African humid period precipitation sustained by robust vegetation, soil and lake feedbacks

Abstract:

The African Humid Period (AHP; ~11,000--5,000 years before present) was the most recent wet interval among several precessionally-paced wet-dry cycles during which an increase in the northern hemisphere summer incoming solar radiation initiates the intensification of the West African Monsoon leading to dramatic land surface changes over northern Africa -- most prominently in the form of a widespread expansion of vegetation over what is presently the Sahara Desert. Owing to the dramatic nature of this transformation and considering the implications it has for the development of Neolithic societies, the AHP puzzle has for a very long time constituted a salient question in climate dynamics and archaeology. Nonetheless, despite nearly four decades of progress on the problem the solution to the puzzle has remained elusive.

While the initiation of the AHP is triggered by the slow and steady increase of insolation, it is now well understood that feedbacks within the earth system, mainly those arising from changes to the land surface are essential in amplifying the insolation driven precipitation increase and yielding an AHP that is comparable to proxy inferences both in terms of the magnitude and spatial extent. On this matter of land surface feedbacks there have been very few studies that have considered the collectivity of all land surface feedbacks together, and those that did had employed models that are very crude by today’s standards. On the basis of these existing results it has been thought that land surface feedbacks by themselves are inadequate to sufficiently enhance the monsoon. Recently [1] we have shown that reasonable, proxy-guided, land surface boundary conditions when implemented within a modern coupled-climate model can indeed yield an AHP that is in excellent agreement with proxies. Our surface property adjustments taken jointly with recently described dust reduction related enhancements to precipitation [2] could be seen to provide a complete solution to the AHP problem.

[1] Chandan and Peltier, 2020, submitted in revised form to GRL

[2] Pausata et al., 2016, EPSL