China's recent weather patterns represent the most severe change in the region's precipitation trends since 950 A.D. Increasingly, the southern parts of the country are experiencing flooding, while northern regions are suffering under drought conditions. Now research published in the current issue of the journal Science suggests that soot, which is generated from industrial activity and incomplete combustion of coal and biomass, may be partly to blame.

Soot particles, also known as black carbon aerosols, affect climate by absorbing sunlight, which warms the surrounding air and limits the amount of solar radiation that reaches the ground. But soot and other aerosol particles are short-lived in the atmosphere, unlike greenhouse gases such as carbon dioxide, and their climate effects are very difficult to quantify. In the new work, Surabi Menon of the NASA Goddard Institute for Space Studies and colleagues used aerosol data collected from 46 ground stations in China to assess four different climate modeling scenarios. Taking factors such as sea surface temperature, greenhouse gases and natural aerosol particles into consideration, the researchers determined that changes in the concentration of black carbon could be the primary driving force behind the observed alterations to the hydrological cycle in the region. "If our interpretation is correct, then reducing the amount of black carbon or soot may help diminish the intensity of floods in the south and droughts in the northern areas of China, in addition to having human health benefits," notes co-author James Hansen of the Goddard Institute.

Additional studies are needed to fully appreciate soot's role in global warming and changing rain patterns, both in China and in other regions. But because of the small soot particles' adverse health effects (their size allows them to enter the lungs, where they can cause respiratory distress), limiting black carbon production will have beneficial effects. "This could be 'low-hanging fruit' in trying to deal with the anthropogenic effects on the climate," comments Michael Bergin of the Georgia Institute of Technology. "From a policy standpoint, the payoff for controlling soot could be on the scale of years rather than centuries."