An iceberg melts off Ammassalik Island in Eastern Greenland in this July 19, 2007 file photo. Arctic ice has shrunk to the lowest level on record, new satellite images show, raising the possibility that the Northwest Passage that eluded famous explorers will become an open shipping lane. The European Space Agency said nearly 200 satellite photos this month taken together showed an ice-free passage along northern Canada, Alaska and Greenland, and ice retreating to its lowest level since such images were first taken in 1978. (John McConnico/AP Photo)

By Deborah Zabarenko, Environment Correspondent, March 18, 2008

WASHINGTON (Reuters) -- The thickest, oldest and toughest sea ice around the North Pole is melting, a bad sign for the future of the Arctic ice cap, NASA satellite data showed on Tuesday.

"Thickness is an indicator of long-term health of sea ice, and that's not looking good at the moment," Walt Meier of the National Snow and Ice Data Center told reporters in a telephone briefing.

This adds to the litany of disturbing news about Arctic sea ice, which has been retreating over the last three decades, especially last year, when it ebbed to its lowest level.

Scientists have said the trend is spurred by human-generated climate change.

Melting Arctic ice does not raise sea levels as the melting of glaciers on Greenland or Antarctica could, but it does contribute to global warming when reflective white ice is replaced by dark water that absorbs the sun's heat.

Using satellites that measure how much ice covers water in the Arctic and Antarctic, Meier and other climate scientists found a steep drop in the amount of perennial ice -- the hardy, thick ice that is over a year old -- in the north.

The oldest Arctic ice that has survived six years or more is the toughest, and even that shrank dramatically, Meier and the other scientists said.

OLD ICE "TOUGH AS NAILS"

Some 965,300 square miles of perennial ice have been lost -- about one and a half times the area of Alaska -- a 50 percent decrease between February 2007 and February 2008, Meier said.

The oldest "tough as nails" perennial ice has decreased by about 75 percent this year, losing 579,200 square miles (1.5 million sq kms, or about twice the area of Texas, he said.

This doesn't mean the Arctic is open water during the winter, but it does mean that in many areas, the stronger perennial ice is being replaced by younger, frailer new ice that is more easily disturbed by wind and warm sea temperatures.

"It's like looking at a Hollywood set," Meier said of an Arctic largely covered with younger ice. "It may look OK but if you could see behind you'd see ... it's just empty. And what we're seeing with the ice cover is it's becoming more and more empty underneath the ice cover."

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