They use it as a floating platform to stand on when they catch seals to eat, and also walk on sea ice to get to their dens.
Already some polar bears, in places like Hudson Bay in Canada, are beginning to struggle because of the effects of global warming.
May have to find new things to eat
The sea ice is melting at a speed of 9% a year, and when it's all gone the polar bears will have to find new ways of living before they become extinct.
Other bears have learnt to change their habits, but polar bears need the fat from seals to get through the winter.
Photos courtesy of Andrew Derocher.
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