The White House letter rejected media reports following the recent release of a major UN report on climate change that it said perpetuate the claim that Bush's interest in the issue is new.

"In fact, climate change has been a top priority since the president's first year in office," the letter said.

"Beginning in June 2001, President Bush has consistently acknowledged climate change is occurring and humans are contributing to the problem," it said, adding excerpts from speeches in 2001 and 2002 as evidence.

"President Bush committed the United States to continued leadership on the issue and since 2001 has dedicated nearly 29 billion dollars to advance climate-related science, technology, international assistance, and incentive programs. This is far more than any other nation."

But the letter coincided with hearings in Congress Wednesday in which experts and politicians accused the Bush administration of repressing public debate over global warming in what one witness branded "a conspiracy of silence."

The hearings were called in the wake of a report last week by the Union of Concerned Scientists and the Government Accountability Project (GAP) which offered evidence of "widespread political interference in federal climate science."

"A few examples of relatively heavy-handed interventions sufficed to send a message ... about White House political sensitivities" to officials producing reports on climate change for the White House-directed Council on Environmental Quality, said Rick Piltz, GAP director of climate science.

Piltz assailed the Bush administration for a "conspiracy of silence" on global warming.

Environmentalists have alleged since Bush came to office in 2001 that his administration has ignored and tried to hide looming evidence of global warming and the key role of human activity in climate change.

They are also strongly critical of Bush's refusal to ratify the Kyoto protocol on global warming, which Bush has criticized as excessively flawed.

While climate change has rarely been mentioned in Bush's major policy speeches during his six years in office, he noted "the serious challenge of global climate change" in his annual State of the Union speech on January 23, as he introduced a program to advance energy conservation and diversification.

The White House letter argued that the United States' performance on cutting emissions of greenhouse gases which influence global warming since 2000 "is among the best in the world."

"According to the International Energy Agency, from 2000-2004, as our population increased and our economy grew by nearly ten percent, US carbon dioxide emissions increased by only 1.7 percent.

"During the same period, European Union carbon dioxide emissions grew by five percent, with lower economic growth."

In Wednesday's Senate hearing, one consultant noted that in June 2006 Bush claimed that there was a debate over whether global warming was manmade or naturally caused, when the large majority of scientists cite human causes.

The UN climate change report last week concluded that human activity was principally responsible for climate change.

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