Deserts cut cross our planet along two fringes parallel to the equator, at 25–35° latitude in both the northern and southern hemispheres. The Desert Biome can be defined climatologically as the sum of all the arid and hyper-arid areas of globe; biologically, as the ecoregions that contain plants and animals adapted for survival in arid environments, and, physically, as large contiguous areas with ample extensions of bare soil and low vegetation cover. A map produced by overlaying areas under these three criteria shows a composite definition of the world’s deserts, occupying almost one-quarter of the earth’s land surface, some 33.7 million square kilometres.
Deserts landscapes are diverse; some are found on a flat shield of ancient crystalline rocks hardened over many millions of years, yielding flat deserts of rock and sand such as the Sahara, while others are the folded product of more recent tectonic movements, and have evolved into crumpled landscapes of rocky mountains emerging from lowland sedimentary plains, as in Central Asia or North America.
Over the last two million years — the Pleistocene period — climatic variations of the earth have transformed the world’s deserts, forcing them to shrink during cold glacial periods and expand during the hot interglacials, leading finally to the current warming and aridization trend of the last 5 000 years, from the mid-Holocene to date. Some of the Ice-Age species still survive in arid mountain ranges, or desert “sky-islands”, as rare relictual organisms.
Most large deserts are found away from the coasts, in areas where moisture from the oceans rarely reaches. Some deserts, however, are located on the west coasts of continents, such as the Namib in Africa, or the Atacama in Chile, forming coastal fog-deserts whose aridity is the result of cold oceanic currents.
The deserts of the world occur in six global bio-geographical realms:
- The Afrotropic deserts are found in the sub-Saharan part of Africa, and in the southern fringe of the Arabian peninsula. Their mean population density is 21 persons per square kilometre, and their human footprint (that is, pressures on the environment resulting from human activities) is relatively high, especially in the Horn of Africa and Madagascar.
- The Australasian deserts comprise a series of lowland arid ecoregions in the Australian heartland, covering in total some 3.6 million square kilometres, of which some 9 per cent is under some degree of environmental protection. Hardly inhabited at all, their mean population density is less than 1 person per square kilometre, and show, by far, the lowest human footprint among the global deserts.
- The Indo-Malay region harbours only two hot lowland deserts — the Indus Valley and the Thar — covering in total 0.26 million square kilometres, of which some 20 per cent receives some level of legal environmental protection. With a mean density of 151 persons per square kilometre, these are the deserts with the most intense human use in the world.
- The Nearctic deserts cover 1.7 million square kilometres in North America, of which 19 per cent is under some level of legal protection. Because of the growth of large urban conglomerates such as Phoenix in the United States, their mean population density is high (44 persons per square kiometre) and their mean human footprint (21) is the second highest of the world’s deserts, especially in the Sonoran and Chihuahuan deserts.
- The Neotropic deserts in South America cover
1.1 million square kilometres, of which only 6 per cent receives legal protection. Their mean population density is 18 persons per square kiometre, and their mean human footprint
(16) is lower than in their North American counterparts, with most pressure concentrating in the Sechura Desert in the coasts of Peru.
- By far, the Paleartic realm concentrates the largest set of deserts in the world, covering a remarkable 16 million square kilometres that total 63 per cent of all deserts on the planet. Their population density is 16 persons per square kilometre, and their mean human footprint (15) is the second lowest on the planet, possibly because of their sheer inaccessibility and extreme aridity. The Sahara, an immense shield-desert, occupies 4.6 million square kilometres, or 10 per cent of the African continent. In sharp contrast with the flat Sahara and Arabian deserts, the deserts of Central Asia present folded mountains with high landscape heterogeneity and enclosed basins, some of which contain large lakes such as the Caspian and Aral Seas.
With summer ground surface temperatures of near 80°C, and only enjoying very ephemeral pulses of rain, species in deserts have evolved remarkable adaptations to harsh conditions, ranging from plants adapted to the fast use of ephemerally-abundant water or to extraordinarily efficient use of scarce water, to behavioural, anatomical, and physiological adaptations in animals. Some species from different deserts show striking resemblances in their appearances despite their differences in phylogenetic origins and biogeographic histories, a phenomenon known as convergent evolution. As a survival strategy, many desert species have symbiotic interactions and cooperate with each other through pollination, fruit dispersal, or by providing protective shade.
True deserts are not the final stage of a process of desertification; they are unique, highly-adapted natural ecosystems, both providing life-supporting services on the planet and supporting human populations in much the same ways as in other ecosystems.