<b>The world's largest land-based hunters, polar bears stand on the edge of rapid decline, unable to adapt quickly to a shrinking habitat as Arctic sea ice melts. Last year the US government added the animal to its threatened species list, but <i></i></b>Tim Skelton<b> examines whether it's too little too late. </b> Images of starving polar bears are a poignant symbol of the state of our planet. When, in May 2008, the US government added the animal to its threatened species list, it became the first large mammal recognised as being in decline as a result of global warming. Polar bears have a major problem. Their habitat is changing fast, and it's threatening their survival. Moreover, it won't just be the bears that are affected. As the Arctic region's top predator, their disappearance would have an impact on the entire food chain. The world's largest land-based hunters, adult male bears can weigh 750kg. They are perfectly adapted to cold climates, with fur designed to trap heat, and furry feet giving good grip on ice. They spend winters on the Arctic sea ice, and are expert seal hunters. But summers are a different story. When the ice melts they spend their time on land fasting, shedding a kilogram a day. This unusual fast/feast cycle worked well until global warming upset the balance. In summer 2007, satellite images revealed that Arctic sea ice retreated to a record level many climatologists had predicted would not happen until 2050. A US Geological Survey report concluded that two-thirds of the polar bear's habitat could disappear by 2050. And some experts believe sea ice may have passed a point of no return, and may disappear entirely during the summer within 25 years. Researchers from NASA and the Canadian Wildlife Service have also published a study on the extent of sea ice since 1978. Focussing on Canada's western Hudson Bay region, they found the ice there was breaking up earlier and earlier, shortening the polar bears' hunting season by three weeks. "If they feed for a shorter time, they're going to accumulate less fat," said Ian Stirling, a polar bear expert with the Canadian Wildlife Service, and co-author of the study. "At the same time, they're going to be on land and fasting for longer." The bears have few food options on land, and must scavenge for whatever they can find. "There are a few sources of nutrition, but not enough to sustain the population," Stirling said. The lack of food has even forced some animals to resort to cannibalism. Another effect of diminishing ice is that bears must swim longer distances across open water, further depleting their energy. This has led to an increase in cases of drowning. Moreover, as females become thinner, their reproductive rates drop and the survival chances of their cubs declines. The average weight of female bears dropped from 290kg in 1980 to 230kg in 2004. The global population of polar bears has actually doubled since 40 years ago. Widespread hunting had driven numbers to a low of 12,000 in the 1960s, and a rebound occurred when strict controls were introduced. Today the global population is thought to be 20,000 to 25,000. But this apparently good news is hugely misleading. Virtually all experts agree a time bomb is ticking, and a rapid decline is imminent. With the Arctic warming faster than anywhere else on the planet, the bear's natural habitat is changing too quickly for them to adapt. Significant falls in local populations have already been observed. Numbers in the western Hudson Bay region declined from 1,200 in 1987 to 950 in 2004, a 22% drop. Unfortunately, because hungry bears congregate around human settlements in the hope of scavenging for food, native Inuit hunters actually see more bears than they used to. Some treat this as evidence the population is growing. Overall, the US Geological Survey predicts two-thirds of the world's polar bear population will disappear by 2050, vanishing from all but their most northerly ranges. By 2080, there may only be a few remaining. The US government's decision to list the bear as "threatened" is a step in the right direction. But it may be too late. The future doesn't look bright. But the polar bears' one remaining trump card may well be their appearance. Despite their ferocious nature, we perceive them as "cute". When we see them on TV, we sit up and take notice. So with documentary films such as Earth bringing the animal's suffering right into our living rooms, their plight has become impossible to ignore. For thousands of years, polar bears have been an integral part of the Arctic. If they are going to be around for another thousand, it's time to act now.