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Science et changements planétaires / Sécheresse. Volume 2, Number 3, 199-210, Septembre 1991, Synthèse

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Author(s) : Pierre Rognon

Summary : During the period of instrumental records that began little more than a century ago, three persistent droughts have been known in the Sahel\; the last one started in 1968 and ended in 1986. It is very difficult to understand all the mechanisms which cause such long droughts, and their long range forecasting is beyond the capacity of the meteorological know-how. However, geologists researching climatic history on the thousand-year scale have discovered a certain number of analogies between the present-day "behaviour" of the Sahel climate and its very-long-term fluctuations. Thus, the progress in historical climatology and in tree-ring records has pointed out that droughts as long as the last Sahelian drought have indeed existed during previous centuries around the Sahara desert and their distribution in time did not present any regular rhythm, comparable to their 30-year quasi-periodic cycle during the present century. The unusual duration of the last drought (1968-1986) is perhaps the beginning of a durable southward shift in climatic zones. But, unfortunately, today there is only one known example for such shifting on the geological time scale. Between 20 000 and 12 500 BP, of course, the Sahara increased in size and shifted in the direction of the Equator. But this shift was related to a period of extreme cooling, when the continental ice-sheet covering North America and Europe had reached their southern-most limit and pushed the other climatic zones towards the South. At this time, the climatic context was quite different from nowadays. If we try to find conditions similar to those of the present day, it is necessary to go back to the last interglacial period, between 125 000 and 70 000 BP. The best records for reconstructing these interglacial climates are to be found in marine cores which show a succession of dry and wet periods according to a 21-23 000 years cycle. They probably are the results, at the low latitudes, of the fluctuations in solar radiation caused by the precession of the equinoxes. In comparison, if such a mechanism were to be transposed into our own interglacial period, it would suggest that the present millennium were very close to a peak of extreme dryness. Nevertheless, it would be interesting to search for the possible consequences of artificial heating due to the green-house effect on the Sahel’s climate natural evolution.

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