Abstract
AbstractDuring the Quaternary, the Sahara desert was periodically colonized 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 that required rainfall regime of the early to middle Holocene is far from being correctly represented in general circulation models. However, intermodel differences related to moisture stress parameterizations, biases in simulated present‐day vegetation, and uncertainties about paleosoil distributions introduce uncertainties, and these are also relevant to Earth system model simulations of African humid periods.
Highlights
During at least the late Quaternary the area of the present-day Sahara desert has periodically transformed to an area of lakes, rivers, and steppe and savannah vegetation
We evaluated the relationship between the qualitative information on past vegetation coverage and climate for the mid-Holocene using three different dynamic vegetation models
Joint UK Land Environment Simulator (JULES) and Sheffield Dynamic Global Vegetation Model (SDGVM) are equidistant from the observations, while LPJ predicts much less vegetation coverage in low-rainfall areas
Summary
During at least the late Quaternary (over the past 1 Myr) the area of the present-day Sahara desert has periodically transformed to an area of lakes, rivers, and steppe and savannah vegetation. The increased rainfall required to stimulate a Green Sahara is taken to equal the transition threshold for desert to steppe biome types or around 200–300 mm yr−1 [Joussaume et al, 1999; Braconnot et al, 2007]. This is based on a modern climate and atmospheric CO2 (350–400 ppm).
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