Is global warming a real threat, or are we confusing climate with weather? Whatever the answer, it is bad news for polar bears.
I have never been unduly worried by the predictions of doom-mongering scientists, particularly on the topic of global warming. Many of the same scientists who are now so concerned about global warming were, after all, warning us only 20 years ago of an impending ice age. Yet while another ice age is indeed inevitable within the next couple of thousand years (it's all to do with the way the earth wobbles around the sun) the evidence for potentially catastrophic effects of global warming is rather less convincing.

The heart of the problem is the "greenhouse effect" which until recently was seen as a benevolent accident of physics that kept our planet warm enough to nurture life. It works like this: of the heat from the sun that enters our atmosphere, an estimated 34 per cent is reflected back into space, mostly by clouds. A further 19 per cent is absorbed by the atmosphere and warms the air directly, but what really keeps us warm is the 47 per cent that reaches the earth's surface. The land and seas then absorb the heat and radiate it back into the atmosphere.

Generally speaking, what happens is that certain gases, particularly carbon dioxide and water vapour, are no hindrance to short-wave radiation from the sun as it passes through the atmosphere on its way to earth, but the same gases reflect and absorb the heat as it comes back, in long- wave radiation from the surface. The sun heats the earth, and the atmosphere stops the heat escaping again into space - just like the walls of a greenhouse.

So as we burn more fuel, we release more CO2 into the air, giving it greater potential to retain heat. Result: global warming, melting ice- caps, rising ocean levels and general disaster. On the other hand, as it gets warmer, we have more cloud, which reflects away more of the sun's warmth into space, making it colder again. And when we add volcanic ash and other pollutants to all that cloud, you arrive at the Seventies prediction of a new ice age around the corner.

Hard evidence of true global warming is difficult to obtain. A few good summers do not add up to a change of climate. In any case, the earth is warming up because we are still recovering from the Little Ice Age which ran from about 1550 to 1850. Recent research has, however, begun to establish a link between temperature rise and the levels of man-made greenhouse gases in the atmosphere.

The latest news looks particularly bad for penguins and polar bears. Edinburgh has had such good weather that penguins in the zoo have needed suntan lotion, while a North Pole expedition has reported that a thaw in the Arctic has led to polar bears' dens collapsing. Yet all is not gloom. Snow, after all, rarely falls when it is really cold - icy air cannot hold sufficient humidity - and snow is the material from which glaciers form. So it's warm weather, not cold, that ushers in an ice age. And there is a good deal of geological evidence to support that view. So don't panic, polar bears. Just wait for those glorious winter snowfalls.