A desert is an arid land area where more water is lost through evaporation than is gained from precipitation. Deserts include the familiar hot, dry desert of rock and sand that is almost barren of plants, the semiarid deserts of scattered trees, scrub, and grasses, coastal deserts, and the deserts on the polar ice caps of the Antarctic and Greenland.
Most desert regions are the result of large-scale climatic patterns. As the earth turns on its axis, large air swirls are produced. Hot air rising over the equator flows northward and southward. The air currents cool in the upper regions and descend as high pressure areas in two subtropical zones. North and south of these zones are two more areas of ascending air and low pressures. Still farther north and south are the two polar regions of descending air. As air rises, it cools and loses its moisture. As it descends, it warms and picks up moisture, drying out the land. This downward movement of warm air masses over the earth have produced two belts of deserts. The belt in the northern hemisphere is along the Tropic of Cancer and includes the Gobi Desert in China, the Sahara Desert in North Africa, the deserts of southwestern North America, and the Arabian and Iranian deserts in the Middle East. The belt in the southern hemisphere is along the Tropic of Capricorn and includes the Patagonia Desert in Argentina, the Kalihari Desert of southern Africa, and the Great Victoria and Great Sandy Deserts of Australia.
Coastal deserts are formed when cold waters move from the Arctic and Antarctic regions toward the equator and come into contact with the edges of continents. The cold waters are augmented by upwellings of cold water from ocean depths. As the air currents cool as they move across cold water, they carry fog and mist, but little rain. These types of currents result in coastal deserts in southern California, Baja California, southwest Africa, and Chile.
Mountain ranges also influence the formation of deserts by creating rain shadows. As moisture-laden air currents flow upward over windward slopes, they cool and lose their moisture. Dry air descending over the leeward slopes evaporates moisture from the soil, resulting in deserts. The Great Basin Desert was formed from a rain shadow produced by the Sierra Nevada mountains. Desert areas also form in the interior of continents when prevailing winds are far from large bodies of water and have lost much of their moisture.
Desert plants have evolved methods to conserve and efficiently use available water. Some flowering desert plants are ephemeral and live for only a few days. Their seeds or bulbs can lie dormant in the soil for years, until a heavy rain enables them to germinate, grow, and bloom. Woody desert plants can either have long root systems to reach deep water sources or spreading shallow roots to take up moisture from dew or occasional rains. Most desert plants have small or rolled leaves to reduce the surface area from which transpiration of water can take place, while others drop their leaves during dry periods. Often leaves have a waxy coating that prevents water loss. Many desert plants are succulents, which store water in leaves, stems, and roots. Thorns and spines of the cactus are used to protect a plant's water supply from animals.
Desert animals have also developed protective mechanisms to allow them to survive in the desert environment. Most desert animals and insects are small, so they can remain in cool underground burrows or hide under vegetation during the day and feed at night when it is cooler. Desert amphibians are capable of dormancy during dry periods, but when it rains, they mature rapidly, mate, and lay eggs. Many birds and rodents reproduce only during or following periods of winter rain that stimulate vegetative growth. Some desert rodents (e.g., the North American kangaroo rat and the African gerbil) have large ears with little fur to allow them to sweat and cool down. They also require very little water. The desert camel can survive nine days on water stored in its stomach. Many larger desert animals have broad hooves or feet to allow them to move over soft sand. Desert reptiles such as the horned toad can control their metabolic heat production by varying their rate of heartbeat and the rate of body metabolism. Some snakes have developed a sideways shuffle that allows them to move across soft sand. Deserts are difficult places for humans to live, but people do live in some deserts, such as the Aborigines in Australia and the Tuaregs in the Sahara.
Desert soils are usually naturally fertile since little water is available to leach nutrients. Crops can be grown on desert lands with irrigation, but evaporation of the irrigation water can result in the accumulation of salts on the soil surface, making the soil unsuitable for further crop production. Burning, deforestation, and overgrazing of lands on the semiarid edges of deserts are enabling deserts to encroach on the nearby arable lands in a process called desertification. Desertification in combination with shifts in global atmospheric circulation has resulted in the southern boundary of the Sahara Desert advancing 600 mi (1,000 km) southward. A desertification study conducted for the United Nations in 1984 determined that 35% of the land surface of the earth was threatened by desertification processes.