Fauna common in the Sahara during the wet phase, found at Tassili in the central Sahara
The Sahara Pump Theory explains how flora and fauna left Africa to penetrate the Middle East and beyond to Europe and Asia. African pluvial periods are associated with a "wet Sahara" phase during which larger lakes and more rivers exist.[1]
One example of the Saharan pump has occurred since the Last Glacial Maximum (LGM). During the Last Glacial Maximum the Sahara desert was more extensive than it is now because the extent of the tropical forests was greatly reduced.[2] During this period, the lower temperatures reduced the strength of the Hadley Cell whereby rising tropical air of the Inter-Tropical Convergence Zone (ITCZ) brings rain to the tropics, while dry descending air, at about 20 degrees north, flows back to the equator and brings desert conditions to this region. This phase is associated with high rates of wind-blown mineral dust, found in marine cores that come from the north tropical Atlantic. Around 12,500 BC, the amount of dust in the cores in the Bølling/Allerød phase suddenly plummets and shows a period of much wetter conditions in the Sahara, indicating a Dansgaard-Oeschger (DO) event (a sudden warming followed by a slower cooling of the climate). The moister Saharan conditions had begun about 12,500 BC, with the extension of the ITCZ northward in the northern hemisphere summer, bringing moist wet conditions and a savanna climate to the Sahara, which (apart from a short dry spell associated with the Younger Dryas) peaked during the Holocene thermal maximum climatic phase at 4000 BC when mid-latitude temperatures seem to have been between 2 and 3 degrees warmer than in the recent past. Analysis of Nile River deposited sediments in the delta also shows this period had a higher proportion of sediments coming from the Blue Nile, suggesting higher rainfall also in the Ethiopian Highlands. This was caused principally by a stronger monsoonal circulation throughout the sub-tropical regions, affecting India, Arabia and the Sahara. The sudden subsequent movement of the ITCZ southwards with a Heinrich event (a sudden cooling followed by a slower warming), linked to changes with the El Niño-Southern Oscillation cycle, led to a rapid drying out of the Saharan and Arabian regions, which quickly became desert. This is linked to a marked decline in the scale of the Nile floods between 2700 and 2100 BC. [3]
During these periods of a wet Sahara, the Sahara and Arabia become a savanna grassland and African flora and fauna become common. During the following inter-pluvial arid period, the Sahara reverts to desert conditions usually as a result of the retreat of the West African Monsoon southwards. Evaporation exceeds precipitation, the level of water in lakes like Lake Chad falls, and rivers become dry wadis. Flora and fauna previously widespread retreat northwards to the Atlas Mountains, southwards into West Africa, or eastwards into the Nile Valley and thence either south-east to the Ethiopian Highlands and Kenya or north-east across the Sinai into Asia. This separates populations of some of the species in areas with different climates, forcing them to adapt, possibly giving rise to allopatric speciation.
The Saharan pump has been used to date four waves of human emigration from Africa, namely:[4]
Homo erectus (ssp. ergaster) into Southeast and East Asia
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