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 Windblown desert dust can choke rain clouds, cutting badly needed rainfall. This new discovery made with the help of NASA satellites suggests that droughts over arid regions, such as central Africa, are made even worse by land management practices that expose new dust and accelerate the growth of deserts. In other words: Dust begets dust!
These findings, reported in the Proceedings of the National Academy of Sciences, present a new view of the decades-long drought in the African Sahel, a semi-arid region of Africa adjacent to the Sahara Desert. The grassy savannas of the Sahel once provided natural pasture land for livestock, but increasing demands on the land due to population growth have lead to desertification, which has been accompanied by increasing levels of airborne dust during the rainy season.
 Dust and other particles in the air cause water droplets in clouds to be smaller, leading to decreased rain. More airborne dust is not necessarily a result of 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. Furthermore, it was the opposite of what scientists expected.
Before Rosenfeld et al.'s research, many scientists thought that large airborne dust particles would speed the formation of rain by forming giant cloud condensation nuclei and larger cloud droplets. Not so.
 "Our laboratory analysis of the desert dust showed that [dust] particles contained very little water-absorbing matter," says co-author Yinon Rudich of the Weizmann Institute. "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. In desert regions, dust storms can often kick up clouds of dust that blanket thousands of square miles of land.
Water droplets that form around dust grains and other aerosols, tend to be smaller than droplets that don't. The size of the water droplets in a cloud determines whether gravity will dominate over the tendency of the air molecules to keep the droplets suspended. Dust and other types of aerosol particles blowing into clouds act as nuclei where water vapour 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 March 2000 dust storm. Droplet sizes steadily increased and the clouds were from the 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 behaviour in clouds over the eastern Mediterranean Sea in March 1998, using data from aircraft and a U.S. weather satellite.
 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."
Source: NASA Press Release
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