The Deserts of the World

A desert is defined by low rainfall, usually less than 250 millimetres a year, rather than by heat alone. The largest desert on Earth is the Antarctic desert, a cold desert of ice, followed by the Arctic desert; the Sahara, at about 9.2 million square kilometres in North Africa, is the largest hot desert. Other major hot deserts include the Arabian, the Gobi in Central Asia, the Kalahari in southern Africa, the Namib on Africa's south-west coast, the Atacama in Chile, the Mojave and Sonoran in North America, and the Australian deserts. Cold deserts also include the high, dry plateaus of Central Asia and the Patagonian desert in Argentina.

Deserts form for different reasons. Subtropical deserts, such as the Sahara and the Australian outback, lie under descending dry air in the subtropical high-pressure belts, roughly 20° to 30° from the equator. Coastal deserts, such as the Atacama and Namib, form where cold ocean currents cool the air and prevent rain. Rain-shadow deserts, such as Patagonia and parts of the Thar's margins, lie behind mountains that strip moisture from winds. Continental deserts, such as the Gobi, lie far from the sea, so moist air has lost its water long before it arrives. Knowing the cause helps predict where deserts will expand or shrink as climate changes.

Life in deserts is adapted to extremes. Cacti store water in fleshy stems, desert animals are often nocturnal or burrowing to avoid daytime heat, and many plants complete their life cycle quickly after rare rain. Human life has also adapted: pastoral nomadism, oasis farming, trade routes and, in modern times, mining, solar energy and tourism. The Atacama, one of the driest places on Earth, is a centre for astronomy because its skies are exceptionally clear, while the Sahara's oases and caravan routes shaped trade between North and West Africa for centuries.

For civil studies, deserts raise questions about limits and adaptation. Desertification, the degradation of drylands through overgrazing, deforestation, poor irrigation and climate change, threatens the livelihoods of hundreds of millions of people, especially in Africa and Asia. At the same time, deserts offer solar energy potential, minerals and space for new infrastructure. The policy challenge is to use drylands without destroying the thin vegetation and fragile soils that hold them together, and to support the people who live there rather than treating deserts as empty land.