On 31 March, the House of Commons science and technology committee, charged with investigating potential wrongdoing by Phil Jones and his colleagues at the University of East Anglia (UEA), concluded that there was "no case to answer" on the allegation of scientific dishonesty. An international panel commissioned by UEA in consultation with the Royal Society agreed. Yet, after a media frenzy back in December over what was inevitably called "Climategate", the vindication of the climate scientists has received very little coverage at all.

Vindication is not as sexy as accusation, and many people are still suspicious. After all, some of those emails, taken out of context, sounded damning. But what they show is that climate scientists are frustrated, because for two decades they have been under attack.

In the late 1970s, scientists first came to a consensus that global warming was likely to result from increasing greenhouse gases released by burning fossil fuels. This idea had been around since the turn of the century, but the development of computer models now made it possible to make quantitative predictions. Almost immediately, a small group of politically connected and conservative scientists began to question these conclusions. As the scientific evidence got stronger, their attacks became more unprincipled. They used data selectively and often misrepresented what was being published in the scientific literature.

In 1992, world leaders gathered in Rio de Janeiro to sign the United Nations Framework on Climate Change. President George W Bush promised to translate the written document into "concrete action". Three years later, the Intergovernmental Panel on Climate Change (IPCC) declared that the human impact on the earth's climate was no longer a prediction, but an observed fact. That's when things started to get ugly.

In the early 1990s, a group of sceptics claimed that Roger Revelle, one of the first climate scientists, had changed his mind about global warming and no longer believed it was a serious problem. The claim was repeated through several news outlets, including the Washington Post. When a graduate student named Justin Lancaster - who had worked closely with Revelle before his death in 1991 - tried to insist that Revelle had not changed his view, he was sued for libel. Lancaster was obliged to settle out of court. The charges were repeated again and again, and echo on the net today.

In 1996, when the IPCC released its second assessment report, stating that the human impact on climate was "discernible", a fossil-fuel-industry-funded group called the Global Climate Coalition accused the IPCC author Benjamin Santer of making unauthorised changes to make global warming appear more certain than it was. The following year, Frederick Seitz, chairman of the libertarian George C Marshall Institute, repeated the charges in the Wall Street Journal in an op-ed piece headlined "A major deception on global warming".

Letters were sent to US members of Congress demanding an investigation and then from Congress to the US energy department, demanding that it withdraw funding from the laboratory that employed Santer.

Massive attack

Had Santer made unauthorised changes to the IPCC report? No: his changes were made in response to peer review. He was doing what every scientist is expected to do - and what IPCC rules required him to do - accepting criticism and using it to make the science clearer. Frederick Seitz was a former president of the National Academy of Sciences, so it was not plausible that he did not know about the peer-review process.

In 2007, the claims were repeated in Unstoppable Global Warming: Every 1,500 Years, a book whose premise is that "human-emitted CO2 has played only a minor role" in contributing to global warming. The authors are Dennis Avery and Fred Singer, a physicist with a track record of challenging scientific evidence who had taken part in the previous attack on Santer.

Both the IPCC and Santer's co-authors took considerable pains to set the record straight, denying that Santer had done anything wrong. Yet, in their book, Avery and Singer reassert that "scientific reviewers discovered that major changes had been made 'in the back room' after they had signed off on the science chapter's contents" and that "Santer single-handedly reversed the 'climate science' of the whole IPCC report". The idea that any one individual could reverse the entire IPCC process is absurd, and yet, like the "Revelle changed his mind" claim, it remains on the internet today.

Climate scientists have been subjected to repeated attacks of this kind. In 2005, Congressman Joe Barton of Texas demanded that Professor Michael Mann, director of the Earth System Science Centre at Pennsylvania State University, produce a huge volume of paperwork relating to his research. In February, Senator James Inhofe of Oklahoma accused a dozen climate scientists of criminal violations of federal law, based on alleged evidence contained in the UEA emails. Recently, Virginia's attorney general, Ken Cuccinelli, went after Mann again, asking for the University of Virginia to produce thousands of pages of documents relating to Mann's work when he taught there.

We, too, have been objects of attack. When one of us (Naomi Oreskes) published a review in the journal Science of the book The Republican War on Science, in which we noted some connections not pursued in that book, Science was threatened with a lawsuit unless it published a rebuttal. (We supplied documents, Science held firm, and the threat went away.)

Blaming scientists for speaking truth to power is an old story. Scientists have long recognised that both the government and public can be
reluctant to accept scientific evidence leading to discomfiting conclusions. In 1949, when the USSR detonated its first atomic bomb, the US had to face the reality that it had lost its monopoly on nuclear weapons. Scientists had been warning of this since 1945, but the success of their predictions did not increase their standing. When they then said that any attempt to stay ahead of the Soviets by building the H-bomb would only speed up the arms race, they were accused of being disloyal. As Harold Urey, who won the Nobel Prize in Chemistry in 1934, wrote: "Because we [scientists] told disagreeable truths, we have even been accused of wishing to give up our progress because we are impractical dreamers or plain traitors."

What is most disagreeable to many "resistors" of global warming is the fear of climate change being used as a warrant for heavy-handed government intervention. There is a parallel with 1949: fear of the Soviet Union then was not fear of a potential invasion, but that the Soviets would export communism to Europe, from where it might spread to the US.

Today, US conservatives and right-wing com­mentators are red-baiting once again. In December, the columnist Charles Krauthammer alleged that "with socialism dead . . . the left was adrift until it struck upon a brilliant gambit: metamorphosis from red to green". Patrick J Michaels, some-time policy scholar at the libertarian Cato Institute, labelled plans for a cap-and-trade system to control greenhouse gases as "Obamunism". The irony is that in 1990, Bush installed a cap-and-trade regime to reduce acid rain because it was an acceptably market-based mechanism. Yet now, when Congress is finally taking the model seriously, conservatives call it communism by other means.

Market failure

“Climategate", and the wider attacks on climate science, had nothing to do with the science itself, and neither did the entire earlier history of global-warming denial we have studied. Scientists have just been an easy target. The real issue is the politics of defending the free market.

Since the mid-1990s, the fossil-fuel industry has made common cause with old cold warriors, maverick scientists and conservative and libertarian think tanks to undermine climate science. The obvious reason is that climate change is what Nicholas Stern calls "the greatest and widest-ranging market failure ever seen". If the free market has failed, then governments will need to act. And that is precisely what various constituencies, from Inhofe to Cuccinelli and a host of think tanks, do not want. It was also what Seitz and his colleagues didn't want. These scientists were passionately anti-communist, and viewed any form of government regulation as a step towards socialism.

No wonder we see the rise of McCarthyite tactics today: the stakes, at least in some people's eyes, are the same. But what these people seem to have forgotten from the 1950s is that McCarthyism didn't just destroy the careers of many innocent people: in the end, it destroyed Joe McCarthy, too.

Naomi Oreskes and Erik Conway are the authors of "Merchants of Doubt: How a Handful of Scientists Obscured the Truth on Issues from Tobacco Smoke to Global Warming", published by Bloomsbury (£20)