The Sahara Desert was once a green savannah thousands of years ago!
The Sahara Desert was once a green savanna about 11 thousand years ago, with elephants and giraffes present! Research explains why.
Has it never crossed your mind that the Sahara Desert used to be a green savannah? But recently, a team of researchers developed a climate model that simulated the effects of atmospheric circulation on the Sahara and the impacts of vegetation in the rains.
Can you imagine that the Sahara desert was once a savannah?
Algeria's Tassili N'Ajjer plateau is the largest national park in Africa, and among its vast sandstone formations is perhaps the largest art museum in the world. More than 15,000 prints and paintings are exhibited there, some up to 11,000 years old, representing a unique ethnological and climatological record.
Interestingly, these images do not portray the arid landscape! Instead, they depict a very vibrant savannah inhabited by elephants, giraffes, rhinos and hippos.
Such rock art is an important record of the past environmental conditions that prevail in the Sahara, the largest hot desert. The engravings are dated from 6 to 11,000 years ago, possibly called the green Sahara or humid period of North Africa. And the existing climatological evidence indicates that during this period, the Sahara had ecosystems of wooded savannah and numerous rivers and lakes, where today Libya, Niger and Chad are.
But this greening, that is, the "greening" of the Sahara did not happen only once. Using marine sediments, the researchers identified that this process occurs approximately every 21,000 years in the last eight million years, providing vegetation corridors that influence the growth, distribution and evolution of the species.
These green areas would have required a large-scale reorganisation of the atmospheric system to bring rain to this arid region. But most climate models were not able to accurately simulate the atmospheric circulation over the Sahara and the impacts of vegetation in the rains. But after all, what is behind this process?
The relationship with change in the Earth's orbit
This was caused by changes in the Earth's orbital precession, that is, a slight oscillation of the planet during rotation, bringing the Northern Hemisphere (HN) closer to the Sun during the summer months. The change causes warmer summers in the HN, and the warmer air is able to retain more moisture, which intensified the strength of the West African monsoon system and moved the African rain belt to the North.
From this, there was an increase in rainfall in the Sahara and resulted in the spread of wooded savannahs throughout the desert, from the tropics to the Mediterranean, providing a vast habitat for plants and animals.
Due to the gravitational influences of the Moon and other planets in the solar system, the Earth's orbit around the Sun is not constant, as it has cyclical variations on time scales of several thousand years. These orbital cycles are called Milankovitch cycles, and influence the amount of energy the Earth receives from the sun.
In cycles of 100,000 years, the shape of the Earth's orbit, called eccentricity, varies between circular and oval, and in 41,000 years the inclination of the Earth's axis (obliquity) has variation. The cycles of eccentricity and obliquity are responsible for boosting the ice ages of the last 2.4 million years.
The Sahara functions as a gate that is responsible for controlling the dispersion of species between northern and Sub-Saharan Africa, on and off the continent. The gate was open when the Sahara was wooded and closed when the deserts prevailed. These results reveal the sensitivity of the Sahara to the Earth's orbit around the Sun, in addition to showing that ice sheets at high latitudes may have restricted the dispersion of species during the glacial periods of the last 800,000 years.
The refinement of climate models may, in the future, be able to identify how global warming will influence the rains and vegetation of the Sahara, in addition to its implications for society.
News reference
Edward Armstrong. The Sahara Desert used to be a green savannah – new research explains why. The Conversation. Available at: <https://theconversation.com/the-sahara-desert-used-to-be-a-green-savannah-new-research-explains-why-216555> Accessed on 20 Dec 2023.