The top of the world is a wonderland. In winter, the temperature often falls to -30°F and the sun never rises. The ocean is surrounded by frozen ground. There are few people or trees, but to polar bears, the Arctic(北极)is home. Polar bears have thick fur, big paws and other features that make them well prepared for life in their tough environment. In fact, they need the Arctic sea ice for survival. But climate change is causing larger and larger areas of summer sea ice to melt(融化). Experts say that if warming patterns continue, the Arctic could be free of summer sea ice by 2050. That may cause two-thirds of the word s 20,000 polar bears to be gone by then too. Polar bears can t survive for long on land. Seals are their main source of food. The only place where polar bears can hunt seals is on the ice. Although these bears are strong swimmers, they are no match for lightning swift seals in the water. A polar bear has brilliantly clever strategies to overcome this disadvantage. In winter the bear waits motionless beside a seal s breathing hole, which is a narrow tunnel through the ice. Often many hours pass before the seal comes up for air and the bear kills it with a powerful blow of its paw. In summer, the polar bears that live on land eat very little and wait for the sea ice to return. With the sea ice forming later in the year and melting earlier, polar bears do not have enough opportunity to hunt and eat. Less sea ice makes it harder for the bears to catch the seals. The bears must swim longer distances between ice packs(大片浮水), and they cant always make it. The ice is also getting thinner. These conditions can cause polar-bear cubs to become separated from their mothers, who provide them with food. Steven Amstrup is the chief scientist of Polar Bear International. The group aims to save the bears and their home. ' The more people who see polar bears and understand their difficult situations, the better the chance well alter our warming path in time to save them,' he says. Which is the best title for the passage?