Bush tried to gag environment expert

One of Nasa's leading climate experts has accused the Bush administration of trying to censor him on global warming and the need for immediate cuts in greenhouse gas emissions.

James Hansen, the director of Nasa's Goddard Institute for Space Studies, said agency officials had ordered that public affairs officers review his lectures, papers and postings on the institute's website as well as vetting requests for interviews from reporters.

The clamp-down followed a lecture he gave last month calling for emission reductions, a move the White House refuses to support. "They feel their job is to be this censor of information going out to the public," said Mr Hansen, who is ignoring the restrictions placed on him.

Nasa's mission statement included the phrase "to understand and protect our home planet," he pointed out, adding: "Communicating with the public seems to be essential because public concern is probably the only thing capable of overcoming the special interests that have obfuscated the topic."

Mr Hansen has been issuing warnings about climate change since 1988 and has had confrontations with politicians from both Republican and Democratic administrations.

He has twice briefed the Vice-President, Dick Cheney, and other senior officials of this administration. But he provoked their ire shortly before the 2004 presidential election when he complained that government scientists were being silenced and said he would be voting for the Democratic candidate, Senator John Kerry.

In an interview with the The New York Times, Mr Hansen said the pressure had been stepped up considerably since his speech last month in San Francisco when he said 2005 was probably the hottest year on record.

Many of the directives to stop talking were made through phone calls so that their was no paper trail, he said. However, other officials at the New York-based institute where Mr Hansen works have supported his claims that there were efforts to restrict what he was saying.

Nasa has denied that it is trying to silence Mr Hansen and said that all its scientists are free to discuss their findings but that matters of policy should be left to policy-makers. It said officials were required to oversee interview requests to ensure an orderly flow of information.