Abstract
1) In the last 150 ka, the tropical arid belt in North Africa (Sahara) and its southern margins (Sahel) successively underwent major changes in their latitudinal range:
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130 ka: extensive lakes throughout present Sahara
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18 ka: Saharan environments down to 14°N
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8–7 ka: Sahelian environments up to 22–23°N.
These continental processes coincide with sea level transgressive and regressive phases and the Milankovitch forcing.
2) Since c 4.500 B.P., another expansion of the Sahara has taken place. The Sahel’s northern limit, now at 17° N is still regressing southwards. These data demonstrate a trend towards an arid scenario and match the insolation curve since 10 ka ago and the future cold trend calculated by A. Berger (1981).
The natural climatic forcing therefore runs counter to an eventual greenhouse effect, as it would be once again associated with temperature and sea-level drops.
The evidence for climatic changes in the north african arid belt thus confirms the predictive model of Kukla (1980) and does not imply any responsibility of man, although deforestation and erosion locally emphasize the trend.
Changes in rainfall, increase in deforestation and erosion recorded both in the tropical belts and in the Mediterranean borders, the possible rise of oceanic vapour and precipitation due to a CO2 greenhouse effect and the recent increase in strength and frequency of the dominant winds in the northern hemisphere are different factors which should be considered carefully and separately before one can imply any valuable theory on future global evolution. In the tropical arid belt enough field data are now available to allow a useful interpretation of environmental changes during the last global climate cycle and to define the part played by man in long trend variations and compare them with shorter term observations.
Successive extensions and shrinkings of the arid zone of North Africa (< 100–150 mm annual precipitation and 4–8 m evapotranspiration) are in evidence for the last 150 ka and are obviously related to marine isotopic trends (Hays and Imbrie, 1986) sea levels (Shackleton et al., 1981) and the astronomical Milankovitch insolation curve at 65°N (Berger, 1981, 1984).
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Petit-Maire, N. (1990). Natural Aridification or Man-Made Desertification? A Question for the Future. In: Paepe, R., Fairbridge, R.W., Jelgersma, S. (eds) Greenhouse Effect, Sea Level and Drought. NATO ASI Series, vol 325. Springer, Dordrecht. https://doi.org/10.1007/978-94-009-0701-0_17
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DOI: https://doi.org/10.1007/978-94-009-0701-0_17
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