Is belief in global-warming science another example of the "madness of crowds"? That strange but powerful social phenomenon, first described by Charles Mackay in 1841, turns a widely shared prejudice into an irresistible "authority". Could it indeed represent the final triumph of irrationality? After all, how rational is it to pass laws banning one kind of light bulb (and insisting on their replacement by ones filled with poisonous mercury vapour) in order to "save electricity", while ploughing money into schemes to run cars on ... electricity? How rational is it to pay the Russians once for fossil fuels, and a second time for permission (via carbon credits) to burn them (see box page 36)? And how rational is it to suppose that the effects of increased CO2 in the atmosphere take between 200 and 1,000 years to be felt, but that solutions can take effect almost instantaneously?

Whether rational or not, global warming theory has become a political orthodoxy. So entrenched is it that those showing any resistance to it are described as "heretics" or even likened to "Holocaust deniers".

Paul Krugman, the Nobel prize-winning economist, professor of economics and international affairs at Princeton University and columnist for The New York Times, has said: "Is it fair to call climate denial a form of treason? Isn't it politics as usual? Yes, it is - and that's why it's unforgivable ... the deniers are choosing, wilfully, to ignore that threat, placing future generations of Americans in grave danger, simply because it's in their political interest to pretend that there's nothing to worry about. If that's not betrayal, I don't know what is."

Another columnist, this time for The Boston Globe, has written: "I would like to say we're at a point where global warming is impossible to deny. Global warming deniers are now on a par with Holocaust deniers, although one denies the past and the other denies the present and future."

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Such pronouncements from these commentators and from other people highly placed in government, international organisations, the press, academia and science make the debate seem closed and the conclusion beyond dispute. Yet the plain fact is that there is something deeply unscientific about the theory of global warming. Despite this, it has gained such widespread, uncritical acceptance that any scientist expressing a doubt often finds his or her actions tarred with accusations of the rankest political and personal motivations.

How this situation came about says much about how science is co-opted to sway public opinion. The case is built, deliberately or not, on misleading images and interpretations that have been perpetuated by parties with a vested interest. It morphs into a tool for governments to intimidate their populations into passive acceptance of very real changes: from the tiny, such as accepting miserable fluorescent light instead of the incandescent light we've been used to; to the major, like welcoming nuclear power plants and obliging rainforest tribes to make way for biofuel plantations.

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Indeed, much of what is presented as hard scientific evidence for the theory of global warming is false. "Second-rate myth" may be a better term, as the philosopher Paul Feyerabend called science in his 1975 polemic, Against Method.

"This myth is a complex explanatory system that contains numerous auxiliary hypotheses designed to cover special cases, as it easily achieves a high degree of confirmation on the basis of observation," Feyerabend writes. "It has been taught for a long time; its content is enforced by fear, prejudice and ignorance, as well as by a jealous and cruel priesthood. Its ideas penetrate the most common idiom, infect all modes of thinking and many decisions which mean a great deal in human life ... ".

But call it what you will, as long as you don't think that by calling it "science" it becomes irrefutable. Because that it ain't.

Consider the presentation in one of the most popular works arguing the case for global warming and the need for action. In Al Gore's film An Inconvenient Truth, the scientists are reduced to a walk-on part: they are, in essence, an audience invited to applaud the decisions of politicians. The former Vice-President unveils as the "scientific" highlight of his presentation a graph offering a clear correlation between CO2 and temperature, as discovered in core samples of polar ice. He goes on to state that as levels of carbon dioxide rise, the Earth's temperature increases because the atmosphere traps more heat from the Sun.

Driving his point home, Gore extends the lines on the graph to terrifying, if distorted, levels (see box, opposite).

To show that these effects are already being felt, the film presents striking images of "global warming", from forlorn boats in dried-up seas to that haunting image at the end of the film of polar bears clinging desperately to a shrinking block of ice (see box, above).

The film - like the theory it is advancing - is not defensible in terms either of factual accuracy or of argumentative logic.

Fine, you may say, but even if the case for global warming really boils down to a few media tricks, how come everyone believes in it? Yet, as Solomon Asch, a social psychologist, discovered in the 1950s via a series of experiments, people are quite prepared to change their minds on even quite straightforward factual matters in order to "go along with the crowd".

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You can't blame folk for doing that. Especially when to do otherwise would mean taking a close look at the scientific issues in climate-change theory. Much of the argument for global warming is based on modelling. The mathematics is sophisticated and certainly intimidating to everyone but experts.

As some of the top climate-change modellers have remarked: "Modellers have an inbuilt bias towards forced climate change because the causes and effect are clear." (That comes from the paper "General circulation modelling of Holocene climate variability", by Gavin Schmidt, Drew Shindell, Ron Miller, Michael Mann and David Rind, published in Quaternary Science Review in 2004.)

And there is an impressive degree of consensus in their predictions. Take the modelling of one of the key components of "greenhouse theory", the degree to which warming of the oceans leads to more water vapour in the atmosphere "trapping" the Sun's heat. Advocates of the theory rely on this to show how a little bit of warming owing to CO2 can create very significant changes in the way the climate system operates.

A paper by Richard Lindzen and Yong-Sang Choi, called "On the determination of climate feedbacks from ERBE data", published in July 2009 in the journal Geophysical Research Letters, examined the modellers' case for CO2-induced global warming. It offered 12 graphs, 11 of them based on the most sophisticated climate models, all but one of which showed that as the temperature of the surface of the seas increases slightly, the amount of heat then trapped in the atmosphere by water vapour increases - a key element in accelerating the "greenhouse effect". We should be worried.

Yet there was that odd graph out, the 12th one. As Lubo? Motl, a sceptical physicist, joked, could it be that this was a tainted model - with its assumptions "tweaked" to fit prejudices by climate-change "deniers" funded by the oil industry? But no - the graph that contradicted all the others was the one based not on a model but on satellite measurements. It showed the Earth's oceans dampening the heating effect.

So what sort of factors mess up the models? Things like changes in ocean currents, changes in the amount of water vapour in the atmosphere, changes in cloud cover - just about everything that determines climate, really. Alas, there is as yet no way to calculate these. And so, the simple fact is, in our climate modellers' own words: "At present, no climate models have included the full range of effects."

Policymakers seem not to be aware of what the modellers know: that the results of their climate simulations are "likely to remain speculative for some time to come" and that people should be "extremely wary of extrapolating results to longer periods".

This demonstrates that the present climate-change models aren't just useless - by offering spurious precision, they are worse than useless.

How, then, does a theory that is incomplete and missing essential data become orthodoxy?

The reports of the United Nations Intergovernmental Panel on Climate Change (IPCC) - whose landmark 2001 report paved the way to ratification of the Kyoto Protocol - are not based on any new or original research, but merely reflect the efforts of participants, including government representatives, to overcome uncertainties in knowledge. Policymakers then use the IPCC reports, nuanced findings or not, to demand that the public change their ways. And most of the public are inclined to fall in line.

Social scientists call it "cascade theory": the idea is that information cascades down the side of an "informational pyramid", like a waterfall. It is easier for people, if they do not have either the ability or the interest to find out for themselves, to adopt the views of others. This is, without doubt, a useful social instinct. As it has been put, cascade theory reconciles "herd behaviour" with rational choice, because it is often rational for individuals to rely on information passed on to them by others.

Unfortunately, it is less rational to follow wrong information, and that is what can often happen. We find people cascading uselessly - like so many wildebeest fleeing a non-existent lion - in so many everyday ways. A lot of economic activity and business behaviour, including management fads, the adoption of new technologies and innovations, not to mention the vexed issues of health and safety regulation, reflect exactly this tendency of the herd to follow poor information.

Some people say that what is needed in response is to encourage a range of views to be heard, even when they are annoying to the "majority" - for instance, one should allow people to contest global warming. Or let teachers in schools and universities decide what they are going to teach. But more people say, on the contrary, that what is needed is stricter control of information to stop "wrong views" being spread. It is that view that is cascading down the pyramid now.

One of the best examples of cascade theory is that of the entirely false consensus that built up in the 1970s around the danger of "fatty foods". In fact, this consensus still exists, even though it has never had any scientific basis.

The theory can be traced back to a single researcher, Ancel Keys, who published a paper saying that Americans were suffering from "an epidemic" of heart disease because their diet was more fatty than their bodies were used to after thousands of years of evolution.

In 1953, Keys added additional evidence from a comparative study of the US, Japan and four other countries. Country by country, this showed that a high-fat diet coincided with high rates of heart disease.

Unfortunately for this theory, it turned out that prehistoric "traditional diets" were not especially low-fat after all - indeed, even the hunter-gatherers of yore, if they relied on eating their prey, would have had more fat in their diet than most people do today. As Science magazine pointed out, in the most relevant period of 100 years before the supposed "epidemic" of heart disease, Americans were actually consuming large amounts of fatty meat, so the epidemic followed a reduction in the amount of dietary fat Americans consumed - not an increase.

Keys' country-by-country comparison had also been skewed, with countries that did not fit the theory (such as France and Italy with their oily, fatty cuisines) being excluded. The American Heart Association (AHA), considered to be the voice of experts, issued a report in 1957 stating plainly that the fats-cause-heart-disease claims did not "stand up to critical examination". The case for there being any such epidemic was dubious, too - the obvious cause of higher rates of heart disease was that people were living longer, long enough to develop heart disease. But it was too late: the cascade had started.

Three years later, the AHA issued a new statement, reversing its view. It had no new evidence but it did have some new members writing the report, in the form of Keys himself and one of his friends. The new report made the cover of Time magazine and was picked up by non-specialists at the US Department of Agriculture, who then asked a supporter of the theory to draw up "health guidelines" for them. Soon, scarcely a doctor could be found prepared to speak out against such an overwhelming "consensus", even if a few specialised researchers still protested. And all this was good enough for the highest medical officer in the US, the Surgeon General, in 1988 to issue a doom-laden warning about fat in foods, and claiming that ice-cream was a health menace on a par with tobacco smoking.

It was a pretty silly theory, and certainly not one based on good evidence. In fact, in recent years, in large-scale studies in which comparable groups have been put on controlled diets (low fat and high fat) a correlation has at last been found. It turns out that the low-fat diet seems to be unhealthy. But no one is quite sure why.

The fact is, science has always been about PR, and as this example shows, it is easy for opinion leaders and experts to be misled. These days, it is not merely fellow researchers but professional marketeers vying to press their agenda and that of their clients (see box, page 34).

At the Kyoto summit in 1997, Fenton Communications, a New York PR firm, was working with "green NGOs and leaders", including Gore and the IPCC, to advise on how to "mainstream the climate threat" and to "harness the public 'tipping point'" on the issue and inspire action, as its website today boasts. And indeed, the public have been well and truly tipped.

The IPCC reports, which are dull but widely used by governments as the basis for their policy discussions, have become steadily more dramatic. (Not for nothing does the head of the IPCC, R.K. Pachauri, have his own dedicated marketing adviser.) Climate Change 2007: The Physical Science Basis says that "numerous long-term changes in climate have been observed (including) changes in Arctic temperatures and ice, widespread changes in precipitation amounts, ocean salinity, wind patterns and aspects of extreme weather including droughts, heavy precipitation, heat waves and the intensity of tropical cyclones".

Yet none of this is science. It certainly offends against the principle that Karl Popper calls "falsification" - in the case of climate change, there is no possibility of falsification. If you listen to proponents of climate-change theory, there is apparently nothing that counts as evidence against it. Increased rainfall in the northern hemisphere is evidence of climate change, but so is decreased rainfall in the southern hemisphere. Melting of ice in the Arctic is evidence of global warming, but cooling of the Antarctic is not evidence against, but attributed to "other effects".

The fact is, the IPCC report's statement quoted above is speculation and fear-mongering. So how did such language get in the report? Alas, it seems that the social and scientific reality is as Feyerabend describes, and that the language of fear has now "penetrated the most common idiom and infected all modes of thinking".

I have seen the effects of this up close, witnessing how truth can go out of the window in the rush to save the planet. I was co-ordinator of a small Yorkshire Friends of the Earth group, charged with protecting, among other things, the local river, the Wharfe, from a water company. In 1995, Yorkshire experienced just slightly less rain than normal, and the local water company found itself faced with the prospect of empty reservoirs. As standpipes went up in the cities of Leeds and Bradford, and trucks brought water in from afar, it desperately turned to the local rivers to try to make up the shortfall. The national press featured large photos of dried-up reservoir beds, waxed lyrical about how British society would soon break down in water wars, and urged its readers to sympathise with Yorkshire Water.

But our local group was not sympathetic because we felt that the company had failed to invest in its reservoirs and infrastructure. We proposed to put an advert in the Yorkshire Post highlighting this. And at this point, an official of Friends of the Earth formally instructed us that this independent line could not be permitted because it was national policy to attribute the shortages of water in the county that summer to runaway climate change. The "small is beautiful", "start locally" element of environmental tradition had disappeared. We were instructed that if we continued to argue that Yorkshire's water shortages were the result of anything other than global warming, we had to do so outside Friends of the Earth.

This highlighted the dangerous tendency of pressure groups to make specific statements for some supposed worthy campaign end. Ten years on, with Yorkshire racked by the usual floods, the director of Friends of the Earth, Tony Juniper, was pleased to warn against companies using climate change as an excuse. In The Observer on 3 July 2005, he said: "This situation emerges with depressing regularity, where you find insufficient capacity to meet people's needs because there's been a minor fluctuation in rain."

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But that wasn't the line in the summer of 1995. Fortunately, our local group ignored the order to abandon our river to a water company and voted to continue to highlight what we saw as the "real causes" of the water supply problems, as an independent group.

Alas, in the climate-change debate, there is a worrying amount of irrationality, incomplete science and skewed presentation. The scientists apparently "cherry-picking" and hiding their data revealed recently in the University of East Anglia email scandal are only following in a long tradition that includes even Galileo "cheating" by saying that the Earth must orbit round the Sun - in a perfect circle. Yet, surely most objectionable of all is the use of gullibility and fear as tactics in campaigns. And if fear requires a world of zero risk, that certainly won't include those mercury-filled light bulbs and nuclear power stations.

Today, global-warming "deniers" have all been told they must fall into line with "the science". But this is not science, this is propaganda. And we are not being asked to be more rational but to suspend our own judgment completely. That, not "runaway climate change", is the most dangerous threat to the world today.

MISLEADING PORTRAITS

There are many ways to fool people, and linking images with complex theories is a good one.

One of the most potent images used to show the impact of rising global temperatures was that of fishing boats stranded in a desert that was once the world's largest freshwater sea. It features in Al Gore's An Inconvenient Truth and in his 1992 book, Earth in the Balance: Ecology and the Human Spirit.

But the Aral Sea is actually not a sea, but rather a huge lake supplied by rivers, which have been gradually choked off since the end of the Second World War by Soviet-era irrigation schemes. Its plight has nothing to do with global warming.

Nor do polar bears.

When Gore used a picture of two polar bears supposedly stranded on a melting iceberg to support his claims about global warming, he chose a photo that had been taken by Amanda Byrd, a marine biology student, on a research cruise in August 2004, a time of year when the fringe of the Arctic ice cap normally melts. The image was later distributed by Environment Canada, a Canadian government department, to media agencies.

With that polar bear picture on the screen behind him, Gore says, "Their habitat is melting ... beautiful animals, literally being forced off the planet. They're in trouble, got nowhere else to go."

However, Byrd says that when she took the picture, the bears didn't appear to be in any danger. An Environment Canada spokesman, Denis Simard, told The National Post, a Canadian newspaper, that you "have to keep in mind that the bears aren't in danger at all. It was, if you will, their playground for 15 minutes ... they were not that far from the coast, and it was possible for them to swim."

The polar bear is still the symbol of the effects of global warming - but it is a cleverly designed marketing symbol, and not a rational, scientific marker.

GRAPHIC SLIPS

Although global warming theorists say they are concerned for the fate of the planet, it does not necessarily mean that their methods are beyond critique. Indeed, much of the "evidence" that makes up the "scientific case" is flawed.

In An Inconvenient Truth, Al Gore unveils as the "scientific" highlight a graph offering a clear correlation between temperature and CO2, as discovered in core samples of polar ice, with these words: "The relationship is actually very complicated, but there is one relationship that is far more powerful than all the others - and it is this. When there is more carbon dioxide, the temperature gets warmer, because it traps more heat from the Sun inside."

He then asks "Do they go together?" and extends the lines on the graph to terrifying levels.

Well, hang on a minute. First of all, historically, CO2 levels and temperatures have not marched in "lock step". Over geological time, the only thing the two variables share is a random walk. The Late Ordovician period saw CO2 concentrations nearly 12 times higher than those of today - and it was also an Ice Age. In fact, over the past 600 million years, only on two occasions have CO2 levels been as low as they are now, at below 400 parts per million.

Even restricting our survey to the past 100,000 years, the relationship is not as Gore and others think. Far from increases in CO2 leading to higher temperatures, the ice-core record shows rises in temperatures preceding (by between 200 and 1,000 years) increases in atmospheric carbon dioxide.

Which is what you would expect. Slightly higher temperatures mean more plant and animal life, and that means more CO2.

Nor do claims of record high temperatures have anything to do with the theory of anthropogenic global warming. The theory concerns the amount of heat radiated back into space at night, not how much heat "gets in" during the day. It is interested only in the supposed warming of the surface of the seas at night under a cocoon of atmospheric CO2 - the greenhouse. And is that happening? Er, no, as recent satellite surveys have found.

But let's return to that graph. It's not that complicated. We can start by looking at the recent history of the climate and the so-called hockey stick curve. The chart, which correlates temperatures and carbon dioxide levels, is called the hockey stick because it depicts the northern hemisphere's temperatures over the past 1,000 years as a fairly regular flat line until the late 20th century, when it curves sharply upwards as runaway warming appears.

The graph contradicted all the previous research into climate, which had indicated that there was a warm period about 1,000 years ago followed by the "Little Ice Age" in the 14th century.

By the time climatologists had pointed out that the graph had missed both the well-known warm periods and ice ages alike, and by the time mathematicians had had a chance to challenge the methodology and sample size, it was too late.

The exact relationship between CO2 and temperature remains elusive.

BIG GREEN MACHINE

Those making the case for global warming present the theory as the unvarnished work of hard-working and sober scientists who have been guided by evidence.

That view would be underlined by anyone reading the glowing endorsement of the "science" behind An Inconvenient Truth published by the website RealClimate.org, which is edited by Gavin Schmidt, a mathematician at Nasa's Goddard Institute for Space Studies.

There, Eric Steig of the University of Washington's Earth and Space Sciences Centre answers the question "How well does the (Gore) film handle the science?" by saying unambiguously: "Admirably, I thought. It is remarkably up to date, with reference to some of the very latest research."

RealClimate's site is hosted by Environmental Media Services (EMS), which sounds very suitable until one discovers that EMS was a creation of Fenton Communications, a New York public relations firm.

The scientists writing on the site are not being paid for their campaigning. But clearly the debate is attracting groups outside science and policy that have interests of their own.

Global warming's believers are quick to note the presence of the fossil-fuel industry behind a variety of lobby groups and reports.

But the green side has its allies, too. Take Fenton, which prefers to be known as "a public interest communications firm". It describes itself as using "sophisticated communications tools that Madison Avenue executives and corporations use and harness[ing] them for progressive change".

And so, of course, it was at the Kyoto summit in 1997 working with people and groups including Al Gore and the Intergovernmental Panel on Climate Change to "highlight" the climate threat.

It has a presence in Gore's circle through Amy "Kalee" Kreider, Gore's communications director and environmental adviser, who can be spotted standing alongside him whenever he testifies on environmental matters to Senate committees, and who was a staffer at Fenton when the firm was advising Gore. Before that, she was a campaigner for Ozone Action in charge of handing out "ozone-friendly" strawberries to the press.

Fenton may hold no sway over Gore, but its involvement shows how sophisticated the forces involved in the debate are.

Fenton's campaigns are never random issues. It specialises in scary stories of the potential human health dangers of many substances such as Alar (a chemical used to control the growth of apples and improve their appearance), plasticisers (chemicals, especially phthalates, used in plastics) and bovine growth hormones.

In 1989, the firm played a pivotal role in the anti-Alar campaign, which resulted in sales of apples and apple products plummeting in the US.

Years after what would come to be known as "the great Alar scare", Fenton's biggest project is climate change. Among its clients are all the most respected names in the environmental business, from Greenpeace USA and Friends of the Earth US to the National Geographic Society and the UN Environment Programme.

Climate change is a big business, and it's not surprising that they want to have the world's biggest "public interest" PR firm running the campaign.

WITH ALL THE GAS AROUND, THERE'S SOME BLOAT

"We are witnessing the birth of the greatest and most complex commodity market the world has seen," wrote The Times' environment editor, Jonathan Leake, in a November 2008 article.

Carbon-trading schemes originate from the Kyoto Protocol on climate change. Governments adhering to the protocol impose limits on the CO2 that companies can emit; the firms are then obliged to buy annual permits to exceed them. Permits are bought from governments or from carbon traders in the City who charge a commission. In terms of dollars, the World Bank has estimated that the size of the carbon market was $11 billion (?6.6 billion) in 2005, $30 billion in 2006 and $64 billion in 2007. The money collected by the UK Treasury, for example, came mainly from UK power companies, with the cost added directly to heating bills.

Meanwhile Russia - because when the Kremlin signed up to the Kyoto treaty it was given an annual emissions limit based on the dirty old Soviet industries - has accumulated emissions permits for about 4 billion tonnes of CO2. Call it a ?50 billion early Christmas present from Western consumers.

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Many companies, too, have found the initiative profitable. SRF Ltd, which produces refrigeration gases at a chemical plant in India, made ?300 million from selling certificates to transnationals including Shell and Barclays. It spent just ?1.4 million on equipment to reduce its emissions, and used the profit to expand production of another greenhouse gas 1,000-fold.