North African climate cycles have a unique history that can be traced back millions of years. The cyclic climate pattern of the Sahara is characterized by significant shifts in the strength of the North African Monsoon. When the North African Monsoon is at its strongest, annual precipitation and consequently vegetation in the Sahara region increase, resulting in conditions commonly referred to as the "green Sahara". For a relatively weak North African Monsoon, the opposite is true, with decreased annual precipitation and less vegetation resulting in a phase of the Sahara climate cycle known as the "desert Sahara". 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. The parameters include the precession of the equinoxes, obliquity, and eccentricity as put forth by the Milankovitch theory. The precession of the equinoxes is regarded as the most important orbital parameter in the formation of the "green Sahara" and "desert Sahara" cycle. A January 2019 MIT paper in Science Advances shows a cycle from wet to dry approximately every 20,000 years.
Orbital Monsoon Hypothesis
Development 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, The hypothesis was later formally proposed and tested by the meteorologist John Kutzbach in 1981. 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.
Insolation 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. Due to variations in heat capacity, continents heat up faster than surrounding oceans during summer months when insolation is at its strongest and cool off faster than the surrounding oceans during winter months when insolation is at its weakest. The wind pattern that results from the continent/ocean insolation temperature gradient is known as a monsoon. Values of summer insolation are more important for a region's climate than winter values. This is because the winter phase of a monsoon is always dry. Thus the flora and fauna of a monsoonal climate are determined by the amount of rain that falls during the summer phase of the monsoon. 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. The result of this cycle of insolation is a waxing and waning in the strength of the monsoonal climates across the globe. A wide range of geologic evidence has shown that the North African Monsoon is particularly susceptible to insolation cycles, and long term trends in monsoonal strength can be linked to slow variations in insolation. However, the abrupt shifts back and forth from the "green Sahara" to the "desert Sahara" are not entirely explained by long term changes in the insolation cycle.
Precession Precession of the equinoxes on Earth can be divided up into two distinct phases. The first phase is created by a wobbling of the Earth's axis of rotation and is known as axial precession. While the second phase is known as apsidal precession or procession of the ellipse and is related to the slow rotation of the Earth's elliptical orbit around the Sun. When combined these two phases create a precession of the equinoxes that has a strong 23,000-year cycle and a weak 19,000-year cycle. Variations in the strength of the North African Monsoon have been found to be strongly related to the stronger 23,000-year processional cycle. 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. The amount of insolation is maximized for the northern hemisphere when the precession cycle is aligned such that the northern hemisphere points toward the sun at perihelion. According to the Orbital Monsoon Hypothesis this maximum in insolation increases the strength of monsoon circulations in the northern 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.
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