Polar bears' hunting season threatened by break-up of ice sheet

The retreat of sea ice in the Arctic is forcing the world's wild polar bear population into an unnatural fast which threatens the species with extinction.

Scientists said yesterday that the earlier annual break-up of sea ice caused by climate change is cutting short the spring hunting season for the bears, which rely on floating banks of ice to reach their prey.

The disappearance of the sea ice in summer months is forcing hungry polar bear populations to spend longer on land, giving a false impression that numbers are increasing as they encroach on human settlements in search of food.

Travel agencies in Canada and the US offering Arctic tours have begun boasting of the increased likelihood of spotting the bears.

But a joint study by the Canadian Wildlife Service (CWS) and Nasa, published in the scientific journal Arctic this week, has found that, far from thriving, the polar bear is at potentially irreversible risk from global warming.

The research into bears in five Arctic regions found that sea ice has begun retreating progressively earlier each year when satellite images from 1979 to 2004 are compared.

Female bears rely on the spring hunting season to build the fat reserves needed to see them through the summer months. The retreating ice means they have not had time to build up normal levels of fat - which can reach a thickness of 12cm.

The study found the spring hunting season was being reduced by nearly three weeks in some places - reducing the fat levels by up to 80kg in each animal.

As females become thinner, they are more susceptible to disease. Their ability to reproduce and the survival chances of their cubs decline significantly.

Claire Parkinson, a Nasa scientist and co-author of the report, said: "Our research strongly suggests that climate warming is having a significant and negative effect on a primary species reliant on the sea ice for survival."

The sea ice provides a waterborne hunting ground for polar bears from which they can find their prey - seals and other marine mammals. The polar bear can detect a seal from 20 miles.

Ms Parkinson said: "Our concern is that if the length of the sea ice season continues to decrease, polar bears will have shorter periods on the ice to feed."