Sea ice in the Arctic (ice that is effectively frozen sea water floating on top of the sea) always melts in the Arctic summer. In 2012 however, it melted more than it ever has done before and the long-term trend is for increasing melting of sea ice in in the Arctic Summer. Polar bears like Peter depend on the ice to be able to find their favourite food - seals! They prefer ringed or bearded seals, but they will eat smaller seals and even walruses. They only eat the fat of the seals, but one seal can provide enough energy for a polar bear to survive for eight days.
With the ice melting more quickly nowadays as a result of climate change and because of changes in ocean currents in the Arctic, polar bears are finding it more and more difficult to find seals in the summer time. When the sea ice melts, the polar bears are forced onto land and the seals don't live on land, which means that more and more bears are going hungry and have to eat land-based prey like geese and bird eggs and the occasional caribou, but these don't have anything like the energy value of a good chunk of seal blubber!