c chemist who is chief scientist of Environmental Defense, a private conservation group.
The debate was spurred last August, when Dr. Hansen and a team of his colleagues at NASA's Goddard Institute for Space Studies, which is based in Manhattan, published ''Global Warming in the 21st Century: An Alternative Scenario,'' in The Proceedings of the National Academy of Sciences.
In the paper, they said that deep cuts in carbon dioxide emissions were probably impossible in the short run, while the other gases and soot were already targets because of their contribution to old-fashioned pollution. In its conclusion, the team used a word rarely found in studies of global warming: ''optimistic.''
The analysis has since been criticized by some longtime colleagues of Dr. Hansen and embraced by industry groups and elected officials who have long voiced doubt about the science behind climate change and the need for big cuts in fossil fuel use.
Just last Thursday, for example, Dr. Hansen's findings were repeatedly cited by Republican senators holding a hearing questioning the need for the Kyoto Protocol, which has been signed by more than 100 countries but has yet to be ratified by any industrial powers. Many of the treaty's provisions focus on cutting carbon dioxide.
Dr. Hansen's critics, including many influential experts on the atmosphere and climate, say that emissions of all of the gases should be curtailed wherever this is possible and not too expensive but that cutting carbon dioxide must still be a top priority.
For his part, Dr. Hansen said that both sides in the debate had largely either misrepresented or misinterpreted his study. He said the fundamental goal was simply to find the most efficient -- and politically and economically palatable -- way to cleanse the atmosphere of human effluents that have the biggest impact on climate.
He said he still hoped that once the dust settled, policy makers from all parts of the spectrum might find merit in the proposal.
''I think it may be possible to get conservatives and liberals to agree on common-sense steps that slow down the climate experiment,'' Dr. Hansen said.
The ''climate experiment'' to which he referred is a substantial rise in the amount of greenhouse gases released into the air by human activities since the start of the Industrial Revolution. So far, the insulating effect of these gases is the equivalent of turning on two one-watt Christmas tree light bulbs over each square yard of the earth's surface.
That does not seem like much additional energy, Dr. Hansen said, but as a comparison he and others have calculated that the extra wattage accounting for the difference between the last ice age and current equable conditions is 6 to 9 watts per square yard.
If greenhouse gas concentrations continue to rise, the human-caused increase in energy bathing the surface could exceed four watts per square yard, a rise that begins to approach the change from the last ice age until now. That is bound to jog climate, Dr. Hansen said.
Dr. Hansen, besides being a respected climate scientist, is an inveterate pot stirrer who in the sweltering summer of 1988 told a Senate committee ''it is time to stop waffling'' about climate change, adding that signs had emerged that humans were causing some of the global warming measured in the 20th century.
His new analysis is clearly intended to provoke his peers to re-examine assumptions made over the last 10 years, particularly by the Intergovernmental Panel on Climate Change, a body of hundreds of scientists working together under the auspices of the United Nations to assess global warming and possible responses. Their first update in five years is due in January.
Most atmospheric scientists have long recognized that more than a dozen greenhouse gases and various kinds of particles emitted by combustion and industry affect the flow of energy to and from the earth. (Most particles reflect sunlight like a parasol, cooling the planet; carbon soot tends to absorb energy and heat up.)
But for the last decade, carbon dioxide has been the main target of c