As the estimated cost of measures proposed by politicians to "combat global warming" soars ever higher - such as the International Energy Council's $45 trillion - "fighting climate change" has become the single most expensive item on the world's political agenda.

As Senators Obama and McCain vie with the leaders of the European Union to promise 50, 60, even 80 per cent cuts in "carbon emissions", it is clear that to realise even half their imaginary targets would necessitate a dramatic change in how we all live, and a drastic reduction in living standards.

All this makes it rather important to know just why our politicians have come to believe that global warming is the most serious challenge confronting mankind, and just how reliable is the evidence for the theory on which their policies are based.

By far the most influential player in putting climate change at the top of the global agenda has been the UN's Intergovernmental Panel on Climate Chan ge (IPCC), set up in 1988, not least on the initiative of the Thatcher government. (This was why the first chairman of its scientific working group was Sir John Houghton, then the head of the UK's Meteorological Office.)

Through a succession of reports and international conferences, it was the IPCC which led to the 1997 Kyoto Protocol, soon to have an even more ambitious successor, to be agreed in Copenhagen next year.

The common view of the IPCC is that it consists of 2,500 of the world's leading scientists who, after carefully weighing all the evidence, have arrived at a "consensus" that world temperatures are rising disastrously, and that the only plausible cause has been rising levels of CO2 and other man-made greenhouse gases.

In fact, as has become ever more apparent over the past 20 years - not least thanks to the evidence of a succession of scientists who have participated in the IPCC itself - the reality of this curious body could scarcely be more different.

It is not so much a scientific as a political organisation. Its brief has never been to look dispassionately at all the evidence for man-made global warming: it has always taken this as an accepted fact.

Indeed only a comparatively small part of its reports are concerned with the science of climate change at all. The greater part must start by accepting the official line, and are concerned only with assessing the impact of warming and what should be done about it.

In reality the IPCC's agenda has always been tightly controlled by the small group of officials at its head. As one recent study has shown, of the 53 contributors to the key Chapter 9 of the latest report dealing with the basic science (most of them British and American, and 10 of them associated with the Hadley Centre, part of the UK Met Office), 37 belong to a closely related network of academics who are all active promoters of the official warming thesis.

It is on the projections of their computer models that all the IPCC's predictions of future warming are based.

The final step in the process is that, before each report is published, a "Summary for Policymakers" is drafted by those at the top of the IPCC, to which governments can make input.

It is this which makes headlines in the media, and which all too frequently eliminates the more carefully qualified findings of contributors to the report itself.

The idea that the IPCC represents any kind of genuine scientific "consensus" is a complete fiction. Again and again there have been examples of how evidence has been manipulated to promote the official line, the most glaring instance being the notorious "hockey stick".

Initially the advocates of global warming had one huge problem. Evidence from all over the world indicated that the earth was hotter 1,000 years ago than it is today.

This was so generally accepted that the first two IPCC reports included a graph, based on work by Sir John Houghton himself, showing that temperatures were higher in what is known as the Mediaeval Warming period than they were in the 1990s.

The trouble was that this blew a mighty hole in the thesis that warming was caused only by recent man-made CO2.

Then in 1999 an obscure young US physicist, Michael Mann, came up with a new graph like nothing seen before.

Instead of the familiar rises and falls in temperature over the past 1,000 years, the line ran virtually flat, only curving up dramatically at the end in a hockey-stick shape to show recent decades as easily the hottest on record.

This was just what the IPCC wanted, The Mediaeval Warming had simply been wiped from the record.

When its next report came along in 2001, Mann's graph was given top billing, appearing right at the top of page one of the Summary for Policymakers and five more times in the report proper.

But then two Canadian computer analysts, Steve McIntyre and Ross McKitrick, got to work on how Mann had arrived at his graph.

When, with great difficulty, they eventually persuaded Mann to hand over his data, it turned out he had built into his programme an algorithm which would produce a hockey stick shape whatever data were fed into it.

Even numbers from the phonebook would come out looking like a hockey stick.

By the time of its latest report, last year, the IPCC had an even greater problem. Far from continuing to rise in line with rising CO2, as its computer models predicted they should, global temperatures since the abnormally hot year of 1998 had flattened out at a lower level and were even falling - a trend confirmed by Nasa's satellite readings over the past 18 months.

So pronounced has this been that even scientists supporting the warmist thesis now concede that, due to changes in ocean currents, we can expect a decade or more of "cooling", before the "underlying warming trend" reappears.

The point is that none of this was predicted by the computer models on which the IPCC relies.

Among the ever-growing mountain of informed criticism of the IPCC's methods, a detailed study by an Australian analyst John McLean (to find it, Google "Prejudiced authors, prejudiced findings") shows just how incestuously linked are most of the core group of academics whose models underpin everything the IPCC wishes us to believe about global warming.

The significance of the past year is not just that the vaunted "consensus" on the forces driving our climate has been blown apart as never before, but that a new "counter-consensus" has been emerging among thousands of scientists across the world, given expression in last March's Manhattan Declaration by the so-called Non-Governmental Panel on Climate Change.

This wholly repudiates the IPCC process, showing how its computer models are hopelessly biased, based on unreliable data and programmed to ignore many of the genuine drivers of climate change, from variations in solar activity to those cyclical shifts in ocean currents.

As it was put by Roger Cohen, a senior US physicist formerly involved with the IPCC process, who long accepted its orthodoxy: "I was appalled at how flimsy the case is. I was also appalled at the behaviour of many of those who helped produce the IPCC reports and by many of those who promote it.

"In particular I am referring to the arrogance, the activities aimed at shutting down debate; the outright fabrications; the mindless defence of bogus science; and the politicisation of the IPCC process and the science process itself."

Yet it is at just this moment, when the IPCC's house of cards is crumbling, that the politicians of the Western world are using it to propose steps that can only damage our way of life beyond recognition.

It really is time for that "counter-consensus" to be taken seriously.