THERE IS A TENDENCY, in popular opinion and some public policy, to view deserts dismissively as unproductive wastelands. The 14th edition of the Encyclopedia Britannica made the case with exuberant eloquence: “Desert, a term popularly applied to any environmentally extreme, deserted, desolate, uninhabitable waste area.” While they are indeed less productive of biomass than other land areas at comparable altitudes and latitudes, deserts and their unique environmental characteristics have long interested physical geographers and ecologists. And while deserts do appear inhospitable, most have long histories and prehistories of human habitation and resourceful human adaptation to desert landscapes. Deserts account for a substantial fraction of the Earth's total land area, occurring in every continent but Europe and dominating northern Africa, Asia Minor, and central and western Australia.
The defining characteristic of a desert is a mean annual precipitation of 10 in (24 cm) or less. Climatologists refine this definition, however, by contrasting mean annual precipitation to rates of evaporation and transpiration, or water lost by plants. Ecologists may further stress adaptations of desert plants and animals to varying conditions of aridity over time. Although the climate of ANTARCTICA fits the climatological definition of a desert, and while it is so considered by some geographers, the special characteristics of the continent are generally excluded from discussions of desert geography and ecologies. The term is applied almost exclusively to land areas, but comparisons to comparably underproductive areas of open ocean can make for worthwhile ecological discussions.
Even if Antarctica is excluded, the global distribution of deserts is quite diverse. Deserts occur at tropical and temporal latitudes, centered on the Tropics of Cancer and Capricorn. Little of the land area at the equator (the exception is the HORN OF AFRICA) is desert. The greatest continuous land area of the world's deserts is found in a band stretching along the TROPIC OF CANCER from the North Atlantic across Africa and continuing through Asia Minor and into regions of KAZAKHSTAN, northeast of the CASPIAN SEA. While most deserts abut oceans along a portion of their boundaries, a small number, including the GOBI DESERT, are LANDLOCKED.
Two climatological factors tend to create conditions of aridity and hyperaridity (very high ratios of evaporation and transpiration, or evapotranspiration, to precipitation). One factor is global atmospheric circulation, augmented by cold ocean currents: Most deserts are found in “horse latitudes,” around 30 degrees latitude, and along the western coasts of continents. Under such conditions, air warmed near the equator circulates aloft to the north and south, cooling, sinking, and compressing as it reaches the higher latitudes. Deserts also occur on the leeward side of mountain ranges, where a “rain shadow” forms as air movement over mountains cools and condenses moisture in the air on the windward side, thus drying the air that passes to the lee.
Although the aridity of deserts is often associated with high surface temperatures, some of the world's deserts occur in regions of lower mean annual surface temperatures. These latter are therefore known as cold deserts.
Among the geomorphological features found in deserts, the best known are sand dunes. Dunes are not found exclusively in desert regions, however; they are products of eolian (wind-driven) processes that can occur under a variety of conditions. Even so, the bulk of the world's sand dunes are found in deserts. Geomorphologists classify dunes according to forms that arise in response to the supply of sand, the persistence of wind direction and speed, and the presence or absence of stabilizing vegetation. Steady winds, very low vegetation, and moderate sand supplies create barchans, dunes having a crescentic shape as viewed from aloft.
Rocky plateaus and plains formed from alluvium with little soil development are more universally typical of desert landscapes. In the deserts of North America, these occur in regions that have been extensively deformed by normal faulting, uplifting alternating blocks of crust relative to others. The resulting basin and range (or graben and horst) structure is characteristic. Basins become loaded with sediments that have eroded from adjacent ranges. Due to low precipitation and therefore low rates of sediment transport, sediments tend to be sorted with finer grains collecting at the centers of basins (known as bolsons), coarser grains at the margins, and the coarsest grains or talus found in the bajada, at the boundary between a plain and the uplifted pediment. The remains of an intermittent or relict lake at the center of a basin are called a PLAYA.
Soil development in desert regions is generally poor. Organic material is sparse, and aridity promotes preservation rather than decomposition. Under some conditions, especially in basin and range landscapes, sediments are reworked chemically, resulting in accumulations of soluble minerals such as gypsum and calcium carbonate. High concentrations of calcium carbonate can cement alluvium at the surface in caliche, or desert pavement.
In spite of the inauspicious conditions for life that obtain in desert regions, thousands of species of organisms have evolved to survive and even to thrive under such conditions. Typical adaptations in desert plants, for instance, include means to reduce transpiration and tissues adapted to store water against periods of drought. There is no one strategy for coping with arid conditions; some plants have taproots to reach down into sources of groundwater, while others have radial systems of roots close to the surface to maximize absorption when rain falls.
Humans have long inhabited, traversed, and found refuge in desert regions throughout the world, deriving sustenance by hunting and gathering, through agricultural practices that include but do not necessarily require irrigation, and as nomadic pastoralists. Growing and stratified or stratifying societies have emphasized agriculture, constructing IRRIGATION networks in many of the world's deserts to support sustained and reliable yields. Efforts to irrigate desert lands have tended to be limited in longevity because of the high rate of evaporation in desert climates, which results in an aggregation of minerals, unhealthful for plant life.