The trap was sprung in February 2006. The White House ordered that Dr Jim Hansen was to be denied the oxygen of publicity forthwith. He was to be banned from appearing in newspapers and on TV and radio. He was effectively to disappear.

It was the kind of treatment that might be reserved for terrorists, criminals or, in a totalitarian regime, for political dissidents.

Hansen, however, was none of these things. The director of Nasa’s renowned Goddard space science laboratories was a dry, rather self-effacing climate change scientist with a worldwide reputation for accurate and high-quality research. What had happened?

“All I had done was to give a talk to the American Geophysical Union, setting out how 2005 had been the warmest year on record,” recalled Hansen, in a visit to London last week.

“But someone at Nasa got a call right from the top, from the White House. They were very annoyed.”

It was not quite all he had done. Hansen had also e-mailed a transcript of the talk to a raft of reporters before he spoke. “I did make sure it hit the headlines,” he recalls modestly. In his talk he declared that humanity, especially Americans and Europeans, were burning fossil fuels so fast that they risked transforming Earth into “a different planet”.

Government scientists were not supposed to say things like that. Shortly afterwards the head of Nasa’s public affairs office, one of George Bush’s political appointees, banned Hansen from speaking to the media.

“Then they also forced us to remove all our data about the latest temperature rises from the website,” says Hansen. “I realised they really were going to stop me communicating.”

It looked like a classic case of a naive scientist being ruthlessly crushed by a government machine.

In reality, however, it was Hansen who laid the trap – and the Bush administration that got caught. A few more calls to the media and soon the story of the lone scientist gagged by the mighty Bush administration hit the front pages all over the world, carrying Hansen’s warning about climate change with it once again.

It is a warning that Gordon Brown appears not to be heeding. Hansen’s visit to London last week was partly inspired by the decision to approve construction of a new coal-fired power station at Kingsnorth in Kent.

This, Hansen wants to warn us, is a recipe for global warming disaster. The recent warm winters that Britain has experienced are a clear sign that the climate is changing, he says.

“We are fast approaching a series of tipping points. Changes such as the melting of the Arctic ice cap, the acidification of the oceans and the global rises in temperature could be approaching the point of becoming irreversible.

“In the face of such threats it is madness to propose a new generation of power plants based on burning coal, which is the dirtiest and most polluting of all the fossil fuels. We need a moratorium on the construction of coal-fired power plants and we must phase out the existing ones within two decades.”

Such warnings will not be popular. Coal provides 25% of global primary energy needs and generates 40% of the world’s electricity. In 2006 about 5.4 billion tons were burnt – a 92% increase over the past 25 years. China alone is building two new coal-fired power stations a year with CO2 emissions rising by about 10% annually.

Coal also offers energy security. Some 70 countries can mine their own coal and there are enough reserves to last at least 150 years.

Hansen, however, has come to the conclusion that coal will destroy the planet. “If we release all that carbon into the air it will be catastrophic,” he says.

At the heart of what Hansen is saying lies a welter of new research into what kind of increase in CO2 can be borne by the Earth’s atmosphere.

In the preindustrial 18th century there were about 280 parts per million (ppm) of CO2 in the air. Since then the 1,000 billion tons of CO2 released by humanity has raised that to 385ppm – with another 49 billion tons being added to that each year.

The global scientific consensus is that humanity can just about afford to let CO2 levels creep up so long as they level off at around 450ppm. This would mean accepting rises in global temperature averaging 2-3C.

Hansen used to accept such ideas – but he is now preparing a new research paper showing that even this limit is far too high.

“If humanity wants to preserve a climate resembling that in which civilisation developed, then the palaeoclimate evidence and ongoing climate change suggest CO2 must be reduced from its current level to between 300-350ppm. A 350ppm target is only achievable by phasing out coal use,” he says.

This new agenda for tackling carbon emissions sounds radical but is rapidly gaining ground among Britain’s own climate change researchers and parliamentarians. What marks Hansen out is his success in getting such ideas heard.

“My original media sin goes back to 1981,” says Hansen.

“I had written a paper for Science [the renowned academic journal] making predictions about climate change, but I thought it might get ignored. So I sent it to a reporter at The New York Times – and he put it on the front page.”

The resulting row saw the Department for Energy, which oversaw research into CO2 emissions, slashing his funding while its head of science launched a furious attack on his work at a scientific conference.

Bloodied but not bowed, Hansen went back to his laboratory, spending the next few years refining his global climate model, a computer simulation of the planet’s climate that allows him, for example, to add extra CO2 to the atmosphere and see what happens.

He also refined his tactics. The next time he wanted to speak out he did not choose a scientific journal. It was June 1988 and America was hit by a roasting summer and droughts.

When a congressional committee asked him to testify on climate change, he told them that 1988 would set a new global temperature record, adding that he was “99% confident” that it was due to the greenhouse effect.

He used similar cunning the following year. Called before a congressional committee hearing looking at climate change, he sent an advance fax to Al Gore, its chairman, suggesting some of the questions that he would like to answer.

“What I told them was that the written evidence submitted in my name did not contain my words. It had been rewritten by the president’s own budget office to support their own agenda.”

This time the resulting storm was so great it saw climate change catapulted into the political arena as never before. George Bush Sr (the father of George W Bush), who was then running for president, promised to “fight the greenhouse effect with the White House effect”.

Billions of pounds were allocated for research worldwide and by 1995 the intergovern-mental panel on climate change had concluded that Hansen was correct – humanity was indeed heating up the planet.

When Bill Clinton and Gore arrived in office, Gore had shown such interest in climate change that Hansen had high hopes – but was again disappointed. “America under Gore and Clinton let down the rest of the world once again. They gave in to the special interests,” says Hansen. “That is the same process we are seeing in the White House now.”

Hansen believes that the governments of Britain and Germany are also proving vulnerable to such lobbying – and that the decision over Kingsnorth is a direct consequence.

“I used to think that the politicians over here had simply not understood how serious climate change really is,” he says.

“Nowadays, however, it is clear they do – but they have just given in.”

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