At dear auntie, www.bbc.co.uk/history/ancient/egyptians/apocalypse_egypt_01.shtml there is an excellent and well thought out article on the collapse of Egypt at the end of dynasty 6. People have said that the very long reign of Pepi II led to infighting among his heirs and led to decentralisation but the author strongly disagrees with this argument and several others. Fekri Hassan says there were a series of low Nile floods and the Faiyum, a lake 65m deep, dried up in the process. This implies it evaporated. He goes on to say that low Nile floods have an origin in global cooling and quotes a 9th century AD example. Global cooling leads to reduced rainfall over Ethiopia and East Africa, he says, and I suppose the same could be said right now as after a couple of prominent La Nina events, on the back of each other, there is drought in the same part of the world. This is not to say it is in any way as serious as it was in earlier epochs – Somalia has a problem and that is political. What Fekri Hassan does not address is what might have caused the global cooling. One theory is that dust and debris had accumulated in the upper atmosphere following a heavy meteoric bombardment (Moe Mandelkehr, The 2300 Event, Denver:2006) but others see it as something akin to the Pleistocene Dansgaard-Oeschger and Heinrich events with an explanation associated with ocean currents.
At http://eltahir.mit.edu/news/climate-change-6000-years-ago-sahara-desert-… is really a piece that belongs on the Climate Change list but is included here as we are already on the subject of climate in North Africa. The dessication associated with the drying out of the Sahara directly affected Egypt. Refugees from the verdant landscape of the Sahara, as it was before 4000BC, migrated towards the Nile Valley, and by 3000BC the desert had become even more inhospitable – and continued to become drier through the next couple of millennia. Now, in this study published in the Journal of Geophysical Research it is accepted that small changes in the orbit of the earth around the Sun were enough to shift the climate of North Africa, dramatically so. However, it seems the paper also says there was a movement of the southern boundary of the Sahara desert. It moved southwards by 500km – but why? Elfatih Eltahir was born in the Sudan and this is why he is so interested in the subject – but needless to say as it has an AGW slant the nasty humans and their cattle were in some way to blame for the amount of dessication. In reality the author does not know what caused the Sahara to shift by 500km over a fairly short period of time. Humans abandoned the Sahara in their droves but a few adapted, the bedouin. It is a trifle disingeneous to blame humans – who were relatively few in numbers, but no doubt they did recklessly make use of the declining vegetation and scrub. It gets cold in the desert at night. That does not explain why the climate shifted in such an obvious manner – which is the crux on what the research was supposed to be about. Merely parotting the consensus position is not enough.