During the Quaternary, the Sahara desert was periodically colonised by vegetation, likely because of orbitally-induced rainfall increases. However, the estimated hydrological change is not reproduced in climate model simulations, undermining confidence in projections of future rainfall. We evaluated the relationship between the qualitative information on past vegetation coverage and climate for the mid-Holocene using three different dynamic vegetation models. Compared with two available vegetation reconstructions, the models require 500-800 mm of rainfall over 20° -25° N, which is significantly larger than inferred from pollen, but largely in agreement with more recent leaf wax biomarker reconstructions. The magnitude of the response also suggests required rainfall regime of the early to mid-Holocene is far from being correctly represented in GCMs. However, inter-model differences related to moisture stress parameterisations, biases in simulated present day vegetation and uncertainties about paleo-soil distributions introduce uncertainties, and these are also relevant to Earth System model simulations of African humid periods.
This article appeared on the Geophysical Research Letters website at http://onlinelibrary.wiley.com/doi/10.1002/2017GL073740/full