http://www.polarbearsinternational.org/polar-bears/bear-essentials-polar-style/human-interaction/climate-change

Climate Change is Killing Polar Bears As We Speak

Polar bears are the top predators in the Arctic, and the reduction of ice means the reduction of their favorite meal; the Ringed seal. They are very dependent on the ice caps floating in the sea. They use them as a platform for hunting and resting. Polar bears use them also for transportation to food sources and for mating. The ice reduction, which is caused by the rising temperature of the sea, means the decrease in seal population and it increases the amount of energy needed to catch them.

The more time needed to catch the seals and other food sources, and just trying to survive means the less they mate. The less they mate the lower the polar bear population becomes. Studies show that the decreases is happening now, in the Hudson Bay the population of the bears have decreased by 22 percent. Some scientist have found out that that if some polar bears haven't drowned from getting to food then they have become cannibalism.