Sahara, Part I: Birth and Evolution of a Desert

For the second part of the Sahara series, see Crossing the Desert.

Tassili Du Hoggar, Ahaggar mountains, in the Sahara of Algeria.
Tassili Du Hoggar, Algeria, by George Steinmetz

Covering a vast area ranging from the Atlantic to the Red Sea, the largest hot desert in the world, the Sahara, is approximately the size of the entire United States or China. But its effects are felt well beyond its borders. Sand storms relentlessly gust through the countries of the Sahel and those of the southern coast. The Saharan sands travel all the way across the Atlantic ocean to fertilize the Amazon forest and across the Mediterranean sea to disperse orange snow over Europe.

What does this desert really look like? How and when did this desert form? Was it always as we know it today?

Geography of the Desert

Such a large area has, of course, a diverse landscape. Ergs, or sand dune regions, only make up around 20% of the desert. They are found in depressed pockets where the sand from other regions’ erosions gets caught and accumulates. Finding the ergs isn’t always an easy task: they move with the wind. The rest of the region is composed of dusty, rocky plains, mountains as high as 3,000 meters, volcanoes, and rocky plateaus.

Map of the geography of the Sahara, as well as North and West African climates
Geography of the Sahara and its surrounding climates

The desert receives less than 100 millimeters of rainfall a year, and its climate never varies: seasons only affect its outer edges. Some areas are naturally more arid than others. In the eastern part of the Sahara, in the Republic of Sudan, the evaporation is so intense that whatever rain falls doesn’t even reach the ground.

One of the most desolate parts of the Sahara is the Tanezrouft, a harsh, barren plain often called the “land of thirst”. There are no plants, no animal life of any sort: its only inhabitants are occasional nomadic Tuareg. A single road runs through connecting the southern city of Gao in Mali to the city of Béchar in Algeria. A fuel stop existed along this route, known as the Bidon 5, or Maurice Cortier. This area spans the borders of Algeria, Mali and Niger, between the Ahaggar mountains and the sandy Erg Chech.

  • 1000 meter high erg in the Ténéré, a region of the Sahara in Niger
    1000 meter high erg in the Ténéré, Niger, by Meteorite Recon.

But the desert isn’t completely barren. Some hardy plants do grow, sparsely and only in certain areas like around the mountains, or near the wadis — dry river beds that collect rainwater in temporary rivers during exceptional monsoon years.

In certain areas where a little rain does make it to the ground, the parched desert blooms with greenery. Plants grow, reproduce, and die, sometimes acting out their entire life cycle in the space of a single day.

At higher altitudes, in the Tibesti or Ahaggar mountains, a few humid tropical plants still cling to life, hidden in the crevices. These are remnants of a time, not so long ago, when the Sahara wasn’t nearly as dry as it is now…

Green and Yellow Sahara

Sahara Lily, a delicate flower growing in the sands of the Sahara.
Sahara Lily (Androcymbium gramineum) by Stefan Heyne.

Before 10,000 BC, the Sahara was the desert that we know today. But between 10,000 and 4,000 BC, the Sahara underwent a massive climactic change, when it became unrecognizable: it was green.

The water levels in the lakes rose, reaching their peak around 6000 BC. Lake Chad, known as ‘Mega-Chad’ in this period, was an inland sea rivaling the size of the modern Caspian sea, that extended over much of the southern Sahara. The soil, now adequately watered, was covered with short vegetation as we see it in the Sahel is today.

Wild animals moved into and repopulated the area — including elephants, rhinoceroses, giraffes, antelopes and hippopotamuses —, and so did humans. Stone tools and grindstones found in the area show that humans thrived on grains and some form of agriculture, but most people were hunter-gatherers, and sometimes, fishermen. Dated to around 6000 to 4000 BC, rock art suggests people captured, and probably tamed wild cattle. The region supported a semi-sedentary lifestyle, with most of the populations only moving to another area for part of the year, and returning at the end of the season.

From 4000 BC onwards, the Sahara’s desertification, which had started in the north, intensified and spread south. As wild animals moved away, the inhabitants came to rely more and more on their small domesticated stock (sheep and goats, mostly) and cattle, which afforded them greater food security as well as a more regular migration pattern between their traditional lands.

After 3000 BC, lakes and swamps started dwindling and disappeared from the landscape, taking with them the major aquatic resources available to the desert-dwellers. Now, with the disappearance of aquatic food sources and lack of irrigation for crops, people depended on their livestock more than ever. The more these animals grazed and trampled the plant cover, the more trees were cut and bush fires were set, the faster the savanna turned into a grass steppe, and then into a semi-desert. Without plants to retain and release moisture into the air, the soil becomes more and more desiccated. Without plants to hold the soil in place, the wind picks up the dirt layers and sandy soil underneath, carrying the particles that chip away at any rocks or mountainsides they encounter, creating even more sand.

Over the next two millennia, people were forced out of the desert as the conditions worsened. Some moved further into the desert massifs, others left the desert entirely, moving north towards the coastal Mediterranean lands, south into the Sahel and savanna regions and east to the Nile river valley.

Though accelerated by human activities, the desertification of the Sahara is a natural evolution of the region. Ever since the desert was born, it has oscillated between a green and yellow Sahara roughly on a 20,000 year cycle. It is projected to naturally become green again in about 15,000 years.

Yellow Sahara in the Ténéré, Niger, with erg sands being blown to the southwest.
Yellow Sahara in the Ténéré, Niger, by the European Space Agency.

This shift from yellow to green and back is caused by the wobble of the Earth on its axis as it orbits around the sun. This wavering tilt of the Earth changes the angle at which solar radiation enters the atmosphere, which has an impact on weather patterns. When the northern hemisphere receives more sunlight, the monsoons intensify, enough to shower the Sahara region and cause it to turn green. When the tilt changes, the monsoons decrease and vegetation starts disappearing. Without sufficient plants to retain water and release it back into the atmosphere, the rain eventually lessens until it comes to a virtual stop.

The other regions of the world aren’t nearly as sensitive to the relatively small wobble of the Earth, so why has the Sahara become so unstable, so extreme? Just as the desert wasn’t always a desert, the Sahara region wasn’t always this changeable.

Birth of the Sahara

Millions of years ago, this region of the world was permanently green, with a tropical climate similar to that of present-day central Africa. The monsoons watered the land in abundance and reliably, forming an environment humid tropical plants could take full advantage of.

To explain the start of the Sahara’s green/yellow cycle, scholars initially proposed the idea that the changeable Sahara we know today was born around 3 to 3.3 million years ago, after the first glacial period in the northern hemisphere, and the subsequent installation of the Glacial-Interglacial cycle. When a lot of the water is locked up in glaciers up north, the monsoon is pushed further south, which would explain the aridification of the Sahara.

But in 2002, archaeologists discovered fossils of sand dunes north of Lake Chad, dating back much further, to 7 million years ago. This finding prompted a study in 2014, showing, through modern climate modeling, that the Sahara is indeed much older than we previously thought and that the birth of a desert is closely entwined with the death of an ocean.

The Sahara desert was never actually underwater, but 40 millions of years ago, it was near an immense inland sea. It’s not so far away in time that the continents were in different positions, they were just about where we know them today. This great sea, named Tethys, was an inland sea, and covered the regions of eastern Europe and all the west of Asia. Over the last 40 million years, the Tethys started shrinking, little by little, because of plate tectonics. Now, the only relics left of it are the Mediterranean Sea, the Black Sea and the Caspian Sea. Over the next few million years from today, the Mediterranean will also disappear because of the northern movement of the African tectonic plaque.

Because of its sheer size, the Tethys Sea had major impacts on the climate around it, especially in generating monsoons. As the sea withdrew, the atmospheric landscape changed drastically. The monsoons in India and Asia intensified, and the African ones changed until zones that were previously well watered, like the Sahara, became desertic around 7 million years ago. This correlates with the evolution of man, between 7 and 2 million years ago, and may have had an impact on the evolution of our species.

Schematic representation of the Tethys sea and the emergence of the Sahara desert over the last 7 million years, as found in a 2014 study based on climactic simulations.
Schematic representation of the Tethys sea and its surrounding paleoclimate in the (a) Late Oligocene [27 to 23 million years ago], (b) Early Miocene [23 to 15 million years ago], (c) Late Miocene [15 to 7 million years ago] and (d) modern times. The schematic shows the West Tethys Sea (WT), the Paratethys Sea (PT), the proto-Mediterranean Sea (PM) and the Arabian penninsula (AP). The hashed areas show the semiarid steppe climate (orange) and the arid desert climate (red). The black line represents the simulated mean Intertropical Convergence Zone (ITCZ) in summer. (Image Source)

Battling the Desert

15,000 years is a long time to wait for the Sahara to turn green again. Over the years and centuries, people have contemplated many ambitious projects to turn the arid region into a vegetal paradise.

The Sahara Sea Project

In the late 1800s, the French, who were already tightening their grip on their African colonies on the eve of the Scramble for Africa, had an ambitious plan to create an artificial inland sea, the Sahara Sea. After the raging success of the Suez Canal project, Ferdinand de Lesseps felt more than up to the task when François Élie Roudaire, a military geographer tasked with mapping the more inaccessible parts of northern Africa, proposed the plan in 1874.

A large lake or sea in the Sahara would increase rainfall through the evaporation cycle, and revive the vegetation in the region. Roudaire, wrote that “the Sahara is the cancer eating away at Africa. We can’t cure it; therefore, we must drown it”.

When Roudaire went to survey the area, he found, to his consternation, that the ancient sea beds he was planning to flood were significantly above sea level. The project would require much more financing then anticipated, for a more modest result. The French government abandoned the plan, and the private company Roudaire created failed to make much progress and dissolved some time after his death.  The idea of a Sahara Sea intrigued Jules Vernes, who wrote of it in two of his books, particularly L’Invasion de la mer (1905; a.k.a. The Invasion of the Sea), whose plot revolves around this idea.

This idea of a Sahara Sea never entirely died, however.  In 1919, there were other plans to insert canals deep into the Tunisian interior. Versions of the plan kept popping up in France as late as the 1950s. Americans proposed using nuclear explosives to create an artificial lake in Tunisia in their Qattara Depression Project, an idea that is being considered again in recent years.

The Great Green Wall Project

Image of a hand planting tree sprouts in the Sahara as part of the Great Green Wall forestation project.
Tree planting, by Tony Karumba.

In the more recent past, one idea to combat climate change, desertification, drought and famine was revived as the Great Green Wall project to enclose the Sahara.

Initially conceived as a project to plant a line of trees to defend against the encroaching desert along its southern border, the project has changed course since then. Scientific consensus says the Sahara is not actually expanding, if anything, the Sahel-Sahara border has been retreating north. More than 80% of the trees initially planted through the initiative died within two months without water or protection. But in Niger, local farmers reclaimed existing trees and nursed them back to health, without any government support. The restorative techniques the farmers used saved around 200 million trees, at the foot of which they grew sufficient crops to feed 2.5 million people.

This shining example of grassroots environmental restoration led the Great Green Wall to move away from forestry and towards supporting the initiatives of local populations, farmers in particular. Now, the tree-shield is taking less the form of a line, and more that of a mosaic of indigenous, hardy trees, flora and crops that can not only curb the punishing dust storms and improve rainfall in the affected regions, but also secure food and protect the cultural heritage of local peoples. More than 20 countries in the Sahelo-Saharan region have banded together under this initiative.

Sources and Further Reading

Books and Articles:

 

Web:

 

Podcast:

  • [French] Martin, N. (2018, March 29). Sahara : une histoire d’eau (avec Contoux, C., Lézine, A.-M. & Ramstein, G.). Retrieved July 13, 2018, from France Culture: La Méthode Scientifique.

 

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