WASHINGTON -- Two private advocacy groups said Tuesday that government scientists have been pressured to downplay the threat of global warming.

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Members of the Union of Concerned Scientists and the Government Accountability Project, a legal-assistance group that represents whistle-blowers, told the House Oversight and Government Reform Committee about "new evidence of suppression and manipulation of climate science."

The groups said 40 percent of the 279 climate scientists who responded to a questionnaire complained that some of their scientific papers have been edited in a way that changed their meaning. And nearly half said in response to another question that at some point they had been told to delete a reference to "global warming" or climate change from a report.

The committee is looking into allegations of political interference as Congress steps up its examination of the Bush administration's climate policy.

The hearings coincide with the anticipated release of the first phase of the Intergovernmental Panel on Climate Change in Paris next week.

The first segment, written by more than 600 scientists and reviewed by another 600 experts and edited by bureaucrats from 154 countries, includes "a significantly expanded discussion of observation on the climate," said co-chair Susan Solomon, a senior scientist for the U.S. National Oceanic and Atmospheric Administration. She and other scientists held a telephone briefing on the report last week.

That report will feature an "explosion of new data" on observations of current global warming, Solomon said.

Solomon and others wouldn't go into specifics about what the report says. They said that the 12-page summary for policymakers will be edited in secret word-by-word by governments officials for several days next week and released to the public on Feb. 2. The rest of that first report from scientists will come out months later.

The full report will be issued in four phases over the year, as was the case with the last IPCC report, issued in 2001.

Global warming is "happening now, it's very obvious," said Mahlman, a former director of NOAA's Geophysical Fluid Dynamics Lab who lives in Boulder, Colo. "When you look at the temperature of the Earth, it's pretty much a no-brainer."