The climate of the Sahara has undergone enormous variations between wet and dry over the last few hundred thousand years. This is due to a 41,000 year cycle in which the axial tilt of the earth changes between 22° and 24.5°. At present (2000 CE), the Sahara is in a dry period, but it is expected that the Sahara will become green again in 15,000 years (17,000 CE).
During the last glacial period, the Sahara was even bigger than it is today, extending south beyond its current boundaries. The end of the glacial period brought more rain to the Sahara, from about 8000 BCE to 6000 BCE, perhaps because of low pressure areas over the collapsing ice sheets to the north.
Once the Ice Age ended, the northern Sahara dried out. In the southern Sahara though, the drying trend was soon counteracted by the monsoon, which brought rain further north than it does today. In this period, there was still a monsoon climate in the Sahara. Monsoons form by heating of air over the land during summer. The hot air rises and pulls in cool, wet air from the ocean, which causes rain. Thus, though it seems counterintuitive, the Sahara was wetter when it received more insolation in the summer. This was caused by a stronger tilt in Earth’s axis of orbit than today (24.5 degree tilt vs the 23.4° tilt today), and perihelion occurred at the end of July around 7000 BCE.
By around 4200 BCE, the monsoon retreated south to approximately where it is today, leading to the gradual desertification of the Sahara. The Sahara is now as dry as it was about 13,000 years ago. These conditions are responsible for what has been called the Sahara Pump Theory.
Posted but not written by: Louis Sheehan
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Feel free to ignore this blog! I am intending to use it as a repository of various writings: drafts, doodles, etc. If there ARE any articles here, they are posted but not written by: Lou Sheehan