Desert Biome
A desert is any place that gets less than 250 millimetres of rain a year. That makes the desert one of the harshest biomes on Earth: too dry for most plants to grow, too exposed to extreme temperatures, too poor in nutrients for big animals. Yet life still finds a way, with cacti, scorpions, lizards and camels all making themselves at home. Deserts cover roughly a third of all the land on Earth and they appear on every continent, including Antarctica.
- RainfallUnder 250 mm/yearIn the driest parts, near zero
- % of Earth's Landapprox. 33%About a third of all land area
- Types4Hot, cold, coastal, polar
- BiggestAntarcticaA cold polar desert, 14 million km²
- Driest placeAtacama (Chile)Parts have not had rain in centuries
- Hottest temp56.7 °CIn Death Valley, USA
How dry is the desert biome?
Yearly rainfall (mm) compared with other biomes.
Rainforests get more than ten times as much rain as deserts. That single difference shapes which plants can grow there.
What is a desert?
The desert is defined by how dry it is, not by how hot. Most hot deserts are baking by day but freezing at night because there are few clouds or plants to hold in heat. Some deserts are cold all year (the Gobi, the high deserts of Tibet, both polar regions). Antarctica is, by area, the biggest desert in the world. Scientists usually split the desert biome into four types: hot and dry deserts like the Sahara, cold deserts like the Gobi, coastal deserts like the Atacama where cold sea currents block rain clouds, and polar deserts like Antarctica and the high Arctic where the air is so cold it cannot hold much moisture.
Soil in the desert is usually thin, gritty and full of minerals. Rain, when it falls, often comes in sudden heavy storms that wash across the bare ground and cause flash floods in dry river beds called wadis. Within weeks of a downpour, a desert can burst into colour as dormant seeds spring into bloom in a brief desert "superbloom".
Where deserts are found
Most of the world's hot deserts sit in two bands either side of the equator, at roughly 30 degrees north and 30 degrees south. This is because of huge global air patterns called Hadley cells. Warm wet air rises at the equator, drops its rain over the rainforests, then flows north and south and sinks back down at around 30 degrees, dry and warm. That sinking dry air is what creates the Sahara, the Arabian, the Kalahari and the Australian deserts.
The major deserts of the world include:
- Sahara (North Africa): the largest hot desert at 9.2 million km².
- Arabian Desert (Middle East): includes the Empty Quarter, the largest sea of sand on Earth.
- Gobi (Mongolia and northern China): a cold high-altitude desert.
- Atacama (Chile): the driest non-polar desert, with parts that have had no recorded rain.
- Kalahari (southern Africa): a semi-desert home to the San people.
- Australian Outback: covers most of central Australia.
- Sonoran and Mojave (south-west USA and Mexico): home of the iconic saguaro cactus and Death Valley.
- Antarctic and Arctic deserts: cold polar deserts, where the snow is technically very low rainfall.
The desert climate
Hot deserts can swing from above 45 °C in the day to below freezing at night. With no clouds and no vegetation to trap heat, the warmth radiates straight back into space once the sun sets. Wind is the desert's constant companion: with no plants to slow it, it can sculpt huge sand dunes hundreds of metres high. The Sahara has dunes up to 180 m tall, taller than most office blocks.
Cold deserts, including the Gobi and Patagonia, can drop below −40 °C in winter. The world's biggest desert by area, Antarctica, almost never gets above freezing. Polar deserts are dry not because they are hot, but because the air is so cold it cannot hold much moisture, so very little snow falls.
Plants of the desert
Desert plants are masters of water-saving. Cacti like the saguaro store water in their thick stems and replace water-losing leaves with sharp spines, which double as protection from thirsty animals. A fully grown saguaro can hold approx. 750 litres of water after a single soaking rain. Succulents like agaves and aloes use the same trick. Other plants, called ephemerals, skip the problem entirely: their seeds lie dormant for years and only sprout, flower and set seed in the short weeks after rain.
Mesquite and acacia trees grow incredibly long taproots, sometimes more than 50 m deep, to reach underground water. The creosote bush of the Mojave produces chemicals that stop other plants growing nearby, claiming the scarce water for itself. Some of these plants can live for thousands of years: a creosote bush ring in California called King Clone is approx. 11,700 years old.
Animals of the desert
Most desert animals are nocturnal (active at night), hiding from the sun in burrows or under rocks by day. Camels are the most famous desert animal: they can drink 100 litres of water in 10 minutes, close their nostrils against blowing sand, and store energy as fat in their humps. Their wide soft feet stop them sinking into the sand. Camels can lose 25% of their body weight to dehydration and survive, where humans would die after losing 15%.
Other iconic desert animals include the fennec fox, with huge ears that radiate heat; the kangaroo rat, which never drinks water and gets all it needs from seeds; the desert tortoise, which can store water in its bladder for months; the Saharan silver ant, which forages in the hottest part of the day when its predators are sheltering; the gila monster, one of the few venomous lizards; and the meerkat of the Kalahari, which lives in family groups and posts lookouts on tall rocks.
People and threats
People have lived in deserts for thousands of years. Nomadic groups like the Bedouin of the Sahara and Arabian Desert, the Tuareg of the central Sahara, the San of the Kalahari, and the Aboriginal peoples of the Australian Outback have all developed cultures and skills perfectly suited to surviving in a place with almost no water. Camels, goats and sheep are the basis of desert herding economies; date palms grown around oases provide a stable food source.
The biggest threats to the desert biome are desertification (when farmland on the desert edge turns into desert through overgrazing or drought), oil and gas drilling, large-scale solar power plants that fragment habitat, and climate change, which is making some deserts hotter and drier and others, surprisingly, wetter. The Sahel, the strip of dry grassland between the Sahara and the African savanna, has been losing approx. 200,000 km² to desertification each decade since the 1970s.
Deeper dive: how do deserts actually form?
Three big patterns explain where almost every desert on Earth sits. The first is the subtropical high. Earth's atmosphere circulates in giant convection loops called Hadley cells. Warm wet air rises at the equator, dumps its rain over the tropical rainforests, then drifts north and south at high altitude. By the time this air sinks back to ground level at around 30 degrees latitude, it has lost almost all its moisture. The sinking dry air is what creates the Sahara, the Arabian Desert, the Kalahari and the great Australian deserts.
The second pattern is the rain shadow. When wet air from the sea meets a mountain range, it is forced upwards. Rising air cools, and cool air cannot hold as much water, so it dumps its rain on the windward side. By the time the air crosses to the other (leeward) side, it is bone dry. The Gobi sits in the rain shadow of the Himalayas; the Patagonian Desert in the rain shadow of the Andes; the Great Basin in the rain shadow of the Sierra Nevada.
The third pattern is the cold coastal current. The Atacama in Chile and the Namib in southern Africa both sit right next to the sea, which should make them wet. But cold ocean currents flow up these coasts, cooling the air above them. Cold air carries very little moisture, so when this dry cool air blows inland it brings no rain. The Atacama receives, on average, just 1 to 3 mm of rain a year; some weather stations have never recorded any.
Polar deserts work on a fourth principle: the air is just too cold to hold much water. Below about −40 °C the amount of moisture even a saturated atmosphere can carry is tiny. That is why the interior of Antarctica receives less precipitation than the Sahara.
For famous individual deserts including the Sahara, the Gobi, the Atacama and Death Valley, see the Deserts section.