ENVIRONMENT
The End of the Green Sahara
~5,500 years ago
Neolithic · Neolithic · Holocene · Northgrippian
About 5,500 years ago, the Sahara dried out after several millennia of savannah and lakes, as the slow shift of the Earth's orbit weakened the monsoon.
The end of the African Humid Period designates the drying of the Sahara that occurred about 5,500 years ago, at the close of several millennia during which the region carried savannah cover, lakes and herding populations, together with a fauna including the hippopotamus. Its cause is orbital: the slow variation of the parameters of the Earth's orbit reduced northern summer insolation and weakened the African monsoon. The archives diverge on the rhythm of the transition, abrupt in marine records, extended over a millennium and punctuated by dry episodes in the sediments of a lake in the Ethiopian rift. Populations withdrew towards the Nile, the Sahel and the oases.
Why it matters
The episode constitutes the reference case of a non-linear climatic response to gradual forcing, a slow inflection of insolation producing a regional transformation of great magnitude. It also intervenes in human history, the concentration of populations along the Nile coinciding with the formation of the Egyptian state. The present-day desert postdates agriculture.
Dating & uncertainty
The two types of archive measure different objects, which explains their divergence. Wind transports Saharan dust over the Atlantic, where it settles in layers: an increased dust content indicates a drier continent, and these records, off Mauritania, place an abrupt end 5,500 years ago. Continental archives extend the transition over a millennium, earlier in the north and east than in the south and west. A marine core integrates the dust of an entire continent, whereas a lake sediment records only its own catchment, which accounts for most of the discrepancy.
Our species’ history as one day
23:34:5125 min 9 s before midnight
if the 315,000 years of Homo sapiens were compressed into a single day