Simulation of an abrupt change in Saharan vegetation in the mid-Holocene

Climate variability during the present interglacial, the Holocene, has been rather smooth in comparison with the last glacial. Nevertheless, there wer e some rather abrupt climate changes. One of these changes, the desertifica tion of the Saharan and Arabian region some 4 - 6 thousand years ago, was p resumably quite important for human society. It could have been the stimulu s leading to the foundation of civilizations along the Nile, Euphrat and Ti gris rivers. Here we argue that Saharan and Arabian desertification was tri ggered by subtle variations in the Earth's orbit which were strongly amplif ied by atmosphere- vegetation feedbacks in the subtropics. The timing of th is transition, however, was mainly governed by a global interplay between a tmosphere, ocean, sea ice, and vegetation.