Cologne, the 9th of May 2008 ??? One of the most intriguing stories in the historical relationship between humans and climate change is the evolution of northern Africa during the past 6000 years from a ???green Sahara??? of tropical savanna grassland dotted with lakes into the world???s largest warm desert today. Some local residents adapted to this inhospitable environment by developing a highly specialized nomadic lifestyle along long-distance trade routes, but farmers and most pastoralists retreated to the present-day Sahel regions and to the Nile Valley. The ensuing cohabitation of desert people with the original inhabitants of the Nile valley stimulated the new social and political structures which eventually gave rise to the classic Egyptian civilization (Kuper & Kröpelin, Science 2006, vol. 313: 803-807).
An important issue about this natural, climate-driven drying of the Sahara is whether it was a slow process developing over several millennia, or alternatively a more rapid process completed within a few hundred years, generated perhaps by strong positive feedback processes between declining rainfall and long-term vegetation dynamics. This question has long remained unresolved because desiccation and wind deflation of lake deposits in the Sahara, and loss of trees suitable for dendroclimatology, has destroyed or disturbed almost all local sources of paleoenvironmental information on this period. Evidence extracted from deep-sea sediments in the adjacent tropical Atlantic Ocean suggested a fairly sudden drying of the Sahara around 5500 years ago. Supported by the the output of some climate modeling work, these data have in recent years become a type example of a so-called regime shift between alternative stable states of ecosystem functioning (in this case a ???green Sahara??? savanna and a ???brown Sahara??? desert).
In the context of climate change, it also appeared to illustrate the non-linear response dynamics that are integral to the global climate system, capable of generating abrupt climate change when pushed beyond a critical threshold or ???tipping point???. The research now published in Science (9 May 2008)* shows that Sahara drying was in fact a gradual process developing between 5600 and 2700 years ago. This conclusion follows from analysis of a wide array of paleoenvironmental indicators extracted from the radiocarbon-dated sediment record of Lake Yoa in northern Chad, one of very few lakes in the Sahara that have been protected against desiccation by continuous groundwater inflow. The very existence of permanent lakes in the hyperarid central Sahara (0-21 mm rainfall per year, versus ~6000 mm evaporation), thousands of years after regional groundwater supplies were last recharged, is unquestionably unique: the much larger Lake Chad dried out repeatedly in recent millennia, therefore its sediment record yields only fragmentary information about the region???s environmental history.
Sponsored by the Deutsche Forschungs Gemeinschaft (DFG) and the Fund for Scientific Research of Flanders, geoarchaeologist Stefan Kröpelin of the Africa Research Unit of the Institute of Prehistoric Archaeology at the University of Cologne (Germany) joined by paleolimnologist Dirk Verschuren from Ghent University (Belgium) obtained intact sediment cores from Lake Yoa, which were subjected to multi-disciplinary analyses by a consortium of experts from Germany, France, Sweden, Canada and the USA. The results of this work document a progressive drying of the regional terrestrial ecosystem between 5600 and 2700 years ago, in response to gradually decreasing tropical monsoon rainfall. This drying followed a logical ecological sequence starting with tropical grassland trees and herbs being replaced by typical Sahel vegetation, followed by loss of grass cover and establishment of the modern desert plant community that is largely restricted to oases. The aquatic ecosystem of Lake Yoa itself experienced a more abrupt transition from freshwater to hypersaline conditions between 4200 and 3900 cal yr BP, when its decreasing water balance stopped groundwater outflow and allowed salts to accumulate in the lake itself. In summary, this new environmental reconstruction from within the Sahara proper strongly contrasts with the generally accepted hypothesis that the ???green Sahara??? which existed between 10,000 and ~6000 years ago had ended abruptly. Consequently, the drying of the Sahara can no longer be used as a type example of abrupt climate and ecological change. However, ample documented examples of abrupt climate change at other times in the past continue to emphasize the potential importance of critical thresholds in the global climate system for the trajectory of anthropogenic climate change.
* S. Kröpelin, D. Verschuren, A.-M. Lézine, H. Eggermont, C. Cocquyt, P. Francus, J.-P. Cazet, M. Fagot, B. Rumes, J. M. Russell, F. Darius, D. J. Conley, M. Schuster, H. von Suchodoletz, D. R. Engstrom (2008): Climate-driven Ecosystem Succession in the Sahara: The Past 6000 Years. Science 320: 765-768.
Contact:
+49 (0)221 9559169