An extended period of drought has persisted in the Sahel since the 1960's.
1. Self-sustaining wet climate equilibrium with deep-rooted perennial grasses
2. Self-sustaining dry climate equilibrium with shallow rooted grasses
Fast or Dependent Variable(s)
Slow or Independent Variable(s)
Disturbance or Threshold Trigger(s)
Variable climate, grazing pressure
External / Internal Trigger
The combination of natural climate variability and the change in land cover have combined to result in dryer climatic conditions in the Sahel. Vegetation type, rooting depth and climate are linked in the model, such that moist growing seasons are associated with abundant root biomass. Less root biomass has less above ground growth and a smaller leaf area. Less water is therefore transpired to the atmosphere, providing less atmospheric moisture to the biosphere, resulting in the system remaining in a dryer state.
Wang, G. L., and E. A. B. Eltahir. 2000. Role of Vegetation Dynamics in Enhancing the Low-Frequency Variability of the Sahel Rainfall. Water Resources Research 36, no. 4: 1013-21.
Higgins, P. A. T., M. D. Mastrandrea, and S. H. Schneider. 2002. Dynamics of Climate and Ecosystem Coupling: Abrupt Changes and Multiple Equilibria. Philosophical Transactions of the Royal Society of London Series B-Biological Sciences 357: 647-55. (D)