also showed that the Arctic ice cap is thinner than ever and the maximum extent of Arctic ice was at an all-time low.
The same month, US researchers warned that the Arctic could be almost ice-free within 30 years, not 90 as scientists had previously estimated.
Arctic ice thinned dramatically since 2004: NASA
Deborah Zabarenko, Reuters 7 Jul 09;
WASHINGTON (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 a year since 2004, for a total of 2.2 feet 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)
