Moist air masses from the Mediterranean caused rainfall, which filled some of the Tibesti mountain's volcanic craters
Descent into the Era Kohor subcrater within the summit caldera of Emi Koussi, the highest mountain in the Sahara, at 3415 m. Whitish sediments can be seen on the crater floor - salt crusts from when the lake dried up.
© Stefan Kröpelin (University of Cologne)
To the point
- Unclear origin: Several thousand years ago, deep crater lakes existed in the Tibesti mountains in the Sahara, but it is unclear where the water that filled them came from
- Interdisciplinary approach: Researchers have solved the mystery using geochemical methods, terrain analyses, as well as climate and hydrological modeling
- Surprising finding: Moist air masses from the Mediterranean region - and not from the south, as previously thought - caused heavy rainfall that fed the lakes
- High-resolution climate simulations: A model resolution of five kilometers made it possible to account for the influence of the steep topography on atmospheric circulation - this is also important for calculating hydrological changes in the future.
The world's largest dry desert, the Sahara, was significantly greener a few thousand years ago than it is today, as evidenced by numerous archaeological and paleobotanical discoveries as well as by landscapes that were once shaped by lakes and rivers. At that time, deep crater lakes existed in Tibesti, the highest mountain range in the Sahara. It probably served as a "water tower" for the surrounding regions and the people who lived there. In some of the volcanic craters, snow-white salt crusts can still be seen today-remnants of the lakes that filled the craters thousands of years ago. Well known are the "Trou au Natron" (the natron pit), known in the local Teda language as Doon Orei ("big hole"), and the Era Kohor crater further south. However, the origin of the water that once filled the lakes remained a mystery.
Interdisciplinary research on the crater lakes
Satellite image of the Tibesti Mountains and reconstruction of the paleolakes in the Trou au Natron and Era Kohor.
© CC BY 4.0 Hoelzmann, Claussen et al. 2025 DOI: 10.1038/s41467-025-62769-9
An interdisciplinary research team led by Philipp Hoelzmann, researcher at the Freie Universität Berlin, and Martin Claussen, director emeritus at the Max Planck Institute for Meteorology, has recently solved this mystery by combining different methods: They used geochemical techniques to analyze and date sediment samples from the Tibesti to reconstruct the dynamics of the paleolakes more accurately. They also investigated the regional paleoclimate of the region using the ICON numerical weather prediction model (ICON-NWP) by creating multi-year simulations with a spatial resolution of five kilometers for the time around 7,000 years ago. The land surface characteristics and sea surface temperature were prescribed based on previous climate simulations performed with the Max Planck Institute Earth System Model (MPI-ESM). This allowed the simulations to capture the dynamics of the topography-related precipitation in the Tibesti for the first time. Using comprehensive remote sensing and terrain analysis, the researchers ultimately evaluated the hydrography of the system and developed a numerical model of the equilibrium water balance.
They found that, 7,000 years ago, precipitation in the northern part of the Tibesti was at least one order of magnitude higher than in the surrounding regions. This was due to the strong uplift of moist air masses on the steep mountain slopes. As the simulations show, these air masses originated in the northeastern Mediterranean region-and not from the south, as previously assumed. The new findings on atmospheric circulation also explain why the "Trou au Natron", located further north, received more rainfall which led to a deeper lake (approx. 330 meters deep) than the Era Kohor further south (approx. 130 meters).
Simulation of moisture transport and precipitation over North Africa. The arrows indicate the direction and strength of moisture transport, and the colors indicate the intensity of precipitation. The orange rectangle shows the investigated region of the Tibesti.
© CC BY 4.0 Hoelzmann, Claussen et al. 2025 DOI: 10.1038/s41467-025-62769-9
Significance of high-resolution climate simulations
The study provides insight into the paleohydrological changes in Tibesti during this African Humid Period. It also demonstrates the importance of using paleoclimate simulations with high spatial resolution to account for the extreme effects of Tibesti's steep slopes on atmospheric circulation. Previous studies using low-resolution climate models did not capture this aspect. However, it is likely to play a role in assessing hydrological changes in the Sahara in the warming climate of the future.
