The Sahara Desert, one of most iconic deserts. Spanning 10 countries across North Africa it is the largest hot desert in the world. The name “Sahara” derives from the Arabic word for desert… so it’s really the desert desert, so I guess it’s better to call it “the Sahara”.
Anyways, back in 2014 I took a trip to Morocco and ended up camping in the Sahara near Tagounite. I came out there because I was Couchsurfing with a very friendly guy in Ouarzazate who said he had a friend out there who could be fun to meet and camp with. Getting there and back was quite a tale though, which involved taking shared taxis through the desert (7+ people in a 5-person car), running out of money and realizing the town I was headed to had no ATM, hitchhiking, a taxi driver asking me if I was coming to Tagounite to “buy a wife”, etc.
The trip ended up being a great experience. I rode a camel into the desert and hung out with some local Berbers who I had some great conversations with over tagine and mint tea. They played music in the evening and I fell asleep on some sand dunes with scarab beetles scurrying around. When I woke up I strolled across some sand dune ridges on my own and watched the sun come up.
But geologically it's quite an interesting place. Believe it or not, it was actually green and covered in plant life and rivers in the not too distant past. But when and how is that possible?
Deserts in themselves are defined as regions of land that are very dry due to the low amount of precipitation (i.e., rain, snow) and very little plant life. The largest desert in the world is technically Antarctica, not the Sahara.
How did this giant (hot) desert develop?
Initially land and marine studies suggest that North Africa became more arid (a process called aridification) since the start of the Quaternary, 2 to 3 million years ago (Zhang et al., 2014). The Quaternary is the most recent of three periods in the Cenozoic Era that spans from 2.58 million years ago to present. It is a period marked with shifts between glacial and non-glacial periods (think the movie Ice Age).
This geologic time scale is a modified version of the timescale found at: geologic time scale. geology (n.d.). https://geology.com/time.htm (last visited: 07-21-2023).
To understand how a Green Sahara could develop in this time period, you need to first look to natural Earth cycles. These cycles include:
Precession (which is the Earth “wobbling” while it rotates, cycling every ~26 thousand years).
Obliquity (the tilt angle of the Earth, which varies from 22.1 and 24.5 degrees, and goes on a cycle every ~41 thousand years).
Eccentricity (the shape of Earth’s orbit around the Sun, which cycles every 100 thousand years).
In modern times, while the Earth is orbiting around the sun, and it's closest to the sun, it's winter in the Northern Hemisphere (which is quite counterintuitive). The Southern Hemisphere gets more direct sunlight. However, in the past, differences in the precession ("wobble") would have positioned the Northern Hemisphere in a more direct path of the sun, causing warmer temperatures in the north. This naturally has an impact on the climate, and very important for our tale, monsoons.
Monsoons are essentially a dramatic seasonal change in the direction of the prevailing winds over a region. Land surfaces heat up more quickly than bodies of water, which leads to pressure differences that impact the direction of wind flow. This changes wind and rainfall patterns and affects whether a region is wet or dry.
Here is a very simplified image that I made to explain how this process normally works on a seasonal time scale (and yes, I know, it looks like a little kid made it!):
So when the natural Earth cycles change, it causes more solar energy (i.e., heat from the sun) to hit the earth (or less if the Earth is in the opposite side of the cycle), which impacts how warm the Northern Hemisphere of the planet is, which, in turn, impacts the monsoons. For the Sahara, this is the African Summer Monsoon. These monsoons would impact the modern Northeast Trade Winds. When the monsoon is at its weakest, the Sahara is a dry desert. When the monsoon is at its strongest, there is an increase in rainfall and more vegetation appears, causing the Sahara to become humid and green (referred to as the “green Sahara”). (Foley et al., 2003). The Sahara has been fluctuating between being a desert and green for at least the past 2 to 3 million years. The most recent Green Sahara is believed to have ended around 5000 years ago (Tierney et al., 2017).
However, the timing of the original formation of the desert is debated, as there is also a theory that it formed 7 million years ago rather than the 2 to 3 million. I’ll try to keep the explanation of this brief, but essentially this involves a prehistoric ocean referred to as the Tethys Sea (Tethys Ocean). This ocean existed between Gondwana (which included Africa) and Laurasia (which included Europe) and was essentially sandwiched between the two continents, as well as microplates, over time. Not entirely, but enough to eventually form the Mediterranean Sea. When the ancient Tethys Sea shrank in the Tortonian period about 7 million years ago it weakened the African summer monsoon, causing arid, desert conditions in northern Africa (Zhang et al., 2014).
Additional factors are believed to have helped the Green Sahara flourish. I will not go into it in detail here (this isn't a Wikipedia article after all!), but a few include albedo feedback (i.e., the amount of sunlight reflected from a surface), Arctic ice sheet fluctuations, changes in sea temperatures and increases in CO2.
So what did it look like?
It would have largely been grassy savannah with shrubs and trees (Tierney et al., 2011). Here is an image that I took of a South African savannah in the wet season in 2023 to give you a very generalized impression:
Although, keep in mind, I heard from others in South Africa that this savannah didn't look this "green" when it was in the dry season.
The landscape would also have contained rivers and lakes (although there is a debate as to if the lakes were mega-lakes or if they were smaller in size with wetlands (Quada et al., 2018)). Evidence for this Green Sahara have been found in many forms, including paleolake deposits, pollen and dust in marine sediment, archaeological remains, etc (e.g., Gasse et al., 2000; Hoelzmann et al., 2001; Kuper et al., 2006). I personally find the archaeological remains particularly interesting, as some of it includes cave art that depicts animals such as elephants, giraffes, hippos and other animals that you would never find in the Sahara today.
So there you have it. Yes, there is scientific evidence that points to it being green through different periods. It's a beautiful fascinatingly part of the world and I hope that you may have the change to experience it as well. Here's a photo of me riding a camel around.
Some articles that I referenced:
Foley, J. A., Coe, M. T., Scheffer, M., & Wang, G. (2003). Regime shifts in the Sahara and Sahel: interactions between ecological and climatic systems in Northern Africa. Ecosystems, 6, 524-532.
Gasse, F. (2000). Hydrological changes in the African tropics since the Last Glacial Maximum. Quaternary Science Reviews, 19(1-5), 189-211.
Hoelzmann, P., Keding, B., Berke, H., Kröpelin, S., & Kruse, H. J. (2001). Environmental change and archaeology: lake evolution and human occupation in the Eastern Sahara during the Holocene. Palaeogeography, Palaeoclimatology, Palaeoecology, 169(3-4), 193-217.
Kuper, R., & Kropelin, S. (2006). Climate-controlled Holocene occupation in the Sahara: motor of Africa's evolution. science, 313(5788), 803-807.
Tierney, J. E., Lewis, S. C., Cook, B. I., LeGrande, A. N., & Schmidt, G. A. (2011). Model, proxy and isotopic perspectives on the East African Humid Period. Earth and Planetary Science Letters, 307(1-2), 103-112.
Tierney, J. E., Pausata, F. S., & deMenocal, P. B. (2017). Rainfall regimes of the Green Sahara. Science advances, 3(1), e1601503.
Quade, J., Dente, E., Armon, M., Dor, Y. B., Morin, E., Adam, O., & Enzel, Y. (2018). Megalakes in the Sahara? A review. Quaternary Research, 90(2), 253-275.
Zhang, Z., Ramstein, G., Schuster, M., Li, C., Contoux, C., & Yan, Q. (2014). Aridification of the Sahara desert caused by Tethys Sea shrinkage during the Late Miocene. Nature, 513(7518), 401-404.
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