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https://en.wikipedia.org/wiki/Sahara#Climate_history
During the
Last Glacial Maximum (LGM) the Sahara desert was more extensive than it is now with the extent of the tropical forests being greatly reduced,
[39] and the lower temperatures reduced the strength of the
Hadley Cell. This is a climate cell which causes rising tropical air of the
Inter-Tropical Convergence Zone (ITCZ) to bring 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. It is associated with high rates of wind-blown mineral dust, and these dust levels are found as expected in marine cores from the north tropical Atlantic. But around 12,500 BCE 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 BCE, 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 BCE 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.
Lake Victoria only recently became the source of the
White Nile and dried out almost completely around 15 kya.
[40]
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 BCE.
[41]