Extinction on the cards: Global warming pushes polar bears towards a cold grave

Nearly all of the 19 sub-populations of polar bears, from the Beaufort Sea off Alaska to the Siberian Arctic, would face being wiped out because the loss of sea ice would force the animals onto land and away from their food supplies for longer periods, the researchers said. Prolonged fasting, and reduced nursing of cubs by mothers, would lead to rapid declines in reproduction and survival.

“There is very little chance that polar bears would persist anywhere in the world, except perhaps in the very high Arctic in one small sub-population” if greenhouse-gas emissions continue at so-called business-as-usual levels, said Peter K Molnar, a researcher at the University of Toronto Scarborough and lead author of the study, which was published Monday in the journal Nature Climate Change. Even if emissions were reduced to more moderate levels, “we still are unfortunately going to lose some, especially some of the southernmost populations, to sea-ice loss,” Dr Molnar said. The fate of polar bears has long been a flash point in the debate over human-caused climate change, used by scientists and environmentalists as well as deniers in their arguments.

By rough estimates there are about 25,000 polar bears in the Arctic. Their main habitat is sea ice, where they hunt seals by waiting for them to surface at holes in the ice. In some areas the bears remain on the ice year round, but in others the melting in spring and summer forces them to come ashore. “You need the sea ice to capture your food,” Dr Molnar said. “There’s not enough food on land to sustain a polar bear population.” But bears can fast for months, surviving on the energy from the fat they’ve built up thanks to their seal diet. Arctic sea ice grows in the winter and melts and retreats in spring and summer. As the region has warmed rapidly in recent decades, ice extent in summer has declined by about 13 percent per decade compared to the 1981-2010 average. Some parts of the Arctic that previously had ice year-round now have ice-free periods in summer. Other parts are now free of ice for a longer portion of the year than in the past.

Dr Molnar and his colleagues looked at 13 of the sub-populations representing about 80 percent of the total bear population. They calculated the bears’ energy requirements in order to determine how long they could survive — or, in the case of females, survive and nurse their cubs — while fasting. Combining that with climate-model projections of ice-free days to 2100 if present rates of warming continue, they determined that, for almost all of the subgroups, the time that the animals would be forced to fast would eventually exceed the time that they are capable of fasting.

In short, the animals would starve. “There’s going to be a time point when you run out of energy,” Dr Molnar said. Compounding the problem is that a longer fasting time also means a shorter feeding period.

“Not only do the bears have to fast for longer and need more energy to get through this, they also have a harder time to accumulate this energy,” he said. While fasting, bears move as little as possible to conserve energy. But sea-ice loss and population declines create new problems — having to expend more energy searching for a mate, for example — that could further affect survival. Even under more modest warming projections, in which greenhouse gas emissions peak by 2040 and then begin to decline, many of the subgroups would still be wiped out, the research showed. Over the years, polar bears have become a symbol both for those who argue that urgent action on global warming is needed and for those who claim that climate change is not happening or, at best, that the issue is overblown.