On the basis of analysis of data collected over the Sahel, India, and China, it is conjectured that the occurrence of above-normal rainfall over the Sahel during the decade of the 1950s and below-normal rainfall during the decades of the 1970s and 1980s is a manifestation of natural variations in the planetary-scale coupled ocean-land-atmosphere climate system. In the light of the results of general-circulation model sensitivity experiments, and the fact that the Sahel region is particularly susceptible to changes in land-surface conditions, it is concluded that strong local atmosphere-land interactions over the Sahel region have contributed toward further reduction of rainfall. Both natural climate variability and human activities degrade the land surface in a way that exacerbates the ongoing drought, and therefore there is no guarantee that natural variability can reverse the present trend.
The West African famine during the late 1960s and the 1970s focused the world's attention on the Sahel. The Sahel is a 200- to 500-km wide band across the southern reaches of the Sahara desert. It is ecologically fragile, though it supports nomads, herders, and sedentary farmers; it has a mean rainfall of 300 to 500 mm per year, which is extremely variable. During the past 25 years, rainfall over the Sahel has been significantly lower than the long-term mean. In the recorded meteorological data for the past 100 years, there is no other region on the globe of this size for which spatially and seasonally averaged climatic anomalies have shown such persistence. (See the paper by Nicholson in this section.)
In this paper we shall attempt to address the following questions concerning the Sahel drought:
What are the causes of the onset of the drought? In particular, what significant roles did the natural variability of the global climate system and of human activities play in the initiation of the drought?
What mechanisms are responsible for the persistence of the Sahel drought?
What are the prospects for reversal of the current trend? In particular, what roles might the natural variability of the global climate system and human intervention play in reversing the current trend?
It is of course difficult, if not impossible, to give precise