National Geographic : 2011 Jul
• o en and had fewer cubs. Fewer cubs survived. When that same year Stirling and his col- leagues published their findings, it was still possible to doubt that warming in the Arctic had already a ected polar bears. In a 1999 in- terview Steven Amstrup, chief scientist at Polar Bears International, who had studied bears in the Beaufort Sea since for the U.S. Geo- logical 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 di cult time I'd been having getting out onto the ice to conduct my autumn eldwork was not just an odd year or two but a prolonged and worsening trend. Shortly therea er we be- gan to see the same biological changes in our bears as well." e 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 de- clined 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, green- house gases have contributed to an average warming of the Earth of about one degree Fahr- enheit. is may seem negligible, but even one degree of warming can noticeably disrupt an en- vironment of ice and snow. It's as if a giant hand has trained a magnifying glass over the Pole. The sea ice above the shallow continental shelves provides the richest sustenance for po- lar bears, but recently the ice has been retreat- ing far from those areas, reducing the summer habitat bears need most to survive. Whether a polar bear lives in Hudson Bay or the Beaufort or Barents Seas, it faces the same problem. Sea ice on which to hunt is available for progres- sively shorter periods, forcing bears to fast for longer periods. And because thinner sea ice is more easily shi ed by winds and currents, bears may be swept into strange territory, forcing them to make longer, more arduous swims in open water to nd favorable sea ice or to get to land. Polar bears are strong swimmers, but swim- ming long distances in open water is draining and can be fatal. In a radio-collared bear with a yearling cub swam an astounding miles to reach the ice o the northern Alaska coast. e cub didn't make it. Researchers count- ing bowhead whales in September spotted four dead polar bears a oat a er a storm in the Beaufort Sea. Scientists estimated that as many as 27 bears may have drowned in that one storm. Females face especially hard times. Malnour- ished males may kill and eat cubs---and even their mothers---behavior scientists believe may become more common as food diminishes. In- creasingly, getting to ancestral denning places on land can be an ordeal. On one island in Svalbard, when the sea has frozen late in the year, scien- tists have seen few, if any, dens the following spring. at's when they'd normally see 20 or more, Jon Aars, of the Norwegian Polar Institute, says. Whether females nd other sites or skip a year of breeding, Aars can't say. From childhood we create a picture of our physical world: e sky is blue, the Arctic is white. But before this century ends---and perhaps much sooner---most of the Arctic is pre- dicted to be blue water every summer. Can a blue Arctic support polar bears? Only in the short run, Amstrup and Stirling say. Currents still cram dri ing sea ice against the Canadian Arctic Islands and northern Green- land in summer, creating pockets that may re- tain enough ice to support polar bears through this century. If we can reduce the warming of the atmosphere, Amstrup says, it will not be too late for polar bears, but "if the world keeps warm- ing, ultimately even those last refuges will fail to sustain the icon of the Arctic." j Susan McGrath is a contributing editor at Audu- bon. Florian Schulz accompanied the MacGillivray Freeman Films team making To the Arctic, a 3-D, IMAX-theater lm to be released worldwide this winter. A companion book of Schulz's work in the Arctic will be published by Braided River.