According to the National Wildlife Federation, polar bears are in danger of extinction due to global warming. In 2008, the bears became the first vertebrate animal to be listed by the United States Endangered Species Act as in danger of becoming extinct due primarily to global warming.
Scientists believe that the rising global temperature has caused the sea ice to begin melting. As the sea ice melts, the polar bears' habitat begins slowly disappearing for longer and longer periods of time. The bears rely on this ice to provide them with hunting grounds for seals, on which they make an almost exclusive diet, and to provide housing. Because of the decrease in ice, the polar bears must swim further and travel longer distances in order to reach the ice during summer months, as well as to migrate, to make their homes and to hunt. Global warming requires polar bears to endure temperature changes up to 100 degrees in difference throughout the year. Unlike other bears, polar bears only nest when the female is pregnant. Because of the freezing arctic temperatures, polar bear cubs have to be born in a warm environment so that they do not die from exposure to the elements as babies. The mother nests in a den with the cubs during the cold winter months during which the cubs subsist on the mother's milk.