№ 132/2025 from Aug 14, 2025

An interdisciplinary research team led by scientists from Freie Universität Berlin and the Max Planck Institute for Meteorology has shown how deep lakes formed over 9,500 years ago in the craters of the Tibesti Mountains and existed there for over 5,000 years. Their study not only sheds light on paleohydrological changes between the Tibesti, located in present-day Chad, and the surrounding Saharan plains during the North African Humid Period, but also demonstrates the importance of high-resolution paleoclimate simulations. The study “Mid-Holocene Extreme Precipitation in the Tibesti, Central Sahara” appeared in Nature Communications and is available online at: https://www.nature.com/articles/s41467-025-62769-9.

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. In 1869, the German explorer Gustav Nachtigal was the first European to report on this deep crater at an altitude of about 2,500 meters during his expedition to the northern part of the Tibesti. The crater is known as the Trou au Natron (“the natron pit”) or in the local Teda language Doon Orei (“big hole”). More recently, Stefan Kröpelin (University of Cologne) initiated a geological exploration of the Trou au Natron to the north and the Era Kohor crater to the south, located within the 3,500-meter-high Emi Koussi summit caldera. Under extremely challenging conditions, the team collected sediment samples from the former lakes and transported them to Germany, where researchers could use geochemical techniques to analyze the sediment at the Laboratory for Physical Geography, Freie Universität Berlin. “Working on these valuable samples from such remote craters was a fantastic opportunity to determine the timing and dynamics of lakes that no longer exist today,” says Philipp Hoelzmann, geographer at Freie Universität Berlin and one of the two first authors of the new study.

Building on these findings, they also evaluated regional paleoclimate simulations of the Sahara and Sahel region at the Max Planck Institute for Meteorology in Hamburg. The simulations use a very high spatial resolution of about five kilometers for a slice of time around 7,000 years ago. “For the first time, we were able to capture the steep orography and the dynamics of precipitation in the Tibesti in a climate model – something that had not been possible before,” says Martin Claussen, head of the modeling group at the Max Planck Institute for Meteorology. The scientists then used remote sensing and terrain analysis to assess the system’s hydrography and developed a numerical model of the equilibrium water balance. Through this new combination of proxy data, remote sensing, water balance estimates, and high-resolution paleoclimate simulations, they were able to show that, around 7,000 years ago, the Tibesti Mountains received at least one order of magnitude more precipitation than the surrounding plains.

The findings came as somewhat of a surprise, the researchers noted, because they indicate that this was due to the moist air masses brought in by northeasterly winds from the Mediterranean region – and not, as previously assumed, by the then-stronger West African monsoon from the south. These air masses also produced precipitation that fed the crater lakes as a result of the strong orographic uplift caused by the Tibesti Mountains.

This study highlights the importance of high-resolution paleoclimate simulations to accurately reflect hydrological changes in a warming climate – not just in the Sahara.

Hoelzmann, P., Claussen, M., Dallmeyer, A. et al. “Mid-Holocene Extreme Precipitation in the Tibesti, Central Sahara.” Nature Communications 16, 7426 (2025). https://doi.org/10.1038/s41467-025-62769-9.