Sahara and Sahel vulnerability to climate changes, lessons from the past
Résumé
Since the Sahelian drought in the 1970s, climate variability in north tropical Africa has been the subject of intensive research focusing on the functioning of the Atlantic monsoon system as well as on past variations in rainfall from historical and natural archives. More severe and longer droughts have occurred in the tropics during the past millennium, including a drought that occurred 200-300 years ago during which the level of Lake Bosumtwi (Ghana) dropped by almost four times as much as it did during the drought of the 1970s. Climate crises of even greater amplitude and developing over only a few years have been recorded during the Holocene leading for dramatic fall of lake levels near the Equator and the complete drying of fresh water bodies in the Sahel and the Sahara. In particular, an “abrupt” climate change has been recorded off the Mauritanian coast at the end of the African Humid Period (AHP) 5500 years ago illustrating the onset of the modern climate regime. Was this change abrupt or gradual, and amplified or not through vegetation change and feedbacks to the atmosphere is still the subject of debate.
In the frame of the French project “SAHELP”, we have used paleohydrological and palynological data between 10 and 28°N in the Sahara and Sahel in order to understand the response of the hydrological system and the vegetation cover to rainfall fluctuations from the onset of the AHP to its end and to catch the regional pattern of moisture availability through time. Special attention have been paid on climate indices (dry spell, wind direction and intensity of events) to characterized the different phases of the climate deterioration. Our work is based on selected “case studies” in different hydrological environment: the mega Lake Chad and Lake Yoa (Chad), Lake In Atei (Algeria) and the Niayes interdunes depressions (Senegal).