Missing a global climate target could spell disaster for these polar bears

A new study shows that polar bears in southern Hudson Bay could become extinct as early as the 2030s as the sea ice they hunt on continues to thin.

“We knew that the loss of Arctic sea ice would spell disaster for polar bears, so this could be the first subpopulation to disappear,” said Julienne Stroeve, the lead author of the study, which was published in the journal on Thursday. Communication Earth & Environment.

Last month, the eastern half of Hudson Bay, home to the world’s most studied polar bears, went ice-free a month earlier than normal.

Polar bears are used to an ice-free season of about four months, during which they rely on fat reserves until the ice recovers and they can hunt blubber-rich seals from the ice floes. However, the presence of sea ice does not guarantee that the bears can hunt; it must be thick enough to support them.

While previous studies looked at the extent of sea ice cover to determine the species’ chances of survival, Dr. Stroeve and her colleagues used climate models from the Intergovernmental Panel on Climate Change most recent report to predict when the remaining ice would be too thin for the bears to hunt successfully.

While there is no consensus on the amount of ice needed to support an adult male polar bear, the study relied on field research to determine a baseline of approximately 10 centimeters.

Polar bears excel at dealing with minimal resources when it comes to ice. They crawl. They wiggle on their bellies. They stretch their limbs as far apart as possible, which distributes their mass more evenly across the ice. Sometimes they even fall through. Normally this is no problem for the bears, which are strong swimmers, but it is a bigger problem when they are hunting seals. Breaking through the ice is like an alarm going off, warning seals of the presence of predators.

Geoffrey York, senior director of research and policy at Polar Bears International and a co-author of the study, said polar bears need thick ice for the sprint they typically make to catch a seal. Sea ice, with its high salt content, is more plastic and resilient than glassy freshwater ice. But other experts said 10 centimeters was too far.

“We always try to find a benchmark that we can use,” says Andrew Derocher, a professor of biological sciences at the University of Alberta. “But 10 centimeters is pretty thin. I can’t land a helicopter on that ice. It has to be about twice as thick before polar bears can really use it.”

Elisabeth Kruger, a manager at the World Wildlife Fund who focuses on the Arctic, said the modeling was less severe than it could have been. “That’s actually quite intimidating,” she said.

The ice-free season now lasts about a month longer than polar bears are used to. Research shows that when the ice-free period extends to six monthsEven the strongest Hudson Bay bears, generally healthy adult males, will struggle to survive.

Polar bears are so-called indicator species, meaning they predict the health and viability of the wider Arctic ecosystem. The simultaneous loss of sea ice and depletion of snow cover is having a significant impact on their preferred diet of ringed seals, which struggle to keep young alive in their birthing dens when snow depth drops below 32 centimetres.

Last year, global temperatures temporarily reached 1.5 degrees Celsius above pre-industrial levels. Under the Paris Climate Pact, countries agreed to try to limit global warming to that level or lower to avert the worst effects of global warming. Although the temperature increase is not permanent, Dr. Stroeve and other scientists that polar bears in this region would not survive if temperatures exceeded 2.1 degrees Celsius above pre-industrial norms.

Today there are approx half as many polar bears in western Hudson Bay as in 1987.

“Our best analysis is that we will still have polar bears until the end of the century,” said Dr. Derocher, referring to the 19 subpopulations that live throughout the Arctic. “But that is very unlikely in Hudson Bay.”

Bears in Hudson Bay are unlikely to leave their habitats even if conditions become untenable. At some point, First Nations and Inuit communities may have to modify their traditional polar bear hunting practices to maintain the bear population. Cities may have to devise ways to discourage bears from seeking human food in times of need to minimize human-bear conflict. Long-term options could include distributing polar bear kibble, but Dr. Derocher said it’s not possible to sustain a subpopulation that way indefinitely.

“Other than dealing with greenhouse gas emissions,” Dr. Derocher said, “there are no possible actions for long-term population management.”

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