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Green Sahara or the origins of ancient Egypt

Green Sahara or the origins of ancient Egypt

The recent weather anomalies and the ongoing debate on the climate change inspire reflection on a wide variety of factors responsible for these procecsses. The only reasonable course of action is to look at both past and current trends and draw conclusions for the future, claims Prof. Mariusz Kędzierski from the JU Institute of Geological Sciences.

 

Cave of the Swimmers (Photo: Paul Ealing 2011 via Compfight cc)

 

This sentence could seem rather vague, because of the enormous network of interconnected causes and effects that determine the climate of our planet. One of the relatively recent examples of a climate change which has not yet been fully understood is the so-called African humid period, related to the Holocene climatic optimum.

Some readers may remember The English Patient, a film which begins with a scene of painting figures of swimmers known from the cave where the main female character dies later in the movie. The Cave of the Swimmers is located in South-West Egypt, on the Gilf Kebir plateau, next to the Libyan border. Besides the abovementioned swimmers, the rock drawings show numerous animals, including elephants, giraffes, antelopes, cattle, and goats as well as hunters. The cave was discovered in 1933 by the Hungarian traveller László Almásy, who later wrote that these drawings showed how the desert plateau had once teemed with life. But before the World War II the awareness of climate changes was low and hence Almásy’s suggestions ware treated as mere fantasies. It seemed inconceivable that the world’s largest desert might have been a Garden of Eden not so long before.

The Gilf Kebir plateau is just one of many such places in Sahara. In mid-19th century, the German explorer of Africa Heinrich Barth spotted Saharan petroglyphs while travelling from Tripoli to Timbuktu. As the first European to notice them, he later recollected that the abundance of life depicted there was in sharp contrast with the desert landscape. He also found it difficult to imagine that Sahara once used to look completely differently.

Charts showing changes that occurred during the last 20 thousand years: A) amount of sunlight reaching the Earth surface B) African Lake Levels C) amount of water flowing out of the Niger river D) amount of dust in marine core sediment collected next to the Mauritanian coast E) changes in the composition of plant wax hydrogen isotope from Lake Tanganika (deMenocal & Tierney, 2012).

Today we know that Sahara has a lot of sites with petroglyphs showing rich animal life. Actually, this region contains the world’s largest deposit of rock art. All the works originated from about 9-6 thousand years ago and they all show Sahara as a lush savanna or bushy steppe. Currently it is also known that the petroglyphs do not lie. Seven thousand year ago Sahara was indeed a green and fertile land. Archaeologists refer to this period as the Neolithic Subpluvial or the African Humid Period. Geologists and geographers classify it as part of the Holocene.

The Holocene started at the end of the last glacial period, that is, 10-11 thousand years ago and formally continues to this day, in spite of the attempts to introduce the concept of Anthropocene. Obviously, the epoch was not climatically homogenous and consisted of several warmer and colder periods. Apart from the ongoing climate changes, which are very short even from the perspective of the Holocene, the hottest period, known as the Atlantic Period is dated from 8-5 thousand years ago, which coincides with the aforementioned African Humid Period.

The results of Saharan lake sediment studies indicate significantly higher water levels and much higher number of lakes in that region in the Atlantic Period. For instance, the currently shrinking Lake Chad was then larger than the Caspian Sea (it used to cover over 400 thousand km2). There were several such megalakes in the Sahara region, such as Fezzan lake in Libya, Chotts lakes w Algeria or Turkana lake in Kenya.

The humid Sahara resembled the Sahel region located more to the south and covered with savanna. It was inhabited by numerous animal species, including the now extinct giraffe-like  Sivatherium or buffalo-like Pelorovis. It is hard to imagine that the Tibesti Mountains – today as dry as bone – were once covered with oak and walnut forests. Alder and Elm trees also grew there in addition to olive trees and junipers. The valleys were filled by rivers teeming with fish. The picture was completed by hunters-gatherers, who in their leisure created rock paintings bearing witness to the Saharan idyll. 

Studies of the coastal sediments of the Atlantic have also shown that the humid Sahara produced much less dust, which proves the presence of thick vegetation significantly reducing wind erosion. The analysis of core sediments from the ocean bottom indicates that the Sahara died up rapidly. In just 100-200 years it started to produce as much desert dust as it does today. The humid savanna environment was replaced by an extremely dry desert. A similar process occurred on the Arabian Peninsula.

Ennedi – one of few surviving relics of the once green Sahara (Photo: _desertsky via Compfight cc)

How did it all happen? The greening up of the Sahara was related to the moving of the monsoon zone further to the north of the equator, so that it covered the Sahara. This coincides with the maximum sunlight period calculated for 20°N based on the changes in Earth’s orbit parameters. The Earth’s orbit undergoes cyclical variations, known as Milankovitch cycles: the eccentricity cycle (about 400 or 100 thousand-year periods), the axial tilt cycle (about 40 thousand years), and the precession cycle (about 26 thousand years). In the case of the African Humid Period, a strong impact of the Earth’s precession on the amount of sunlight reaching the planet’s surface has been observed.    

It has been calculated that 10 thousand years ago, during the advent of the Holocene, 480 W/m2 reached 20°N. Currently it is 480 W/m2. Greater insolation and prolonged summer led to the rise in temperature differences between the land and ocean. In the summer, the Sahara heated up faster than the ocean and became hotter, which resulted in the formation of a wide belt of low pressure over the land, sucking up humid air from the tropical Atlantic. In winter, an opposite phenomenon occurred, since land areas cool down faster than oceans (thermal inertia). Simple models taking into account only the effect of the atmosphere indicate that the increase of summer insolation by 7% (which occurred in the Atlantic Period) led to the increase of monsoon precipitation by at least 17%.

The presented charts clearly illustrate that the climate change in the Sahara occurred rapidly. The shift from the over five thousand year long humid period to an arid period took just a little more than 100 years. Petroglyphs are one of few things that remained from what once was a Garden of Eden. The population that once inhabited Sahara was forced to move to other more welcoming places. Some of these people settled down in the Nile valley and gave birth to the ancient Egyptian civilization. And this is the most important lesson for the future – the human history is largely determined by climate changes.

Original text by Prof. Mariusz Kędzierski from the JU Institute of Geological Sciences is available on the website www.nauka.uj.edu.pl

Sources:

  • top left photo: pteroglyphs from Gilf Kebir. Photo: Paul Ealing 2011 via Compfight cc
  • deMenocal, P., Ortiz, J., Guilderson, T., Adkins, J., Sarnthein, M., Baker, L., & Yarusinsky, M. (2000). Abrupt onset and termination of the African Humid Period: Quaternary Science Reviews, 19 (1-5), 347-361 DOI: 10.1016/S0277-3791(99)00081-5
  • deMenocal, P. B. & Tierney, J. E., 2012. Green Sahara: African Humid Periods Paced by Earth's Orbital Changes. Nature Education Knowledge 3(10):12.
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