Where the sun burns on the bare ground today, a garden of Eden stretched until around 5000 years ago. What climatic effects were behind this phenomenon of the once green Sahara? Research results from fossil plant remains shed light on this question. These and climate models show that the once green landscape was only possible because two rainy seasons overlapped in the so-called African humid phase. The results are also important for predicting future developments in the context of climate change, say the researchers.
It covers 26 times the area of Germany – today the Sahara is the largest dry desert on earth. Few animals and plants can survive in the barren sea of sand and stone. But that was not always the case: In the period from 14,500 to around 5000 years ago, the Sahara was characterized by bodies of water and a lush flora in which the typical savannah animals of Africa grazed. The once friendly Sahara was also home to people. This is documented by rock paintings in various places such as the so-called “Swimmer’s Cave”, where giraffes and crocodiles as well as people bathing are shown. This time is known as the green Sahara or the African wet phase.
So far, scientists have assumed that the arm of the African summer monsoon reached further north and donated the rain. The reason for this is the position of the earth’s axis at the time, which changes slightly due to the slow gyroscopic movement of our planet. The slope increased solar radiation over North Africa in the era of the green Sahara, which was blamed for a shift in the monsoons. But even with this effect, the former vegetation pattern in the region has not yet been plausibly explained: Climate models showed that the foothills of the summer monsoon would hardly have been enough to ensure permanent plant cover.
Searching for traces in a sediment archive
In order to gain more information about the former climatic conditions in North Africa, an international team of researchers has therefore now examined a sediment drill core that comes from the small Tislit Lake, which is more than 2000 meters above sea level in the Moroccan Atlas Mountains. As the scientists explain, remains of plants from the region have been brought into and deposited in the water over the course of thousands of years. Above all, fossil pollen and traces of plant wax in the datable sediment layers allow conclusions to be drawn about the development of the vegetation and thus also about the climate history, the researchers explain.
By examining the sediment drill core, which dates back up to 18,500 years, they were initially able to document the increase in the amount of rain in the African wet phase based on the changes in the plant growth. But did the foothills of the summer monsoon from the south donate this rainfall to the North African region? Apparently no: “The pollen grains found explicitly indicate that there was Mediterranean vegetation and not subtropical or even tropical,” says senior author Enno Schefuß from MARUM – Center for Marine Environmental Sciences at the University of Bremen. This points to a special precipitation pattern, because Mediterranean plants are adapted to rain in winter, which means they can survive the drought in summer. “That means that the reconstructions of the monsoon from previous studies have to be reconsidered,” says Schefuß. As he and his colleagues explain, the findings show that during the time of the green Sahara, in addition to the limited northward shift of the summer monsoon, there was a southward shift of the west wind belt. This influx apparently brought more rainfall in winter than today in the North African region in the area of the lake.
Winter and summer rains explain the green
When the scientists integrated this scenario into climatic models, a plausible explanation emerged for the then green core zone of the Sahara: A continuous vegetation cover could only develop there with precipitation in two seasons – the plants would have a long dry phase after a short rainy season however not survived. “At that time there was winter rain on the northern edge of the Sahara, the monsoon on the southern edge and the two rain systems overlapping in the zone in between – so there was precipitation in summer and winter,” explains first author Rachid Cheddadi from the University of Montpellier. The result of the vegetation simulations on the basis of this scenario was clear – a green Sahara was formed, say the scientists.
They describe their results as a paradigm shift in understanding the background to the phenomenon. In doing so, they emphasize that research into the former environmental conditions in North Africa is not only important for understanding the climatic past: it can also provide important information about future climate and vegetation developments in the region, as well as tips for archaeologists, settlement patterns and Investigate migration routes. This is why the scientists now want to stay on the ball: They are planning an expedition with the research vessel RV METEOR, during which further high-resolution sediment archives are to be obtained from deposits near the coast off Morocco.
Source: MARUM – Center for Marine Environmental Sciences at the University of Bremen, specialist article: PNAS, doi: 10.1073 / pnas.2024898118