The Bush administration's skepticism about the dangers posed by global warming collided yesterday with the views of the government's own scientists, who told a Senate hearing that -- if anything -- the problem may be worse than their limited computer programs can predict. Seven leading scientists -- including representatives from the National Oceanic and Atmospheric Administration, NASA and the U.S. Geological Survey -- told the panel recent evidence suggests that the so-called "greenhouse effect" is accelerating and may already be moving out of the reach of human intervention. "There is virtually no scientific controversy" about atmospheric changes that are gradually raising the Earth's temperature, said Stephen H. Schneider of the National Center for Atmospheric Research in Boulder, Colo. "That's not a speculative theory." In another development, Sir Crispin Tickell, Britain's ambassador to the United Nations, announced his nation's support for an international convention to address global warming, leaving the United States alone among major Western economic powers in opposition to such an initiative. Twenty-two nations, including five of the seven members of the Western economic summit, called for such a convention in March. Among those testifying yesterday was an atmospheric scientist who disavowed his own written testimony because it was altered by the Office of Management and Budget. The White House confirmed a report in the New York Times that an OMB official altered testimony by James E. Hansen, director of NASA's Goddard Institute for Space Studies, to avoid the impression "that there is unanimity within the government on this issue." Hansen, after giving the Senate Commerce subcommittee on science, technology and space a copy of his edited testimony, said the changes made it appear that he did not believe that global warming will lead to more frequent droughts. "I don't object to review of policy statements," Hansen testified. "My only objection is being forced to alter the science." Another witness, Jerry D. Mahlman of NOAA, testified that OMB officials also had attempted to alter testimony he delivered in February. Mahlman said he resisted the changes, which he found "objectionable as well as unscientific." White House press secretary Marlin Fitzwater said that a "fourth-level or fifth-level" staffer at OMB altered Hansen's testimony, but defended it as a routine effort to see that prepared testimony conforms to administration policy. "Our concern is that unless we base decisions on sound scientific data, we could end up being forced to agree with reductions in global warming gases that are neither realistic nor economically sound," he said. Global warming, caused by a buildup of carbon dioxide, methane and other gases, has emerged as a major environmental issue with worldwide implications. The gases are trapping more heat near the Earth, like a greenhouse, and posing the threat of climate change, intensified storms and the inundation of coastal areas as oceans rise. In the 1988 campaign, President Bush vowed that his administration would address the issue, saying he would "combat the greenhouse effect with the White House effect." In recent weeks, however, top budget and White House officials have advocated more research, arguing that too little is known about the economic impact of controlling greenhouse gases. Last week, White House Chief of Staff John Sununu rejected a proposal to put the United States in the lead in establishing an international convention on global warming. The convention had been supported by Secretary of State James A. Baker III and Environmental Protection Agency Administrator William K. Reilly, who were unable to persuade the rest of the administration to go along. U.S. delegates would have proposed the convention at a meeting in Geneva this week of the Intergovernmental Panel on Climate Change, a UN group chaired by the U.S. "We're sitting in the chair of this panel and we're trying to drive the train from the caboose," said Jessica Tuchman Matthews of the World Resources Institute, a Washington-based policy group. Fitzwater said there is disagreement between the EPA, the State Department and the White House on the state of science surrounding the greenhouse effect. But there was no sign of disagreement among the scientists at yesterday's hearing. Scientists have estimated that global mean temperatures could rise 4 to 9 degrees Fahrenheit by the middle of the next century if greenhouse gases continue to accumulate at current rates. By way of comparison, the Earth's mean temperature is about 9 degrees Fahrenheit warmer now than it was during the last ice age, when much of the United States was covered with mile-thick glaciers. "I have no hesitation in calling that catastrophic," Schneider said. Schneider, Hansen and other scientists said they are not certain how quickly warming will occur or how specific areas of the planet will be affected. Among the major uncertainties, they said, is how much excess heat oceans can absorb and whether greater evaporation will create more cloud cover, which can have a cooling effect. But current predictions about the extent of global warming may also understate the problem, they said, just as early models of stratospheric ozone depletion from chlorofluorocarbons understated the rate of loss. It was not until a large "hole" appeared in the ozone layer over Antarctica that scientists discovered that ice crystals were accelerating the chemical destruction of ozone. Biologist George M. Woodwell, director of the Marine Biology Laboratory at Woods Hole, Mass., testified that recent data show that the accumulation of carbon dioxide in the atmosphere is accelerating. A research station in Hawaii, which has documented a steady increase of about 1.5 parts per million of carbon dioxide each year since 1957, showed a "surge" in the last 18 to 24 months. The new rate of increase is about 2.5 parts per million a year, he said. "I'm suggesting that the warming of the Earth is increasing the decay of organic matter in the Earth," Woodwell said. "It may be mobilizing carbon from the Earth, and that has not been worked into the climatologists' models." Ralph Cicerone of the National Center for Atmospheric Research said a similar acceleration could result if temperatures become warm enough to speed decay in northern wetlands and tundra areas, which are rich in organic material that decays very slowly because of long cold seasons. "These feedback processes have yet to be studied adequately," Cicerone said. "They may be important, or they may not. But none is included in the climate models."