Effect of nitrogen limitation and soil processes on mid–Holocene greening of the Sahara

Abstract. The so–called Green Sahara (GS), wet and vegetative Sahara region in the mid–Holocene, provides useful information on our climate simulation because it is consequence of complex interaction between biophysical and climatic processes. It is still a challenge to simulate the GS in terms of vegetative extent and precipitation using the current climate models. This study attempts to simulate the Green Sahara by using the state–of–the–art earth system model CESM that incorporates the nitrogen cycle and the soil–albedo–precipitation feedback. Our study focuses on the impact of soil biophysical properties and soil nitrogen on the simulation of the GS. With changes in the Earth’s orbit and dust in the mid–Holocene, the model simulates increased precipitation in North Africa, but does not capture the extent of the GS. Further analysis shows that the mid–Holocene greening is simulated better if the amount of soil nitrogen and soil texture are properly modified during the GS period through their influence on photosynthesis and surface albedo and their consequent enhanced albedo– and evapotranspiration–precipitation feedbacks. Our findings suggest that future climate simulation needs to consider consequent changes in soil nitrogen and texture with changes in vegetation cover and density for proper climate simulations.