While these measurements are in decades, predictions of global warming involve centuries. There is wide speculation that greenhouse gases, including about half of the nearly seven billion tons of carbon dioxide that human activity produces each year, are drifting into the atmosphere and acting like a radiation blanket over the Earth's surface.

In 1995 a United Nations panel placed the blame on the industrial gases. A global treaty that would set targets on greenhouse emissions could be signed following December's "Earth Summit" in Kyoto, Japan. However, such targets could prove costly to the industrialized nations. And there is wide disagreement about whether the human-made emissions have actually contributed to the small rise in global temperatures, or whether the temperature increase is due to natural climate cycles.

There is a problem with most computer simulations of global warming based on human-made emissions. The results say that the amount of heat-trapping gases should cause a much sharper increase in warming than scientists have recorded. To account for this conundrum, scientists look to the "sulfate paradigm" theory. They suggest that burning fossil fuels creates a haze of sulfates in the atmosphere. This haze acts like a grimy shield, reflecting sunlight back into space, offsetting some warming from the greenhouse effect.

But Hobbs has now complicated their assumptions. In May he and Dean Hegg, research professor of atmospheric sciences at the UW, reported on their airborne analysis of the chemical content and reflective properties of the haze. To their surprise they found the sulfates were outnumbered by carbon particles--from burning forests and fossil fuels as well as natural sources. Since carbon particles absorb more light than do sulfates, they have less tendency to cool the Earth.

So if this haze is not the cooling factor that lessens the greenhouse effect, what is? Hobbs speculates that the answer lies with microscopic particles from industrial emissions. These particles act as seeds for the formation of water droplets in clouds. And clouds, being reflectors of sunlight, cool the global surface.

Hobbs thinks that once it is understood how human-made emissions are affecting the atmosphere, the extent of global warming can be calculated more accurately. Then it becomes a political problem. "We will be able to provide governments with various scenarios for them to consider. If we continue to burn fossil fuels by `X' amount, then this will be the consequence. But if we decrease our use of fossil fuels by `Y' amount, then this will be the benefit."

That will be a struggle for this and future generations. Other prediction problems also lie ahead, such as a 23,000-year climate cycle linked to the procession of the equinoxes, and a 41,000-year cycle connected with changes in the tilt of the Earth's axis of rotation.

But these are for future millenniums to worry about. * David Brand is a science writer for the UW Office of News and Information. He was previously a reporter and editor for Time and the Wall Street Journal.

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