How Deserts Form: Types, Causes, and Geography Explained
Discover how deserts form, the different types of deserts, their geographic distribution, the role of atmospheric circulation, and the process of desertification.
What Is a Desert?
A desert is a region that receives very little precipitation — typically defined as less than 250 millimeters (10 inches) of rainfall annually — and where evaporation and transpiration exceed precipitation. Deserts cover approximately one-third of the Earth's land surface (about 50 million square kilometers), making them the most extensive terrestrial biome. While popular imagination associates deserts with endless sand dunes, only about 20% of the world's deserts are sandy (called ergs). Most deserts are rocky plateaus, gravel plains, salt flats, or even ice-covered polar regions. Understanding how deserts form requires examining atmospheric circulation, geography, ocean currents, and increasingly, human activity that drives desertification.
Deserts exist on every continent, from the vast Sahara in Africa to the frozen interior of Antarctica. Their formation is governed by several distinct mechanisms that often work in combination.
Major Causes of Desert Formation
Subtropical High-Pressure Zones
The most common mechanism for desert formation is the global atmospheric circulation pattern known as Hadley cells. Near the equator, intense solar heating causes air to rise, cool, and release moisture as tropical rainfall. This dried air then flows poleward at high altitude before descending at approximately 20–30 degrees latitude north and south of the equator. As this air descends, it compresses, warms, and absorbs moisture — creating zones of persistent high pressure characterized by clear skies and extreme aridity.
This mechanism explains why the world's great subtropical deserts — the Sahara, Arabian, Kalahari, Australian, and Sonoran — all lie in bands roughly between 15 and 35 degrees latitude.
Rain Shadow Effect
When moist air masses encounter a mountain range, they are forced upward (orographic lift), cooling and releasing precipitation on the windward side. By the time the air descends on the leeward side, it has lost most of its moisture, creating an arid "rain shadow." This process explains the formation of deserts adjacent to major mountain ranges:
- Patagonian Desert: In the rain shadow of the Andes Mountains in southern Argentina.
- Great Basin Desert: In the rain shadow of the Sierra Nevada in the western United States.
- Gobi Desert: Partially in the rain shadow of the Himalayas and other Central Asian mountain ranges.
- Atacama Desert: Created by the rain shadow of the Andes combined with the cold Humboldt Current.
Cold Ocean Currents
Cold ocean currents flowing along coastlines cool the air above them, reducing its capacity to hold moisture and suppressing precipitation over adjacent land. This mechanism contributes to some of the world's driest deserts:
- Atacama Desert (Chile): The Humboldt Current (Peru Current) cools the adjacent air. Parts of the Atacama have recorded no rainfall for periods exceeding 400 years, making it the driest non-polar desert on Earth.
- Namib Desert (Namibia): The cold Benguela Current creates fog but suppresses rainfall.
- Baja California deserts: Influenced by the cold California Current.
Continentality (Interior Location)
Regions far from oceanic moisture sources receive less precipitation simply due to distance. Moisture-bearing air masses lose their water content as they travel inland, creating arid interior regions. The Gobi Desert and Central Asian deserts (Karakum, Kyzylkum) exemplify this pattern.
Types of Deserts
Classification of Desert Types
| Type | Cause | Examples | Typical Location |
|---|---|---|---|
| Subtropical | Hadley cell circulation | Sahara, Arabian, Kalahari | 15–35° latitude |
| Rain shadow | Orographic effect | Patagonian, Great Basin, Gobi | Leeward side of mountains |
| Coastal | Cold ocean currents | Atacama, Namib | Western coasts near cold currents |
| Continental interior | Distance from moisture | Gobi, Karakum, Taklamakan | Deep continental interiors |
| Polar | Extreme cold limits moisture | Antarctic, Arctic | Above 60° latitude |
Polar deserts are often overlooked but are the world's largest. Antarctica is technically the largest desert on Earth, receiving less than 50 mm of precipitation annually across most of its interior. The Arctic polar desert similarly receives minimal precipitation, though warming temperatures are rapidly changing Arctic conditions.
The World's Major Deserts
Largest Deserts by Area
| Desert | Area (km²) | Location | Type |
|---|---|---|---|
| Antarctic | 14,200,000 | Antarctica | Polar |
| Arctic | 13,900,000 | Arctic region | Polar |
| Sahara | 9,200,000 | North Africa | Subtropical |
| Arabian | 2,330,000 | Middle East | Subtropical |
| Gobi | 1,295,000 | Mongolia/China | Continental/rain shadow |
| Kalahari | 900,000 | Southern Africa | Subtropical |
| Patagonian | 673,000 | Argentina | Rain shadow |
| Great Victoria | 647,000 | Australia | Subtropical |
The Sahara Desert alone is roughly the size of the contiguous United States. It was not always a desert — paleoclimate evidence shows that the Sahara experienced a "Green Sahara" period approximately 5,000–11,000 years ago, with lakes, rivers, and savanna vegetation supporting human populations and megafauna. Cyclical changes in Earth's orbital parameters (Milankovitch cycles) alter African monsoon patterns, periodically transforming the Sahara between desert and grassland.
Desert Ecosystems
Despite their harsh conditions, deserts support remarkably adapted life:
- Plants: Desert plants have evolved strategies including deep root systems (mesquite roots can reach 50+ meters), water storage (cacti, succulents), reduced leaf surface area (spines), and dormancy during dry periods. The creosote bush of the Sonoran Desert can survive years without rain.
- Animals: Desert animals conserve water through nocturnal behavior, concentrated urine, metabolic water production, and burrowing. The kangaroo rat can survive its entire life without drinking water, obtaining moisture from seeds.
- Microorganisms: Biological soil crusts — communities of cyanobacteria, algae, fungi, and lichens — stabilize desert soils, fix nitrogen, and retain moisture. These crusts are extremely fragile and can take decades to recover from disturbance.
Desertification: Human-Caused Desert Expansion
Desertification is the degradation of land in arid, semi-arid, and dry sub-humid areas, primarily caused by human activities and climatic variations. The United Nations estimates that desertification threatens approximately 2 billion people across 100 countries, with 12 million hectares of productive land lost to desertification annually.
Key drivers of desertification include:
- Overgrazing: Livestock removing vegetation faster than it can regenerate, exposing soil to erosion.
- Deforestation: Clearing trees and shrubs for fuel or agriculture removes root systems that stabilize soil and retain moisture.
- Unsustainable irrigation: Over-extraction of groundwater and salinization from irrigation can render agricultural land infertile. The Aral Sea in Central Asia shrank by 90% due to irrigation diversion, leaving behind a salt desert.
- Climate change: Rising temperatures and changing precipitation patterns are expanding arid zones. The Sahara has grown by approximately 10% since 1920, partly due to climate change.
Combating desertification requires integrated approaches including sustainable land management, reforestation, improved irrigation techniques, and addressing the root causes of climate change. Africa's Great Green Wall initiative, aiming to restore 100 million hectares of degraded land across the Sahel region, represents one of the most ambitious responses to this global challenge.