Simple tipping or complex transition? Lessons from a green Sahara

Little does the monotonous and hostile environment of today's sahara Desert tell us about its colorful past. However, climate and vegetation reconstructions reveal that several thousand years ago, parts of the sahara resembled a flourishing garden with extensive vegetation and lakes (Jolly et al. 1998). this green sahara was only the most recent of many green episodes in North Africa's history. the most important driver of these landscape transformations was the permanent change in the Earth's orbital parameters (Kutzbach 1981). When the distance between Earth and sun is smallest during boreal summer, the larger solar irradiation intensifies the West African monsoon and thus increases rainfall. While it is obvious that rainfall is beneficial for vegetation, climate models indicate that vegetation can also enhance rainfall. First, the dark vegetation absorbs more sunlight than the bright desert and provides energy for convection (Charney et al. 1975). second, the evaporation from vegetation and lake surfaces feeds the water back into the atmosphere (Rachmayani et al. 2015). Vegetation and rainfall are therefore linked in a self-amplifying process, a positive feedback. the stronger this feedback, the more abrupt the transition from a green sahara to a desert (Fig. 1).