Humans as Agents in the Termination of the African Humid Period
The termination of the African Humid Period (AHP) induced the most significant, broad-scale landscape change that occurred on the African continent during the Holocene. Although variable in tempo and spatial distribution, rainfall profoundly decreased across most of the northern half of Africa and plant and animal communities reorganized into new ecological niches (Gasse, 2000; Hély et al., 2014; deMenocal, 2015).
Human communities likewise changed their settlement and subsistence practices, shifting from foraging to agriculture-dependent economies (Marshall and Hildebrand, 2002; Blanchet et al., 2015). The termination of the AHP is hypothesized to have spurred ecological conditions favorable for the rise of complex social systems in northern Africa and southwestern Asia in which irrigation systems, agricultural resources and food redistribution networks were managed by hierarchical leaders (Kuper and Kröpelin, 2006; Gatto and Zerboni, 2015).
The orthodox view of the termination of the AHP, occurring variably across northern Africa between 8000 and 4500 years ago, is that orbital-induced weakening of monsoon advection over the Sahara caused desertification of inland regions, which enhanced albedo (the reflectance of the sun's rays off the earth's surface), accelerated dust entrainment and created a terrestrial-atmospheric feedback loop that further reduced the precipitation potential (Kutzbach et al., 1996; Gasse, 2000; Prentice and Jolly, 2000).
The degree of albedo depends on vegetation cover, which retards sunlight penetrating to the ground. Simulations and proxy records show that an abrupt weakening of monsoon strength around 8200 years ago was not permanent, and there was a partial recovery to humid conditions.
In this paper, the termination of the AHP is reviewed with the perspective that humans are potentially effective agents for inducing large-scale changes in vegetation, which can, in turn, force the crossing of ecological tipping points. Incipient pastoral economies are documented in prehistoric and historic contexts as reducing vegetal biomass. In the context of the termination of the AHP, a regime shift in vegetation was amplified by reduced inland monsoon flow and reduced precipitation. Thus, in this interpretation humans are not merely passive recipients of climatic variability, but could have been active agents in broad-scale landscape change.
Review ARTICLE in" Frontiers in Earth Science ", 26 January 2017 |
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