Desert biomes stretch across every continent on Earth, covering roughly a third of the planet’s land surface. They cluster along two broad belts near the Tropics of Cancer and Capricorn, where atmospheric patterns suppress rainfall, but they also appear deep inside continents, in the rain shadows of mountain ranges, and along coasts cooled by frigid ocean currents. Some of the largest are brutally hot; others are frozen year-round. Their global distribution follows a logic rooted in how the atmosphere moves heat and moisture, which makes them more predictable than they might seem on a map.
Why Deserts Form Where They Do
The single biggest driver of desert placement is a planet-scale pattern of air circulation. Warm, moist air rises near the equator, drops its moisture as tropical rain, then flows outward at high altitude toward the poles. As it cools and descends in the subtropics, roughly between 20° and 35° latitude in both hemispheres, it creates zones of persistent high pressure where clouds rarely form and rain seldom falls. This large-scale loop of rising and sinking air is the mechanism responsible for the band of great deserts that wraps around the globe in both hemispheres.
1Annals of the New York Academy of Sciences. The Hadley circulation in a changing climateThat atmospheric engine explains why the Sahara, the Arabian Desert, the deserts of Australia, the Kalahari, and the Sonoran all sit at roughly the same latitudes. But not every desert fits the subtropical pattern. Rain shadow deserts form when mountain ranges block moisture-laden winds, wringing out precipitation on the windward slopes and leaving the leeward side parched. Cold ocean currents also play a role: where frigid water wells up along a coast, it chills the air above it, suppressing evaporation and rainfall even at low latitudes. The Atacama in South America and the Namib in southwestern Africa both owe their extreme dryness partly to cold currents offshore.
The Sahara and the Arabian Peninsula
The Sahara dominates any conversation about deserts. Stretching roughly 9 million square kilometers from the Atlantic coast of Africa to the Red Sea, and from the Mediterranean south to the Niger River and the Ethiopian highlands, it is the world’s largest warm desert by a wide margin. Most of the Sahara experiences an arid or hyperarid climate with almost no measurable precipitation. Its northern fringe receives occasional winter storms of Mediterranean origin, while the southern edge picks up some moisture from the West African monsoon in summer.
East of the Sahara, the Arabian Desert extends across most of the Arabian Peninsula. The Rub’ al Khali, or Empty Quarter, in the southern portion is the largest contiguous sand desert on Earth and one of the most inhospitable. Together, the Sahara and the Arabian Desert form a nearly continuous belt of aridity stretching from the northwest corner of Africa deep into the Middle East, interrupted only by the narrow ribbon of the Nile Valley and the Red Sea.
Central and East Asian Deserts
The interior of Asia hosts some of the most extreme deserts on the planet, though they differ from their subtropical counterparts in important ways. The Gobi, spanning northern China and southern Mongolia, is a cold desert where winter temperatures plunge well below freezing and snowfall is common. It sits at relatively high latitude and high elevation, far from any ocean, and owes its dryness to sheer continental distance from moisture sources rather than to the subtropical high-pressure belt.
To the west, the Karakum and Kyzylkum deserts sprawl across Turkmenistan and Uzbekistan. Farther south, the Thar Desert straddles the India-Pakistan border and marks the eastern edge of the subtropical desert belt in Asia. The Taklamakan, hemmed in by the Tian Shan and Kunlun mountain ranges in western China, is one of the most landlocked places on Earth. These Asian deserts vary enormously in temperature, sand cover, and elevation, but they share the common thread of being starved of moisture by distance, mountains, or descending air.
North American Deserts
The southwestern United States and northern Mexico contain four recognized warm deserts, each with a distinct character. The Mojave, centered on southeastern California and southern Nevada, is the smallest but includes Death Valley, one of the hottest locations regularly measured on the planet. The Sonoran, spanning southern Arizona and the Mexican state of Sonora, is famous for its iconic saguaro cacti and receives enough summer monsoon rain to support surprisingly rich plant and animal life. The Chihuahuan, the largest North American desert, stretches from central Mexico into southern New Mexico and West Texas and sits at higher elevation than the others, making it cooler on average. The Great Basin Desert of Nevada and Utah is a cold desert where sagebrush dominates and winter snow is a major source of moisture.
Recent research tracking vegetation and drought across the Mojave, Sonoran, and Chihuahuan deserts found that all three regions experienced more frequent and more severe drought conditions in the early 21st century compared to previous decades.
2Remote Sensing. Assessing Vegetation Response to Multi-Scalar Drought across the Mojave, Sonoran, Chihuahuan Deserts and Apache Highlands in the Southwest United StatesSouth American Deserts
South America’s western coast is home to the Atacama, often described as the driest non-polar desert on Earth. Stretching along a narrow strip of northern Chile and southernmost Peru, it occupies a zone squeezed between the Andes and the cold Humboldt Current flowing north along the Pacific. Some weather stations in the Atacama’s core have recorded no measurable rainfall for years at a stretch, and parts of it are so barren that they have been used as stand-ins for the Martian surface.
The Atacama’s extreme aridity has deep roots. Geological evidence suggests that while the region has been generally dry for millions of years, the shift to true hyperaridity did not happen until the late Pliocene, roughly 3 million years ago. Interestingly, the researchers who established that timeline concluded that the rain shadow cast by the Andes played a smaller role than often assumed, and that the cold Humboldt Current, though important in maintaining general aridity, was not the trigger for the final push into the driest conditions.
3Geology. Late Pliocene age for the Atacama Desert: Implications for the desertification of western South AmericaEast of the Andes, the Patagonian Desert covers a vast plateau in southern Argentina. It is a cold desert shaped by the same rain shadow principle: moisture-laden Pacific winds dump their water on the Chilean side of the mountains, leaving Patagonia dry and windswept. Farther north, the Monte Desert occupies western Argentina with scrubby vegetation adapted to seasonal drought.
African Deserts Beyond the Sahara
While the Sahara draws most of the attention, southern Africa has its own impressive arid regions. The Namib Desert, running along the Atlantic coast of Namibia and Angola, is one of the oldest deserts on Earth, with some estimates placing its origin at 55 to 80 million years ago. Like the Atacama on the opposite side of the Atlantic, the Namib’s coastal aridity is reinforced by a cold ocean current, in this case the Benguela. Inland, the Kalahari spreads across Botswana, Namibia, and parts of South Africa. The Kalahari is technically a semi-arid savanna rather than a true desert in some classification systems, because it receives more rainfall than classic deserts, but its sandy soils and sparse vegetation give it a distinctly desert character for much of the year.
Australian Deserts
Australia is the driest inhabited continent, and its interior is dominated by a mosaic of named deserts that together cover most of the landmass west of the Great Dividing Range. The Great Victoria Desert is the largest, followed by the Great Sandy Desert, the Tanami, the Simpson, and the Gibson. These merge into one another without sharp boundaries, and Australians often refer loosely to the entire interior as “the outback.” The position of Australia squarely within the subtropical high-pressure belt, combined with relatively flat terrain that offers little in the way of orographic rainfall, makes aridity the default condition across the continent’s interior.
What makes the Australian deserts distinctive is their biological richness relative to their aridity. Many Australian desert plants and animals evolved in prolonged isolation on a continent that has been drifting through arid latitudes for tens of millions of years, producing species found nowhere else. Spinifex grasses, mulga trees, and a startling diversity of lizards and marsupials thrive in conditions that look lifeless at first glance.
Polar Deserts
By strict meteorological definition, a desert is any region that receives less than about 250 millimeters of precipitation per year. Under that criterion, the largest desert on Earth is not the Sahara but Antarctica. The interior of the Antarctic ice sheet receives so little snowfall that it qualifies as a polar desert, even though the surface is buried under kilometers of ice accumulated over millions of years. The Arctic, too, includes large swaths of polar desert, particularly on the Canadian Arctic Archipelago and in northern Greenland, where cold air holds very little moisture and precipitation is minimal despite the presence of ice and snow.
Polar deserts challenge the popular image of a desert as a hot, sandy wasteland, but they share the defining feature: water is scarce in a form that living things can use. The biological communities that survive in polar deserts are typically limited to mosses, lichens, and microorganisms, clinging to exposed rock or surviving beneath translucent stones where a thin greenhouse effect provides just enough warmth.
How Desert Life Adapts
The plants and animals that occupy desert biomes have evolved an extraordinary range of strategies to cope with water scarcity and temperature extremes. Among plants, the convergence is striking: species on different continents that share no recent ancestry have independently evolved similar solutions, including thick waxy coatings to limit water loss, the ability to store water in swollen stems or leaves, and deep or wide-spreading root systems to capture whatever moisture the soil holds. Researchers studying the genetic basis of drought adaptation have found that distantly related plant lineages have converged on similar gene regulatory networks to survive in dry environments, suggesting that the menu of possible solutions to extreme aridity is surprisingly limited at the molecular level.
4PubMed Central. Convergent evolution of gene regulatory networks underlying plant adaptations to dry environmentsAnimals rely on behavioral strategies as much as physiological ones. Many desert mammals are nocturnal, avoiding the heat of the day entirely. Some rodents and reptiles obtain all of the water they need from food rather than drinking. Birds in desert environments often concentrate near the scarce water sources, and some species can fly dozens of kilometers daily to reach them. The interplay between desert organisms and their environment is more dynamic than the bare landscape suggests.
Water Beneath the Sand
One of the least intuitive facts about deserts is that many of them sit on top of vast underground water reserves. The Sahara, despite its surface aridity, conceals enormous aquifers. Much of this water was deposited thousands of years ago during wetter climate periods and is often called “fossil water.” But satellite-based measurements of water volume changes beneath the northern Sahara revealed that these aquifers are not entirely fossil: they receive, on average, about 1.4 cubic kilometers of recharge per year, based on data from 2003 to 2010. That represents roughly 40 percent of the water being pumped out, mainly for irrigation supporting oasis agriculture in Algeria and Tunisia. The recharge does not keep pace with withdrawals, but its existence means that careful management could extend the life of these aquifers considerably rather than simply draining a fixed reserve.
5Geophysical Research Letters. Sub-saharan water: Not just fossil waterOases have sustained human communities in deserts for millennia, and their existence almost always traces back to groundwater reaching the surface through springs or shallow wells. In the Arabian Peninsula, traditional falaj irrigation systems channel underground water to farms with remarkable efficiency. In the Sahara, oasis towns like Siwa in Egypt and Ghardaia in Algeria have persisted for centuries by managing fragile water supplies. The tension between growing demand for irrigation water and the slow rate at which desert aquifers recharge is one of the defining resource challenges for arid-region communities worldwide.
When the Sahara Was Green
The Sahara was not always a desert. Between roughly 14,500 and 5,000 years ago, much of North Africa experienced what paleoclimatologists call the African Humid Period. Lakes formed in what is now open sand, grasslands and even scattered woodlands spread across regions that today receive almost no rain, and human communities thrived in areas that are now uninhabitable without modern technology. Rock art depicting hippos, crocodiles, and cattle in the central Sahara is one of the most vivid reminders of this dramatically different past.
Climate modeling work has explored what triggered the greening. Simulations covering the last 21,000 years found that neither changes in Earth’s orbital geometry (which shifted the distribution of sunlight across the seasons) nor the retreat of the Northern Hemisphere ice sheets alone was sufficient to trigger the rapid onset of the humid period. Only when both factors acted together did the models produce the observed shift. A feedback loop between vegetation and rainfall amplified the change: as plants spread, they darkened the land surface, increased moisture recycling, and drew in more monsoonal rain, which supported still more vegetation.
6Journal of Climate. Mechanisms for the Onset of the African Humid Period and Sahara Greening 14.5–11 ka BPThe collapse of the green Sahara was not a simple reversal. Evidence from lake sediments and pollen records suggests that the drying happened unevenly, with some areas reverting to desert over centuries while others held out longer. The speed and geographic patchiness of the transition remain active areas of research, and understanding them matters because the same feedback loops between vegetation and rainfall operate today along the Sahel, the semi-arid band on the Sahara’s southern edge.
Measuring Extreme Desert Heat from Space
Deserts hold the record for the highest surface temperatures on Earth, but pinning down exactly how hot they get is harder than you might expect. Weather stations measure air temperature a meter or two above the ground, and most of the planet’s hottest desert areas are too remote and too harsh to maintain ground stations. Satellites can observe every part of the surface equally, but what they measure is skin temperature, the temperature of the ground itself rather than the air above it, which can be dramatically higher.
7ScholarWorks at University of Montana. Satellite Finds Highest Land Skin Temperatures on EarthSatellite surveys have identified the Lut Desert in southeastern Iran as one of the hottest spots on the planet’s surface, with skin temperatures exceeding 70°C (about 159°F) in some readings. The Sahara, the Gobi, and the Sonoran are also among the hottest regions measured this way. The gap between skin temperature and air temperature can be 20°C or more over bare desert soil at midday, which is why the official air-temperature record holders (like Death Valley’s famous 56.7°C reading from 1913, still debated by some meteorologists) are always lower than the satellite-derived surface readings from the same regions. For anyone trying to understand just how punishing desert environments can be, the surface temperature is arguably the more relevant number, since it determines how hot the ground is under your feet, how quickly exposed skin burns, and how much heat radiates back up into the air at night.
Coastal Fog Deserts
A handful of deserts worldwide receive almost no rain yet are not entirely dry, thanks to fog. The Namib is the best-known example. Cold upwelling water along the coast generates persistent fog banks that roll inland, sometimes penetrating dozens of kilometers into the desert. Plants and animals in the Namib have evolved remarkable strategies to harvest this fog: the darkling beetle, for instance, tilts its body into the wind on dune crests so that fog droplets condense on its back and trickle down to its mouth. Fog nets, mimicking this principle, have been installed in parts of Chile’s Atacama coast and in Namibia to collect usable water from the air in regions where drilling wells is impractical.
The Atacama’s coastal strip, despite its extreme aridity at ground level, supported human communities along the shoreline for thousands of years. Archaeological work documents maritime communities stretching back through the Holocene who relied on the ocean for food while living on one of the driest coasts in the world.
8Maritime Communities of the Ancient Andes. Marine Communities in the Atacama DesertFog deserts blur the line between true desert and something else entirely. They receive negligible rainfall, yet the moisture delivered by fog sustains ecosystems that would be impossible in an inland desert of equivalent aridity. They remind us that the simple metric of annual precipitation, while useful for classifying biomes on a map, does not always capture the full picture of how water reaches living things in the driest corners of the planet.