Cite as: Rewald B, Eppel A, Shelef O, Hill A, Degu A, Friedjung A, Rachmilevitch S. 2012. Hot desert environments. In: Bell EM ed. Life at extremes - Environments, organisms and strategies for survival. New York, USA, CABI Publishing. 1 Hot desert environments Boris Rewald, Amir Eppel, Oren Shelef, Amber Hill, Asfaw Degu, Avital Friedjung and Shimon Rachmilevitch French Associates Institute for Agriculture and Biotechnology of Drylands, Blaustein Institutes for Desert Research, Ben-Gurion University of the Negev. Phone: +972-86563435; Fax: +972-86596742; e-mail: rewald@rootecology.de, amireppel@gmail.com, shelefo@bgu.ac.il, bamerbright@gmail.com, asfaw1621@yahoo.com, avitush@hotmail.com, rshimon@bgu.ac.il 1. Hot desert environments 1.1 Formation and geographical distribution Hot deserts cover between 14 and 20% of the Earth's surface, around 19-25 million km 2 (see Middleton and Thomas, 1997; Peel et al., 2007 for distribution of deserts). Most hot deserts, such as the Sahara of North Africa and the deserts of the south-western United States, Mexico and Australia, occur along the tropics in both the Northern and Southern hemispheres (between approximately 10° and 30-40° latitude). They are created as a result of global Hadley air circulation (Warner, 2004). The sun’s radiation results in hot air rising and the accumulation of moisture around the equator; as the air moves away it cools, starts to descend and at this point all of the moisture is lost as rainfall in the tropics. As the air subsides and becomes compressed it also becomes warmer, so that the relative humidity in desert air is decreased - even through the absolute amount of water vapour held may be substantial, as evidenced by dew-fall in the cold hours before dawn (Parsons and Abrahams, 2009). Because most of the water in the atmosphere is derived from evaporation from the seas, there is often an aridity gradient on large continents, i.e. the land closer to the sea often receives a larger share of this water. Regions lying deep within a continent may become drylands simply because the air currents reaching them have already traversed vast land distances and lost most moisture (Warner, 2004). Continentality is a major factor, especially for the Taklimakan Desert in Central East Asia and the Australian deserts. Further factors accounting for the distribution of deserts are a low inland relief and high coastal ranges (i.e. creating a ‘rain shadow’), the presence of cool ocean water close offshore and/or aloft of a tropical jet-stream leading to the formation of coastal deserts (Goudie and Wilkinson, 1977). A good example is the Atacama Desert, Chile where the air circulation from the sea is drying out the land. Notably, coastal deserts can be only found on the western fringes of continents and are not as hot as subtropical deserts (Warner, 2004). The current boundaries and extents of hot deserts are likely to continue to expand over the next century due to a combination of global warming and poor land management (e.g., overgrazing, indiscriminative clearing of the vegetation cover, and salinization; Laity, 2008). 1.2 Types of hot deserts A defining characteristic of a hot desert is aridity. According to the Koppen-Geiger climate classification, deserts are regions with an annual precipitation <250 mm (Peel et al., 2007). However, annual precipitation can be misleading because water loss is just as important a component of the water budget. Thus, the United Nations Environmental Program’s