That’s all fine and dandy for panda bears, but the outlook is grimmer for their northern (non)cousins, polar bears. Polar bears face a number of threats — widespread habitat fragmentation, pollution, excessive hunting — but the most serious menace of all is climate change, according to a report issued yesterday by the World Wildlife Fund. Rising temperatures at the top of the world are melting the massive ice sheets where polar bears hunt for their prey; computer models predict a 60 percent decline in summer sea ice in the next 50 years, which would lengthen the ice-free season by 90 days. For the bears, that translates to a shorter hunting season and diminished fat reserves to ensure summer survival. Polar bears are the world’s largest land carnivore; there are currently about 22,000 of them in Canada, Alaska, Russia, Norway, and Greenland.