In 1992, in a publication dating from before the appearance of the notion of “man-made global warming”, two of the world's greatest tropical climatologists, the French Yves Tardy and Jean-Luc Probst, explained in a few lines that the reason for the alternation of drought-rain cycles in the Sahel is due to variations in the “intertropical front”:
«The climate in Africa follows the position of the FIT (Intertropical Front) or ITCZ (Intertropical Convergence Zone). We can distinguish two scenarios:
1- When the ITF is maintained in a southern position, either because the mobile polars, originating from the South Pole, are less active than usual, or because their northern counterparts coming from the North Pole are on the contrary longer and more strongly active, the rainfall deficit is widespread in the Sahel of West Africa (…) This is the case for the years 1942, 1944, 1948, 1970, 1971, 1972 and 1973. This situation can be read very well on the fluctuation curve of the Senegal and Niger rivers (…).
2- When the FIT rises high towards the North under the push of mobile anticyclones originating from the South Pole, we record excess rainfall over West Sahelian Africa (…).
Thus, with the movements of the FIT which are under the influence of the rise towards the North of polar air masses coming from the South Pole or the descent towards the South of polar air masses coming from the North Pole, we can easily grasp the relationship that may exist between temperature fluctuations and those of humidity, as well as the effect of competition between the Northern Hemisphere and the Southern Hemisphere»
Current research has integrated the variations of the FIT into the long history of the Saharo-Sahelian climatic cycles, which provides insight over more than two million years as demonstrated by Mathieu Dalibard (2011) in his dedicated thesis. to African climate change.
According to Dalibard, the African climate varies according to three major cycles:
1- The cycles depending on the variation of the Earth's orbit, or “eccentricity cycles”, would fluctuate between 400,000 and 100,000 years.
2- The cycles depending on the inclination of the earth's axis, or “obliquity cycles”, would fluctuate between 54,000 and 41,000 years ago.
3- The cycles depending on the variation of the axis of rotation of the Earth, or “precession cycles”, would fluctuate between 23,000 and 19,000 years.
This succession of cycles, by definition independent of any human activity, allows us to understand why, several hundred million years ago, the Sahara and the Sahel were covered by a glacier, then by the ocean. Why, a hundred million years ago, it was an immense humid equatorial forest roamed by dinosaurs, before slowly transforming into a tropical forest, then into a wooded savannah.
Closer to us, this long-term climatic movement, tending for 5,000 years towards drying, was interspersed with remissions which gave rise to a succession of dry and humid episodes through which the establishment of populations took place.
Even closer to us, the 20th century experienced four major droughts between 1909 and 1913, between 1940 and 1944, between 1969 and 1973, and between 1983 and 1985 (Retaille, 1984; Ozer et alii, 2010; Maley and Vernet, 2013 ). During the 1960s, a “hot” period, increasing rainfall briefly caused the Sahelian zone to move northwards, which resulted in a retreat of the desert. And yet, we were then at the peak of global industrialization and the pollution that results from it.
Then, from the 1970s, with decreasing rainfall, the desert expanded again and the Sahel shrank, with the average isohyets descending 100 to 150 kilometers towards the South. The consequences of this new cycle are currently aggravated, but not caused, by Sahelian demographic pressure.
The analysis of these contradictory natural phenomena is extremely complex. She does not tolerate shortcuts or preconceived ideas. Their understanding comes neither through anathemas nor through slogans, but through the study of the long and even very long duration.
Bibliography
– Carré, M et alii., (2018) «Modern drought conditions in Western Sahel unprecedented in the past 1600 years». En ligne.
– Dalibard, M., (2011) “Climatological changes in the African intertropical zone during the last 165,000 years”. Thesis in climatic paleontology, Claude Bernard University, Lyon 1.
-Leroux, M., (1994) “Meteorological interpretation of climate changes observed in Africa for 18,000 years”. Geo-Eco-Trop1994,16, (1-4), pp.207-258.
– Leroux, M., (2000) “The dynamics of weather and climate”. Paris.
– Vehicle, B., (2023) “History of the Sahel from its origins to the present day”. Paris.
– Maley, J and Vernet, R., (2013) “People and climatic changes in north-tropical Africa, from the end of the Neolithic to the dawn of the modern era”. Africas, debates, methods and historical fields, vol 4.
– Ozer, P et alii., (2010) “Desertification in the Sahel: history and perspectives”. BSGLg, 2010, 54, pp 69-84.
– Retaille, D., (1984) “Drought and droughts in the Sahel. Geographic information”, 1984, 48, pp 137 to 144.
– Tardy, Y and Probst, JL., (1992) “Droughts, climate crises and teleconnected climate oscillations over the past hundred years”. Drought, 1992; 3:25-36.
