June 16, 2022

These polar bears have found a way to survive without sea ice


A previous version of this article incorrectly said the polar bears were isolated in southeast Greenland for hundreds of thousands of years. The bears have been isolated for hundreds of years. The article has been corrected.

For centuries, sea ice was deemed essential for polar bears to survive. Generations of the Arctic bears ventured onto the slabs of floating ice to travel long distances and prey on unsuspecting seals. As climate change shrank sea ice concentrations, many of these populations struggled to thrive.

But now researchers have found a new genetic population of polar bears in Greenland that don’t rely on sea ice to hunt, rewriting how we think about the sea bears and their ability to adapt to a warming planet. Scientists described their discovery of this 20th subpopulation of polar bears in a study released Thursday in the journal Science.

“This was just a wholly unexpected finding,” said lead author Kristin Laidre in an interview. “They are the most genetically isolated polar bears in the world, and they’re different from all the other currently accepted 19 subpopulations around the Arctic.”

Much of the population’s uniqueness comes from their remote location in the southeast corner of Greenland. The Greenland ice sheet borders them to the west, while open ocean borders them to the east, limiting their travel and interaction with other polar bear populations. The team isn’t certain how the bears got there, but the data suggested they have probably been isolated in the region for hundreds of years. Their unique genetic makeup may have evolved over several hundred years of isolation.

Laidre said this new subpopulation — estimated to number in just the hundreds — lives at the most southern reaches of polar bear distribution, technically in the subarctic region. As a result, this region also experiences shorter sea ice seasons than other polar bear habitats on the island.

“They’re very local bears. They don’t move very far. They stay in the same fjord for years,” said Laidre. “They have sea ice on average about 100 days per year, and we know that’s just way too short for a polar bear to survive.”

Instead of relying exclusively on sea ice, the polar bears adapted and hunt from glacial ice protruding from the ice sheet. While other polar bear populations must move to new locations during the ice-free seasons, these bears move to the back of the fjords against the glacier fronts. They use these glaciers as platform to hunt seals all year round.

The study’s authors say the discovery of this unusual behavior is enlightening, especially as climate change continues to shrink the region’s sea ice.

“When we’re looking to the future and we’re looking at an ice-free Arctic, we ask, ‘Where are the places that polar bears can hang on? Where might they be able to survive or persist?’ ” Laidre said.

Rising global temperatures have reduced Arctic sea ice concentrations by 13 percent each decade since 1979. Climate models project sea ice conditions in the heavily polar bear-populated regions in the High Arctic will deteriorate even further later this century. The sea ice season could become as meager as currently seen in this region of southeast Greenland, which is ice-free for more than eight months a year.

Earth is now losing 1.2 trillion tons of ice each year. And it’s going to get worse.

“What’s neat about this population is that they’re really living in a habitat that we thought was beyond the physiological capacity of these bears to survive,” said Beth Shapiro, a study author and evolutionary biologist at the University of California at Santa Cruz. Shapiro said the team doesn’t know if the bears hold a specific genetic mutation that helps them adapt to this habitat, but would like to investigate any links in the future.

Despite this adaptation, the bears are not immune from climate change. Just as the Greenland ice sheet is losing mass each year, glaciers around the ice sheet are also retreating. But projections show the southeast edge of the ice sheet and nearby glaciers not retreating as quickly as other highly bear-populated areas.

“Some changes that we anticipate with climate change may occur more rapidly than we expected, while others may occur more slowly,” Twila Moon, a study author and researcher at the National Snow & Ice Data Center at the University of Colorado at Boulder, said in an email. “As sea ice continues to decline, glacial ice may remain available for longer.”

Greenland ice sheet experiences record loss to calving of glaciers and ocean melt in 2021

Even if glacier ice is available longer, the researchers said there are few places in the Arctic where this type of glacial ice is accessible to polar bears. Such environments only exist in this region of Greenland and in Svalbard, Norway.

“There are a limited number of places in the Arctic where this type of glacial ice is available, however, so using glacial ice is not an option for many Arctic polar bear populations,” Moon wrote.

John Whiteman, who serves as the chief research scientist at nonprofit Polar Bears International and was not involved in the study, agreed that this finding does not change the fate of polar bears.

“This paper reinforces that polar bears are exclusively reliant on ice; what is unique here is that the source of the ice is a glacier rather than sea ice,” Whiteman said in an email. “This strategy does not offer a long-term home for polar bears.”

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