On Thin Ice

By Susan McGrath, Staff Writer, National Geographic
1/27/2013 10:02:33 AM

In August 1881 the naturalist John Muir was sailing off Alaska aboard the steamer Thomas Corwin, searching for three vessels that had gone missing in the Arctic. Off Point Barrow he spotted three polar bears, "magnificent fellows, fat and hearty, rejoicing in their strength out here in the bosom of the icy wilderness."

Were Muir to sail off Point Barrow in August today, any polar bears he'd see would not be living in a wilderness of ice but swimming through open water, burning precious fat reserves. That's because the bears' sea-ice habitat is disappearing. And it's going fast.

Polar bears ply the Arctic niche where air, ice, and water intersect. Superbly adapted to this harsh environment, most spend their entire lives on the sea ice, hunting year-round, visiting land only to build maternal birthing dens. They prey mainly on ringed and bearded seals (it's been said that they can smell a seal's breathing hole from more than a mile away) but sometimes catch walruses and even beluga whales.

Ashore on Svalbard, a male polar bear investigates a whale's backbone. Fat reserves from hunting ringed and bearded seals, and sometimes walruses, must carry bears through lean summers. This photo and others can be found in the July 2011 issue of National Geographic magazine, on newsstands June 28. (Florian Schulz/National Geographic)

Sea ice is the foundation of the Arctic marine environment. Vital organisms live underneath and within the ice itself, which is not solid but pierced with channels and tunnels large, small, and smaller. Trillions of diatoms, zooplankton, and crustaceans pepper the ice column. In spring, sunlight penetrates the ice, triggering algal blooms. The algae sink to the bottom, and in shallow continental shelf areas they sustain a food web that includes clams, sea stars, arctic cod, seals, walruses-and polar bears.

Experts estimate the world's polar bear numbers at 20,000 to 25,000, in 19 subpopulations. Bears in Svalbard (the Norwegian archipelago where Florian Schulz made most of these photographs), the Beaufort Sea, and Hudson Bay have been studied the longest. It was in western Hudson Bay, where ice melts in the summer and freezes back to shore in the fall, that the creatures' predicament first came to light.

Ian Stirling, now retired from the Canadian Wildlife Service, has monitored polar bears there since the late 1970s. He found that they gorged on seals in the spring and early summer, before breakup, then retreated to land as the ice melted. In a good year, breakup found bears packing a thick layer of fat. Ashore, the bears entered a state known as walking hibernation, their metabolisms on idle to hoard their fat stores. "Until about the early 1990s at Hudson Bay," Stirling says, "bears were able to fast through the open-water season of summer and fall because hunting on the spring sea ice was so good."

During subsequent years of bear-watching, Stirling and a colleague, Andrew Derocher, began to see an alarming pattern. They observed that although the bears' population held steady, the animals were getting thinner. The western Hudson Bay bears were missing vital weeks of peak seal hunting, and the later winter freeze-up was extending their fast. By 1999 the biologists had correlated a steady decline in most measures of polar bear health with a decline in sea ice. Bears didn't grow as large, and some came ashore notably skinnier. Females gave birth less often and had fewer cubs. Fewer cubs survived.

"When the female saw him," Schulz said, "she huffed at her cubs, and then they just pinned their ears back and ran." Leaping over floes, they kept going long after they'd made good their escape. This photo and others can be found in the July 2011 issue of National Geographic magazine, on newsstands June 28. (Florian Schulz/National Geographic)

When that same year Stirling and his colleagues published their findings, it was still possible to doubt that warming in the Arctic had already affected polar bears. In a 1999 interview Steven Amstrup, chief scientist at Polar Bears International, who had studied bears in the Beaufort Sea since 1980 for the U.S. Geological Survey, said he hadn't yet seen the kind of changes Stirling had. Or had he? "My aha! moment," Amstrup recalls, "was when I realized the difficult time I'd been having getting out onto the ice to conduct my autumn fieldwork was not just an odd year or two but a prolonged and worsening trend. Shortly thereafter we began to see the same biological changes in our bears as well."

The world didn't know it yet, but during the summer in the Arctic Ocean, sea ice had been melting earlier and faster, and the winter freeze had been coming later. In the three decades since 1979 the extent of summer ice has declined by about 30 percent. The lengthening period of summer melt threatens to undermine the whole Arctic food web, atop of which stand polar bears.

Data have since bolstered the early warning signs. Since Muir set out in the Corwin, greenhouse gases have contributed to an average warming of the Earth of about one degree Fahrenheit. This may seem negligible, but even one degree of warming can noticeably disrupt an environment of ice and snow. It's as if a giant hand has trained a magnifying glass over the Pole.

For more information on how Arctic warming is impacting Polar Bears, continue reading the article from the July issue of the National Geographic magazine.

credit: National Geographic

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