Thank you girls for your replies.
I found some more information here:
http://www.defenders.org/wildlife_and_habitat/wildlife/polar_bear.php
Climate change, which is causing the loss of older stable sea ice and the thinning, disappearance and moving offshore of older sea ice, is reducing essential polar bear habitats and is the great threat to their survival. Loss of sea ice leads to higher energy requirements to locate prey and a shortage of food. This causes higher mortality among cubs and reduction in size among first year cubs and adult males. For example, in Alaska 42% of cubs now reach 12 months of age, down from 65% 15 years ago. Another threat is human-caused mortality. Some bears are attracted by unsecured garbage and animal carcasses. This can eventually lead to conflicts between people and bears and lethal removal of the bear. Illegal killing (poaching) of bears also remains another factor causing their decline.