Green Sahara Riddles

Several thousand years ago, Sahara and Sahel were much wetter and greener than today. Subtle and steady variations in the Earth’s orbit are supposed to have triggered changes in the West African monsoon which were amplified by feedbacks between climate, ocean and primarily the land surface. While these processes are understood in principle, reconstructions and simulations differ. Some missing feedbacks are generally blamed for the deviation of model results from reconstruction. But the issue might be simpler: most models do not capture the steep and high mountains in the Sahara. High-resolution simulations help to solve the riddle of deep crater lakes in the Tibesti, the highest and steepest mountain in the central Sahara. Lakes were presumably abundant in the Sahara, but it is not yet known, how widespread lakes were and what their effect on the climate was. First attempts to predict the interaction between climate, dynamic lakes and vegetation are presented. Some 25 years ago, we predicted a “collapse” of the “green Sahara” that should have occurred some 5.5000 years ago. New, more details predictions and reconstructions of abrupt changes in the Sahara are shown, and a suggestion is made to answer the question of what “abrupt” really means. Reconstructions indicate that a greening of the Sahara had occurred rather regularly in the more distant past. A tentative answer to the question of why the past green Saharas differed in their amplitude and extent is given including a prediction for Saharan greening of the last 800,000 years.