Could The Sahara Be Made Green

Some models that involve both vegetation and climate have suggested the hidden potential for far more extreme changcs in the climate of the Sahara than we have witnessed over the past century. These models have conccntratcd so far on just the western half of the Sahara desert. They tend to find that if you were to blanket the whole of the western Sahara desert in a leafy cover of grass or bushes, the climate of the region would be transformed. The low albedo, the greater roughness, the capturc of rainfall and its evaporation from leaves would result in monsoon rains that normally stay to the south of the desert coming farther north. According to these models, the rain made by all this vegetation would actually be enough to sustain the vegetation cover itself and the Sahara (at least, the western Sahara, and perhaps the whole Sahara) would vanish! It would be replaced by the sort of open cover of small bushes and scattered patches of grass that we see just along the northern and southern edges of the Sahara at present, something more like "semi-desert" than the desert of the present. The higher rainfall /ones to the south would also move farther north, bringing much moister climates to areas that are nowadays scrub and semi-desert. According to these models, then, the imaginary "green Sahara" and the present-day "brown Sahara" are both equally probable, equally stable in the present-day world and it is just by chance that we have one rather than the other. Some people have suggested that, given this possibility, we should set out to create a greener and more useful Sahara region for ourselves by progressively planting trees and other vegetation inwards from the edges of the desert.

However, climate modeling is a complex business and different groups' models often come up with different conclusions from one another. Some models (e.g., one put together by Hans Renssen and colleagues) set up in slightly different ways suggest that the "green" Sahara is not actually a possibility in the present-day global climatc system: that even if we blanketed the whole desert in vegetation the feedbacks it set up would not manage to bring in the rains needed to keep the vegetation going. Given the uncertainties, any large-scale exercise in climate engineering that sets out to transform the Sahara through planting vegetation would risk becoming an expensive failure.

Continue reading here: Box 52 Simulating climate GCMs and mesoscale models

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