Birth of the Sahara

13 Nov 2013

This should actually be, 'the birth of the modern Sahara' as apparently the desert has come and gone on a number of occasions. At ... more clues to the episode separating the Pre Dynastic from the Old Kingdom periods in Egypt. In an analysis of a sediment core off Aden Peter de Menocal and Jessica Tierney found that the Sahara dried out in as a little as a hundred years - 5000 years ago. Trees and savannah grassland were part of the Sahara landscape prior to 3000BC - but at that point the climate changed abruptly. Since then the Sahara desert has continued to dry out - and vegetation has become somewhat rare.

Seems like we are going backwards to an earlier view, and one generally accepted by most Egyptologists (in the past). In recent years the Milankovitch model has coloured descriptions of the switch from a wetter regime to a dry almost rainless environment, visualised as a rather strung out and prolonged affair. Basically, the theory was that the monsoon system gradually became less and less able to sustain vegetation and rivers in the Sahara, as a result of orbital changes as the Earth goes round the Sun. As these things are prone to smoothing, all it takes are a few glitches, or dry periods, and the data begins to look as if rainfall was declining over a long period of time. A similar problem exists with the curve produced for sea level changes in NW Europe - from the end of the Ice Age. A series of glitches, or periods of rapid sea level change, have produced a long and beautifully rounded curve. It is only at the point the curve comes largely to a halt, around 3000BC, that the rate of sea level change is accepted as coming to an end (although most of it occurred prior to 6000BC). So it is with rainfall in the Sahara - an assumption that it was drying out gradually when in fact there were several periods of dry weather interspersed within the wetter climate, most notably at 6000BC and around 4200BC, and when the data is smoothed it seems to support the idea that the Sahara was drying out gradually as a result of a declining monsoon system.

Peter de Menocal is the author of a number of important papers in the past and in his latest collaboration with Tierney, published in the journal Science, he acknowledges the Milankovitch model by saying the shift to a wetter climate was triggered by more sunlight falling on Earth's northern hemisphere as Earth's cyclic orientation toward the Sun changed. He is therefore sticking rigidly to the consensus explanation for the wetter Sahara climate, even though its geographical position, wedged between the tropics and the temperate zone, more or less demands a dry climate. Be that as it may, the paper says that around 3000BC the Sahara became very dry over a comparitively short period - and this fact is displayed in the sediment core off the Horn of Africa. It so happens this coincides with a prominent low growth tree ring event and a crisis in civilisation across western Asia. It also included long distance migration of groups of people. For example, horse mounted barbarians from the steppes - bringing the horse and the Royal tombs of Ur.

What occurred in the Sahara around 3000BC was an abrupt switch in rainfall - suggesting a reduced monsoon for some unexplained reason. Dry periods prior to 3000BC did not apparently result in a permanent dry climate - although the very wet climate of the early Holocene had largely faded by 3000BC, the period 6000-3000BC being more like the savannah zone of the Sahel, immediately south of the Sahara. This is why it looks so much like a decline of the monsoon system when in all likelihood something more complex was going on.The significance of the paper is that de Menocal and Tierney have proved that the abrupt climate switch could not have had anything to do with the monsoon system. There was something else going on that affected the level of rain produced by the monsoon system - and what was true of North Africa and the Near and Middle East must also have affected India and China.