National Public Radio: The threatened polar bear has become an icon for the potentially devastating effects of global warming. The animal depends on sea ice for its survival, and this ice is disappearing, no more so than in the Chukchi Sea — the remote stretch of ocean between Alaska and Russia. U.S. biologists don’t know much about the bears that live there, but they’re trying to learn as much about the population as they can before their habitat melts away. One hundred miles from land in the …