limate 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 host of human sources, including coal mines, leaking pipelines, landfills, belching cattle and rice paddies.
Dr. Hansen's study concluded that efforts to shift to rice-farming methods where fields are drained more frequently could cut releases. Also, his team pointed to Australian research showing that changing feeding patterns or mixing certain anti-gas additives in feed for tropical cattle cut by 40 percent their release of this gas without interfering with growth. With two billion cows, sheep, and goats belching away -- half of them in the tropics -- there is plenty of potential to cut into the greenhouse effect, he and supporters say.
Finally, the Goddard analysis emphasizes the importance of reducing ozone, an evanescent gas that is not on the list of greenhouse gases considered under the Kyoto Protocol because it is not directly produced by human activities.
Ozone, a poisonous molecule consisting of three oxygen atoms, is continually formed and destroyed when sunlight hits air, particularly when that air is laden with nitrogen oxides, hydrocarbons, or other emissions from burning fuels or forests. This is the lung-searing brew called smog.
Separately, ozone is formed high in the stratosphere, where it shields the planet from harmful radiation. Wherever it exists, it traps heat.
Dr. Hansen said that as with carbon soot, as China, Mexico and other industrializing countries start to enact pollution controls to cut smog, they will also indirectly be lessening their greenhouse impact.
But other scientists say the situation is not that clear-cut. One reason ozone has largely been left out of greenhouse equations so far is that its relationships with other greenhouse gases and other compounds in the air are almost impossibly complex.
For example, extremely reactive hydroxyl radicals, OH, which are created as ozone encounters other compounds, destroy methane molecules. In essence, one greenhouse gas eats the other.
''OH is the detergent of the atmosphere, scrubbing everything,'' said Dr. Monika Mayer, a research sci