Okay, we have now looked at the Younger Dryas and the 8,200 year event - those two are pretty famous and are expected to be in the vocabulary of every geography/geology/earth science student. However, asking any of them whether they have heard about the 5.9 kiloyear event, I would expect hardly any nods. And yet it is one of the most intense aridification events during the Holocene!
You might find it hard to believe, but once upon a time the Sahara desert was actually a steppe, covered in grasses and shrubs! Pretty attractive for human hunters, it is exactly where the buffaloes and goats got domesticated. Then, around 3900 BC, an abrupt decrease in Saharan vegetation took place:
According to Hoelzmann et al. (1998) orbital variability was responsible for the monsoon strength in Africa and other parts of the tropics. Therefore, as rainfall gradually decreased eventually conditions became too dry for the plants and a transition to dusty, desert conditions took place.
So what about the humans?
Brooks (2006) explored the links between mid-Holocene socio-cultural and environmental change in 6 key regions: central Sahara, Egypt, Mesopotamia, South Asia, northern China and coastal Peru. He argued that the 5.9 kiloyear event (coinciding with Bond Event 4) has triggered worldwide migration to river valleys and has eventually led to the emergence of the first complex, highly organised, state-level societies in the Afro-Asiatic monsoon belt and northern South America in the 4th millennium BC.
List of references:
Brooks, N. (2006) 'Cultural Responses to Aridity in the Middle Holocene and Increased Social Complexity', Quaternary International, 151, 1, 29-49.
Hoelzmann, P., D. Jolly, S. P. Harrison, F. Laarif, R. Bonnefille and H.-J. Pachur (1998) 'Mid-Holocene Land-Surface Conditions in Northern Africa and the Arabian Peninsula: A Data Set for the Analysis of Biogeophysical Feedbacks in the Climate System', Global Biogeochemical Cycles, 12, 1, 35-51.
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