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Wednesday, October 17, 2007

Wetter Climate Fueled Human Migrations

PNAS 104:16416-21
East African megadroughts between 135 and 75 thousand years ago and bearing
on early-modern human origins
CA Scholz cs.2007

The environmental backdrop to the evolution and spread of early Homo sapiens in East Africa is known mainly from isolated outcrops and distant marine sediment cores. Here we present results from new scientific drill cores from Lake Malawi, the first long and continuous, high-fidelity records of tropical climate change from the continent itself. Our record shows periods of severe aridity between 135 and 75 ka, when the lake's water volume was reduced by at least 95 %. Surprisingly, these intervals of pronounced tropical African aridity in the early late-Pleistocene were much more severe than the Last Glacial Maximum, the period previously recognized asone of the most arid of the Quaternary.
From these cores and from records from Lakes Tanganyika (East Africa) and Bosumtwi (West Africa), we document a major rise in water levels and a shift to more humid conditions over much of tropical Africa after c.70 ka. This transition to wetter, more stable conditions coincides with diminished orbital eccentricity, and a reduction in precession-dominated climatic extremes. The observed climate mode switch to decreased environmental variability is consistent with terrestrial and marine records from in and around tropical Africa, but our records provide evidence for dramatically wetter conditions after 70 ka. Such climate change may have stimulated the expansion and migrations of early modern human populations.

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