Some of America's leading scientists have accused Republican politicians of intimidating climate-change experts by placing them under unprecedented scrutiny.

A far-reaching inquiry into the careers of three of the US's most senior climate specialists has been launched by Joe Barton, the chairman of the House of Representatives committee on energy and commerce. He has demanded details of all their sources of funding, methods and everything they have ever published.

Mr Barton, a Texan closely associated with the fossil-fuel lobby, has spent his 11 years as chairman opposing every piece of legislation designed to combat climate change.

He is using the wide powers of his committee to force the scientists to produce great quantities of material after alleging flaws and lack of transparency in their research. He is working with Ed Whitfield, the chairman of the sub-committee on oversight and investigations.

The scientific work they are investigating was important in establishing that man-made carbon emissions were at least partly responsible for global warming, and formed part of the 2001 report of the Intergovernmental Panel on Climate Change, which convinced most world leaders - George Bush was a notable exception - that urgent action was needed to curb greenhouse gases.

The demands in letters sent to the scientists have been compared by some US media commentators to the anti-communist "witch-hunts" pursued by Joe McCarthy in the 1950s.

The three US climate scientists - Michael Mann, the director of the Earth System Science Centre at Pennsylvania State University; Raymond Bradley, the director of the Climate System Research Centre at the University of Massachusetts; and Malcolm Hughes, the former director of the Laboratory of Tree-Ring Research at the University of Arizona - have been told to send large volumes of material.

A letter demanding information on the three and their work has also gone to Arden Bement, the director of the US National Science Foundation.

Mr Barton's inquiry was launched after an article in the Wall Street Journal quoted an economist and a statistician, neither of them from a climate science background, saying there were methodological flaws and data errors in the three scientists' calculations. It accused the trio of refusing to make their original material available to be cross-checked.

Mr Barton then asked for everything the scientists had ever published and all baseline data. He said the information was necessary because Congress was going to make policy decisions drawing on their work, and his committee needed to check its validity.

There followed a demand for details of everything they had done since their careers began, funding received and procedures for data disclosure.

The inquiry has sent shockwaves through the US scientific establishment, already under pressure from the Bush administration, which links funding to policy objectives.

Eighteen of the country's most influential scientists from Princeton and Harvard have written to Mr Barton and Mr Whitfield expressing "deep concern". Their letter says much of the information requested is unrelated to climate science.

It says: "Requests to provide all working materials related to hundreds of publications stretching back decades can be seen as intimidation - intentional or not - and thereby risks compromising the independence of scientific opinion that is vital to the pre-eminence of American science as well as to the flow of objective science to the government."

Alan Leshner protested on behalf of the American Association for the Advancement of Science, expressing "deep concern" about the inquiry, which appeared to be "a search for a basis to discredit the particular scientists rather than a search for understanding".

Political reaction has been stronger. Henry Waxman, a senior Californian Democrat, wrote complaining that this was a "dubious" inquiry which many viewed as a "transparent effort to bully and harass climate-change experts who have reached conclusions with which you disagree".

But the strongest language came from another Republican, Sherwood Boehlert, the chairman of the house science committee. He wrote to "express my strenuous objections to what I see as the misguided and illegitimate investigation".

He said it was pernicious to substitute political review for scientific peer review and the precedent was "truly chilling". He said the inquiry "seeks to erase the line between science and politics" and should be reconsidered.

A spokeswoman for Mr Barton said yesterday that all the required written evidence had been collected.

"The committee will review everything we have and decided how best to proceed. No decision has yet been made whether to have public hearings to investigate the validity of the scientists' findings, but that could be the next step for this autumn," she said.

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