Polina the Polar Bear sat on an iceberg in the Arctic Ocean, north of Alaska. Polina was waiting for her mother, Bearnice, to return with some tasty seal fat for lunch. Bearnice was hunting, waiting for seals to come up for air through holes in the ice, the same way Polar Bears have hunted in the winter for centuries. Except for one big difference.
A lot more of the ice had melted last summer, and this winter it hadn't refrozen as usual. A combination of warmer weather and changes in water and air currents was making the ocean warmer than anyone could remember, and Bearnice had to swim farther every day to find a good hunting spot.
As Polina waited for her mother to bring food back to her iceberg, the ice shrank bit by bit until she was sitting on very thin ice. And one afternoon, it cracked.