The Sahara Desert is the world's largest warm desert region and covers an area of approximately 9 million square kilometers, from the Atlantic coast of Africa to the Red Sea and from the Mediterranean Sea to the Niger River and the margins of the Ethiopian highlands. Most of the Sahara Desert experiences an arid or hyperarid climate, with scant precipitation. Rainfall is mainly from winter cyclonic storms on its northern margins, whereas the southern Sahara receives summer monsoonal rainfall. The history of climate change and variability in the Sahara is a clear indication of the sensitivity of a major desert region to climate change on all timescales (Hoelzmann, 2005).

Changes in Saharan climates also affected global biophysical systems via variations in the flux of windblown dust, for which the Sahara is the major global source. The major past climate changes in the Sahara Desert were the result of cyclic changes in the Earth's orbit and are therefore not good analogues for future climates in the region, although some authors have suggested that the early-to-mid Holocene humid period may be an appropriate model for the response of the Sahara region to future climate change and global warming.

Information on the history of climate in the Sahara comes from deposits of lake basins, fossil rivers, springs, and sand dunes, supplemented by the record of marine sediments offshore of the western Sahara. Desert conditions appear to have prevailed in the Sahara for at least 7 million years, evidenced by pollen in ocean cores and sand dune deposits in Mali dating back that far, but these conditions were interrupted on many occasions by periods of increased rainfall.

The present extent of the Sahara Desert dates to a period lasting from six thousand to four thousand years ago, when the region was desiccated following an interval of much increased rainfall that lasted for several thousand years--the African Humid Period. Reconstructions of past environments from the region show that savanna vegetation covered most of the region in the early-to-mid Holocene, with perennial or intermittent streams occupying now dry wadis and large lakes in topographic basins, such as the Bodele Depression. Human and animal populations greatly increased at this time.

These lakes gradually dried out starting about six thousand years ago, resulting in increased dust flux. Increased rainfall throughout the Sahara was the result of the insolation maximum in Northern Hemisphere low latitudes caused by cyclical changes in Earth's orbit. This phenomenon promoted a much stronger West African monsoon circulation. The African Humid Period in the Sahara was, however, not uniformly wetter: A significant dry spell occurred around eight thousand years ago.

Prior to the Last Glacial Maximum (LGM), the northern Sahara experienced increased and more effective precipitation from winter storms, giving rise to groundwater recharge in the period from 45,000 to 23,500 years ago (Brooks, 2005). Studies of noble gases in groundwater indicate temperatures were 2œ-3œ Celsius lower at that time than they are today in the northern parts of Sahara. They were at least 5œ-6œ Celsius lower in the southern Sahara. Other periods of wetter climate in the Sahara are poorly dated but appear to have occurred during several interglacial periods. At the time of the LGM, the Sahara experienced intense aridity and sand dune formation that extended to areas far to the south of the modern limits of the Sahara, including Mauritania, Mali, and Niger, where linear dunes formed between twenty five thousand and twelve thousand years ago. Dust flux to ocean sediments also increased at this time. Glacial age aridity in the Sahara was accompanied by an intensified trade wind circulation and the virtual absence of monsoon circulations.

Analysis of climatic data and historical records show the sensitivity of the southern Sahara and its margins to climate variability, with major droughts occurring at regular intervals. In recent decades, such climate variability has been linked to Atlantic Ocean surface temperatures. Warmer waters off Africa result in convection over the ocean rather than the land and reduced monsoon rainfall in a wide area from Senegal to Ethiopia.

Global climate models differ in their predictions of the direction and magnitude of future change in the Sahara, as in many other arid regions, in large part because prediction of precipitation in global climate models is less reliable than are estimates of future temperatures. In the Sahara, there is support in many climate model predictions for increased rainfall in southern and southeastern areas (including the Sahel) but strong drying in the northern and western areas. Some models, however, suggest that strong drying will occur throughout the region. The differences between model predictions for the Sahara show the complexity of forcing factors for this region, as well as the possible influence of feedbacks between land surface conditions and the atmosphere, which may affect rainfall total, effectiveness, and spatial distribution.

References

1. Brooks, N., et al. "The Climate-Environment-Society Nexus in the Sahara from Prehistoric Times to the Present Day." Journal of North African Studies 10, nos. 3/4 (2005): 253-292.

2. Hoelzmann, P., et al. "Palaeoenvironmental Changes in the Arid and Sub Arid Belt (Sahara- Sahel-Arabian Peninsula), from 150 Kyr to Present." In Past Climate Variability through Europe and Africa, edited by R. W. Battarbee, F. Gasse, and C. E. Stickley. Dordrecht, the Netherlands: Springer, 2004.

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