Polar bears will suffer as longer summers thin Arctic ice

Rising temperatures have caused the Arctic ice cap to thin, which is bad news for polar bears.

British scientists say they've conclusively shown longer summers have led to the thinning.

Polar bears rely on a short window in spring to eat seals on the ice. That food tides them over for the rest of the year. But thinner ice cuts into the seal-hunting time.

"As the sea ice has broken up, the bears are forced ashore earlier, which means they don't come ashore in quite as good condition," said bear biologist Prof. Andrew Derocher at the University of Alberta.

The British research linking higher temperatures and thinner ice challenges computer models that point to wind patterns as the cause of the change. Submarine data suggests the ice has thinned as much as 40 per cent since the 1960s.

Until now, researchers have resorted to computer models to explain why the Arctic ice is melting.

But the British researchers used new data and reached a different conclusion.

"We compared our radar measurements of the thickness of the ice to data from another satellite which tells us how long the summers in the arctic were over the same period," said physics Prof. Seymour Laxon of University College London. "We found an almost perfect match."

The study appears in Thursday's issue of the journal Nature.

Laxon says the data indicate Arctic summers have lengthened by almost half a day each year since the early 1990s. Each extra half day was linked to a two-centimetre thinning of ice the following winter.

Derocher says the thinning ice increases the chances of bear-human encounters, because hungry bears are curious.

Polar bears are listed as a species of concern in the Canadian Arctic, home to about half of the world's population.