HERE'S something that's been puzzling me for a long time, and perhaps a kind reader will help me out. Pour yourself a gin and tonic, almost to the top of the glass. Then put in enough ice to take the level of the liquid to the very brim. Now the difficult bit: fight the temptation to take a slurp, and just sit and watch while the ice melts. Does the liquid overflow? I think not.

But that's not what's puzzling me. You don't have to be a great physicist to grasp what Archimedes was on about when he had his Eureka moment in the bath and came up with the principles of water displacement and buoyancy. The floating ice in the drink displaces its own weight in gin and tonic -- and so, as it melts, the level of liquid in the glass stays the same.

No. My problem is that if Archimedes was right -- and I don't think anyone disputes that he was -- then why are we all supposed to fly into a terrific flap about global warming and the effect of the melting polar ice-caps on sea levels?

Oh, I know that some of the polar ice rests on land. But the great bulk of it is just floating in the Arctic and Antarctic oceans, quietly displacing its own weight in sea-water. So why are we constantly being warned that, as it melts, great swathes of East Anglia and other low-lying coastal areas will disappear beneath the waves?

Only yesterday, the European Space Agency said that the minimum ice extent in the Arctic -- the smallest area of ice recorded annually-- had shrunk by a mind-boggling 970,000 square miles since the early Eighties, from 3.09 million to only 2.12 million.

Scaremongering
A few weeks ago, said the agency, a ship could have sailed unhindered from northern Europe to the North Pole.

Wow! As far as I know, however, Wells-next-the-Sea in Norfolk is still next-the-sea and not under it. I find that very encouraging. If the Norfolk coast can survive the melting of nearly a million square miles of the Arctic, without suffering any perceptible ill effects, then there doesn't seem to be all that much to worry about.

In fact, I've often thought that when I'm rich I'd like to buy a weekend cottage near Wells -- and no amount of scaremongering by the ecologists will make me change my mind. But then putting sea-levels aside -- and strictly between you and me -- I'm not sure how far I go along with much else of what the scientific Establishment has to say about global warming and climate change.

I'm not denying that it's happening. It's pretty clear that average temperatures are creeping upwards at this particular moment in history. But then temperatures, like sea-levels, have been going through periods of ups and downs since the dawn of time. What I'm not so sure about is precisely what is causing the present trend -- and how much of it can be attributed to the behaviour of mankind.

The ecology lobby and the scientific Establishment seem very sure indeed. They believe the current global warming is being caused by the massive amounts of carbon dioxide and other greenhouse gases that have belched from factories, power stations and machines since the Industrial Revolution.

I grant you that it's a very plausible theory, and I'm inclined to believe that there may be at least a little truth in it. But it's certainly not the only one.

Even if greenhouse gases do cause global warming -- and it's still an if -- they have had plenty of other sources since time immemorial, from erupting volcanoes to billions of animals breaking wind (no, really, the digestive systems of horses and cattle, in particular, produce colossal amounts of methane). Yet somehow we still seem to need our pullovers in the winter.

What is quite clear is that global warming, like every other branch of meteorology, is an extremely inexact science. For years, scientists have been telling us that before long, pineapples and bananas will be growing all over the British Isles.

Now up pops another set of academics from the University of Colorado to tell us that northern Europe will actually become cooler as the ice caps melt. So back to the garden centre we must go to swop our palm trees for Arctic tundra.

The eco-warriors are so sure that modern man is to blame for global warming that some of them seem to have become a little unhinged.

In yesterday's Guardian, for example, environmentalist George Monbiot wrote: 'Almost everywhere, climate change denial now looks as stupid and unacceptable as Holocaust denial.'

Does he realise how doltish and offensive he is being, equating those who are sceptical about the environmentalists' wilder claims with the crypto-Nazis who maintain that the most hideous and best documented crime of the 20th century never took place?

But Mr Monbiot is far from being the only eco-warrior who seems to have taken leave of his senses. This week, it emerged that Britain's leading scientists of the Royal Society have written to the oil giant ExxonMobil, demanding that its board stops funding researchers who attempt to undermine the Establishment's consensus on climate change.

What an astonishing way for men and women who dare to call themselves 'scientists' to behave. Almost every worthwhile advance in science throughout history sprang from testing and challenging the received wisdom of the day -- that the Earth was flat, that the Sun revolved around it, that the best way to cure a patient of fever was to slit his arm and drain off a couple of pints of blood...

Isn't there at least a possibility that the scientists are either wrong or wildly exaggerating when they say that industrial effluent is threatening the very survival of our species? If so, wouldn't it be just as well to test the theory from every angle before we commit ourselves to lives of post-industrial poverty and crippling taxes on fuel?

The Royal Society is behaving more like a bunch of religious maniacs than a group of eminent scientists when it tries to stop ExxonMobil from dividing a paltry £1.5 million grant between 39 groups of eco-sceptic researchers.

Taxpayers
The question surely arises if they believe their theories are proven beyond doubt, then what are they afraid of?

It is not as if they are failing to get their message across. The entire bien-pensant Establishment -- from the Government, the Civil Service and the teaching profession to the Guardian and the BBC -- preaches Man's responsibility for global warming as an article of faith.

Nor is it as if the eco-lobby is starved of funds, while the wicked oil industry lavishes money on the sceptics. On the contrary, research aimed at establishing that global warming is man-made has become a multibillion-dollar worldwide industry -- financed very largely by taxpayers.

Indeed, scientists and academics can barely get a grant to study anything these days, until they have chanted the new creed of the Establishment: 'I believe that civilisation is about to be wiped out by man-made global warming, passive smoking and avian flu.'

Could it be that the Royal Society is trying to silence all opposition to its consensus because it has the teeniest suspicion that it may be wrong?

I have a theory that climate change is caused by massive blasts of hot air from Broadcasting House, Television Centre, Whitehall and the offices of the Guardian. If ExxonMobil would like to fund the large quantities of gin and tonic I will need for my research, all cheques will be gratefully received c/o the Daily Mail.