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

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

At Britain's Meteorological Office in Bracknell, England, Peter Rowntree is studying the possible role soil moisture might have. Less soil moisture results in less evaporation, he explains, which means that more solar energy goes into heating the environment than into evaporating moisture. This contributes to a warming of air near the dry ground. Since warm air holds more moisture, its dew point, or the temperature at which water condenses, is therefore increased.

With less moisture returning to the air and a higher dew point, there will be less chance of rain. Dennett adds that the near-surface heating of air also tends to reduce the air's upward movement, itself a factor in reducing rainfall, since the water removed from the soil doesn't get a chance to climb high enough into the atmosphere to reach the cool temperatures where rain clouds form. And recent experiments run on the Meteorological Office's 11-layer general circulation (computer) model tend to confirm all this.

In a control experiment using the general circulation model, Rowntree looked at the rainfall that would be associated with soil moisture levels characteristic of well-vegetated land. To simulate Sahel conditions, the experiment was then rerun with only 6.6 percent as much soil moisture --a value Rowntree says "implies that essentially you have bare soil, or you only have vegetation with shallow roots.' An experimental run calculated how rainfall would change over a nine-month period beginning in March.

"We observed a rainfall decrease during northern summer--that's June, July and August--of about 1 mm a day,' Rowntree says. "That's something like the observed decreases in rainfall that occurred over the last few years.' While this doesn't prove that rainfall has been reduced in the Sahel because widespread overgrazing and devegetation have denuded the ground, and have thereby reduced the soil's ability to retain water, "I do think that is the suggestion,' Rowntree told SCIENCE NEWS. Some type of feedback mechanism must be prolonging the dry period, he says, because the probability that this drought could sustain itself for 17 years purely by chance is roughly 1 chance in 130,000.

In addition to reducing a soil's ability to retain moisture, devegetation would be expected to increase the soil's reflectance of spectral energy, since a dry soil is not as black as a wet one. This energy reflectance, or albedo, would contribute to near-surface air heating and the further baking out of soil moisture. In experiments by a number of different researchers 10 years ago, computer models suggested rainfall would decrease over areas where albedo increased.

Robert Chervin, with the National Center for Atmospheric Research in Boulder, Colo., says his experiments showed that such albedo increases resulted in statistically significant reductions in rainfall for the affected region, and corresponding increases in rainfall in adjacent areas. The affected regions were thousands of kilometers in size, he says, "so we're not talking about city-block-size changes.'