Dynamics of the ‘Green’ Sahara and its termination in an ensemble of transient Holocene model simulations

Abstract:

The Holocene ’greening’ of the Sahara and the subsequent, possibly rapid desertification that followed are fascinating examples of natural environmental change. In North Africa this was driven by a gradual decline in summer insolation over the Holocene that is generally held to have been reinforced by land-atmosphere feedbacks. Most general circulation models (GCMs) cannot effectively reproduce the rainfall required for this greening, and so the transient dynamics of this period are not well understood. Here we examine a suite of transient simulations of the Holocene that use the coupled GCM HadCM3. In these simulations we have varied both the forcing (ice-sheets, greenhouse gas levels, solar input and volcanic eruptions) and the model physics, specifically the parametrisations of convective entrainment and vegetation moisture stress. HadCM3 shows a ‘greening’ of much of the Sahara region only when both parametrisation changes are included. The modelled response over North Africa is therefore largely determined by the parametrisations that appear to be loosely constrained by modern observations. The simulated ‘greening’ is followed by several rapid oscillations in vegetation cover and precipitation that culminate in an abrupt collapse of the monsoon in North Africa at around 6000 years before present. The spatio-temporal pattern of this appears consistent with the reconstructed response of the hyrdrological cycle, with abrupt behaviour further north and a more gradual transition elsewhere. We also evaluate the importance of stochastic forcing from solar input and volcanic eruptions and the influence of the ‘greening’ on the large-scale circulation. We conclude with a discussion of how novel methods of inferring suitable parameter values may help inform on both the sensitivity of monsoons, and the potential for abrupt dynamics in the land-atmosphere system.