Droughts Aggravated By Dust In The Wind

The findings, reported in the Proceedings of the National Academy of Sciences, present a new view of the decades-long drought in the African Sahel, which has been accompanied by increasing levels of airborne dust during the rainy season.

The higher dust frequency is not necessarily a result of the decreased rainfall, but rather its cause, according to scientists from Israel's Hebrew University and the Weizmann Institute. "This impact of desert dust on rainfall was not known before," says lead author Daniel Rosenfeld, Hebrew University, Jerusalem. "Due to the large sizes of some of these dust particles, it had been assumed that desert dust would enhance precipitation rather than decrease it."

Scientists had expected that the largest dust particles would form giant cloud condensation nuclei, which produce larger cloud droplets that speed the formation of rain. "Our laboratory analysis of the desert dust, however, showed that the particles contained very little water-absorbing matter," says co-author Yinon Rudich of the Weizmann Institute, Rehovot. "As a result, even large dust particles form relatively small cloud droplets."

The research shows dust actually amplifies the process of creating deserts. Activities that expose and disrupt topsoil, such as grazing and agricultural cultivation, can increase the amount of dust blown into the air. More dust reaching rain clouds produces less rainfall, which exacerbates the drought conditions and contributes to the desertification of the landscape.

Dust and other types of aerosol particles blowing into clouds act as nuclei where water vapor can condense to form cloud droplets. If a lot of dust enters a cloud, the available water is spread over many small droplets. These small droplets grow more slowly through collisions with one another to the size of a raindrop, and the cloud yields less rainfall over the course of its lifetime.

What the researchers saw in two separate cases, using different satellite observations, was that cloud droplets were smaller as dust concentrations increased.

NASA's Tropical Rainfall Measuring Mission (TRMM) spacecraft captured images of clouds over the Atlantic Ocean off the coast of northern Africa during a major dust storm last March. Droplet sizes steadily increased the farther the clouds were from dust-filled air. Rain was falling only from the dust-free clouds even though all the clouds contained equal amounts of water.

The researchers also observed similar behavior in clouds over the eastern Mediterranean Sea in March 1998, using data from aircraft and a U.S. weather satellite.

TRMM is a joint U.S.-Japanese mission and part of NASA's Earth Science Enterprise, a long-term research program designed to study the Earth's land, oceans, air, ice and life as a total system. Each day, the TRMM spacecraft observes the Earth's equatorial and tropical regions, including the southernmost United States and all of Africa.

Rosenfeld has used TRMM observations in two other recent studies to show that aerosols from biomass-burning smoke and urban air pollution also reduce rainfall. Combined with the negative impact of desert dust, Rosenfeld believes the aerosol rainfall suppression effect can have a major impact on regional and global climate.

"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 Rosenfeld. "Still, this is perhaps the climate-change effect with the greatest socio-economic impact on water-scarce areas."