How much this doubled CO2 concentration will raise Earth�s global mean temperature, however, remains quite uncertain and is the subject of intense research - and heated debate.

In a paper to be published in the November issue of the Journal of the Air and Waste Management Association, Stephen Schwartz, an atmospheric scientist at the U.S. Department of Energy�s Brookhaven National Laboratory, argues that much of the reason for the present uncertainty in the climatic effect of increased CO2 arises from uncertainty about the influence of atmospheric aerosols, tiny particles in the air.

Schwartz, who is also chief scientist of the Department of Energy�s Atmospheric Science Program, points out that aerosols scatter and absorb light and modify the properties of clouds, making them brighter and thus able to reflect more incoming solar radiation before it reaches Earth�s surface.

"Because these aerosol particles, like CO2, are introduced into the atmosphere as a consequence of industrial processes such as fossil fuel combustion," says Schwartz, "they have been exerting an influence on climate over the same period of time as the increase in CO2, and could thus very well be masking much of the influence of that greenhouse gas."

However, he emphasizes, the influence of aerosols is not nearly so well understood as the influence of greenhouse gases.

As Schwartz documents, the uncertainty in the climate influence of atmospheric aerosols limits any inference that can be drawn about future climate sensitivity - how much the temperature would rise due to CO2 doubling alone - from the increase in global mean temperature already observed over the industrial period.

The global warming of 0.5 degrees Celsius (0.9 degrees Fahrenheit) that has taken place since 1900 suggests that, if there were no aerosol influence, the effect of CO2 doubling on mean global temperature would be rather low - a rise of 0.9 degrees Celsius (1.6 degrees Fahrenheit).

But, the likelihood that aerosols have been offsetting some of the warming caused by CO2 all along, says Schwartz, means that the observed 0.5-degree-Celsius temperature rise is just the part of the CO2 effect we can "see" - the tip of the greenhouse "iceberg."

So the effect of doubling CO2, holding everything else constant, he says, might be three or more times as great.

"Knowledge of Earth�s climate sensitivity is central to informed decision-making regarding future carbon dioxide emissions and developing strategies to cope with a greenhouse-warmed world," Schwartz says.

However, as he points out, not knowing how much aerosols offset greenhouse warming makes it impossible to refine estimates of climate sensitivity.

Right now, climate models with low sensitivity to CO2 and those with high sensitivity are able to reproduce the temperature change observed over the industrial period equally well by using different values of the aerosol influence, all of which lie within the uncertainty of present estimates.

"In order to appreciably reduce uncertainty in Earth�s climate sensitivity the uncertainty in aerosol influences on climate must be reduced at least threefold," Schwartz concludes. He acknowledges that such a reduction in uncertainty presents an enormous challenge to the aerosol research community.

An editorial accompanying the paper credits Schwartz with presenting "a unique argument challenging the research community to reduce the uncertainty in aerosol forcing of climate change in order to reduce the uncertainty in climate sensitivity to an extent that would be more useful to decision makers."

The editorial also suggests that, "Schwartz�s calculations are not only of interest for the issue of climate change but may serve as a paradigm for environmental issues in general."

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