To return to the original question which set Otterman and others wondering: Are humans a large part of the blame for dry periods in the Sahel? Could they sometimes be the "kick" that sets the climate system tumbling towards a dry state after overgrazing takes place? The conclusion of all the models so far is that humans and their animals do not in fact have a big influence on the Sahel vegetation-climate system. While overgrazing may occur, it tends to alter details of structure and composition of the vegetation, towards thornier and less edible species, more than it affects the overall vegetation coverage. There is always at least some effect on overall vegetation coverage from grazing, and in this sense humans may have some small part to play in reinforcing droughts in the Sahel. However, it is an effect that is dwarfed by, and very difficult to disentangle from, the other vegetation-climate processes that cause wet and dry cycles in that region.
Interestingly, there is at least a possibility that events in the Sahel could be influenced by humans changing the vegetation outside the region, hundreds of miles to the south in West Africa. One model suggests that if the forests in West Africa are cleared, the loss of re-evaporation of water cuts off the supply of moisture for rainfall over the Sahel. There has already been a considerable amount of forest loss in West Africa in the last few decades and it is not certain how this might have affected the recent climate history of the Sahel. It seems that most of those working on this area presently feel that the forest removal has not had much effect, and that the variability in the Sahel is mainly due to variable sea surface temperatures in the Atlantic plus internally generated vegetation-climate feedbacks in the Sahel.
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