A leading architect of the nation's science policy cautioned yesterday that the United States should continue to move slowly in placing new controls on the production of greenhouse gases thought to contribute to global warming.
"Our scientific understanding of climate change is far from certain," said D. Allan Bromley, the White House science adviser, speaking at the opening of "Chemrawn VII," a five-day meeting in Baltimore of atmospheric scientists from around the world.
"Fundamental questions of great importance remain unanswered," said the former Yale physicist, addressing several hundred scientists at the Stouffer Harborplace Hotel. Too little is known about the chemistry of the atmosphere, he added, to take "drastic" action.
But some scientists at the conference said Dr. Bromley's remarks, which included a reference to recent tree-ring data that supports the warming theory, sounded less skeptical than previous White House pronouncements.
And Michael Oppenheimer of the Environmental Defense Fund predicted that President Bush would change his "intransigent" policy over the next year under pressure from other industrial nations and the presidential elections.
Dr. Oppenheimer said yesterday's resignation of White House Chief of Staff John H. Sununu would also help change the president's mind. Mr. Sununu is a vocal greenhouse theory skeptic.
Unlike most other developed countries, the United States has not moved toward limiting emissions of carbon dioxide, the most widespread greenhouse gas, from autos and power plants.
Scientists generally agree that agribulture, transportation and industry over the past 150 years have dramatically increased the atmosphere's load of certain gases that tend to trap the sun's heat.
But there is a dispute over whether those gases have already led to warmer global temperatures. Scientists also disagree over how far and how fast temperatures are likely to rise in coming decades and what the local effect of higher temperatures would be.
Dr. Bromley cited recent evidence that ozone-destroying chlorofluorocarbons, once thought to be potent greenhouse gases, make no net contribution to warming.
"Preliminary" findings from research now under way, he said, suggest that another major greenhouse suspect, methane, may also have no net warming effect.
He stressed alternatives to emission controls on greenhouse gases, which he called the "lifeblood" of industry.
One was the construction of a new generation of safer and more reliable nuclear power plants. "The nuclear option in my opinion is the only technology available to us that can provide the large block of electricity that we require," he said.
Another was the creation of new "sinks" for greenhouse gases after they are released -- such as a White House program to plant 1 billion carbon dioxide-absorbing trees a year on 1.5 million acres of vacant land.
And he said the United States should encourage Third World countries to use more efficient coal-burning power plant technology rather than try to impose "premature and likely ineffective controls on fuel use."
Dr. Bromley conceded that recent studies suggesting that people could adapt to greenhouse gas-induced warming with relative ease were controversial but said, "they provide a welcome balance to 'the sky-is-falling' rhetoric all too common elsewhere."
Some scientists at Chemrawn -- an acronym for chemical research applied to world needs -- were disappointed by Dr. Bromley's remarks. Glen E. Gordon, an atmospheric chemist with the University of Maryland, said the White House should shift its support behind energy conservation.
"Planting a billion trees sounds impressive," he said. But in terms of the amount of carbon dioxide they absorb, he added, the effects are "not all that significant."
Dr. Oppenheimer of the Environmental Defense Fund said it would be "imprudent to let uncertainties provide a barrier to action."