Sahel region of Mali

The idea that changes in insolation (solar heating) caused by long-term changes in the Earth’s orbit are a controlling factor for the long-term variations in the strength of monsoon patterns across the globe was first suggested by Rudolf Spitaler in the

late nineteenth century,[27] The hypothesis was later formally proposed and tested by the meteorologist John Kutzbach in 1981.[28] Kutzbach’s ideas about the impacts of insolation on global monsoonal patterns have become widely accepted today as the underlying driver of long-term monsoonal cycles. Kutzbach never formally named his hypothesis and as such it is referred to here as the “Orbital Monsoon Hypothesis” as suggested by Ruddiman in 2001.[27]

Sahel region of Mali
During the last glacial period, the Sahara was much larger than it is today, extending south beyond its current boundaries.[29] The end of the glacial period brought more rain to the Sahara, from about 8000 BCE to 6000 BCE, perhaps because of low press

ure areas over the collapsing ice sheets to the north.[30] Once the ice sheets were gone, the northern Sahara dried out. In the southern Sahara, the drying trend was initially counteracted by the monsoon, which brought rain further north than it does today. By around 4200 BCE, however, the monsoon retreated south to approximately where it is today,[31] leading to the gradual desertification of the Sahara.[32] The Sahara is now as dry as it was about 13,000 years ago.[25]