Deserts of the World

A desert is most commonly defined as a region that receives less than 250 millimeters of precipitation in an average year. Some classification systems tighten the definition by also considering the potential evaporation rate, since a dryland is only truly arid when water leaves the landscape faster than it can arrive. By either measure, drylands account for approximately one third of Earth's land surface, a share that has remained remarkably stable on continental scales even as local boundaries have expanded and retreated over geologic time.

The popular image of a desert is dominated by the Sahara and the Arabian Peninsula: dune fields, camel caravans, and blazing sun. In reality, geographers recognize several distinct families of desert. Hot subtropical deserts form along the great belts of descending dry air near 30 degrees north and south of the equator. Cold winter deserts, such as the Gobi or the Great Basin, endure long snowy winters and short growing seasons. Coastal deserts sit alongside cold ocean currents and are often shrouded in fog rather than baked by sun. Rain-shadow deserts appear on the leeward side of high mountain ranges, where descending air has already lost its moisture. Polar deserts, finally, include the vast Antarctic continent and the Arctic archipelagos, where extreme cold locks water away as ice and makes liquid moisture scarce at ground level.

Classification can therefore be framed in several ways: by latitude (tropical, temperate, polar), by temperature (hot, cold), by moisture regime (arid, hyper-arid), and by geographic setting (subtropical, rain-shadow, coastal, continental, polar). These categories overlap. The Atacama, for example, is simultaneously subtropical in latitude, coastal in setting, and hyper-arid in moisture, while the Gobi is cold, continental, and shaped by both distance from the ocean and the Himalayan rain shadow.

Deserts are not lifeless. They support distinctive plant and animal communities, some of the world's oldest human societies, irreplaceable paleontological records, and significant reserves of minerals, oil, and gas. They also serve as sentinels for climate change: small shifts in temperature or rainfall can push marginal lands across the threshold into full aridity, a process the United Nations tracks under the broad heading of desertification.