NEW DISCOVERIES
The Sahara Desert used to be a green savannah – new research explains why
Edward Armstrong
Current Address : University of Helsinki Reprinted From : The Conversation
A lgeria ’ s Tassili N ’ Ajjer plateau
is Africa ’ s largest national park .
Among its vast sandstone formations is perhaps the world ’ s largest art museum . Over 15,000 etchings and paintings are exhibited there , some as much as 11,000 years old according to scientific dating techniques , representing a unique ethnological and climatological record of the region .
Curiously , however , these images do not depict the arid , barren landscape that is present in the Tassili N ' Ajjer today . Instead , they portray a vibrant savannah inhabited by elephants , giraffes , rhinos and hippos .
This rock art is an important record of the past environmental conditions that prevailed in the Sahara , the world ’ s largest hot desert .
These images depict a period approximately 6,000-11,000 years ago called the Green Sahara or North African Humid Period . There is widespread climatological evidence that during this period the Sahara supported wooded savannah ecosystems and numerous rivers and lakes in what are now Libya , Niger , Chad and Mali .
This greening of the Sahara didn ’ t happen once . Using marine and lake sediments , scientists have identified over 230 of these greenings occurring about every 21,000 years over the past eight million years . These greening events provided vegetated corridors which influenced species ’ distribution and evolution , including the out-of-Africa migrations of ancient humans .
These dramatic greenings would have required a large-scale reorganisation of the atmospheric system to bring rains to this hyper arid region . But most climate models haven ’ t been able to simulate
Figure 1 . Rock carving of an elephant at the Tassili N ' Ajjer National Park , Tadrart Rouge , in Algeria . Eric Lafforgue / Getty Images
how dramatic these events were .
As a team of climate modellers and anthropologists , we have overcome this obstacle . We developed a climate model that more accurately simulates atmospheric circulation over the Sahara and the impacts of vegetation on rainfall .
We identified why north Africa greened approximately every 21,000 years over the past eight million years . It was caused by changes in the Earth ’ s orbital precession - the slight wobbling of the planet while rotating . This moves the Northern Hemisphere closer to the sun during the summer months .
This caused warmer summers in the
Northern Hemisphere , and warmer air is able to hold more moisture . This intensified the strength of the West African Monsoon system and shifted the African rainbelt northwards .
This increased Saharan rainfall , resulting in the spread of savannah and wooded grassland across the desert from the tropics to the Mediterranean , providing a vast habitat for plants and animals .
Our results demonstrate the sensitivity of the Sahara Desert to changes in past climate . They explain how this sensitivity affects rainfall across north Africa . This is important for understanding the implications of present-day climate change ( driven by human activities ). Warmer temperatures in the future may also en-
27 Grassroots Vol 24 No 1 March 2024