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James Hansen, the Nasa scientist who first warned the US Government about global warming, yesterday delivered a withering critique of the way the White House has interfered with climate scientists working for the space agency.

Dr Hansen, the director of Nasa's Goddard Institute of Space Studies in New York, said that the space agency's budget for studying the Earth's climate has been slashed and that its scientists have been systematically gagged about speaking of their concerns.

In detailed written testimony delivered yesterday to the US House of Representatives, Dr Hansen said that there has been creeping politicisation of climate change with the effect that the American public has been left confused about the science of global warming.

"During my career I have noticed an increasing politicisation of public affairs at headquarters level, with a notable effect on communication from scientists to the public," Dr Hansen writes in his testimony.

"Interference with communication of science to the public has been greater during the current Administration than at any time in my career," he says.

"In my more than three decades in government, I have never seen anything approaching the degree to which information flow from scientists to the public has been screened and controlled as it has now," he says.

Political appointees within the public affairs office at Nasa headquarters were accused by Dr Hansen of interfering in scientific statements and of blocking reports that link rising temperatures or melting sea ice with global warming.

He says that instructions and reprimands were often made orally so that there was no paper or electronic record of the interference, which allowed press relations personnel to dismiss gagging allegations as hearsay.

"My suggestion for getting at the truth is to question the relevant participants under oath, including the then Nasa associate administrator for earth sciences who surely is aware of who in the White House was receiving and reviewing press releases that related to climate change," Dr Hansen says.

On one occasion when Dr Hansen gave a lecture to the American Geophysical Union about the record global temperatures in 1995, the White House called Nasa headquarters to complain of the resulting media attention.

Dr Hansen said that one participant in a three-way telephone conference between the White House, Nasa headquarters and Nasa Goddard described the heated conversation as a "shit storm".

"The upshot was a new explicit set of constraints on me, including the requirement that any media interviews be approved beforehand and that headquarters have the right of first refusal on all interviews," he says.

"It became clear that the new constraints on my communications were going to be a real impediment when I was forced to take down from our website our routine posting of updated global temperature analysis."

Since then, Nasa has slashed its budget for the study of Earth sciences, which has been cut by 20 per cent compared to an increase of between 1 and 3 per cent in other areas of Nasa's science spending.

The agency's "mission statement" has also been changed so that there is no longer a reference to saving our home planet, Dr Hansen says.

"That part has been deleted, a shocking loss to me, as I had been using that phrase to justify speaking out about the dangers of global warming," Dr Hansen says.

"There is little doubt that the Administration's downplaying of evidence about global warming has had some effect on public perception of the climate change issue.

"The impact is to confuse the public about the reality of global warming, and about whether that warming can be reliably attributed to human-made greenhouse gases," he says.