Multi vegetation model evaluation of the Green Sahara climate regime

* First systematic dynamic vegetation model evaluation of the Green Sahara climate regime such as the mid-Holocene and Last Interglacial.
* Three different dynamic vegetation models show a range of rainfall required to green the Sahara, largely caused by intermodel differences in moisture stress formulations.
*All models require that mid-Holocene rainfall likely exceeded that inferred from pollen assemblages, averaging 500-800mm over 20-25ºN.

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 20ª-25º 15 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.