Climate and Africa: why the land goes dry - part 1

Science News, May 4, 1985 by Stefi Weisburd, Janet Raloff

Though the Atlantic Ocean is the ultimate source of much of Africa's rain, Chervin points out that even in western Africa, local soil evaporation also contributes substantially to precipitation. And it is this evaporative component that could be affected by human activity.

Rain--even in the driest desert--starts as ice crystals. But water will not freeze unless it contains the proper type of foreign particles to act as ice nuclei. For years people had assumed ice nuclei could be just about anything, such as dust. But research has shown "that "clean' dust, without biological materials from plants or bacteria, was useless,' explains Russell Schnell, an atmospheric scientist with the National Oceanic and Atmospheric Administration in Boulder.

Schnell discovered that nature produces the nuclei, in the form of lipoproteins contained in the coats of several species of bacteria that live on plants. They allow frost to form on unprotected crops. But more important, some of these bacteria also help plant matter to decay, and in doing so can shed the lipoproteins onto vegetative litter on the soil surface, making the organic litter itself into ideal ice nuclei. Schnell believes that by devegetating the Sahel--for firewood, crops and animal fodder--humans have eliminated the required food source for the particular bacteria needed to initiate the precipitation process in clouds above them. One result might be for the rains to start falling a little farther south, he says, "providing a self-feeding desert situation.'

In field studies in the Sahel several years ago, Schnell found that as he got closer and closer to overgrazed areas, there were fewer and fewer ice nuclei in soil and vegetation. Moreover, he says, not only have experiments shown that these nuclei can be carried up from the ground and into clouds in a span of time as short as 20 minutes, "but we have also been able to but these things into clouds and make them rain.' The rub, Schnell says, is that no one can yet conceive of a definitive experiment to prove that the removal of these biogenic ice nuclei are implicated in Africa's precipitation deficit by more than just coincidence.

Plant pathologists David Sands and Al Scharen at Montana State University in Bozeman are among those currently investigating the bacteria-rainfall connection in Africa. Focusing on the needs of farmers in north Africa--Morocco, Tunisia and Egypt--they're trying to characterize which plants serve as the best hosts to these bacteria. Their goal is to identify "bioprecipitation support crops' that drought-afflicted farmers could use as a sort of ground-based cloud-seeding program.

One means of weighing the relative impact on the African climate of these land changes or any other factors, like sea surface temperatures, is to run computer experiments using general circulation models, such as the one used by Rowntree. These models, of which there are about a dozen in the world, consist of a series of equations relating changes in wind patterns, barometric pressure and temperature of the atmosphere. The dynamics of the atmosphere and climate can be explored by altering various boundary conditions such as SSTs, surface properties like albedo and roughness, the chemical composition of the atmosphere, and even astronomical parameters such as distance to the sun. By letting the computer keep track of all the details as it integrates the equations, scientists can test their ideas on the physical mechanisms that brought drought to Africa and then allowed it to stay.