In Friday’s issue of the journal Science, the researchers, led by Stefan Kröpelin, a geologist with the Institute of Prehistoric Archaeology at the University of Cologne in Germany, report that the climate transition occurred gradually. In particular, the changing types of pollen that fell on the water and drifted to the bottom tell a story of how the surrounding terrain shifted from trees to shrubs to grasses to sand — “where today you don’t find a single piece of grass,” Dr. Kröpelin said.
The findings run counter to a prevailing view that the change happened abruptly, within a few centuries, about 5,500 years ago, marking the end of the “African Humid Period” when monsoon rains poured down on the region. That view arises from ocean sediment cores drilled off the coast of Africa, to the west of Mauritania. In 2000, analysis of the cores by researchers led by Peter B. deMenocal of Columbia University’s Lamont-Doherty Earth Observatory showed a sudden rise in the dust blown off Africa at that time.
Dr. Kröpelin did not dispute the ocean core data, but said it had been “overinterpreted.”
It's got an Egyptian connection, what with the aridification (presumably) causing people to move out of the now-desert Sahara and into the Nile valley. It would affect models of how quickly this migration could have happened.