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:
Afrotropic deserts are found in the sub-Saharan part of Africa, and in the southern fringe of the Arabian Peninsula. Pressures on the ecosystem from humans are relatively high, especially in the Horn of Africa and Madagascar.
The Australasian deserts comprise a series of lowland arid ecoregions in the Australian heartland. Hardly inhabited, their mean population density is less than 1 person per square kilometer. They have by far, the lowest human footprint among the global deserts.
The Indo-Malay region has two hot lowland deserts: the Indus Valley and the Thar. These are the deserts with the most intense human use in the world.
The Nearctic deserts cover 1.04 million square miles in North America. Because of the growth of large urban conglomerates such as Phoenix in the United States, their mean population density is high.
The Neotropic deserts in South America cover 684,000 million square miles, of which only 6 per cent receives legal protection..
By far, the Paleartic realm concentrates the largest set of deserts in the world, covering a remarkable 9.9 million square miles that total 63 per cent of all deserts on the planet and are known for their sheer inaccessibility and extreme aridity. The Sahara occupies 9 million square miles, or 10 per cent of the African continent. In contrast, the deserts of Central Asia have folded mountains with high landscape heterogeneity and enclosed basins.