[1] Variations in the climate of the Sahara region can, at the simplest level, be attributed to the changes in insolation because of slow shifts in Earth's orbital parameters.
[3][4] The idea that changes in insolation caused by shifts in the Earth's orbital parameters 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,[5] The hypothesis was later formally proposed and tested by the meteorologist John Kutzbach in 1981.
[5] Insolation, which is simply a measure of the amount of solar radiation received on a given surface area in a given time period, is the fundamental factor behind the Orbital Monsoon Hypothesis.
[5] Over periods of tens to hundreds of thousands of years the amount of insolation changes in a highly complex cycle that is based on orbital parameters.
[2][7][8] The relationship between the precession cycle and the strength of the North African Monsoon exists because procession affects the amount of insolation received in a given hemisphere.
On the opposite end of the spectrum, when the Northern Hemisphere is pointed toward the sun during aphelion, there is a minimum in insolation and the North African Monsoon is at its weakest.
Evidence for the impact of obliquity on the intensity of the North African Monsoon has been found in records of dust deposits from ocean cores in the Eastern Mediterranean that occur as a result of Aeolian processes.
[2] This evidence requires complex feedback mechanisms to explain since the strongest impact of obliquity on insolation is found in the high latitudes.
For example, when the orbit of the Earth is highly elliptical one hemisphere will have hot summers and cold winters, corresponding to a larger than average yearly insolation gradient.
In the Eastern Mediterranean layers of sapropels can be found in marine sediment cores that align with periods of maximum insolation in the precession cycle over Northern Africa.
During periods of high insolation the increased strength and northward progression of the North African Monsoonal Front causes very heavy rain along the upper and middle reaches of the Nile River basin.
These rains then flow northward and are discharged into the Eastern Mediterranean, where the large influx of nutrient rich fresh water causes a steep vertical salinity gradient.
[11] One of the key pieces of evidence linking the formation of sapropels to enhance discharge from the Nile River is the fact that they have occurred during both interglacial and glacial periods.
[13] Another key piece of evidence for a processional control on the North African Monsoon can be found in the deposits of freshwater diatoms in the tropical Atlantic.
Finally, when the prevailing northeasterly winds arrive during winter, the freshwater diatom deposits in the dried lake beds are picked up as dust and carried thousands of kilometers out into the tropical Atlantic.
For instance some ocean cores directly off the western coast of Africa show a mix of freshwater lake and river diatom species.
So for a core to accurately represent the diatom cycle of the Sahara it must be recovered from a region of the tropical Atlantic that has sufficient distance from the coast such that the impacts of river outflows are minimized.
When insolation in North Africa is at its peak during the precession cycle the easterly trade winds over the equatorial Atlantic are strongly diverted toward the Sahara.
On the other end of the spectrum when insolation in North Africa is at a minimum due to the precession cycle, the diversion of the easterly trade winds is relatively weak.
[16] The proof that this pattern of periodic weakening of the eastern equatorial Atlantic upwelling exists is found in deposits of surface dwelling planktic organisms in ocean sediment cores.
Such cores show that the relative abundance of warm and cold water planktic species vary with a consistent beat of 23,000 years, matching the 23,000-year precession insolation cycle.
The African Humid Period was also characterized by a network of vast waterways in the Sahara, consisting of large lakes, rivers, and deltas.
These river and lake systems provided corridors that allowed many animal species, including humans, to expand their range across the Sahara.
The onset and termination of the African Humid Period both occurred when the insolation cycle reached a value of roughly 4.2% higher than today.