Shrinking Arctic Sea Ice Thinner, More Vulnerable

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Using satellites to track sea ice as it moves around, scientists can keep tabs on specific patches of ice over several years and count the ice's age. With this method, they saw that perennial ice had reached a record low for the winter.

Through the 1980s and early 1990s, about 50 to 60 percent of the sea ice was perennial when the ice reached its largest extent in March.

But this year, the amount of perennial sea ice dropped to only 30 percent of the total.

"We've lost about 50 percent of the perennial ice cover," Meier said.

"And the old ice [that is more than five years old] has decreased even more, by about 75 percent," he added.

That older ice "is as tough as nails," Meier said.

"It's very resistant to short-term melting."

But now there is very little of it left, making Arctic sea ice more vulnerable, the researchers said.

Another Low Year

So-called seasonal ice—which is less than a year old—now dominates the Arctic.

"It reaches a maximum thickness of about 4 feet (1.3 meters), and it's salty," said Seelye Martin of NASA.

"The thinness and saltiness means it's more flexible and more vulnerable to winds and currents," which can break up the ice or push it out of the Arctic, Martin added.

The researchers said they can't predict whether this coming summer will set a new record low for the extent of summer sea ice.

"It's definitely shaping up to be another very low year," Meier said.

(Related story: "Arctic Sea Ice Gone in Summer Within Five Years?" [December 12, 2007].)

But exactly how much melts away will depend on the summer temperature, winds, and other factors, Meier said.

"That's weather, and something we can't predict at this point," he added.

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