Scientists have gathered preliminary evidence that sulfur dioxide, an air pollutant produced by burning some fossil fuels and a major contributor to acid rain, also may be cooling the planet and counteracting global warming.

The perverse possibility that one pollutant may tend to cool the Earth while others tend to warm the planet is likely to present policy-makers with a profound dilemma over what to do about the apparently linked problems of global warming and acid rain.

Among researchers, there is growing awareness that man-made aerosols such as sulfur dioxide may play a significant and perhaps crucial role in climatic change.

The findings have emerged in a spate of recent papers and technical presentations. Scientists who study climatic change have begun to suspect that while "greenhouse" gases trap heat and warm the planet, other pollutants may be counterbalancing the greenhouse effect.

Sulfur dioxide appears to be having this effect by modifying clouds so they become more effective in shading the planet from the sun's rays and bouncing sunlight back into space.

Because of this effect, scientists fear that reducing acid rain may accelerate potential global warming. But failing to control acid rain will cause further harm to lakes, streams and forests and will perhaps only delay an inevitable worldwide warming that, instead of coming slowly over the next century, could arrive quickly in what one scientist called "a tremendous heat pulse."

The magnitude of the "sulfur effect" remains unknown, but some climate experts believe the cooling produced by sulfur pollution may offset half the warming that most scientists predict will be caused by all greenhouse gases or that caused by carbon dioxide alone.

"It's the wild card in climate change," said Robert Cess of the State University of New York in Stony Brook.

"It's the billion-dollar question," said George Kukla of Columbia University's Lamont Doherty Laboratory.

Until recently, researchers focused on the potential warming effects of carbon dioxide, whose concentration in the atmosphere is steadily increasing because of the burning of fossil fuels and destruction of forests. Carbon dioxide and certain other gases act like the glass in a greenhouse, letting sunlight reach Earth's surface but then holding in the heat that results. Experts fear that the accumulation of such greenhouse gases, which is expected to double in the next century, will trap more and more heat and raise temperatures several degrees, setting off a chain of events that could include rising sea levels and mass extinction of plants and animals.

But now many scientists are beginning to turn their attention to a group of man-made aerosols, microscopic particles that drift in the air -- especially sulfur dioxide, which is produced by the burning of fossil fuels, in particular coal.

When sulfur dioxide enters the atmosphere, the particles act as reflectors that bounce some of the sun's rays back into space. Robert Charlson of the University of Washington and colleagues have suggested that the cooling effect of the particles alone might mask half the warming caused by greenhouse gases.

However, sulfur dioxide particles also serve as the nuclei on which water vapor condenses to form tiny droplets. The result is a droplet of dilute sulfurous acid. Because the droplets are smaller than normal, the clouds look brighter. The brighter the clouds, the more they reflect the sun's energy back into space.

Cess of the State University of New York compares the effect to sugar. A bowl of powered sugar, with its tiny particles, is a brighter white than a bowl of ordinary granular sugar. "It is the same with clouds," Cess said.

There is also evidence of increased cloud cover. James Angell of the National Oceanic and Atmospheric Administration, for example, reported recently that cloud cover over the United States appears to have increased between 2 and 3 percent since 1950.

"There's some pretty convincing observations from satellites that the aerosols are producing thicker clouds and they are reflecting more solar radiation back into space," said Kukla of Columbia. "We are finding that the cooling impact of man-made aerosols could be quite large."

Scientists have long suspected that sulfur affects climate. Benjamin Franklin, for instance, noted that sulfur aerosols produced by volcanoes reflect sunlight back to space. Franklin went on to suggest that the eruption of Laki Volcano in Iceland produced unusually cold weather in 1783-84. The much larger Tambora eruption in Indonesia in 1815 resulted in the "year without a summer," although much of that was due to the clouds of pulverized rock and other material ejected into the atmosphere.

Paul Mayewski of the University of New Hampshire and colleagues have examined ancient ice from Greenland and reported last month that the quantity ofman-made sulfur in the atmosphere now rivals the amount of sulfates produced by the large eruptions of Laki and Tambora and trapped in the ice.

"One way to look at it is that acid rain is good. But I'd never want to say that," Mayewski said. "But I would say that we're underestimating the effect of warming due to carbon dioxide because we're dumping so much sulfur dioxide into the system."

Back in the 1970s, researchers referred to the sulfur loading in the atmosphere as a "human volcano." In recent years climate experts thought the carbon dioxide would overwhelm the sulfur pollution. Now researchers are not so sure.

Based on observed sulfur loading in the atmosphere and computer climate simulations, Tom Wigley of the University of East Anglia in England has reported that sulfur clouds already may have "significantly offset the temperature changes that have resulted from the greenhouse effect."

If the sulfur effect is real, it would explain a central enigma in the climate record. Despite the fact that the world on average appears to have warmed about 1 degree Fahrenheit in the last century, the warming has not been uniform all over the world. From 1940 through the 1980s there has been little or no net warming in the Northern Hemisphere, where most of the Earth's people, industry and sulfur pollution are. There has, however, been warming in the Southern Hemisphere.

"Some of us have been wondering where all the warming was," said Patrick Michaels of the University of Virginia. "The sulfur explanation ties up a lot of disparate facts. It shows why the Northern Hemisphere hasn't warmed and why the Southern Hemisphere has shown more of a magnified greenhouse effect."

Unlike the buildup of greenhouse gases such as carbon dioxide, which stays in the atmosphere for tens of years and spreads evenly over the globe, the effects of sulfur are short-lived and regional. The sulfur particles stay aloft for only days or weeks before they fall to the ground in rain or snow.

If the role of sulfur cooling proves to be large, and this is still far from certain, some researchers say it could be necessary to continue burning fossil fuels in order to produce sulfur dioxide to fight the carbon dioxide-driven warming.

"I would not be surprised if somebody suggested concentrating fossil fuel power plants on the eastern margins of continents, which would put a lot of sulfates into the atmosphere, which would rain out over the oceans, which have a tremendous capacity to absorb acidity," Michaels said. This plan would make sense, Michaels said, because the prevailing winds blow from west to east.

Such a scheme, however, flies in the face of arguments by many researchers and environmentalists who have been calling for a commitment by the United States and other governments to reduce their fossil-fuel burning.

James Hansen and Andrew Lacis of the National Aeronautics and Space Administration recently reported that continuing to produce sulfur pollution to slow warming would commit earthlings to "a Faustian bargain." Sooner or later, the two said, "fossil fuels would run out, whereupon a huge carbon-dioxide-induced warming would begin." This is because the carbon dioxide gas would linger for decades after the sulfur dioxide particles precipitated out.

Moreover, Hansen and Lacis said that while sulfur pollution may play a significant role in climate change, its effect probably will not be large enough or last long enough to prevent warming entirely. But the magnitude is not yet known. Given the uncertainties, the two scientists recommend limiting the ultimate magnitude of the "experiment" by taking steps -- such as increasing energy efficiency and slowing deforestation -- that make sense on other grounds regardless of whether global warming takes place.