The fate of polar bears has long been a flashpoint 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,” 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 per cent 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.
Molnar and his colleagues looked at 13 of the subpopulations representing about 80 per cent 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 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,” 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.
Groups including the Cato Institute, a libertarian research organisation that challenges aspects of climate change, have called concerns about the bears unwarranted, arguing that some research shows that the animals have survived repeated warm periods. But scientists say during earlier warm periods the bears probably had significant alternative food sources, notably whales, that they do not have today.
Poignant images of bears on isolated ice floes or roaming land in search of food have been used by conservation groups and others to showcase the need for action to reduce warming. Occasionally, though, these images have been shown to be not what they seem.
After a video of an emaciated bear picking through garbage cans in the Canadian Arctic was posted online by National Geographic in 2017, the magazine acknowledged that the bear’s condition might not be related to climate change. Scientists had pointed out that there was no way of knowing what was wrong with the bear; it might have been sick or very old.
The New York Times