Aride und humide Phasen in der Sahara Afrikas während des Holozäns in einer stark vereinfachten schematischen Darstellung. Verändert nach Kevin White & David J. Mattingly (2006) Versunkene Seen in der Sahara.- SdW, September 2006, S. 51 (Original-Abb. von Barbara Aulicino).

Der folgende Text wurde der Website Fezzan Project - Palaeoclimate and environment entnommen. Die umfangreichen Literaturangaben können im Originaltext eingesehen werden. Texthervorhebungen und zusätzliche Absätze durch den Verfasser.

• Bitte beachten Sie, dass die jeweiligen humiden und ariden Phasen nicht unbedingt den gesamten Raum der Sahara betrafen, z.B. in der zweiten holozänen Feuchtphase die nördlichen Zonen der Sahara nur sehr geringe zyklonale Niederschläge erhielten (vgl. Sie bitte den Text unten!) [date of access: 28.08.06]

The most important wet periods in terms of archaeology and the development of human society occured during the Holocene (the last 10,000 years). By about 10 ka [cf. humide Phase A], rainfall was plentiful and most of the Sahara was vegetated; in the south, vegetation zones were displaced some 400 km north of their present-day positions, and fauna from the equatorial regions had migrated north into the Sahara (...).

Between about 10 ka and 8 ka, it is believed that rainfall in the Sahara was generated by the interaction between mid-latitude weather systems and the inter-tropical convergence zone, where warm dry Saharan air meets cooler moist air originating over the eastern tropical Atlantic (...). It is the northwards migration of this moist oceanic air in the form of the West African Monsoon that today brings rainfall to the Sahel (the semi-arid transition zone between the hyper-arid Sahara and the humid equatorial regions) in summer.

Significantly, it is not thought that the West African Monsoon penetrated any further north than today between 10 ka and 8 ka. The rainfall-generating systems during this period were the result of semi-permanent low pressure regions sustained by remnant ice-sheets over North America and northern Europe, and resulted in precipitation throughout the Sahara from south to north (...). The monsoon would have remained active over the Sahel.

The wet episode described above was interrupted by century-scale arid episode sometime around 8 ka, which was most probably due to the collapse of the remnants of the Laurentide Ice Sheet in North America (...). This would have caused a massive injection of cold fresh water into the North Atlantic, altering oceanic and atmospheric circulation and lowering sea surface temperatures, which would have reduced the moisture content of the atmosphere by suppressing evaporation from the ocean surface.

Reduced surface temperatures would have reduced the intensity of atmospheric convection and hence its capacity to sustain rainfall-generating weather systems. These changes represented the transition to full interglacial conditions in the northern hemisphere.

Sometime after 8 ka, wetter conditions returned to much of the Sahara as the northern hemisphere warmed, and were certainly fully established by 6.5 ka [cf. humide Phase B]. However, the northernmost parts of the Sahara remained dry.

The most likely explanation for this situation is that the transition to full interglacial conditions was associated with increased solar heating of the northern hemisphere due to changes in the tilt of the Earth (...). This would have intensified the West African Monsoon, which may have penetrated to some 30 degrees north, some 10 degrees further north than at present (...).

However, the disappearance of the ice sheets vastly reduced the interaction between mid-latitude and tropical weather systems that previously had generated rainfall in the northern Sahara (as well as much of the central and southern Sahara).

The two Holocene wet phases thus represented very different climatic regimes; it is probably that the perennial vegetation of the first phase gave way to semi-arid seasonal savannah in the second, and that in the latter the survival of human populations required greater ingenuity.

There is widespread evidence that the onset of the hyper-arid conditions that characterise the Sahara today occurred at around 5 ka (...). It is believed that the desiccation occurred in two phases, and was the result of changes in the Earth's orbital parameters which resulted in reduced solar heating of the northern African landmass that caused a weakening of the West African Monsoon.

Groundwater levels remained high in some areas after the onset of hyper-aridity, and lakes are likely to have persisted in some regions. Open water bodies and near-surface groundwater would have sustained a reduced human population in many regions; (...) has found evidence of human activity and open water bodies in the Fezzan outside of the Wadi al-Ajal as late as about 3 ka.

The Garamantian civilisation in the Wadi el-Agial appears to have developed soon after this time (...), and it is likely that people settled in the wadi in increasing numbers as access to water became more and more difficult in other parts of the Fezzan. Where such oasis refuges did not exist, people would have migrated to the the Saharan margins and the Nile Valley; it is plausible that a large influx of Saharan refugees was one of the factors that led to the development of Egyptian Dynastic civilisation; certainly a knowledge of astronomy and various religious themes appear to have been common to Pharaonic Egypt and pre-Dynastic Saharan cultures (...)."