Cutting greenhouse gas emissions enough over the next few decades may stabilize the rapidly shrinking Arctic sea ice sufficiently to provide a sustainable habitat for polar bears, a paper in the Dec. 16 Nature reports. And if emissions do keep rising, another new study finds, the only species that has officially been declared threatened by the U.S. government due to global warming may still be able to hang on for a while in a few pockets of the northern Arctic.

Polar bears need sea ice to hunt their prey, but the frozen skin that floats atop the Arctic Ocean has been thinning and shrinking in recent decades as global temperatures rise. Between 1979 and 2010, Arctic sea ice cover at the end of the summer melt season dropped an average of 11.5 percent per decade. Many researchers think that end-summer Arctic ice could be almost entirely gone by the middle of this century.

In 2007, the U.S. Geological Survey reported that two-thirds of the world’s 25,000 polar bears could disappear within 50 years if greenhouse gas emissions continued unabated. The following year, interior secretary Dirk Kempthorne relied on that report when putting the bear, Ursus maritimus, on the government’s list of threatened species.

Steven Amstrup, an Anchorage-based senior scientist with Polar Bears International in Bozeman, Mont., who was a coauthor on the 2007 USGS report, decided to look at whether cutting emissions could preserve enough of the Arctic sea ice to save polar bears from extinction.

In the Nature paper, his team studied five scenarios for how much atmospheric levels of greenhouse gases would rise over the next century.

Using a widely accepted climate model, the researchers analyzed potential futures for several measures of sea ice habitability — such as the amount of sea ice extending over continental shelves, the number of months each year those shelves are free of ice and the distance between that ice and the more northerly pack ice that bears also use to hunt.

The results don’t support the idea that Arctic sea ice is headed for a catastrophic “tipping point” beyond which the ice disintegrates completely, Amstrup says. Instead, if greenhouse gas emissions and hence temperatures can be stabilized, the sea ice stabilizes too.

“If we act, it isn’t too late to save the polar bear,” Amstrup says.

As it thins, sea ice reaches a point where it becomes more responsive to the water temperature below and is better able to regrow in the winter , says Marika Holland, a sea ice specialist at the National Center for Atmospheric Research in Boulder, Colo. This enhanced growth helps stabilize the shrinking ice.

Still, the more people can limit greenhouse gas emissions, the less melting will happen in the first place, says Amstrup.

Even if emissions keep rising, sea ice will stick around in certain areas of the Arctic longer than others, Stephanie Pfirman, an Arctic specialist at Barnard College in New York and the nearby Lamont-Doherty Earth Observatory, and her colleagues will report in San Francisco December 16 at the annual meeting of the American Geophysical Union. The work meshes nicely with the new Nature paper, she said: “They’re asking what happens if we act to mitigate. We’re looking at the base case: What if we don’t act?”

Winds and ocean circulation regularly pile ice up in the Canadian Arctic archipelago and north of Greenland, Pfirman said. Sea ice is thick there today and may persist long after it has melted elsewhere, the researchers propose.

Polar bears aren’t the only creatures that may depend on those last remnants of ice cover, Pfirman noted. An entire ecosystem, including seals and walruses, depends on sea ice. On December 3, the National Oceanic and Atmospheric Administration proposed listing four subspecies of ringed seal and two populations of bearded seal as threatened because of shrinking ice. It is the first such proposal since the polar bear based solely on the threat of climate change.

By 2100 only the northern fringes of Canada and Greenland — the same areas Pfirman’s group identified in its polar bear study — will have snow deep enough to shelter ringed seal pups, suggests research presented at the AGU meeting by Brendan Kelly, a marine-mammal specialist at NOAA’s National Marine Laboratory in Juneau, Alaska, and Cecilia Bitz, a sea ice physicist at the University of Washington in Seattle. Each spring, these seals make snow caves atop the ice to shelter their newborn pups; to do so, they need snow at least 50 centimeters deep.

But in a greenhouse world, these high Arctic pockets will last only so long, says Robert Newton, an oceanographer at Lamont-Doherty.

“This [refuge] is not expected to last forever,” Newton says. “As the planet keeps warming, it will eventually be lost.”

Image: NASA

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