In the mid-nineteenth century, during a journey between Tripoli and Timbuktu, German explorer Heinrich Barth discovered paintings and rock engravings depicting hunting scenes and animals. These traces, dated between 11,000 and 5,500 years ago, demonstrate that the Sahara was vastly different back then. Indeed, tropical vegetation and rivers covered what is now desert: the Sahara was "green."

However, approximately 5,500 years ago, the region experienced extremely rapid aridification, giving way to the desert as we know it today. An international team, including Thibaut Caley from the EPOC laboratory (CNRS and University of Bordeaux), demonstrated the role played by falling temperatures in the higher latitudes of the Northern Hemisphere in the disappearance of the Green Sahara. Preceded and followed by arid climatic conditions, this African humid period which lasted approximately 6,000 years was exceptional. However, its end remains poorly understood, particularly the speed and synchronicity of aridification across the entire Sahara and Sahel region.

To clarify this question, Thibaut Caley and his colleagues first analyzed marine sediments in the Gulf of Guinea. They focused particularly on the wax coating plant leaves, which is found in sedimentary deposits. The stable hydrogen isotope composition of these waxes allows for reconstruction of the intensity of the hydrological cycle (this composition can be linked to the composition of rainwater used by plants). Using this indicator, researchers showed that precipitation decreased significantly between 5,800 and 4,800 years ago in the Cameroon region and in the central Sahel-Sahara. A similar observation was established in northeastern Africa, confirming a global phenomenon. Furthermore, the drop in Lake Chad's water level by roughly one hundred meters around 5,200 years ago and increased dust in northwestern Africa around 5,500 years ago are also signs of severe drought.

Study of Atmospheric Phenomena

To understand what happened, researchers studied the atmospheric phenomena affecting moisture supply to the region. Moisture sources for the Sahel and Sahara are, on one hand, the Atlantic Ocean and, on the other hand, the monsoon coming from central Africa. Precipitation volumes and their seasonal character are modulated by two atmospheric currents, the tropical easterly jet (TEJ) and the African easterly jet (AEJ). The first evolves at high altitude near the equator while the second is situated at lower altitude but further north. If the TEJ slows down, conditions become more arid and, conversely, a stronger AEJ causes dry conditions.

But What Phenomenon Triggered the Green Sahara's Aridification?

The answer could lie in the higher latitudes of the Northern Hemisphere. Indeed, many indicators show that summer temperatures in the region extending from Greenland to the Norwegian Sea would have dropped between 6,000 and 5,000 years ago. This could be due to a slowdown of ocean currents in the Atlantic Ocean that bring warm, salty water from lower latitudes northward (known as thermohaline circulation) or to an expansion of the Northern Hemisphere's polar vortex, which brings cold wind further south.

To understand how this phenomenon could have influenced climatic conditions in the Sahara, researchers used a numerical climate model that reproduced conditions from that era with a cooling, ranging from 0.5°C to 2.5°C, of the North Atlantic. Researchers demonstrated that temperature anomalies then manifested themselves as far as northern Africa, which had the effect of slowing the TEJ, reducing precipitation. Furthermore, the drop in ground temperatures in the Sahara also blocked the northward advance of the monsoon, leading to a decrease in precipitation in the Sahel. Researchers also showed that these conditions strengthened the AEJ, further intensifying the region's aridification. Thus, a temperature variation in the higher latitudes of the Northern Hemisphere could have triggered a cascading effect with feedbacks that ultimately led to the disappearance of the Green Sahara. More broadly, as Thibaut Caley emphasizes, "this research also supports the hypothesis that future temperature changes in the higher latitudes of the Northern Hemisphere could have significant repercussions on the Saharan hydrological cycle and consequently on the populations of this region."