The lack of a global climate target could spell disaster for these polar bears

Polar bears in southern Hudson Bay could disappear 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 could be the first subpopulation to disappear,” said Julienne Stroeve, 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, broke free of ice a month earlier than usual.

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

While previous studies looked at the extent of sea ice cover to determine species survival, Dr. Stroeve and her colleagues used climate models from the latest Intergovernmental Panel on Climate Change 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 establish a baseline 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 squeeze in the stomach. They extend their limbs as much as possible, spreading their mass more evenly over the ice. Sometimes they still fall. This is usually not a problem for bears, which are strong swimmers, but it is more of a problem if they are hunting seals. Crashing through the ice is like an alarm going off, alerting the seals to the presence of predators.

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

“We’re always trying to find a metric to use,” said 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 for the polar bears to 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. “It’s actually quite scary,” she said.

The ice-free season is now about a month longer than polar bears are used 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 are known as an indicator species, meaning they predict the health and resilience of the wider Arctic ecosystem. The concomitant loss of sea ice with reduced snow cover significantly affects their preferred diet of ringed seals, which find it difficult to keep pups alive in their birth dens if snow levels drop under 32 centimeters.

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 below to avoid the worst effects of global warming. While the increase in temperature is not permanent, Dr. Stroeve and other scientists said polar bears in the region could not survive if temperatures rose 2.1 degrees Celsius above the pre-industrial standard.

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

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

Hudson Bay bears are unlikely to move from their habitats, even when conditions become unstable. At some point, First Nations and Inuit communities may have to change their traditional polar bear harvest just to maintain bear populations. Cities may need to find ways to deter bears from foraging for humans during hard times to minimize human-bear conflict. Longer-term possibilities may include the distribution of polar bear jumps, but Dr. Derocher said it was not possible to maintain a subpopulation that way indefinitely.

“Beyond addressing greenhouse gas emissions,” said Dr. Derocher, “there are no possible actions for long-term population management.”