On the Role of Dust-Climate Feedbacks During the Mid-Holocene

A reduction in dust over North Africa during the mid-Holocene "Green Sahara" period could have amplified precipitation, helping reconcile climate model simulations with paleo-precipitation reconstructions. Here we test this using general circulation model simulations including interactive dust. We calculate a dust-precipitation amplification factor using three different dust configurations to evaluate the sensitivity to dust optical properties and particle size range. The resultant amplification ranges from -20% to 50%. With more absorbing dust properties, there is a large negative net radiative effect and hence a larger impact on the hydrological cycle. With the inclusion of particles greater than 1 mu m in radius, the precipitation amplification is reversed. Based on the simulations which best match observed Saharan dust properties, we conclude that there was a limited enhancement of precipitation due to reduced dust during the mid-Holocene, meaning other aspects of the climate system should be the focus of future research. Plain Language Summary From around 11,000-4,000 years ago, the area that is the modern day Sahara desert was covered in vegetation-the "Green" Sahara. This was brought about by long-term variations in the orbit of the Earth and the resultant changes in the distribution of incoming sunlight. However, when this period is simulated with climate models, the simulated rainfall increase is not adequate to maintain vegetation. We also know that this time period showed much reduced atmospheric dust from the desert. Here we study the effect of this dust reduction using a coupled climate-dust model. We find that the reduction in dust had little impact on rainfall change, because dust has opposing effects on heat and radiation in the atmosphere that approximately balance over North Africa. To understand this response, we configured different versions of our climate-dust model and show that the result is highly dependent on the physical characteristics of the dust particles that are given to the model. With the more realistic and up to date measurements from Saharan particles, the rainfall feedback is small. This means that future research needs to focus on rainfall processes in the atmosphere to better understand this "Greening" of the Sahara.