20．概要写作．阅读下面短文，根据其内容写一篇60词左右的内容概要．
With global warming and melting ice，it isn't easy being a polar bear anymore．Some studies have predicted that polar bears could die out by the end of the century．The good news is not all researchers think the bears are absolutely disappearing．Scientists at the American Museum of Natural History（AMNH） have published a new paper indicating that things might not be as bad for polar bears as some scientists expect．
To understand the reason for the researchers'cautious optimism，we must first understand the factors that are threatening the polar bear's existence．Polar bears consume a diet of mainly young seals．In order to hunt these seals，polar bears need to rest at the top of sea ice---the same ice that is increasingly melting for most of the year thanks to climate change．In another 50years，experts expect that the Arctic will be too warm for sea ice to form for half of the year，leaving polar bears without a reliable food source and in serious danger of starvation．
As it turns out，alternative food sources for the polar bears aren't completely out of the question．For as long as biologists have tailed after the animals，they've seen polar bears eating animals found on land like caribou（驯鹿） and snow geese---as well as the snow geese's eggs．
Can polar bears actually survive off these alternative food sources for long periods of time？To figure this out，researchers calculated the nutrients that a caribou and snow geese diet would provide．They found that even adult male polar bears would be able to obtain more calories than they would burn in hunting these meals．Moreover，the food would provide the food necessary to avoid starvation during the summer months．
Unfortunately，not all polar bears have tended to seek food on land．That said，the researchers expect that necessity would push more polar bears to hunt on land to avoid starvation．They also expect that the bears could learn from their fellow bears how to hunt on land until the practice becomes second nature．
