Scientists at US government agencies and the independent but federally funded National Centre for Atmospheric Research reported at least 435 instances of political interference in their work over the past five years, signaling a 'system-wide epidemic,' the group said.
US agencies restricted media contacts by climate scientists, who 'routinely encounter difficulty' in gaining approval for official press releases that highlight global warming, said the Government Accountability Project, a watchdog group that co-authored the report.
The evidence was presented at a congressional hearing in Washington, part of efforts by the new Democratic Party majority in both chambers to step up pressure on the Bush administration over its policies.
Henry Waxman, a Democrat who chairs the Oversight and Government Reform Committee in the lower House of Representatives, called the session to expose the administration's reluctance to acknowledge global warming.
'It appears there may have been an orchestrated effort to mislead the public about the threat of global climate change,' he told the panel.
The White House had no immediate response.
The hearing came as hundreds of climate experts met in Paris to complete a new UN report on global warming, the first in six years by the Intergovernmental Panel on Climate Change (IPCC).
The report, due to be presented on Friday, is expected to provide new details on the link between global warming and emissions from the burning of fossil fuels on Earth.
Witnesses called for the House hearing included Drew Shindell, a NASA climate expert who challenged the US space agency's public relations policy after officials softened his findings on Antarctic warming in 2004.
'While it was frustrating for me to see my work suppressed, even more importantly it is a disservice to the public to distort or suppress the information needed for decision-making,' Shindell said in prepared testimony.
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