How Geography Turned the Sahara Green

in Greek mythology Python was the son of the god Helios who drove the Sun across the sky in a chariot every day Python begged and pleaded for his father to allow him to drive the chariot just for one day across the sky despite knowing how difficult this task truly was Helios eventually agreed and gave the reins over to fight on but being a mortal and much lighter than his immortal father Python lost control nearly immediately and the horses rampaged across the sky with the Sun still in the chariot threatening to come too close to the earth and burn the entire planet up to prevent this Zeus cast a thunderbolt striking pathan dead the son of Helios fell out of the sky and the earth was saved though it was said that during the wild ride the son was brought to close to Africa earning the skin of the Ethiopians which literally means burnt skin and scorched most of the lands here drying up all the rivers and lakes and turning much of North Africa from a pleasant farmable land into a desert in 1850 the German scholar Heinrich Barth was appointed to an expedition to explore the African interior leaving from Tripoli in modern day Libya the expedition was immediately met with the great challenge that is the Sahara Desert while others in the expedition were focused on opening commercial relations with the people here earth they used the trip to study the continent something which at the time hadn't really been done Europeans knew so little about Africa in fact that many of the places Barth came to greeted him as the first European to ever visit it was during this journey through the Sahara that Heinrich discovered something that didn't make any sense to him here in the barren desert with in some of its caves and rocky outcrops Heinrich discovered a number of prehistoric paintings indicating that people had once lived here even more confusing however was that these paintings didn't show the desert landscape that Barth was currently exploring but rather most of them depicted a grassland where numerous elephants giraffes hippos Oryx and antelopes were to be found despite conventional knowledge at the time telling that the Sahara Desert had always been well a desert the barth have discovered evidence to the contrary and wrote how the art he found bears testimony to a state of life very different from that which we are accustomed to see now in these regions what birth was describing and what he had found the first evidence for was the African humid period a time when North Africa was covered not by the sprawling sand dunes of the Sahara but rather in grasslands and even some forests or in other words a time when the Sahara Desert was really more like the Saharan savanna although he was able to figure out that the Sahara hadn't always been a desert Barth was unable to figure out why what he was missing at the time are known now as Milankovitch cycles what are these well you see while it might feel like the Earth's orbit is stable and unchanging on timescales covering thousands of years it can actually change quite a lot as the earth interacts with the gravity of the Moon the Sun and even Jupiter and Saturn the Earth's orbit cycles in three main ways axial eccentricity axial obliquity and axial precession today we only need to look at two first we need to look at orbital eccentricity this is basically how circular are really non circular of a path the plane it takes while going around the Sun and very slowly over the course of a hundred thousand years our eccentricity can change from zero point zero zero zero zero five five which is very nearly circular all the way up to zero point zero six seven nine which is relatively elliptical and then second there's axial precession with regards to the climate of the Sahara this is by far the most influential of these two cycles this is where Earth's rotational axis spins in a circle the same way the top of a gyroscope moves in a circular motion as it spins today our orbital eccentricity is about zero point zero one seven which is still mildly eccentric and means our orbit looks less like this and more like this because our orbit isn't a perfect circle that means the earth can be at different distances to the Sun at different times of the year when the earth is here in its orbit what's called the perihelion it is closest to the Sun and therefore receives more of this energy when it's all the way out here at its aphelion or furthest point from the Sun in its orbit it receives less energy the difference between these two isn't enormous and actually today when the earth is closest to the Sun it's actually winter in the Northern Hemisphere and the southern hemisphere is pointed out towards the Sun but due to axial precession this hasn't always been the case roughly ten thousand years ago this would have been reversed so that when the earth was at its closest to the Sun it was the northern hemisphere that pointed towards it what this resulted in was the northern hemisphere of receiving approximately seven percent more solar radiation than it does today now typically more sunlight would mean a higher average temperature which would usually make a place like this drier and even more barren but here in North Africa this increase in solar radiation triggered something even more influential the West African monsoon you see land has a lower thermal inertia than water or in other words land will heat up faster than the adjacent ocean will this resulted in the land in North Africa becoming much hotter in the summertime than the surrounding ocean because the land was hot the air directly above the land also became hot more so than the air over the ocean which stayed relatively cool when things like masses of air get hot they expand meaning they become less dense than the surrounding air and that means that the hot air over North Africa would have been less dense than the air over the Atlantic Ocean when fluids like air experienced density gradients like this they tend to move from high density to low which translates into massive air movement from the Atlantic Ocean over to northern Africa in a process known as wind today the winds typically blow east to west here from Africa to the ocean these are called the trade winds and their effects can be seen from space as dust from the Sahara gets blown out over the ocean but ten thousand years ago because of Earth's orbit these trade winds would have experienced a seasonal reversal and would have blown in the opposite direction bringing the moist ocean air over North Africa during the summer months and creating the African monsoon season the still happened today in places like India and on this map you can actually see their monsoon season as India starts off nearly entirely dry but for one season is just entirely covered in rain using current climate models a 7% increase in solar radiation would when including ocean feedbacks shows as high as a 50% increase in average annual precipitation over North Africa as a result of northern Africa at this time would receive an average of 500 millimetres of rain per year for a real-world comparison out bring Hindi a back up if we look at a map of India's average precipitation and select only the color that represents 500 millimetres we can see that a large part of India's center and southern region are in this range moreover India and the Sahara occupied just about the same exact latitude meaning with similar weather patterns much of the Sahara would look like what this area of India looks like now which if you're unfamiliar with the interior of India is mostly semi-arid tropical savanna so then if we use India as a reference and paint the Sahara to match I got something that looks like this though exactly how green each region was would have depended on the season becoming more Brown during the dry seasons and a lush green right after the monsoons for an average greenish brown color a small side note this greening of the Sahara actually extended much further into the Arabian desert as well as a result an estimated 13 point eight million square kilometers of desert would have been transformed into a savanna that's more surface area than all of Canada which is a lot there's only one problem with this and that's that a 50% increase in rainfall might sound like a lot but for a place like the central Sahara where the average annual precipitation can be as low as zero point zero nine centimeters a year a 50% increase would result in a grand total of zero point one four centimeters a year which is basically still nothing but even in the most remote places within the Sahara we've still found fossil and human evidence suggesting that it had been transformed as well so just looking at the orbital cycles of the earth isn't actually enough to Blayne how the Sahara became fully green instead now we need to talk about ecological facilitation which is when the presence of one species makes it possible for another species to also be present like a tree creating shade allowing for a more Sun sense that have planted like grass to grow underneath there are many different ways plants can facilitate the growth of others but perhaps the most ecologically far-reaching would be the effects of transpiration you see as rainfall increase to near the ocean and produced vegetation around the edges of the desert having more plants here actually increased the amount of moisture in the surrounding areas this works because plants don't only grow on the surface of the earth but also extend roots deep into the ground and retrieve water that would otherwise remain unused so as plant life expanded along the edges of the Sahara this opened up access to the stores of water beneath the surface and while plants do take in a great deal of water most of this water and I mean typically between 97 to 99 percent of the water taken up by plants isn't actually used by the plant at all but rather is released or transpired back into the atmosphere so if at first vegetation was only able to expand here on the very edge of the Sahara due to root uptake and the transpiration of water moisture was redistributed deeper into the Saharan interior allowing for essentially a domino effect to take place where some additional water allowed for the growth of more coastal plants which led to greater groundwater withdrawal which leads to increased transpiration which leads us back to the beginning and now we have a positive feedback loop but not even these Milankovitch cycles combined with the environmental engineering of plants were fully enough to transform the entire Sahara into savanna so lastly besides these there's a third and final major contributor geography or more specifically topography if we look at an actual topographic map of the continent we can see the Sahara actually has some variability in its elevation even featuring some mountains and even more importantly some depressions it was these depressions which collected the runoff from the West African monsoon and became lakes the biggest of these paleo mega lakes who would have been an expansion of the modern Lake Chad which would have reached a size comparable with the Caspian Sea the world's biggest Lake today the same thing happened over here across modern day Libya where Lake phezzan formed then again over here between Algeria and Tunisia where the chops Lake fill then further south and entirely within modern-day Algeria was another named Annette in lastly two places within the Nile Valley filled as well to become Lake Turkana and Lake Darfur besides these numerous small lakes and water systems dotted the entire land but were most concentrated in the western end these six new great lakes alongside the plentiful smaller ones served as large stores of water for the interior of the Sahara essentially transporting and holding excess moisture in the interior and together with the Earth's weird orbit and transpiration allowed the central Sahara to sustain a year-round vegetation unfortunately however as we all know Africa isn't like this today and that's because starting roughly five to seven thousand years ago Earth's axial precession began to point the southern hemisphere out during its perihelion instead of the northern hemisphere as a result Africa received less solar radiation and the entire cycle broke down returning the Saharan savanna back into the Saharan desert so after learning this the question most people want to ask next is what does the future look like for the Sahara to answer this I think it helps to look back even further in time starting around 22,000 years ago and going up to 11,000 years ago the earth was more or less in the same place it is now in terms of its axial precession but for some even more complicated reasons this meant Africa as a whole was actually far drier than it is today the Sahara this brown section was even larger than it is now reaching 200 miles further south while the Congo rainforest this dark green area barely remained a forest at all but starting roughly 10,000 years ago we have the African human period and of course the entire Sahara was Green that's what I've been talking about this whole time but this wasn't the only difference to be found across Africa further south of the Congo rainforest had expanded as far west Sierra Leone and Guinea and as far south as Botswana today the Congo rainforest is the second largest rainforest in the world behind the Amazon of course and measures 1.78 million square kilometers but back then the Congo rainforest would have been over four times bigger covering an area of around 7.8 million square kilometers for some perspective on how big that really is the biggest force on earth today is the Amazon rainforest which covers well maybe a bit less now that all these fires are burning but around 5.5 million square kilometres that means that during this African human period the Congo rainforest would have been bigger than any modern forest which I don't know is just crazy to think about the only real desert remaining on the continent was to be found in the southern Namib Desert which was so dry that it actually remained mostly unchanged during this time but around seven to five point five thousand years ago the Earth's axial precession swung back the other way an Africa dried out until we got to where we are today if we look at these events in sequence we can see a clear pattern one that points to a future regretting of the Sahara after all the axial precession of the earth is cyclical and therefore the process of greening and into certifying in Africa is also cyclical but then the next question must be when can we expect a return of the Saharan savanna and well if we purely used the procession of the earth as a guide the next time the North hemisphere will face the Sun during the Earth's perihelion is in about thirteen thousand years so not anytime soon but if you're like me and a little impatient well you're in luck because the earth is currently going a different type of global warming due to another process as humans continue to drive global temperatures up models show well just what you'd expect northern Africa receiving an increase in moisture and a potential expansion of vegetation from the south the degree to which this happens is of course up in the air until we figure out how much we want to warm the planet but a recent study found that human induced climate change could at most lead to 45% of the Sahara being covered by vegetation so if you really want to see the entire desert gun beneath grass and forest you'll just have to wait the thirteen thousand years hey so I hope you enjoyed the video and considering you're still watching I'd say you did if you want to help keep this channel running maybe you should check out my patreon where you can donate to do just that if you do you could even get your name up here like all these generous people if not well then you might want to at least subscribe so that you can see all future videos I make I should be coming out with another one next week so stay tuned thanks [Music]

Channel: Atlas Pro

Views: 1,906,014

Rating: 4.9165468 out of 5

Keywords: education, geography, science, atlaspro, sahara, africa, egypt, climate, desert, savanna, savannah, grass, grassland, sahel, african

Id: CM_QS984JKI

Length: 15min 17sec (917 seconds)

Published: Fri Sep 27 2019

Reddit Comments

[u/commander_nice]

This video is way too long for what it's trying to say. There's so much extraneous information.

TL;DW: The Sahara was not always desert, evidenced by ancient paintings in the region depicting a different climate (and I'm sure geologic evidence). It had large lakes, watersheds, grasslands, and even forests. Plants trap water in the ground which allows more plants to grow. And some of the water taken up by a plant evaporates and can then end up elsewhere on the continent. Also, the Congo rain forest was much bigger. And this all happened because Earth's orbit goes through cycles lasting ~25,000 years; the Earth was tilted the other way at perihelion. So the Sahara absorbed more solar radiation which made it hotter which brought in wind and (therefore) rain from the Atlantic ocean.

[u/MakeItMike3642]

so you are saying we should use giant boosters to change the axial precession and fight climate change?

[u/CrimsonBuc90]

could axial precession and eccentricity give some thought to the idea that humans arent causing climate change and it does have to do with natural cycles the earth goes though?