Paws of polar bears sustaining ice-related injuries in a warming Arctic

The study led by the University of Washington was published Oct. 22 in the journal Ecology. It's the first time that such injuries have been documented in polar bears.

The researchers suggest several mechanisms for how the shift from a climate that used to remain well below freezing to one with freeze-thaw cycles could be causing ice buildup and injuries.

"In addition to the anticipated responses to climate change for polar bears, there are going to be other, unexpected responses," said lead author Kristin Laidre, a senior principal scientist at the UW Applied Physics Laboratory and a professor in the UW School of Aquatic and Fishery sciences. "As strange as it sounds, with climate warming there are more frequent freeze-thaw cycles with more wet snow, and this leads to ice buildup on polar bears' paws."

Between 2012 and 2022, Laidre and co-author Stephen Atkinson, a wildlife veterinarian, studied two populations of polar bears living above 70 degrees north latitude and saw the injuries.

In the Kane Basin population, located between Canada and Greenland, 31 of 61 polar bears showed evidence of icing-related injuries, such as hairless patches, cuts or scarring.

In the second population in East Greenland, 15 of 124 polar bears had similar injuries. Two Greenland bears at separate locations in 2022 had massive ice balls stuck to their feet.

"I'd never seen that before," Laidre said. "The two most affected bears couldn't run -- they couldn't even walk very easily. When immobilizing them for research, we very carefully removed the ice balls. The chunks of ice weren't just caught up in the hair. They were sealed to the skin, and when you palpated the feet it was apparent that the bears were in pain."

Researchers have studied these two polar bear populations since the 1990s but haven't reported these types of injuries before. Consultations with lifetime Indigenous subsistence hunters and a survey of the scientific literature suggests this is a recent phenomenon.

Polar bears have small bumps on their foot pads that help provide traction on slippery surfaces. These bumps, which are larger than those on the pads of other bear species like brown and black bears, make it easier for wet snow to freeze to the paws and accumulate. This problem also affects sled dogs in the North.

The authors hypothesize three possible reasons for increasing ice buildup on polar bears' paws -- all related to climate warming. One is more rain-on-snow events, which creates moist, slushy snow that clumps onto paws and then freezes to form a solid once temperatures drop.

A second possibility is that more warm spells are causing the surface snow to melt and then refreeze into a hard crust. The heavy polar bears break through this ice crust, cutting their paws on its sharp edges.

The final possible reason is that both these populations live on "fast ice" connected to the land, near where freshwater glaciers meet the ocean. Warming in these environments leads to thinner sea ice, allowing seawater to seep up into the snow. This wet snow can clump onto bears' feet and then refreeze to form ice. Also, unlike other areas, polar bears living at glaciers' edges rarely swim long distances in spring, which would help thaw and dislodge accumulated ice chunks because the water is warmer than the air.

While the bears are clearly affected by the ice buildup, the researchers are cautious regarding broader conclusions about the health of the two populations.

"We've seen these icing-related injuries on individual polar bears," Laidre said. "But I would hesitate to jump to conclusions about how this might affect them at a population level. We really don't know."

Melinda Webster, a research scientist at UW's Applied Physics Laboratory, recently published a separate study analyzing snow cover on Arctic sea ice over recent decades.

"The surface of Arctic sea ice is transforming with climate change," Webster said. "The sea ice has less snow in late spring and summer, and the snow that does exist is experiencing earlier, episodic melt and more frequent rain. All these things can create challenging surface conditions for polar bears to travel on."

Asked what can be done to help the polar bears, Laidre had a simple response: "We can reduce greenhouse gas emissions and try to limit climate warming."

The field observations of polar bears were funded by the governments of Canada, Denmark, Nunavut and Greenland. Laidre is also affiliated with the Greenland Institute of Natural Resources.