After a complex Last Glacial and deglaciation periods when abrupt dry/wet shifts coincide remarkably with cold/warm events in Greenland, a period considerably wetter than today established rapidly at the onset of the Holocene. Between ca. 11.5-11 and 6-5 ka BP (early-mid Holocene), the Sahara was wet and green, with numerous lakes and rivers. Lake Chad covered an area of 350,000 km2 (against 20,000 today). Closed East African lakes extent several tens or hundreds meters above their present-day level. All data indicate an intensification of the monsoon and a northward migration of 500-600 km of the tropical rainfall belt. Paleoprecipitation estimates based on hydrological modelling of individual lake systems suggest an increase of 30 to >> 100% compared to modern. Many records document, however, a short-term but marked arid event around 8.5-8 ka BP possibly linked to a minor cooling recorded in Greenland ice cores.
The major step of the North Africa aridification is abrupt, but its exact timing depends on the region. Its started earlier in central Sahara (as soon as 8 ka BP as some sites), than southward where it appears more gradual (7-4.5 ka BP). It is followed by wet episodes of minor amplitude, (e.g., around 4-3.5 ka). High resolution records suggest increasing climate instability during the Late Holocene.
Orbitally-induced changes in summer insolation has been the primary cause of changes in monsoonal rainfall. Nevertheless, neither the amplitude nor the abruptness of the observed hydrological changes can be explained by orbital forcing alone. The role of linkages with high latitude climate through oceanic and atmospheric circulation, significant positive feedbacks from the ocean, vegetation and soil moisture is discussed through comparison with marine records and General Circulation Model simulations.
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