Modeling climate-vegetation interactions during the last interglacial: The impact of biogeophysical feedbacks in North Africa

Abstract

Previous climate model simulations of the early Last Interglacial (LIG) underestimated the temperatures compared to proxy-based reconstructions. One possible reason for this underestimation is that in these simulations the vegetation was prescribed to a pre-industrial state. To study the impact of climate-vegetation interactions, we performed a series of model experiments using the iLOVECLIM climate model, in which either VECODE or LPJ-GUESS was coupled as vegetation component. We specifically assessed the evolution of vegetation during the LIG and the magnitude of dynamical vegetation feedbacks. Our results show a relatively high vegetation cover (>70%) in the Sahara during the early LIG when the summer insolation at 20°N was high. This early stage is followed by an accelerated desertification phase after 123 ka BP. The rates of desertification in the Sahara peak at 122 ka BP, responding to the fast decline in 20°N July insolation. This desertification is accelerated when the magnitude of positive vegetation feedbacks on precipitation cannot offset the moisture deficit due to decreased summer insolation.Simulations including the LIG vegetation feedback suggest warmer conditions than simulations with prescribed pre-industrial vegetation, but they still slightly underestimate the temperature suggested by proxy records. This is particularly the case in the high latitude regions and the tropics, where the 125 ka BP vegetation cover is significantly higher than the pre-industrial state. The magnitude of vegetation feedbacks in North Africa to local climate peaks in the early LIG and decreases during the LIG, corresponding to temporal vegetation changes. During the early LIG, experiments with dynamical vegetation suggested a doubling in the amount of precipitation (∼60 cm/yr) in comparison to experiments with fixed pre-industrial vegetation. In addition, adding dynamical vegetation causes higher surface temperatures by about 2.5 °C in North Africa. At a global scale, a vegetated Sahara during the early LIG leads to an increase in surface temperature and a decline in surface air pressure due to local feedbacks, thereby enhancing mid-latitude westerlies as a result of increased latitudinal temperature and pressure gradient, leading to an increase in the amount of heat transported by the atmosphere from tropical regions to the Arctic. This green Sahara feedback provides 30% of the total contribution of global vegetation feedbacks to high latitudes warming.