Michael McCarthy: The symbol of nature's battle with climate change

The giant panda for the age of climate change: such has become the fate, in emblematic terms, of Ursus maritimus, the great white bear of the north. As the panda logo of what was the World Wildlife Fund, 50 years ago, came to symbolise all threatened animals, so the image of a polar bear on a melting ice floe has come to stand for the dire consequences of global warming.

There have been wild exaggerations: that all polar bears are immediately threatened, or even on the brink of extinction. In reality, some of the world's 19 polar bear sub-populations are doing well; some are increasing. But others are not, and with the more southerly groups, such as that in Hudson Bay, the effects of the increasingly rapid melting of the sea ice are now working through: these animals have grown measurably smaller and weaker, and the population has dropped by a quarter in 30 years.

The point about the new study from the University of Alberta is that it is science rather than modern mythology, based on mathematical models rather than guesswork; it simply plots how reduced time hunting for seals translates into reduced polar bear body fat, energy, ability to bear pups and to survive.

And the hunting time is reducing, by about a week every decade, and is likely to continue to reduce, if the sea ice continues to shrink, as it is likely to do, unless the laws of cause and effect are suspended, and our pumping out of hugely increasing amounts of a gas we know retains the sun's heat in the atmosphere turns out to be effect-free.