Regime shifts in the Sahara and Sahel: Interactions between ecological and climatic systems in Northern Africa

Foley, J.A. and Coe, M.T. and Scheffer, M. and Wang, G. (2003) Regime shifts in the Sahara and Sahel: Interactions between ecological and climatic systems in Northern Africa. Ecosystems, 6 (6). pp. 524-532.

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The Sahara and Sahel regions of northern Africa have complex environmental histories punctuated by sudden and dramatic “regime shifts” in climate and ecological conditions. Here we review the current understanding of the causes and consequences of two environmental regime shifts in the Sahara and Sahel. The first regime shift is the sudden transition from vegetated to desert conditions in the Sahara about 5500 years ago. Geologic data show that wet environmental conditions in this region—giving rise to extensive vegetation, lakes, and wetlands—came to an abrupt end about 5500 years ago. Explanations for climatic changes in northern Africa during the Holocene have suggested that millennial-scale changes in the Earth’s orbit could have caused the wet conditions that prevailed in the early Holocene and the dry conditions prevalent today. However, the orbital hypothesis, by itself, does not explain the sudden regime shift 5500 years ago. Several modeling studies have proposed that strong, nonlinear feedbacks between vegetation and the atmosphere could amplify the effects of orbital variations and create two alternative stable states (or “regimes”) in the climate and ecosystems of the Sahara: a “green Sahara” and a “desert Sahara.” A recent coupled atmosphere-ocean-land model confirmed that there was a sudden shift from the “green Sahara” to the “desert Sahara” regime approximately 5500 years ago. The second regime shift is the onset of a major 30-year drought over the Sahel around 1969. Several lines of evidence have suggested that the interactions between atmosphere and vegetation act to reinforce either a “wet Sahel” or a “dry Sahel” climatic regime, which may persist for decades at a time. Recent modeling studies have indicated that the shift from a “wet Sahel” to a “dry Sahel” regime was caused by strong feedbacks between the climate and vegetation cover and may have been triggered by slow changes in either land degradation or sea-surface temperatures. Taken together, we conclude that the existence of alternative stable states (or regimes) in the climate and ecosystems of the Sahara and Sahel may be the result of strong, nonlinear interactions between vegetation and the atmosphere. Although the shifts between these regimes occur rapidly, they are made possible by slow, subtle changes in underlying environmental conditions, including slow changes in incoming solar radiation, sea-surface temperatures, or the degree of land degradation.

Item Type: Article
Uncontrolled Keywords: Sahel; Sahara; regime shifts; resilience; climate–vegetation interactions; drought; Holocene
Author Affiliation: Center for Sustainability and the Global Environment (SAGE), Gaylord Nelson Institute for Environmental Studies, University of Wisconsin, Madison, 1710 University Avenue, Madison, Wisconsin 53726, USA
Subjects: Atmosperic Science > Climatology
Divisions: General
Depositing User: Mr T L Gautham
Date Deposited: 10 Jan 2019 08:07
Last Modified: 10 Jan 2019 08:08

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