Evidence, gathered by the researchers, suggests that the Sahara desertification, the largest change in land cover and climate in the last 11,000 years, came in two spurts. The first transition occurred between 6,700 and 5,500 years ago. The second, which was more severe than the first, took place between 4,000 to 3,600 years ago. Summer temperatures increased sharply and precipitation decreased, according to carbon-14 dating. The civilizations of the time and their socio-economic systems were devastated.
Feedbacks are changes in temperature or rainfall that in turn create changes in vegetation.
The changes in vegetation affect the atmosphere and so on. They concluded that oceans, as well as land-use practices of the people who cultivated the land of the Sahara were not significant enough to cause the desertification. Atmosphere and vegetation feedbacks in the subtropics were clearly to blame, they say. Further investigation is necessary, according to the researchers, to determine more precisely the specific effects of latitude and oceanic feedback, as compared with biospheric feedback, on the timing of the event. These changes, say the researchers, may explain why civilizations were founded along the Nile, Euphrates and Tigris rivers.