Abstract:

We ran a sequence of climate model experiments for 6000 years ago, with land-surface conditions based on a realistic map of palaeovegetation, lakes and wetlands, to quantify the effects of land-surface feedbacks in the Saharan region. Vegetation-induced albedo and moisture flux changes produced year-round warming, forced the monsoon to -N two months earlier, and shifted the precipitation belt 300 km northwards compared to the effects of orbital forcing alone. Albedo and moisture flux changes due to lakes and wetlands produced localised changes in evaporation and precipitation, but caused no further extension of the monsoon belt. Diagnostic analyses with biome and continental hydrology models showed that the combined land-surface feedbacks, although substantial, could neither maintain grassland as far north as observed ( N) nor maintain Lake "MegaChad" (330,000 km). Additional climate system interactions (for example, involving altered sea-surface temperature patterns) may be required to explain why orbital forcing effects were amplified to the extent shown by terrestrial palaeorecords.