From green savannah to great desert

a postdoctoral research fellow at the University of Helsinki in Finland, explores research into a period 6 000 to 11 000 years ago called the Green Sahara or North African Humid Period.

- Edward Armstrong,

Algeria’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 to 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-ofAfrica 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 how dramatic these events were.