Satellite imagery has shown that the Arctic ice cap is thinning and already is nearly 30 percent smaller than it was 25 years ago. In the winter of 2004-5, the Arctic’s perennial ice, which usually survives the summer, shrank by 280,000 square miles, the size of Turkey. This past August, a crack opened in the ice pack from the Russian Arctic to the North Pole, an event never seen before.

Arctic ice reflects sunlight; its absence may accelerate global warming. The intricate chemistry that occurs in the rich Arctic waters could go haywire with unaccustomed heat and sunlight. “People tend to think there’s not much life in the Arctic. But it’s an incredibly diverse ecosystem,’’ says Gary Stern, the chief scientist on the Amundsen. He was aboard when the ship was deliberately frozen in Franklin Bay in 2003. They spent the long winter doing experiments on the ice.

This year is “amazing. No ice,’’ Stern says.

Estimates vary widely on when the passage will be open to shipping all summer because of the ceaseless warming. The Canadian Ice Service conservatively predicts the southerly drift of even a shrunken ice pack will keep the passage clogged for most of this century. Other experts predict it will be open as soon as 2020; Canada’s defense agency says 2015. Those who visit regularly say the evidence is before their eyes. “You can see it. You come every year and you see less and less ice,’’ says Marie Emmanuelle Rail, 30, a researcher who has been working in the Arctic for five years.

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