National Geographic: It's a tough time to be a polar bear. The Arctic predatorswhich depend on constantly diminishing amounts of sea ice to catch marine mammals such as sealsare declining in number, and fast. There are fewer than 25,000 polar bears left in the wild, according to the nonprofit organization Polar Bears International. Near the southern Beaufort Sea (map), for instance, the population has dropped about 40 percent between 2001 to 2010, from 1,500 to 900 bears. And as their habitat shrinks, they've have...