Reigning greens

Satellite observations suggest vegetation encourages rainfall in Africa. A study led by P M Cox, a scientist with UK's Natural Environment Research Council, says vegetation accounts for around 30 per cent of annual rainfall variation in Africa's Sahel region.

For the study, published in the journal Geophysical Research Letters (Vol 33, No 16, Aug 16, 2006), the researchers combined rainfall records from 1982 to 1999 with a dataset developed from nasa satellites giving a measure of the vegetation coverage in Sahel over the same period. The researchers first examined how well historical rainfall levels predicted the future rainfall pattern for each month over an 18-year period. They then used the green factor (vegetation) in the analysis to see if it affects rainfall prediction. They found when they included vegetation as a factor in the analysis, it revealed change in subsequent rainfall levels by 30 per cent. The finding suggests that rain encourages greenery, which in turn creates more rainfall.

The researchers reason that plants are thought to transfer moisture from the soil into the air by evaporation, and hold water in the soil close to the surface, where it can also evaporate. On the other hand the darker surfaces of plants absorb more solar radiation compared to sandy deserts that can create convection and turbulence in the atmosphere. This in turn affects rainfall. The researchers believe that the results should help scientists understand rainfall patterns in the area, which is threatened with increasing desertification and periods of drought. The result adds to the importance to preserve green spaces in dry regions to help prevent deserts from growing.