The Sahara Desert, now a vast expanse of arid landscapes, was once a flourishing green savannah, as evidenced by ancient rock art found in Algeria’s Tassili N’Ajjer plateau. The depictions showcase a vibrant ecosystem teeming with elephants, giraffes, rhinos, and hippos, offering a glimpse into a period called the Green Sahara or North African Humid Period, which occurred approximately 6,000–11,000 years ago.
Scientists have identified over 230 instances of these “greenings” happening about every 21,000 years over the past eight million years, shaping the Sahara into wooded savannahs with numerous rivers and lakes spanning present-day Libya, Niger, Chad, and Mali. These events influenced species distribution and played a role in the out-of-Africa migrations of ancient humans.
A breakthrough in climate modeling has enabled researchers to understand why these greenings occurred. A team of climate modelers and anthropologists developed a more accurate climate model that simulates atmospheric circulation over the Sahara and considers the impact of vegetation on rainfall. Their findings point to changes in Earth’s orbital precession, the slight wobbling of the planet while rotating, as the key driver behind the Sahara’s periodic greening.
During the Green Sahara periods, Earth’s orbital precession brought the Northern Hemisphere closer to the sun during summer, resulting in warmer temperatures. Warmer air, capable of holding more moisture, intensified the West African Monsoon system, shifting the African rainbelt northwards. This increased rainfall in the Sahara, fostering the growth of savannah and wooded grassland across the desert from the tropics to the Mediterranean. The recurring wetter periods in north Africa every 21,000 years are linked to Milankovitch cycles, variations in Earth’s orbit influenced by gravitational forces from the moon and other planets. The precession cycle, specifically, appears to be the dominant driver of the Sahara’s humid phases, influencing seasonal contrasts and rainfall patterns.
Interestingly, the study reveals that these humid periods did not coincide with ice ages when large glacial ice sheets covered polar regions. The cooling effect of the ice sheets countered the influence of precession, suppressing the expansion of the African monsoon system during those periods. The Sahara, acting as a gateway, controlled the dispersal of species between north and sub-Saharan Africa. It was open during the Green Sahara phases and closed during desert conditions. The study emphasizes the sensitivity of this ecological gate to Earth’s orbit and suggests that high-latitude ice sheets may have restricted species dispersal during glacial periods.
Understanding the alternation of humid and arid phases in Africa’s past has significant implications for the dispersal and evolution of species, including humans. Furthermore, the research provides insights into potential future greening in response to climate change and its environmental impact, offering a tool for predicting how climate warming might influence rainfall and vegetation in the Sahara region.
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