WASHINGTON - Federal scientists have been pressured to play down global warming, advocacy groups testified yesterday at the Democrats' first investigative hearing since taking control of Congress.

The hearing focused on allegations that the White House for years has micromanaged the government's climate programs and has closely controlled what scientists have been allowed to tell the public.

"It appears there may have been an orchestrated campaign to mislead the public about climate change," said Rep. Henry A. Waxman (D., Calif.). Waxman is chairman of the Oversight and Government Reform Committee and a critic of the Bush administration's environmental policies, including its views on climate.

Climate change also was a leading topic yesterday in the Senate, where presidential contenders for 2008 lined up at a hearing called by Sen. Barbara Boxer. They expounded - and at times tried to outdo one another - on why they believed Congress must act to reduce heat-trapping "greenhouse" gases.

"This is a problem whose time has come," Sen. Hillary Rodham Clinton (D., N.Y.) proclaimed.

"This is an issue over the years whose time has come," echoed Sen. John McCain (R., Ariz.).

Sen. Barack Obama (D., Ill.) said, "For decades far too many have ignored the warning" about climate change. "Will we look back at today and say this was the moment we took a stand?"

At the House hearing, two private advocacy groups produced a survey of 279 government climate scientists showing that many of them say they have been subjected to political pressure aimed at downplaying the climate threat. Their complaints ranged from a challenge to using the phrase global warming to raising uncertainty on issues on which most scientists basically agree, to keeping scientists from talking to the media.

The survey and separate interviews with scientists have "brought to light numerous ways in which U.S. federal climate science has been filtered, suppressed and manipulated in the last five years," Francesca Grifo, a senior scientist at the Union of Concerned Scientists, told the committee.

Grifo's group, along with the Government Accountability Project, which helps whistle-blowers, produced the report.

Drew Shindell, a climate scientist with NASA's Goddard Institute for Space Studies, said that climate scientists frequently had been dissuaded from talking to the media about their research, though NASA's restrictions have been eased, he said.

Prior to the change, interview requests of climate scientists frequently were "routed through the White House" and then turned away or delayed, Shindell said. He described how a news release on his study forecasting a significant warming in Antarctica was "repeatedly delayed, altered and watered down" at the insistence of the White House.

Some Republican members of the committee questioned whether science and politics ever can be kept separate.

"I am no climate-change denier," said Rep. Thomas M. Davis 3d of Virginia, the top Republican on the committee, but he questioned whether "the issue of politicizing science has itself become politicized."

"The mere convergence of politics and science does not itself denote interference," Davis said.

Administration officials were not called to testify. In the past the White House has said it had only sought to inject balance into reports on climate change. President Bush has acknowledged concerns about global warming, but he strongly opposes mandatory caps of greenhouse-gas emissions, arguing that approach would be too costly.

U.N. Urged to Hold A Climate Summit

The top U.N. official for

the environment asked Secretary-General Ban Ki-moon yesterday to convene an international summit to combat climate change, an official said.

Ban met with the executive director of the U.N. Environment Program, Achim Steiner, who recommended the summit take place later this year, an official close to the talks said. Ban was sympathetic to the idea, said the official on condition of anonymity because the talks were not public.

Discussions about the summit come as an authoritative report, compiled by 2,000 climate experts and other scientists, is expected to be released Friday in Paris, warning that human-caused global warming is destined to get much worse. The report is by the Intergovernmental Panel on Climate Change, which was created by the United Nations in 1988 and releases assessments every five to six years.

Kenya has agreed to serve as host for a possible summit, Ban said. He is

to meet Kenyan President Mwai Kibaki today to discuss such a meeting.

- Associated Press