What is a Desert? Definitions from Different Perspectives
1. Climatic Perspective
From a climatic perspective, a desert is defined by its extremely low annual precipitation. Specifically, deserts receive less than 10 inches (25 centimeters) of rain per year. This scarce rainfall results in dry conditions that prevent the growth of most types of vegetation. The aridity leads to significant temperature fluctuations, with hot deserts experiencing extremely high daytime temperatures and cold deserts experiencing frigid temperatures, particularly in winter. For example, the Sahara Desert can reach daytime temperatures up to 122°F (50°C), while the Gobi Desert can see winter temperatures drop below -40°F (-40°C).
2. Geographic Perspective
Geographically, deserts are identified by their location and physical features. They can be found on every continent, demonstrating their widespread nature. Deserts can be categorized into different types based on their geographic and climatic conditions:
3. Biological Perspective
From a biological perspective, a desert is defined by its sparse vegetation and specially adapted wildlife. The extreme dryness and temperature fluctuations mean that only plants and animals adapted to these harsh conditions can thrive. Desert plants, like cacti, have developed features such as thick, waxy skins and deep roots to conserve water. Desert animals, like camels, have evolved to survive with minimal water and withstand extreme temperatures. For instance, camels can drink large amounts of water at once and go for extended periods without drinking again.
4. Ecological Perspective
Ecologically, deserts are characterized by their unique ecosystems and the role they play in the Earth’s climate system. Despite their harsh conditions, deserts support a variety of life forms that are specially adapted to the environment. The low precipitation and extreme temperatures create a challenging habitat where only the most resilient species can survive. Deserts also influence global weather patterns and play a role in atmospheric processes. For example, the dust from deserts can affect air quality and climate patterns across large distances.
How Deserts Formed
Deserts likely existed much earlier, during periods of global arid climate, in areas shielded from rain by mountain ranges or in the centers of vast continental regions. This was primarily before the evolution of angiosperms (flowering plants, which most current desert plants belong to). Today’s deserts have only a few primitive plants from ancient desert vegetation, such as the unusual conifer relative, welwitschia, found in the Namib Desert of southwestern Africa. Welwitschia features two leathery, strap-like leaves that grow continuously from the base of a large, mainly underground woody stem, eroding at their ends over time. This desert also supports various plants and animals uniquely adapted to arid conditions, suggesting it may have a longer history of aridity compared to other deserts.
Deserts have experienced significant migration of plant species, especially during drier climatic intervals over the past two million years. This movement is evident in the floristic similarities observed between northern and southern desert regions in Africa and the Americas. For instance, the creosote bush (Larrea tridentata), now common in North American hot deserts, likely migrated from South America around the end of the last Ice Age, about 11,700 years ago.
Migration between separate desert regions has been facilitated for plants adapted to saline soils, as such conditions are found in both deserts and coastal areas. Coastal regions have thus served as migration corridors for salt-tolerant plants. Additionally, buoyant seeds can be transported between coasts by ocean currents. An example of this is the saltbush or chenopod family of plants, which is believed to have reached Australia through coastal migration, initially colonizing coastal areas before spreading into the inland deserts.
Deserts are diverse and variable environments, making it challenging to define them concisely. Their defining feature is a severe lack of moisture for plants, caused by an imbalance between precipitation and evapotranspiration. This issue is compounded by erratic rainfall, low atmospheric humidity, high daytime temperatures, and strong winds.
- Precipitation: Annual rainfall in deserts ranges from nearly zero in some South American coastal deserts and Libyan deserts to about 600 mm (24 inches) in Madagascar. Most recognized deserts receive less than 400 mm annually. Some consider regions with 250 to 400 mm of rainfall as semideserts. These areas are often barely arable, primarily used for grazing livestock.
- Temperature Extremes: Deserts are known for their extreme temperature variations. Daytime temperatures can become dangerously high, posing risks of dehydration and heatstroke. At night, the lack of humidity and cloud cover causes rapid cooling, with temperatures sometimes dropping to 4°C (40°F) or lower. In the Chihuahuan Desert, temperatures can fluctuate drastically within a single day, soaring above 37°C (100°F) during the day and plunging below freezing (0°C or 32°F) at night.
- Playas and Basins: Many deserts lack drainage systems leading to rivers or oceans. Instead, rainwater collects in large depressions called basins, forming shallow lakes that eventually evaporate, leaving behind salt flats or playas. These expansive, salt-covered areas, such as the Black Rock Desert in Nevada, are often used for car racing. In 1997, British pilot Andy Green set a land speed record in this desert, reaching 1,228 kilometers per hour (763 miles per hour) in the ThrustSSC, the first car to break the sound barrier.
- Hadley Cells: Deserts are often located in subtropical regions, influenced by the Hadley cell circulation pattern. Near the Equator, solar energy heats the air, causing it to rise, cool, and condense into rain. This air then moves towards the subtropics where it descends, having lost most of its moisture. This descending dry air contributes to desert conditions. Local variations can occur, such as increased precipitation on the east sides of continents or in rain shadows behind mountain ranges.
- Rainfall Variability: Desert rainfall is typically scarce and can be highly variable. Some deserts may go years without rain, such as Cochones, Chile, which experienced no rain for 45 years between 1919 and 1964. When rain does occur, it can be intense but brief, like the 14 mm recorded in just seven minutes at Mashʾabe Sade, Israel, in 1979. Rain often falls during specific seasons, with equatorial deserts receiving most of their rain in summer and temperate deserts in winter. Local convectional cells can cause heavy but localized showers.
- Fog and Dew: In coastal deserts, such as the Namib Desert in southwestern Africa and deserts along the coasts of California and Peru, fog provides an important source of moisture. Fog droplets settle on plants and soil, and dew can also be significant in some areas. However, in central continental deserts, atmospheric humidity is usually too low for significant fog or dew formation.
- Potential Evaporation: Deserts have high potential evaporation rates, typically between 2,500 and 3,500 mm per year, with extreme cases like Death Valley reaching up to 4,262 mm. The lack of vegetation amplifies the effects of wind erosion.
- Winds: Winds in deserts can reach speeds of about 100 km/h (60 mph), transporting sand and dust over great distances. These winds can induce significant erosion, shaping desert landscapes and moving dunes. Sandstorms can even affect areas far from the desert, like the U.S. state of Florida.
- Landforms and Erosion: Deserts are known for their unique landforms, including dunes, bare peaks, mesas, and buttes. Erosion by water and wind shapes these features, creating alluvial fans and extensive salt flats, such as those in the Black Rock Desert.
Water in the Desert
In deserts, rain is the primary source of water, but it is infrequent and often unreliable. Consequently, many desert inhabitants rely on groundwater stored in aquifers beneath the surface. This groundwater originates from precipitation, such as rain, snow, or hail, which seeps into the ground and can remain stored for thousands of years.
When rain does fall in deserts, it often comes down with remarkable intensity. The desert landscape bears witness to this phenomenon with dry stream channels, known as arroyos or wadis, that traverse the surface. These channels can transform into raging torrents with startling speed after a storm, even if the storm originated many kilometers away.
When groundwater rises to the surface, it can form springs or seeps, creating fertile green areas known as oases, or cienegas. The Sahara Desert, for example, is dotted with about 90 major inhabited oases, supported by some of the world’s largest underground water reserves. These oases provide essential water, food, and shelter for people, animals, and plants.
In the absence of surface springs, drilling into the ground to access aquifers is common. Many desert cities, ranging from the American Southwest to the Middle East, depend on these underground water sources to meet their needs. Rural communities, like the kibbutzim in Israel’s Negev Desert, also rely on aquifers for water used in agriculture and fish farming.
Local communities can also create smaller-scale water management systems, such as artificial wadis in the Middle East. These manmade channels can carry freshwater during rainy seasons, and in some places like Yemen, they are even used for recreational activities like whitewater rafting.
Water rights in deserts can lead to disputes, particularly when water sources cross state or national boundaries. The Colorado River Basin has been a focal point for such conflicts. Historical agreements did not account for the needs of Native American tribes or the environmental impacts of dams. With growing populations and agricultural demands, states within the basin continue to negotiate water rights to address future needs and potential droughts.
Floristic Links and Plant Diversity in Deserts
Desert regions often share related plant species, indicating floristic links among them. While identical species are rarely found across different desert regions—except where humans have introduced them—there are notable exceptions. For instance, the prickly saltwort (Salsola kali) is found in deserts across Central Asia, North Africa, California, and Australia, as well as in various saline coastal areas.
Floristic similarities are particularly evident in regions where there are no significant barriers such as oceans or dense vegetation that could impede plant migration. For example, despite the climatic differences between the hot deserts of North Africa and the cooler, arid regions of Central Asia, there are noticeable plant connections between these areas. Similarly, there are distinct floristic links from the northern to the southern deserts in Africa and the Americas. In contrast, Australia’s more isolated desert flora shows fewer similarities to other desert regions’ plant life.
The daisy family (Asteraceae) stands out as the most diverse plant family in deserts, particularly abundant in Australia, southern Africa, the Middle East, and North America. However, except for the widely distributed genera Artemisia (wormwood) and Senecio, different genera from the daisy family are typically found in different desert regions. Grasses dominate in deserts like those in Iran, the Sahara, and the Thar Desert of India, but the daisy family also shows considerable diversity here. Another well-represented family in desert environments is the bean family (Fabaceae).
Certain plant families are more locally significant in specific desert regions. In Africa, the ice plant (Aizoaceae) and lily (Liliaceae) families are prominent. The cabbage family (Brassicaceae) spans from the Sahara to Iran, while the carnation family (Caryophyllaceae) is prevalent in the Middle East. In Australia, the myrtle (Myrtaceae), protea (Proteaceae), and casuarina (Casuarinaceae) families are notable. These families also occur in other vegetation types within their regions, showing adaptations to arid conditions.
Some plant families are common in desert environments but less prominent in other ecosystems. The chenopod family (Chenopodiaceae), for example, is diverse in arid regions of Australia, North America, and from the Sahara to Iran, India, and Central Asia, but is scarce in other environments. The cactus family (Cactaceae) is notably prevalent in the Americas but absent elsewhere. Similarly, the Frankeniaceae family, which thrives in salty habitats, is diverse in deserts from North Africa to Central Asia and western South America but less common in other ecosystems.
Desert Fauna: Regional Distinctions and Adaptations
Desert animals exhibit significant regional distinctions, particularly among larger species. Australia, being the most geographically isolated continent, has a notably unique desert fauna. Australian deserts are characterized by a high diversity of reptiles, such as skinks and geckos, in contrast to other desert regions, which typically have more diverse mammalian populations. Australian deserts also have fewer mammals overall, and many of these are marsupials rather than the rodents that dominate other deserts. Notable marsupials include kangaroos, wallabies, bandicoots, and the burrowing marsupial mole. Unfortunately, many smaller Australian desert mammals have recently become rare or extinct. The European rabbit, introduced by humans, is now a common sight in many Australian deserts. Additionally, camels, which were introduced to Australia, are now found there in a completely wild state.
In the hot deserts of the Old World, the large herbivorous mammals such as camels, donkeys, goats, sheep, and horses are predominantly domesticated. Wild herbivores like gazelles, ibexes, and oryxes are much less common. Smaller burrowing rodents and reptiles are more diverse and widespread. Large carnivores, including foxes, hyenas, and various cats such as leopards and lynxes, are present, though the lion, once a prominent species, is now extinct in these regions.
Desert birds also exhibit various adaptations to their environment. Many desert birds are nomadic, moving to areas where recent rains have created temporary food sources. Seed-eating birds, such as finches and pigeons, are common in many deserts, although Australia has fewer finches and more desert-dwelling parrots, such as the budgerigar. Carnivorous birds can obtain water from their prey, while seed-eaters must find surface water and may travel long distances to do so.
Natural resource extraction
Deserts are rich in mineral resources, often found across their surfaces, which contribute to their distinct colors. For instance, the red hue of many sand deserts is due to laterite minerals. In the harsh desert climate, geological processes can lead to the concentration of minerals into valuable deposits. Groundwater can leach ore minerals and redeposit them in concentrated forms depending on the water table. Additionally, evaporation in desert lakes tends to concentrate minerals, forming dry lake beds or playas that are rich in various minerals. These evaporites can include gypsum, sodium nitrate, sodium chloride, and borates.
The Great Basin Desert in the U.S. has historically been known for its evaporites, with borax being transported from Death Valley to the nearest railway by “20-mule teams.” The Atacama Desert in Chile is particularly noted for its rich deposits of sodium nitrate, which has been mined for explosives and fertilizer since around 1850. Other significant desert minerals include copper found in Chile, Peru, and Iran, as well as iron and uranium in Australia. Deserts also provide many other metals, salts, and commercially valuable rocks like pumice.
Oil and gas deposits often originate from the remains of microorganisms that decomposed on the bottoms of shallow seas under anoxic conditions. Over time, these remains become covered by sediment, which eventually transforms into oil and gas. Many deserts today were once covered by shallow seas, or they have experienced the movement of tectonic plates that brought hydrocarbon deposits to their current locations. For instance, significant oilfields like Ghawar are located beneath the sands of Saudi Arabia.
Geologists also propose that some oil deposits in deserts were formed by aeolian processes in ancient desert environments. This theory suggests that winds may have played a role in concentrating and preserving hydrocarbon deposits in what are now major oil fields, such as those found in the United States.
Farming
Traditional farming systems in North Africa have adapted to the challenges of desert environments, with irrigation playing a crucial role in overcoming water scarcity. Techniques such as drip irrigation, using organic residues or animal manures as fertilizers, and other traditional management practices help build soil fertility and protect against erosion. Once the soil’s fertility is established, continued crop production helps prevent destruction from wind and erosion.
Plant growth-promoting bacteria have been found to enhance plant resistance to stress, and these rhizobacterial suspensions can be introduced into the soil near plants. Research indicates that these microbes help combat desertification by creating “islands of fertility,” which support higher crop yields despite harsh environmental conditions. For instance, a field trial in the Sonoran Desert tested the effects of rhizobacteria and the nitrogen-fixing bacterium Azospirillum brasilense on different tree species to restore degraded lands. Although the results were only partially successful, the trial highlighted the potential of these techniques in des
ert agriculture.
Sand and dust storms
Sand and dust storms are common in arid regions where vegetation does not cover the land. These storms often originate at desert margins, where the finer materials have already been removed by previous winds. As a steady wind blows, it causes fine particles on the exposed ground to vibrate. When the wind reaches higher speeds, it lifts some of these particles into the air. This process can initiate a chain reaction, where lifted particles strike and dislodge additional particles.
The airborne particles can move in different ways based on their size, shape, and density: suspension, saltation, or creep. Suspension involves very fine particles, less than 0.1 mm (0.0039 in) in diameter, which can be lifted to altitudes of up to 6 km (3.7 mi). These particles can reduce visibility and remain airborne for days, traveling up to 6,000 km (3,700 mi) with the trade winds.
In stronger winds, denser clouds of dust can form, creating a dramatic billowing effect. Such storms can darken the sky to the point of resembling night at ground level. For example, a 2001 dust storm in China involved an estimated 6.5 million tons of dust covering 134 million km² (52 million sq mi), with an average particle size of 1.44 μm.
On a smaller scale, dust devils can form in calm conditions. These are short-lived whirlwinds created when hot air near the ground rapidly rises through a cooler, low-pressure air pocket above, lifting particles in a swirling column.
