Polar bears: Walking on thin ice

Polar bears are under attack. Global warming and chemical pollution have affected their breeding and feeding patterns so severely that the big beasts of the Arctic could soon be extinct. Peter Marren reports

It's the old, old story. Back in the Sixties, agricultural pesticides such as DDT were used on an enormous scale without much regard to the environmental consequences. The result, in Britain, was a crash in the numbers of most of our top predators, such as otters, eagles and peregrines, after dangerous levels of pesticides accumulated in their body fat. DDT and other persistent chemicals have long been effectively banned. But, just as we hurried into things then, so our generation's rush to make a safer world for ourselves and our children has liberated a new generation of toxic chemicals into the environment.

The new DDT is known as polybrominated diphenyl ether (PBDE). Manufactured in the United States, it was widely used in the 1990s to coat electrical appliances, sofas, carpets and car seats to make them flameproof. The problem is that this chemical was designed to last the lifetime of the product, but in fact it lasts much longer. When sofas, carpets and car seats were thrown away, PBDE entered the rivers, the oceans and even the atmosphere.

The most prominent victim of the 1990s rush to safety is the much-loved polar bear. The Arctic, where all the world's polar bears live, is one of the great sinks of the planet. Chemical pollutants such as PBDE are carried towards the Arctic Ocean by the great continental rivers of Russia and Canada. PBDE already in the sea are wafted northwards by the currents. Even PBDE molecules in the air are carried there by winds, where they condense in the cold and fall to the ground in snow or hail.

Essentially the chemicals "biomagnify" as they move through the food chain from plankton to predator, so that long-lived top carnivores such as the polar bear accumulate the most concentrated amounts of them. In the case of DDT, the chemicals affected not only the general health of the animal but fatally reduced its capacity to breed. The chemically induced thin eggshells of polluted birds smashed before they could hatch.

Now Derek Muir of Environment Canada has found worryingly high amounts of PBDE in the body tissue of polar bears from right across the Arctic. They are higher in some areas than others, with Norway's Spitzbergen population having 10 times the level of the chemical as those in Alaska. Similar large quantities of PBDE have been found in Arctic killer whales. The long-term environmental effect of PBDE is unknown, but researchers suspect that it will damage the polar bear's immune systems, brain functions and bone strength. It also messes up the bear's sex hormones. One female bear on Spitzbergen had both male and female organs, a phenomenon often linked to chemical pollution.

This disaster could not have come at a worse time for the polar bear. The big beast of the Arctic, the largest land carnivore in the world, is already under stress from the results of climate change. Most of the bear's food comes from the sea. Its Latin name, Ursus maritimus, means "sea bear". They spend their time roaming the sea-ice along the frozen shores of the great northern land masses of Europe and America, hunting for seals in the channels and cracks where the ice is thinnest. During the Arctic summer, the bears follow the receding ice northwards. This means often having to swim from one ice floe to the next. This is normally not a problem for the sea bear, which can swim up to 15 miles in a calm sea.

Unfortunately, climate change has made life harder for polar bears. The sea ice is melting earlier and receding further northwards than ever before. The bears must therefore swim up to 60 miles in rough seas to reach it, and many are drowning on the way. Polar bears are strong swimmers, using their front legs to doggy-paddle with the back legs held flat as a rudder. They can swim for several hours without tiring, but they are built for paddling near the shore, and are vulnerable in heavy seas.

Off the north coast of Alaska the average summer temperature has increased by 2 to 3C since the 1950s. Last September the ice had retreated a record 160 miles from the Alaskan coast, and scientists from the US Geological Survey found 10 polar bears swimming offshore with another 40 stranded on land. A few days later, after a storm and fierce seas, they found four drowned bears in the water, a fate that had probably befallen many more.

Polar bears rely on regular supplies of fresh meat, and can eat 100lb or more of meat and blubber in a single sitting. However, the "fat time" when the bears feast on seals in the teeming seas of the Arctic spring is being squeezed by climate warming. Because the ice is breaking upearlier now than ever before, young bears have less time to feed up and store the fat they need for survival on land.

On land, polar bears are clumsy hunters and rely on the fat reserves that they are able to build up in the more plentiful times. Hunger brings them into contact with human settlements, where they rummage through dustbins and rubbish dumps. Recently researchers working for the Worldwide Fund for Wildlife (WWF) in north-east Russia found the first-known evidence of cannibalism in the species.

WWF researchers on Spitzbergen in 2002 found polar bears in poor condition with few signs of seal kills. Only one maternity den, where polar bears give birth to their pair of cubs, was found, compared with up to 30 recorded in years gone by.

The reason for the reproductive downturn may be that the bears no longer have the energy to breed. Observations in the Barents Sea area by Mette Mauritzen at the University of Oslo found that the polar bears are living on the equivalent of a treadmill. As the ice floes are pushed around by winds and currents, the bears instinctively walk against the drift to avoid losing contact with the main ice pack. However, climate change studies predict stronger winds in this area. This would result in the bears consuming more energy to stay in touch with the pack ice, with less left over for breeding.

Observations like these have led to a campaign to list the polar bear internationally as an endangered species. Some scientists predict that the polar bear could become extinct in the wild over the next 100 years. But the plight of the polar bear is only a symptom of a wider threat to the Arctic from global warming and chemical contamination. PBDE has been banned in Canada, but is still legal in the EU. As Norway's environment minister put it, pollution by PBDE "is one of the greatest global environmental threats".

The bear facts

Polar bears are found throughout the Arctic. Bear tracks have been found near the North Pole but most polar bears live along the edge of the land in Greenland, Canada and Russia, hunting in the sea-ice near the shore.

The world population is estimated at between 21,000 and 28,000 animals. They increased after hunting was restricted in the 1970s, but have started to decline.

Polar bears are the world's largest carnivorous land animals. A male can weigh up to 650kg.

They can live as long as 20-30 years, though 15-18 is more usual. One bear in a zoo reached 41.

They can see and hear about as well as us. However, their sense of smell is far better: they can sniff out a seal from miles away.

They normally walk at a sedate 3mph. But don't try running away - they can reach up to 26mph over short distances.

Polar bears are covered in thick, waterproof fur on top of an insulating layer of blubber up to 11cm thick. Underneath the white fur their skin is actually black.

Polar bears feed mainly on seals. They also scavenge the carcasses of whales and walruses, as well as varying their diet with fish, birds, eggs, kelp, berries and human rubbish.

In warm weather, polar bears are in danger of overheating. They find somewhere cool to rest or go for a swim, though bears have been spotted sunbathing on their backs with their feet in the air.

Since 1973 polar bears have been protected. In Canada, Greenland and Alaska regulated hunting by native people is permitted for food, clothing and handicrafts. Bears can also be shot in defence of people or property.