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| Figure 1: An oasis in the Sahara desert (source) |
In 2008, a team of palaeontologist set out on a trip to hunt for dinosaur bones to the Ténéré desert, northern Niger, on the southern flanks of the Sahara, and their story was detailed by Gwin (2008). Today, the landscape of the Ténéré is barren with rolling sand dunes and average summer temperature can reach 40°C. Sereno et al. (2008) details how instead of finding dinosaur bones, they came across a Stone Age graveyard full of human bones! Among the sand dunes, they found clay pots, beads, stone tools, arrowhead; a clear evidence of a Neolithic society. They found bones of water-loving animals including the bones of crocodiles, hippos, turtles and fish which today are found nowhere near the dry, barren region - evidence indicating a possible palaeolake. Rock engravings were found with images of animals and vegetation, indicating a lush, greener region (figure 2). In addition to this, it came clear to the archaeologists that the human bones were not of the same age. The earliest bones belonged to the fishing-based Kiffian tribe, dated between 10,000 and 8,000 years ago, which indicates a wetter climate when the Kiffian occupied the region. The youngest bones belonged to the nomadic, herding-based Tenerian tribe that occupied the region a couple of thousand of years after the Kiffian disappeared, between 6,500 and 4,500 years ago. Some of the Kiffian skeletons reached 6 foot 8 inches in height, were thick-bones and well-muscled as is expected of a strenuous fishing life. In comparison, the skeletons of the Tenerian tribe appeared were smaller, and some of the skeletons were laid to rest on a bed of flowers as indicated from the presence of pollen. Clearly, between 10,000 and 4,500 years ago, regions of the Sahara desert was lush enough to sustain a fishing culture which transitione to sustaining a herding lifestyle. But after 4,500 years ago, this region of the Sahara could no longer sustain life, and the tribes vanished.
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| Figure 2: a rock engraving with an image of a giraffe, an animal which today cannot survive in this region (source) |
Gasse et al. (1990), and many other studies identify, an African Humid Period (AHP) approximately 12,000 years ago marking a change into a lush, vegetated Sahara which terminated 5,5000BP. This is also referred to by Sereno et al. (2008) as the 'Green Sahara'. The mechanism behind this transition to a dry Sahara is still debated, yet three potential mechanism have been proposed to solve 'The Green Sahara problem'. Firstly, Berger (1978) suggests that it occurred to due orbital forcing which caused the Earth to be closest to the sun in the Northern hemisphere in the summer and a significant gradient difference between the equator and the poles to form, pushing the ITCZ and the rains northward across the Sahara during the AHP. A recent paper by Shanahan et al., (2015) supports the work of Berger and demonstrated that a change in incoming solar energy caused an abrupt collapse of the African Monsoon, terminating the AHP. Secondly, Charney (1975) suggests that a decrease in vegetation cover cause the termination of the AHP by increasing the difference in albedo between the Sahara and the oceans, strengthening the zonal circulation winds and decreasing the rainfall across the Sahara. A more recently proposed theory is that of Swann (2014) which suggests that the decline in forests across Europe caused a southward shift in the ITCZ and altered precipitation patterns. Understanding what happened during this period might be useful to see how the Sahara region, and other deserts across the world, might react to future climate change.
How this climate change and ecological transition effected the human occupant of the Sahara during the Holocene has been explored by Manning and Timspon (2014). By studying 1101 archaeological sites, they demonstrate a major shift in human behaviour and population between 10,500 and 5,500 years ago corresponds to the African Humid Period (figure 3). The paper admits that the lack of available palaeoclimate data and the assumptions and limitations associated with palaeoclimate proxies question the reliability of the correlation test and the uncertainty in the findings. Timing issues arise between the demographic shift in 11,000BP and the increase in aeolian dust in the sedimentary record at 12,500BP, and this temporal delay questions the understanding of the interplay between humans, the environment and the climate. Nonetheless, this study, along with many others, show how climate change was the prime driver behind the demographic shift and the major collapse in population 5,200 years ago. Understanding the timing and nature of this change in the Sahara has major implications for understanding the links between climate change and human population today.
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| Figure 3: still frames from a model simulating the population density across the Sahara between 12,700 and 2,900 years ago. An exponential increase is seen between 10,200 and 7,500BP and a rapid decrease is seen between 4,900 and 2,900BP. (source) |