Missing a Global Climate Target Could Spell Disaster for These Polar Bears

Polar bears in the Southern Hudson Bay could go extinct as early as the 2030s because the sea ice that helps them hunt for food is thinning, a new study suggests.

“We’ve known that the loss of Arctic sea ice would spell disaster for polar bears, so this might be the first subpopulation that disappears,” said Julienne Stroeve, the lead author of the study, which was published Thursday in the journal Communications 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 usual.

Polar bears are used to an ice-free season of about four months when they rely on fat reserves until ice reforms and they can hunt blubber-rich seals from the floes. But the presence of sea ice doesn’t guarantee the bears will be able to hunt; it needs to be thick enough to support them.

While earlier studies looked at the expanse of sea ice coverage to determine the survivability of the species, Dr. Stroeve and her colleagues used climate models from the Intergovernmental Panel on Climate Change’s most recent report to project when the remaining ice would be too thin for the bears to hunt successfully.

While there is no consensus on how much ice is needed to support an adult male polar bear, the study relied on field research to determine a base line of about 10 centimeters, or just under four inches.

Polar bears excel at dealing with minimal resources when it comes to ice. They crawl. They shimmy on their bellies. They extend their limbs as far apart as possible, spreading their mass more evenly over the ice. Sometimes they still fall through. That’s not usually a problem for the bears, who are strong swimmers, but it’s a bigger problem if they’re hunting seals. Crashing through the ice is like an alarm going off, alerting seals to the presence of predators.

Geoffrey York, senior director of research and policy at the Polar Bear Institute and co-author of the study, said polar bears need thick ice for the sprint they typically need to catch a seal. Sea ice, with a high salt content, is more plastic and resilient than glass-like freshwater ice. But other experts said 10 centimeters was pushing it.

“We always try to look for a metric to use,” said Andrew Derocher, 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 needs to be about twice that thick for polar bears to be really using 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 pretty daunting,” she said.

The ice-free season is now about a month longer than what polar bears are habituated to. Studies show that when the ice-free period extends to six months, even the hardiest Hudson Bay bears, generally healthy adult males, will struggle to survive.

Polar bears are what is known as an indicator species, meaning they predict the health and viability of the broader Arctic ecosystem. The concurrent loss of sea ice with depletion in snow cover significantly affects their preferred diet of ringed seals, which have a hard time keeping pups alive in their birthing dens if snow levels drop below 32 centimeters.

Last year, global temperatures temporarily hit 1.5 degrees Celsius above preindustrial 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. While the temperature rise isn’t permanent, Dr. Stroeve and other scientists said polar bears in this region could not survive if temperatures surpassed 2.1 degrees Celsius above the preindustrial benchmark.

Today, there are about half as many polar bears in Western Hudson Bay as there were in 1987.

“Our best analysis is that we’ll 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’s very unlikely in Hudson Bay.”

Hudson Bay bears are unlikely to move from their habitats, even when conditions become untenable. At some point, First Nations and Inuit communities might have to change their traditional polar bear harvest just to preserve the bear population. Towns might have to figure out ways to deter bears from seeking human food during times of distress to minimize human-bear conflict. Long-term possibilities could include distributing polar bear kibble, but Dr. Derocher said that it wasn’t possible to sustain a subpopulation that way indefinitely.

“Beyond dealing with greenhouse gas emissions,” Dr. Derocher said, “there are no possible actions for long term management of the population.”