A diagram of the Sahara Desert.
Thousands of years ago, the Sahara Desert was a green world, just like a prehistoric mural in Niger, with giraffes, crocodiles and even humans swimming in it. However, this is only a small piece of the historical puzzle. A recent study reveals the history of environmental change in North Africa over the past 160,000 years.
The analysis, published Jan. 28 in the journal Nature Geoscience, shows that the rivers that originally dried up on North African land have gone through several cycles of filling up, drying up and filling up again. At the moment, this region is one of the few driest on Earth.
Together with researchers from Korea, the Netherlands and the United States, the Helmholtz Centre for Ocean Research (GEOMAR) in Kiel, Germany, organized an international expedition to the Gulf of Sirte, a bay in the Mediterranean Sea north of Libya, aboard the Dutch expedition vessel Pelagia in 201112 to drill the sea floor in the Gulf of Sirte. The Gulf of Sirte, north of Libya, was used to drill samples from the seafloor.
Cécile Blanchet, the principal investigator, said, “We guessed that when the Sahara desert was an oasis, those rivers that are now dry were in a river-filled state and the sand from the rivers would have been washed into the Gulf of Sirte. So looking at the sediments underneath the Gulf of Sirte helps us understand the previous history of these rivers, the point in Time when they filled up again and the environmental factors required for the desert to become green again.”
They used a new method of “piston coring” to obtain several cylindrical, 10-meter-long sediment samples from the seafloor. Blanchette said, “You can imagine that we used a giant cylinder tool to insert into the seafloor to get these samples. The seafloor silt has many layers and contains a variety of rocky debris from the next African continent, plant remains, and a large number of organisms and shells that live in the local waters. These sediments will tell us the history of past climate change there.”
By analyzing these sediments, combined with the analysis of computer simulation models, the researchers recovered the history of climate change in North Africa over the past up to 160,000 years.
Their research shows why there used to be enough rain in the heart of the Sahara desert to resurrect these rivers. Blanchette said, “We found that subtle changes in the Earth’s orbit, changes in the thickness of the polar ice caps, led to alternating changes between wet and dry periods of high precipitation in this region.”
The study shows that these changes affect a region that extends from North Africa to the Mediterranean coast, with each cycle lasting about five thousand years. Such dramatic changes in climate in North Africa are also responsible for the historical migration of North African populations.