North African climate cycles
Encyclopedia
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 subsequent 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
due to slow shifts in Earth’s orbital parameters. These parameters include the precession of the equinoxes
, obliquity, and eccentricity
as put forth by Milankovitch theory. The procession of the equinoxes is regarded as the most important orbital parameter in the formation of the “green Sahara” and “desert Sahara” cycle.
, 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 regions 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.
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.
. The current tilt of the Earth’s axis is roughly 23.5°. However, over long periods of time the tilt of the Earth’s axis of rotation changes because of the uneven distribution of mass across the planet and gravitational interactions with the sun
, moon
, and planets. Due to these interactions the tilt of the Earth’s axis of rotation varies between 22.2° and 24.5° on a 41,000 year cycle.
Modulation of the precession driven insolation cycle is the primary impact of obliquity on the North African Monsoon. 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. This evidence requires complex feedback mechanisms to explain since the strongest impact of obliquity on insolation is found in the high latitudes. Two possible mechanisms for the existence of an obliquity tracer found in the Eastern Mediterranean Aeolian dust deposits have been proposed. The first of which suggests that at times of higher obliquity the temperature gradient between the poles and the equator in the southern hemisphere is greater during boreal
summer (summer in the northern hemisphere). As a result of this gradient the strength of the North African Monsoon increases. A second theory that may explain the existence of an obliquity signature in the North African climate record suggests that obliquity maybe related to changes in the latitude of the tropics. The latitudinal extent of the tropics is roughly defined by the maximum wondering path of the thermal equator
. An area that today is located between the Tropic of Capricorn
and the Tropic of Cancer
. However as the obliquity changes, the overall wondering path of the thermal equator shifts between 22.2° and 24.5° north and south. This wondering may affect the positioning of the North African Summer Monsoon Front and thus impact the perceived strength of the North African Monsoon. Further confirmation of the impacts of obliquity on the North African Monsoonal have been provided through a global fully coupled atmosphere–ocean–sea ice climate model
, which confirmed that precession and obliquity can combine to increase precipitation in North Africa through insolation feedbacks.
. At the same time the other hemisphere will have warm summers and cool winters due to a smaller than average yearly insolation gradient.
Like obliquity, eccentricity is not considered to be a primary driver of the strength of the North African Monsoon. Instead eccentricity modulates the amplitude
of the insolation maximums and minimums that occur due to the precession cycle. Strong support for the modulation of the precession cycle by eccentricity can be found in Aeolian dust deposits in the Eastern Mediterranean. Upon close examination it can be shown that periods of low and high hematite
fluxes correspond to both the 100,000 year and 400,000 year eccentricity cycles. It is believed that this evidence for the eccentricity cycles in the dust record of the Eastern Mediterranean indicates a stronger northward progression of the North African Monsoonal Front during times when the eccentricity and precession insolation maximums coincide. The modulating effect of eccentricity on the precession cycle has also been shown using a global fully coupled atmosphere–ocean–sea ice climate model.
is tempered by the slow melting of polar ice sheets. Thus the full strength of the monsoonal pattern is not observed until the polar ice sheets have become so small that there impact on the development of yearly monsoons is minimal. The second alternative solution proposes that relatively cool tropical oceans left over from glaciation may initially slow the development of monsoons globally, since colder oceans are less potent sources of moisture.
is shut off and the water column becomes stably stratified. Once this stable stratification occurs, bottom waters in the Eastern Mediterranean quickly become depleted in oxygen and the large influx of pelagic organic matter from the nutrient rich surface waters is preserved as sapropel formations. 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. Therefore the formation of sapropels must be linked to fresh water discharge from the Nile River and not melt water from dissipating ice sheets.
, which has a maximum depth of 11 m and an area of only 1,350 km2. Satellite imagery
of the shorelines of ancient Lake Megachad reveal that the lake has existed under two distinctive wind regimes, one northeasterly and southwesterly. The northeasterly wind regime is constant with today’s wind patterns and is characteristic of weak monsoonal flow. Meanwhile, the southwesterly wind regime is characteristic of a stronger monsoonal flow.
zone can also be used to support a cycle of the North African Monsoon that is regulated by the precession cycle. 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. This diversion weakens the equatorial upwelling zone in the eastern equatorial Atlantic, resulting in warmer waters in the pelagic. 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. Due to this the region of upwelling in the eastern equatorial Atlantic remains strong and the waters in the pelagic zone are cooler. 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.
. Meanwhile, the Sahel
region south of the Sahara was mostly savanna. Today the Sahara region is mostly desert and the Sahel is characterized by steppe conditions. The African Humid Period was also characterized by a network of vast waterways in the Sahara, consisting of large lakes, rivers, and deltas. The four largest lakes were Lake Megachad, Lake Megafezzan, Ahnet-Mouydir Megalake, and Chotts Megalake. Large rivers in the region included the Senegal River
, Nile River, Sahabi River, and Kufra River. These river and lake systems provided corridors that allowed many animal species, including humans, to expand their range across the Sahara.
Climate
Climate encompasses the statistics of temperature, humidity, atmospheric pressure, wind, rainfall, atmospheric particle count and other meteorological elemental measurements in a given region over long periods...
pattern of the Sahara
Sahara
The Sahara is the world's second largest desert, after Antarctica. At over , it covers most of Northern Africa, making it almost as large as Europe or the United States. The Sahara stretches from the Red Sea, including parts of the Mediterranean coasts, to the outskirts of the Atlantic Ocean...
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 subsequent 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
Insolation
Insolation is a measure of solar radiation energy received on a given surface area in a given time. It is commonly expressed as average irradiance in watts per square meter or kilowatt-hours per square meter per day...
due to slow shifts in Earth’s orbital parameters. These parameters include the precession of the equinoxes
Precession of the equinoxes
In astronomy, axial precession is a gravity-induced, slow and continuous change in the orientation of an astronomical body's rotational axis. In particular, it refers to the gradual shift in the orientation of Earth's axis of rotation, which, like a wobbling top, traces out a pair of cones joined...
, obliquity, and eccentricity
Eccentricity
Eccentricity or eccentric may refer to:* Off-center* Eccentricity , odd behavior on the part of a person, as opposed to being "normal"* Eccentricity of a vertex in a graph...
as put forth by Milankovitch theory. The procession of the equinoxes is regarded as the most important orbital parameter in the formation of the “green Sahara” and “desert Sahara” cycle.
Development
The idea that changes in insolation caused by shifts 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 capacityHeat capacity
Heat capacity , or thermal capacity, is the measurable physical quantity that characterizes the amount of heat required to change a substance's temperature by a given amount...
, 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
Monsoon
Monsoon is traditionally defined as a seasonal reversing wind accompanied by corresponding changes in precipitation, but is now used to describe seasonal changes in atmospheric circulation and precipitation associated with the asymmetric heating of land and sea...
. Values of summer insolation are more important for a regions 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
Procession 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 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.
Obliquity
Obliquity, otherwise known as tilt, refers to the angle that the Earth’s axis of rotation makes with a line that is perpendicular to the Earth’s orbital planeOrbital plane
Orbital plane may refer to:*Orbital plane *In anatomy, it refers to a specific area of the maxilla...
. The current tilt of the Earth’s axis is roughly 23.5°. However, over long periods of time the tilt of the Earth’s axis of rotation changes because of the uneven distribution of mass across the planet and gravitational interactions with the sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
, moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...
, and planets. Due to these interactions the tilt of the Earth’s axis of rotation varies between 22.2° and 24.5° on a 41,000 year cycle.
Modulation of the precession driven insolation cycle is the primary impact of obliquity on the North African Monsoon. 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. This evidence requires complex feedback mechanisms to explain since the strongest impact of obliquity on insolation is found in the high latitudes. Two possible mechanisms for the existence of an obliquity tracer found in the Eastern Mediterranean Aeolian dust deposits have been proposed. The first of which suggests that at times of higher obliquity the temperature gradient between the poles and the equator in the southern hemisphere is greater during boreal
Boreal
Boreal may refer to*Boreal ecosystem**Boreal climate, the climate found in a region of boreal forests, and designated Dfc, Dwc or Dsc in the Köppen climate classification scheme.*boreal forest*Boreal forest of Canada*Boreal Bluet...
summer (summer in the northern hemisphere). As a result of this gradient the strength of the North African Monsoon increases. A second theory that may explain the existence of an obliquity signature in the North African climate record suggests that obliquity maybe related to changes in the latitude of the tropics. The latitudinal extent of the tropics is roughly defined by the maximum wondering path of the thermal equator
Thermal equator
The thermal equator is a belt encircling the Earth, defined by the set of locations having the highest mean annual temperature at each longitude around the globe...
. An area that today is located between the Tropic of Capricorn
Tropic of Capricorn
The Tropic of Capricorn, or Southern tropic, marks the most southerly latitude on the Earth at which the Sun can be directly overhead. This event occurs at the December solstice, when the southern hemisphere is tilted towards the Sun to its maximum extent.Tropic of Capricorn is one of the five...
and the Tropic of Cancer
Tropic of Cancer
The Tropic of Cancer, also referred to as the Northern tropic, is the circle of latitude on the Earth that marks the most northerly position at which the Sun may appear directly overhead at its zenith...
. However as the obliquity changes, the overall wondering path of the thermal equator shifts between 22.2° and 24.5° north and south. This wondering may affect the positioning of the North African Summer Monsoon Front and thus impact the perceived strength of the North African Monsoon. Further confirmation of the impacts of obliquity on the North African Monsoonal have been provided through a global fully coupled atmosphere–ocean–sea ice climate model
Climate model
Climate models use quantitative methods to simulate the interactions of the atmosphere, oceans, land surface, and ice. They are used for a variety of purposes from study of the dynamics of the climate system to projections of future climate...
, which confirmed that precession and obliquity can combine to increase precipitation in North Africa through insolation feedbacks.
Eccentricity
Eccentricity is a measure of the deviation of the Earth’s orbit from a perfect circle. If the Earth’s orbit is a perfect circle then the eccentricity would have a value of 0, and eccentricity value of 1 would indicate a parabola. The Earth has two cycles of eccentricity that occur on cycles of 100,000 and 400,000 years. Over the years the Earth’s eccentricity has varied between 0.005 and 0.0607, today the eccentricity of Earth’s orbit is approximately 0.0167. While the value of eccentricity does impact the distance of the Earth from the Sun, its primary impact on insolation comes from its modulating effect on the procession cycle. For example when the orbit of the Earth is highly elliptical one hemisphere will have hot summers and cold winters, corresponding to a larger that average yearly insolation gradientGradient
In vector calculus, the gradient of a scalar field is a vector field that points in the direction of the greatest rate of increase of the scalar field, and whose magnitude is the greatest rate of change....
. At the same time the other hemisphere will have warm summers and cool winters due to a smaller than average yearly insolation gradient.
Like obliquity, eccentricity is not considered to be a primary driver of the strength of the North African Monsoon. Instead eccentricity modulates the amplitude
Amplitude
Amplitude is the magnitude of change in the oscillating variable with each oscillation within an oscillating system. For example, sound waves in air are oscillations in atmospheric pressure and their amplitudes are proportional to the change in pressure during one oscillation...
of the insolation maximums and minimums that occur due to the precession cycle. Strong support for the modulation of the precession cycle by eccentricity can be found in Aeolian dust deposits in the Eastern Mediterranean. Upon close examination it can be shown that periods of low and high hematite
Hematite
Hematite, also spelled as haematite, is the mineral form of iron oxide , one of several iron oxides. Hematite crystallizes in the rhombohedral system, and it has the same crystal structure as ilmenite and corundum...
fluxes correspond to both the 100,000 year and 400,000 year eccentricity cycles. It is believed that this evidence for the eccentricity cycles in the dust record of the Eastern Mediterranean indicates a stronger northward progression of the North African Monsoonal Front during times when the eccentricity and precession insolation maximums coincide. The modulating effect of eccentricity on the precession cycle has also been shown using a global fully coupled atmosphere–ocean–sea ice climate model.
Lag
One key issue with the Orbital Monsoon Hypothesis is that a detailed inspection of climate record indicates that there is a 1000 to 2000 year lag in the observed North African Monsoon maximum compared to the predicted maximum. This issue occurs because the Orbital Monsoon Hypothesis assumes that there is an instantaneous response by the climate system to changes in insolation from orbital forcing. Fortunately there are a number of fixes for this problem. The most reasonable fix can be shown through a simple analog to today’s climate. Currently the peak in solar radiation occurs on June 21, but the peak of the summer monsoon in North Africa occurs a month later in July. A one month lag such as this should be represented by roughly a 1500 to 2000 year lag in the monsoonal circulation maximum, because a July insolation maximum in a 19,000 to 23,000 year precession cycle occurs roughly 1500 to 2000 years after the June insolation maximum. Two other possible explanations for the observed lag in the data have been put forward. The first suggest that the development of the monsoons in the subtropicsSubtropics
The subtropics are the geographical and climatical zone of the Earth immediately north and south of the tropical zone, which is bounded by the Tropic of Cancer and the Tropic of Capricorn, at latitudes 23.5°N and 23.5°S...
is tempered by the slow melting of polar ice sheets. Thus the full strength of the monsoonal pattern is not observed until the polar ice sheets have become so small that there impact on the development of yearly monsoons is minimal. The second alternative solution proposes that relatively cool tropical oceans left over from glaciation may initially slow the development of monsoons globally, since colder oceans are less potent sources of moisture.
Sapropels
Sapropels are dark organic rich marine sediments that contain greater than 2% organic carbon by weight. 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. Such an alignment can be explained by a link to the North African Monsoon. 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. As a result thermohaline convectionConvection
Convection is the movement of molecules within fluids and rheids. It cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids....
is shut off and the water column becomes stably stratified. Once this stable stratification occurs, bottom waters in the Eastern Mediterranean quickly become depleted in oxygen and the large influx of pelagic organic matter from the nutrient rich surface waters is preserved as sapropel formations. 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
Interglacial
An Interglacial period is a geological interval of warmer global average temperature lasting thousands of years that separates consecutive glacial periods within an ice age...
and glacial periods. Therefore the formation of sapropels must be linked to fresh water discharge from the Nile River and not melt water from dissipating ice sheets.
Paleolakes
Evidence for the existence of large lakes in the Sahara can be found and interpreted from the geologic record. These lakes fill as the precession cycle approaches the insolation maximum and are then depleted as the precession cycle nears the insolation minimum. The largest of these paleolakes was Lake Megachad, which at its peak was 173 m deep and covered an area of roughly 400,000 km2. Today the remnants of this once massive lake are known as Lake ChadLake Chad
Lake Chad is a historically large, shallow, endorheic lake in Africa, whose size has varied over the centuries. According to the Global Resource Information Database of the United Nations Environment Programme, it shrank as much as 95% from about 1963 to 1998; yet it also states that "the 2007 ...
, which has a maximum depth of 11 m and an area of only 1,350 km2. Satellite imagery
Satellite imagery
Satellite imagery consists of photographs of Earth or other planets made by means of artificial satellites.- History :The first images from space were taken on sub-orbital flights. The U.S-launched V-2 flight on October 24, 1946 took one image every 1.5 seconds...
of the shorelines of ancient Lake Megachad reveal that the lake has existed under two distinctive wind regimes, one northeasterly and southwesterly. The northeasterly wind regime is constant with today’s wind patterns and is characteristic of weak monsoonal flow. Meanwhile, the southwesterly wind regime is characteristic of a stronger monsoonal flow.
Freshwater diatoms
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. Ocean cores from the tropical Atlantic have been found to have distinct layers of the freshwater diatom Aulacoseira Granulata also known as Melosira Granulata. These layers occur on a 23,000 year cycle that lags the maximum in precession insolation by roughly 5000 to 6000 years. To explain this cycle freshwater diatoms deposits we have to look inland at the Sahara region of Africa. Around the time of the insolation maximum in the precession cycle the North African Monsoon is at its strongest and the Sahara region becomes dominated by large monsoonal lakes. Then as time progress toward the insolation minima, these lakes begin to dry out due to weakening North African Monsoon. As the lakes dry up thin deposits sediment containing freshwater diatoms are exposed. 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. From this series of events the reason for 5000 to 6000 year delay in the freshwater diatom deposits is evident, since the North African Monsoon must become sufficiently weak before the monsoonal lakes in the Sahara begin to dry up and expose potential freshwater diatom sources. One key factor that must be noted with freshwater diatom deposits is species identification. For instance some ocean cores directly of 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.Eastern equatorial Atlantic upwelling
Observed variations in the strength of the eastern equatorial Atlantic upwellingUpwelling
Upwelling is an oceanographic phenomenon that involves wind-driven motion of dense, cooler, and usually nutrient-rich water towards the ocean surface, replacing the warmer, usually nutrient-depleted surface water. The increased availability in upwelling regions results in high levels of primary...
zone can also be used to support a cycle of the North African Monsoon that is regulated by the precession cycle. 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. This diversion weakens the equatorial upwelling zone in the eastern equatorial Atlantic, resulting in warmer waters in the pelagic. 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. Due to this the region of upwelling in the eastern equatorial Atlantic remains strong and the waters in the pelagic zone are cooler. 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.
Climatology
The African Humid Period occurred between 14,800 and 5,500 years ago, and was the last occurrence of a “green Sahara”. Conditions in the Sahara during the African Humid Period were dominated by a strong North African Monsoon, resulting in larger annual rainfall totals compared to today’s conditions. With the increased rainfall, the vegetation patterns North Africa were nothing like what we see today. The majority of the Sahara region for instance was characterized by expansive grasslands, also known steppeSteppe
In physical geography, steppe is an ecoregion, in the montane grasslands and shrublands and temperate grasslands, savannas, and shrublands biomes, characterized by grassland plains without trees apart from those near rivers and lakes...
. Meanwhile, the Sahel
Sahel
The Sahel is the ecoclimatic and biogeographic zone of transition between the Sahara desert in the North and the Sudanian Savannas in the south.It stretches across the North African continent between the Atlantic Ocean and the Red Sea....
region south of the Sahara was mostly savanna. Today the Sahara region is mostly desert and the Sahel is characterized by steppe conditions. The African Humid Period was also characterized by a network of vast waterways in the Sahara, consisting of large lakes, rivers, and deltas. The four largest lakes were Lake Megachad, Lake Megafezzan, Ahnet-Mouydir Megalake, and Chotts Megalake. Large rivers in the region included the Senegal River
Sénégal River
The Sénégal River is a long river in West Africa that forms the border between Senegal and Mauritania.The Sénégal's headwaters are the Semefé and Bafing rivers which both originate in Guinea; they form a small part of the Guinean-Malian border before coming together at Bafoulabé in Mali...
, Nile River, Sahabi River, and Kufra River. These river and lake systems provided corridors that allowed many animal species, including humans, to expand their range across the Sahara.