Modeling global terrestrial vegetationclimate interaction

By coupling an atmospheric general circulation model asynchronously with an equilibrium vegetation model, manifold equilibrium solutions of the atmosphere^biosphere system have been explored. It is found that under present-day conditions of the Earth's orbital parameters and sea-surface temperatures, two stable equilibria of vegetation patterns are possible: one corresponding to present-day sparse vegeta-tion in the Sahel, the second solution yielding savannah which extends far into the south-western part of the Sahara. A similar picture is obtained for conditions during the last glacial maximum (21 000 years before present (BP)). For the mid-Holocene (6000 years BP), however, the model ¢nds only one solution: the green Sahara.We suggest that this intransitive behaviour of the atmosphere^biosphere is related to a westward shift of the Hadley^Walker circulation. A conceptual model of atmosphere^vegetation dynamics is used to interpret the bifurcation as well as its change in terms of stability theory