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A 16-metre-long sediment core from an oasis lake in Chad reveals that a former wet phase was disrupted by short-term droughts in northern Africa / publication in ‘Nature’
The analysis of a sediment core from an oasis lake in Chad provides new insights into the history of precipitation in the Sahara. The study, led by the University of Cologne, shows that a prolonged wet phase, which lasted from 14,800 to 5,500 years ago, was interrupted by short-term droughts. Such drought events could also occur in a similar manner in the future. The results were published under the title ‘Decadal-scale droughts disrupted the African Humid Period in the Sahara’ in the scientific journal Nature. In addition to the University of Cologne, research institutions in Germany, France, Belgium, Chad and China contributed to the study through further laboratory analyses and climate modelling.
In the 1970s and 1980s, the Sahara and the adjacent Sahel region experienced a severe drought that led to devastating famines. Precipitation increased significantly in the following decades, attributed to the current global warming, which has caused increased evaporation and a shift in the West African monsoon. The result is a spreading of plants, a phenomenon described by the term 'Greening Sahara'.
A green Sahara has occurred frequently in recent Earth history, whenever shifts in the Earth’s orbital parameters have led to stronger solar radiation in the northern hemisphere and, consequently, higher precipitation in northern Africa. The last of these so-called African Humid Periods occurred between 14,800 and 5,500 years ago. It is known from geological and archaeological data that a savannah existed in the Sahara at that time, with lakes and rivers, a diverse wildlife and flourishing human cultures. However, it was not yet sufficiently understood how stable or unstable the wet period was.
This question has now been answered thanks to a 16-metre-long sediment core that was drilled by geologists from Cologne, in collaboration with partners from Chad, in Lake Yoa, an oasis lake in the centre of the Sahara that formed 10,800 years ago. Despite the extreme aridity of the desert, Lake Yoa still exists today because a permanent inflow of groundwater prevents it from drying out. This allowed a continuous sediment sequence to accumulate at the bottom of the lake, the composition of which provides an archive for the climatic and environmental history of the region with unrivalled accuracy.
“The geoscientific analysis of the sediment core has shown for the first time that the last African Humid Period was interrupted at least three times by dry events, around 9,300, 8,200 and 6,300 years ago,” says lead author Dr Florence Sylvestre from the Institute de Recherche pour le Développement (France). Professor Dr Martin Melles from the University of Cologne, also lead author, adds: “The reconstructed drought events coincide, at least in part, with times for which archaeological findings indicate deteriorating living conditions for the population at the time.”
A more detailed analysis of the dry event about 8,200 years ago, based on counting the annual layers within the sediment core, revealed that this event at Lake Yoa lasted 77 years, from 8,229 to 8,152 years before present. Climate modelling has shown that it is causally linked to a simultaneous cooling that took place in the North Atlantic. This cooling has been known for some time. It is attributed to a massive inflow of freshwater into the Atlantic due to the draining of a huge ice-dammed lake in North America. This has weakened the oceanic overturning circulation in the Atlantic, including the Gulf Stream.
Oceanographic data indicate that the overturning circulation in the Atlantic is currently weakening as well, although this time the rapidly increasing melting of the ice masses on Greenland as a result of human-made climate change is assumed to be the cause. However, it remains unclear whether history is now repeating itself. “This is not transferable one-to-one because the boundary conditions today are not comparable with those 8,200 years ago, for example in terms of greenhouse gas concentrations, the extent of continental glaciation or global sea levels,” says Professor Melles. “But our results show the impact that changes in the Atlantic can have on precipitation in northern Africa, with the speed, magnitude and spatial extent of drought events.” The researchers therefore conclude that further efforts are needed to predict future precipitation trends in the Sahara with greater precision and reliability.
The fieldwork and numerous analyses of the sediment core were carried out at the University of Cologne as part of the German Research Foundation´s Collaborative Research Centre ‘Our Way to Europe’, which was funded from 2009 to 2021.
Professor Dr Martin Melles
Institute of Geology and Mineralogy
+49 221 470 2541
mmelles@uni-koeln.de
https://www.nature.com/articles/s41586-026-10336-7
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