At www.physorg.com/print212152097.html … a study of lake sediments from a dried up lake in northern Chad in the Sahara has shown evidence that the lake was dessicated by a slow process and progressively, beginning in around 6000 years ago (4000BC) and reaching the present condition around AD900. Geochemical and the analysis of sediments were done on a yearly basis to determine when the Sahara went dry. It seems there is a widely held belief by scientists that the Sahara dried up due to a change in the earth's orbit that affected solar insolation, or the amount of electromagnetic energy the earth receives from the sun. Basically, this means the amount of sunlight reaching different regions of the earth at any given time and at any moment in the tilt of the orbit of the earth. Climate scientist Gavin Schmidt, widely known for his activities at the http://realclimate.com blog, blamed the drying up of the Sahara on a change of tilt, consistent with the Milankovitch calculations, involving orbital tilt and precession. Most scientists appear to support the Milankovitch effect as the primary reason for climate change such as the Ice Ages – and tilt changes over a 41,000 year cycle and precession over 26,000 years. However, at this point modern AGW theory rears its head, and we are told that incredibly, AGW can actually speed up the otherwise gradual changes in axial tilt. Yes, the current melting of the ice in Greenland is already causing the tilt to change more rapidly than the Milankovitch methodology allows. Change in tilt and insolation have an impact on weather patterns such as the monsoon. During the greening of the Sahara the monsoon rains were much more powerful, it is alleged, but when tilt changed the monsoon rains decreased – and vegetation began to disappear. When there were no plants to retain water and release it back into the atmosphere the rainfall levels plummeted. Eventually, it produced the huge desert that is the Sahara today.
However, it seems that most of this might be guesswork as it relies a great deal on modelling – and on top of that has been added the computer simulation of Gavin Schmidt and buddies. He thinks the change in climate in the Sahara took place more quickly than envisaged by your average scientist. The problem is there has been very little research done on the ground, the lake sediments above being an exception to the rule. Most of it is based on modelling techniques and that is of course the province of climate scientists as well – and people like Schmidt are seeking paleo evidence to support modern alarmism. Schmidt has come out and dated the transition from a green to a dry environment at 5500 years ago. He assures his disciples that a small change in the orbit led to an abrupt collapse in the eco-system. He appears to prefer computer simulation to actual field research – which is why he can ignore the paper above.