rals 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 climate experts and diplomats trying to stabilize climate. Its concentration in the atmosphere has risen by a third from levels seen before the Industrial Revolution; once added to the air, it takes many decades to disappear; and levels could double from pre-industrial amounts by the end of the new century, scientists project. Also important, it is relatively easy to measure the emissions and their sources and ''sinks,'' and processes that remove them from the air are fairly well understood.
But Dr. Hansen noted that carbon dioxide emissions are not continuing their relentless climb at anywhere near the pace set in the last century, as fuels are used more efficiently and dirty ones, like coal, are supplanted by cleaner ones, like natural gas.
Rather than seeking relatively costly additional cuts in carbon dioxide now, he said, the best return on an environmental investment in the short term would be to look at the other greenhouse culprits on his list. Then, later in the new century, when technologies have matured that offer sources of energy without new streams of carbon dioxide, the emphasis can shift back to that gas.
Here is his thinking on the best targets for action now.
First there is black carbon -- microscopic soot particles released from inefficient diesel engines, unfiltered power plants or burning forests. These tiny particles, besides harming lungs, absorb heat and also -- like other particles in the air -- form seeds for cloud droplets. They tend to make clouds darker, Dr. Hansen says, reducing clouds' tendency to reflect sunlight and make the earth's surface cooler.
He said that if continuing efforts to reduce conventional pollution from fossil-fuel burning spread from wealthy countries to developing countries, the amount of this heat-trapping substance in the air would quickly drop.
Methane is on most lists of priorities for reducing the greenhouse effect, and Dr. Hansen said it had, if anything, been underplayed as an important target. Methane comes both from natural sources like bogs and from a h