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Greening Sahara

25 July 2021
Climate change

There are some interesting reads in the latest issue of Current World Archaeology [108 August 2021] – see past issues at www.world-archaeology.com – but one that I always make a beeline for is Chris Catling's column – see page 60 and 61. It concerns climate change and archaeology – in the Sahara. During Late Pleistocene. For example, he quotes a recent paper that claimed there was evidence that violence broke out due to climate change, after archaeologists re-examined a cemetery on the Nile that dated back to 13,400 years ago. On my calculation that would be slap bang in the middle of the Bolling-Alleroed warm period, often presented as Dansgaard-Oeschger warming following the Oldest Dryas event. However, no doubt my calculation is wrong – but in any case, we have a cemetery of people that are described as hunters and fishers [dodging the crocodiles in the Nile] that left behind a lot of skeletons that had traces of lesions, healed or otherwise. The new study, instead of lumping them all together as the result of a single violent incident, say the bodies were inhumed over an extended period of time – reflecting tensions arising with their neighbours [presumably because they had access to good hunting and fishing grounds]. So we now have frequent conflict, or skirmishes, rather than a short and sweet single encounter. The statement, 'most probably due to climate change' is quoted by Catling. Are archaeologists now advocates of global warming – peering into their crystal balls?

However, what really caught my attention was the next research paper mentioned by Catling, by Enno Schefuss and Cheddadi in PNAS. They analysed pollen and leaf material from a sediment core from a lake in the High Atlas of Morocco – mentioned on the News a few weeks or months ago. The Green Sahara is said to define a period between 14,500 to 5000 years ago, when large areas of North Africa were well populated and the Sahara was green with vegetation and ample water in rivers and lakes etc. The duration of the greening is open to a bit of movement on either end but that is roughly the mainstream position. It is unclear if the Sahara was a desert prior to 14,500 years ago – but that is a different story. At this point, Catling casually throws in the line, 'until now, researchers have attributed this phenomenon to the tilting of the earth's axis ….' bringing summer monsoon rain further north. Unfortunately, after an initial spurt of interest, I realised he was not really talking about physical movement at the axis of rotation. They are of course referring to the slow gradual tilt of the earth as it orbits around the Sun, as used by Milankovitch in his very clever set of calculations. If there had been a shift in the axis of rotation at some point after the Late Glacial Maximum, that might well explain the greening of the Sahara – but no, this is not what was implied. Various earlier papers have suggested the monsoon track by moving northwards brought enough rain to bear on the Sahara, causing vegetation to grow and rivers to flow. We may note this also applies to the Sinai, the Negev, and the Arabian peninsular, and most probably, what is now Pakistan and Baluchistan. Indeed, most of Iran. It seemed like a good theory and various paleoclimate scientists recited the mantra until a recent study, a year or so ago, realised that a shift of the Monsoon was not enough. The Sahara is a big place – and a shift in the monsoon track could not have affected all of it. In this latest study the team sought to find out what was required to make the whole of the Sahara green – and our quandary is – are they right, or not.

In other words, the Milankovitch style tilt of the earth was not enough to explain the situation. The sediment core and its pollen and leaf material was Mediterranean in nature – more commonly associated with Italy and Greece in the modern world. These plants rely on winter rains – and the summer monsoon rain would not have favoured their existence. The authors continue by saying this means there was a move southwards by the westerlies, a belt of winter rain fall, much as modern Mediterranean type plants rely on them. Or at least, the Mediterranean climate may account for the greening of North Africa – but once again, not for the whole of the Sahara [as crocodiles, giraffe and elephants were part of the fauna]. So, did the monsoon track move northwards just as the westerlies track moved south? If so, why? And how? I look forwards to the sequel.

Catling has a knack for making the reader think, dredging out the odd sentence of a research author that one might themselves, or ourselves, have missed. He also has an excellent take on pre-coinage prehistoric money systems, noting how well it worked, and why.

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