Orbital and non-orbital drivers of late Quaternary African Humid Periods

Variations in the Earth's orbit are recognised as the main trigger for the hydrological changes that led to the periodic 'greening' of the Sahara region over the late Quaternary. However, the frequency and amplitude of the greening events as seen in the geological records cannot be predicted from orbital theory alone. To understand the changes in the proxy data it is also important to consider feedback mechanisms that arise from the complexity of the interactions between the vegetation, land, atmosphere and ocean components in the region. Yet discrepancies between state-of-the-art computer simulations of greening during African Humid Periods (AHPs) and proxy data still remain. We hypothesize that the effects of additional internal forcing from other climate drivers like atmospheric levels of greenhouse gases (GHGs) and extension of ice sheets may have had a greater impact than previously thought. Using two climate models of varying complexity and spatial resolution, CLIMBER-2 and MPI-ESM, we simulate several of the greening events in the Sahara within the last glacial cycle and study the effects of the orbital, GHGs and ice sheets forcings for every greening response. The results from CLIMBER-2 suggest that the critical insolation at the Tropics required for AHPs onset depends on atmospheric levels of GHGs, while the results from MPI-ESM show that the spatial pattern that develops during AHPs varies with all three forcing factors. These findings highlight the role that GHGs may play for the future of Saharan climate, when low--eccentricity orbits concur with high levels of atmospheric GHGs.