ost 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 scientist in the climate program at the Massachusetts Institute of Technology. She published a study last month in The Journal of Geophysical Research -- Atmospheres in which she and several colleagues concluded that future trends for ozone and methane would largely cancel each other out.
Generally, she said, models show that although ozone should increase in cities, on average around the globe levels should drop, and this will allow methane to persist longer in the air because there will be fewer hydroxyl radicals there to destroy it.
The effects of all of this on climate, she concluded in the paper, are likely to be a wash.
Dr. Ralph J. Cicerone, an atmospheric scientist and chancellor of the University of California at Irvine, said he agreed with Dr. Hansen that the complexity of chemistry in the atmosphere should not deter efforts to cut ozone, methane and other substances that contribute both to pollution and climate problems.
He pointed to recent work showing that although levels of ozone and nitrogen oxides, which help create ozone, have been dropping over the United States and western Europe, they are relentlessly rising elsewhere.
One study, Dr. Cicerone said, showed that nitrogen oxide concentrations in China, South Korea, Taiwan and Japan rose 60 percent from 1997 to 1998. There is plenty of concrete benefit to be gained, with little of the potential pain that might come from putting the same effort into reducing carbon dioxide emissions.
But other scientists fear that any shift away from reducing burning of coal and oil would only impede long-term progress toward limiting the human influence on climate.
''The most important thing is to stabilize all these emissions,'' said Dr. Thomas P. Ackerman, chief scientist in the Energy Department's atmospheric radiation measurement program in Richland, Wash. ''And there's no way you're going to do that without attacking the fossil-fuel problem. We're a society that likes to have simple solutions, but there are no easy answers here.''
For his part, Dr. Hansen seems