Polar bears are the largest land carnivores, reaching mythic proportions: 12 feet high, 2,000 pounds. They have 42 ivory teeth, and their paws are 12 inches wide, with curved, nonretractable, lethal claws. Polar bears have no animal enemies: Nothing they meet is bigger or fiercer than they are. They are supreme.

They are supreme, but they are dying, victims of something too large for even them to combat. Their habitat – the Arctic ice cap – is vanishing, because of global warming. Their presence on this planet will depend on our response to their plight.

Most of us will never see a polar bear in its native habitat, which adds to its glamour and mystery. Polar bears – ursus maritimus – are found throughout the circumpolar Arctic, ranging like ghostly shadows across the sea ice off the coastlines of Siberia, Greenland, Canada, Iceland and Alaska. In the course of 100,000 years or so, they have adapted brilliantly to life in a punishing, icy moonscape, where temperatures drop to 90 degrees below zero. Their soft, dense fur has turned to misty silver for camouflage. The most pelagic (maritime) of their kind, polar bears are insulated by a thick layer of fat and a smooth, water-repellent coat. They swim for long distances through frigid seas; one was tracked swimming for 60 miles without stopping.

Polar bears prey on marine mammals, mostly seals. They hunt by stalking and by “still hunting” – waiting, poised and motionless, by an ice hole, for an unwary seal to surface. Sea ice is the bears’ habitat, a perforated platform on which to encounter their prey. It is the bears’ home territory, their own vast, shifting, unstable continent. Sea ice – thick, solid and plentiful – is essential to the bears’ survival.

But sea ice itself is endangered. As the planet warms, the rising temperatures are melting the tundra, the glaciers, and the ice caps, destroying the polar bears’ world. The whole arctic landscape, that misty, fantastical world of glaciers, icebergs, floes and pack ice, is vanishing. At summer’s end, the great white bears now find themselves far out to sea, trapped on melting ice floes. They set out to swim, but the distances are too great even for them: Polar bears are drowning. In the last five years, their population has dropped 20 percent, with no relief in sight. The melting arctic ice reduces the hunting range and puts the whole population at risk. Among reproductive females, body weight is dropping, reducing the ability to bear offspring.

Ursus maritimus may be doomed. Even if polar bears are classified as “endangered,” there is no way to protect a portion of the landscape as a wildlife preserve for them. Their habitat is ice and snow, and the entire frozen Arctic Ocean is melting. So in our lifetimes we may lose that great pale shadow that has lived so vividly in the north of our imaginations – enormous, shimmering, powerful, at once fearsome and comforting – padding silently across the glittering wastes. The polar bear may soon be something we’ll tell our children about, like snow.

Global warming is no longer an ominous possibility, but a reality: There is no longer substantive disagreement within the scientific community about its existence. The Bush administration’s own 2004 report stated that greenhouse gases are the “result of human activities and are now higher than they have been for about 400,000 years.” It’s happening, now, and because of us.

The United States is the leading producer of greenhouse gases. It’s also a model of energy policy for the developing world. Our resolute refusal to accept our own scientific counsel, our insistent denial of the consequences of our actions, provide a dangerous example for the developing world. We may deeply regret setting such an example, since the developing world will soon outstrip us in greenhouse-gas production.

In the meantime, the rest of the world is held hostage to our fossil-fuel-fueled agenda, defenseless against U.S. policies. Indigenous Arctic people and polar bears are helpless before our destruction of their way of life. Glaciers are melting worldwide, in Africa, the Andes, the Himalayas, and the Alps, but no one there can demand a change in U.S. policy. Americans, too, are affected. Important aspects of our own economy – travel, communication, shipping, hotels, insurance and agriculture, not to mention public health – are increasingly affected by the severe, unseasonable and unpredictable weather that is increasingly common, and which has been identified as a result of global warming.

It is not just the Arctic that we are losing; it is the notion of the whole world as we first knew it: icy and blizzarding, cracking with cold, at the poles; deep and green and humid, lush with life, in the tropical rain forests; vivid and dense, teeming with birds and fish in the river deltas. Rich, gorgeous, strange, full of a life beyond our control: That was the world we were given, the world we understood was ours. How can we give it up? How can we knowingly destroy it?

Maybe the loss of that last, great, dreamlike animal will rouse us. Maybe now we’ll take a stand and insist on preventing the changes that threaten something so distant, so close, so essential.