Scientists long thought that large dust particles in the air helped promote rainfall, providing seeds of a sort in clouds around which condensation could form. But in fact dust may have the exact opposite effect. "Our laboratory analysis of the desert dust...showed that the particles contained very little water-absorbing matter," says Yinon Rudich of the Weizmann Institute in Rehovot, Israel, and co-author of the study. "As a result, even large dust particles form relatively small cloud droplets." In turn, a cloud produces less rain during its lifetime.

The problem is particularly severe in deserts, which by definition get little precipitation and often have an abundance of dust storms. It is also a cause for concern in arid agricultural regions, where grazing and the disruption of topsoil can create dust that then reduces precipitation. More dust, less rain and even more dust can become a vicious cycle that ultimately creates increasingly more desert lands.

The researchers, who published their findings in this week's Proceedings of the National Academy of Sciences, looked at satellite images and aircraft observations of cloud behavior over the eastern Mediterranean Sea and the Atlantic Ocean off the coast of northern Africa during a major dust storm. They found that the clouds hit by dust storms generally contained smaller droplets and made less rain than those unaffected by dust.

"The recent observations of the impact on precipitation of all kinds of aerosols, each with a major human contribution, show a major climate change issue that has nothing to do with greenhouse gases," says lead author Daniel Rosenberg of Hebrew University in Jerusalem. "Still, this is perhaps the climate-change effect with the greatest socioeconomic impact on water-scarce areas."