Deserts are areas that receive very little precipitation. People often use the adjectives &#;hot,&#; &#;dry,&#; and &#;empty&#; to describe deserts , but these words do not tell the whole story. Although some deserts are very hot, with daytime temperatures as high as 54°C (130°F), other deserts have cold winters or are cold year-round. And most deserts , far from being empty and lifeless, are home to a variety of plants, animals, and other organisms. People have adapted to life in the desert for thousands of years.
One thing all deserts have in common is that they are arid, or dry. Most experts agree that a desert is an area of land that receives no more than 25 centimeters (10 inches) of precipitation a year. The amount of evaporation in a desert often greatly exceeds the annual rainfall. In all deserts , there is little water available for plants and other organisms .
Deserts are found on every continent and cover about one-fifth of Earth&#;s land area. They are home to around 1 billion people&#;one-sixth of the Earth&#;s population.
Although the word &#; desert &#; may bring to mind a sea of shifting sand , dunes cover only about 10 percent of the world&#;s deserts . Some deserts are mountainous. Others are dry expanses of rock, sand , or salt flats.
Kinds of Deserts
The world&#;s deserts can be divided into five types&#;subtropical, coastal, rain shadow, interior, and polar. Deserts are divided into these types according to the causes of their dryness.
Subtropical Deserts
Subtropical deserts are caused by the circulation patterns of air masses. They are found along the Tropic of Cancer, between 15 and 30 degrees north of the Equator, or along the Tropic of Capricorn, between 15 and 30 degrees south of the Equator .
Hot, moist air rises into the atmosphere near the Equator . As the air rises, it cools and drops its moisture as heavy tropical rains. The resulting cooler, drier air mass moves away from the Equator . As it approaches the tropics, the air descends and warms up again. The descending air hinders the formation of clouds, so very little rain falls on the land below.
The world&#;s largest hot desert , the Sahara, is a subtropical desert in northern Africa. The Sahara Desert is almost the size of the entire continental United States. Other subtropical deserts include the Kalahari Desert in southern Africa and the Tanami Desert in northern Australia.
Coastal Deserts
Cold ocean currents contribute to the formation of coastal deserts . Air blowing toward shore, chilled by contact with cold water, produces a layer of fog. This heavy fog drifts onto land. Although humidity is high, the atmospheric changes that normally cause rainfall are not present. A coastal desert may be almost totally rainless, yet damp with fog .
The Atacama Desert, on the Pacific shores of Chile, is a coastal desert . Some areas of the Atacama are often covered by fog . But the region can go decades without rainfall. In fact, the Atacama Desert is the driest place on Earth. Some weather stations in the Atacama have never recorded a drop of rain.
Rain Shadow Deserts
Rain shadow deserts exist near the leeward slopes of some mountain ranges. Leeward slopes face away from prevailing winds.
When moisture-laden air hits a mountain range , it is forced to rise. The air then cools and forms clouds that drop moisture on the windward (wind-facing) slopes. When the air moves over the mountaintop and begins to descend the leeward slopes, there is little moisture left. The descending air warms up, making it difficult for clouds to form.
Death Valley, in the U.S. states of California and Nevada, is a rain shadow desert . Death Valley , the lowest and driest place in North America, is in the rain shadow of the Sierra Nevada mountains.
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Interior Deserts
Interior deserts , which are found in the heart of continents , exist because no moisture-laden winds reach them. By the time air masses from coastal areas reach the interior, they have lost all their moisture. Interior deserts are sometimes called inland deserts .
The Gobi Desert, in China and Mongolia, lies hundreds of kilometers from the ocean. Winds that reach the Gobi have long since lost their moisture. The Gobi is also in the rain shadow of the Himalaya mountains to the south.
Polar Deserts
Parts of the Arctic and the Antarctic are classified as deserts . These polar deserts contain great quantities of water, but most of it is locked in glaciers and ice sheets year-round. So, despite the presence of millions of liters of water, there is actually little available for plants and animals.
The largest desert in the world is also the coldest. Almost the entire continent of Ant arctica is a polar desert , experiencing little precipitation . Few organisms can withstand the freezing, dry climate of Ant arctica .
Changing Deserts
The regions that are deserts today were not always so dry. Between and BCE, for example, the Sahara had a much milder, moister climate . Climatologists identify this period as the &#;Green Sahara.&#;
Archaeological evidence of past settlements is abundant in the middle of what are arid , unproductive areas of the Sahara today. This evidence includes rock paintings, graves, and tools. Fossils and artifacts show that lime and olive trees, oaks, and oleanders once bloomed in the Sahara. Elephants, gazelles, rhinos, giraffes, and people used stream-fed pools and lakes.
There were three or four other moist periods in the Sahara. Similar lush conditions existed as recently as 25,000 years ago. Between the moist periods came periods of dryness much like today&#;s.
The Sahara is not the only desert to have dramatic climate change. The Ghaggar River, in what is now India and Pakistan, was a major water source for Mohenjo-daro, an urban area of the ancient Indus Valley Civilization. Over time, the Ghaggar changed course and now only flows during the rainy monsoon season. Mohenjo-daro is now a part of the vast Thar and Cholistan deserts .
Most of Earth&#;s deserts will continue to undergo periods of climate change .
Desert Characteristics
Humidity &#;water vapor in the air&#;is near zero in most deserts . Light rains often e vaporate in the dry air, never reaching the ground. Rainstorms sometimes come as violent cloudbursts. A cloudburst may bring as much as 25 centimeters (10 inches) of rain in a single hour&#;the only rain the desert gets all year.
Desert humidity is usually so low that not enough water vapor exists to form clouds . The sun&#;s rays beat down through cloudless skies and bake the land. The ground heats the air so much that air rises in waves you can actually see. These shimmering waves confuse the eye, causing travelers to see distorted images called mirages.
Temperature extremes are a characteristic of most deserts . In some deserts , temperatures rise so high that people are at risk of dehydration and even death. At night, these areas cool quickly because they lack the insulation provided by humidity and clouds . Temperatures can drop to 4°C (40°F) or lower.
In the Chihuahuan Desert , in the United States and Mexico, temperatures can vary by dozens of degrees in one day. Daytime temperatures in the Chihuahua can climb beyond 37°C (100°F), while nighttime temperatures can dip below freezing (0°C or 32°F).
Winds at speeds of about 100 kilometers per hour (60 miles per hour) sweep through some deserts . With little vegetation to block it, the wind can carry sand and dust across entire continents and even oceans. Windstorms in the Sahara hurl so much material into the air that African dust sometimes crosses the Atlantic Ocean. Sunsets on the Atlantic coast of the U.S. state of Florida, for example, can be tinted yellow.
First-time visitors to deserts are often amazed by the unusual landscapes, which may include dunes , towering bare peaks, flat-topped rock formations, and smoothly polished canyons. These features differ from those of wetter regions, which are often gently rounded by regular rainfall and softened by lush vegetation .
Water helps carve desert lands. During a sudden storm, water scours the dry, hard-baked land, gathering sand , rocks, and other loose material as it flows. As the muddy water roars downhill, it cuts deep channels, called arroyos or wadis. A thunderstorm can send a fast-moving torrent of water&#;a flash flood&#;down a dry arroyo . A flash flood like this can sweep away anything and anyone in its path. Many desert regions di scourage visitors from hiking or camping in arroyos for this reason.
Even urban areas in deserts can be vulnerable to flash floods . The city of Jeddah, Saudi Arabia, sits in the Arabian Desert . In , Jeddah was struck by a sudden thunderstorm and flash flood . Roads and buildings were washed away, and more than 100 people died.
Even in a desert , water and wind eventually wear away softer rock. Sometimes, rock is carved into tablelike formations such as mesas and buttes. At the foot of these formations, water drops its burden of gravel, sand , and other sediment, forming deposits called alluvial fans.
Many deserts have no drainage to a river , lake, or ocean. Rainwater, including water from flash floods , collects in large depressions called basins. The shallow lakes that form in basins eventually e vaporate , leaving playas, or salt-surfaced lake beds. Playas , also called sinks, pans, or salt flats , can be hundreds of kilometers wide.
The Black Rock Desert in the U.S. state of Nevada, for instance, is all that remains of the prehistoric Lake Lahontan. The hard, flat surface of desert salt flats are often ideal for car racing. In , British pilot Andy Green set the land speed record in Black Rock Desert &#;1,228 kilometers per hour (763 miles per hour). Green&#;s vehicle, the ThrustSSC, was the first car to break the sound barrier.
Wind is the primary sculptor of a desert &#;s hills of sand , called dunes . Wind builds dunes that rise as high as 180 meters (590 feet). Dunes migrate constantly with the wind . They usually shift a few meters a year, but a particularly violent sandstorm can move a dune 20 meters (65 feet) in a single day.
Sandstorms may bury everything in their path&#;rocks, fields, and even towns. One legend holds that the Persian Emperor Cambyses II sent an army of 50,000 men to the Siwa Oasis in western Egypt around 530 BCE. Halfway there, an enormous sandstorm swallowed the entire group. Archaeologists in the Sahara have been unsuccessfully looking for the &#;Lost Army of Cambyses&#; ever since.
Water in the Desert
Rain is usually the main source of water in a desert , but it falls very rarely. Many desert dwellers rely on groundwater, stored in aquifers below the surface. Groundwater comes from rain or other precipitation , like snow or hail. It seeps into the ground, where it can remain for thou sands of years.
Underground water sometimes rises to the surface, forming springs or seeps. A fertile green area called an oasis , or cienega, may exist near such a water source. About 90 major, inhabited oases dot the Sahara. These oases are supported by some of the world&#;s largest supplies of underground water. People, animals, and plants all surround these oases, which provide stable access to water, food, and shelter.
When groundwater doesn&#;t seep to the surface, people often drill into the ground to get to it. Many desert cities, from the American Southwest to the Middle East, rely heavily on such aquifers to fill their water needs. Rural Israeli communities called kibbutzim rely on aquifers to furnish water for crops and even fish farming in the dry Negev Desert .
Drilling into aquifers provides water for drinking, agriculture, industry, and hygiene. However, it comes at a cost to the environment. Aquifers take a long time to refill. If desert communities use groundwater faster than it is replenished, water shortages can occur. The Mojave Desert , in southern California and Nevada, for instance, is sinking due to aquifer depletion. The booming desert communities of Las Vegas, Nevada, and California&#;s &#;Inland Empire&#; are using water faster than the aquifer is being refilled. The water level in the aquifer has sunk as much as 30 meters (100 feet) since the s, while the land above the aquifer has sunk as much as 10 centimeters (4 inches).
Rivers sometimes provide water in a desert . The Colorado River , for instance, flows through three deserts in the American Southwest: the Great Basin , the Sonoran, and the Mojave. Seven states&#;Wyoming, Colorado, Utah, New Mexico, Nevada, Arizona, and California&#;rely on the river for some of their water supply.
People often modify rivers to help distribute and store water in a desert . The Nile River ecosystem dominates the eastern part of the Sahara Desert , for instance. The Nile provides the most reliable, plentiful source of freshwater in the region. Between and , the government of Egypt constructed a massive dam on the Upper Nile (the southern part of the river , near Egypt&#;s border with Sudan). The Aswan Dam harnesses the power of the Nile for hydroelectricity used in in dustry . It also stores water in a manmade lake, Lake Nasser, to protect the country&#;s communities and agriculture against drought.
Construction of the Aswan High Dam was a huge engineering project. Local desert communities can divert rivers on a smaller scale. Throughout the Middle East , communities have dug artificial wadis , where freshwater can flow during rainy seasons. In countries like Yemen, artificial wadis can carry enough water for whitewater rafting trips during certain times of the year.
When deserts and water supplies cross state and national borders, people often fight over water rights. This has happened among the states in the Colorado River Basin , which have negotiated for many years over the division of the river &#;s water. Rapidly expanding populations in California, Nevada, and Arizona have compounded the problem. Agreements that were made in the early 20th century failed to account for Native American water rights . Mexican access to the Colorado, which has its delta in the Mexican state of Baja California, was ignored. Desert agriculture , including cotton production, demanded a large portion of the Colorado. The environmental impact of dams was not considered when the structures were built. States of the Colorado River Basin continue to negotiate today to prepare for population growth, agricultural development, and the possibility of future droughts .
Life in the Desert
Plants and animals adapt to desert habitats in many ways. Desert plants grow far apart, allowing them to obtain as much water around them as possible. This spacing gives some desert regions a desolate appearance.
In some deserts , plants have unique leaves to capture sunlight for photosynthesis, the process plants use to make food. Small pores in the leaves, called stomata, take in carbon dioxide. When they open, they also release water vapor . In the desert , all these stomata would quickly dry out a plant. So desert plants typically have tiny, waxy leaves. Cactuses have no leaves at all. They produce food in their green stems.
Some desert plants, such as cactuses , have shallow, wide-spreading root systems. The plants soak up water quickly and store it in their cells. Saguaro cactuses , which live in the Sonoran Desert of Arizona and northern Mexico, expand like accordions to store water in the cells of their trunks and branches. A large saguaro is a living storage tower that can hold hundreds of liters of water.
Other desert plants have very deep roots. The roots of a mesquite tree, for example, can reach water more than 30 meters (100 feet) underground.
Mesquites , saguaros, and many other desert plants also have thorns to protect them from grazing animals.
Many desert plants are annuals , which means they only live for one season. Their seeds may lie dormant for years during long dry spells. When rain finally comes, the seeds sprout rapidly. Plants grow, bloom, produce new seeds, and die, often in a short span of time. A soaking rain can change a desert into a wonderland of flowers almost overnight.
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Animals that have adapted to a desert environment are called xerocoles. Xerocoles include species of insects, reptiles, birds, and mammals. Some xerocoles avoid the sun by resting in scarce shade. Many escape the heat in cool burrows they dig in the ground. The fennec fox, for example, is native to the Sahara Desert . Fennec fox communities work together to dig large burrows , some as large as 93 square meters (1,000 square feet). Dew can collect in these burrows , providing the foxes with fresh water. However, fennec foxes have adapted so they do not have to drink water at all: Their kidneys retain enough water from the food they eat.
Most xerocoles are nocturnal. They sleep through the hot days and do their hunting and foraging at night. Deserts that seem desolate during the day are very active in the cool nighttime air. Foxes, coyotes, rats, and rabbits are all nocturnal desert mammals. Snakes and lizards are familiar desert reptiles. Insects such as moths and flies are abundant in the desert . Most desert birds are restricted to areas near water, such as river banks. However, some birds, such as the roadrunner, have adapted to life in the desert . The roadrunner, native to the deserts of North America, obtains water from its food.
Some xerocoles have bodies that help them handle the heat. A desert tortoise&#;s thick shell insulates the animal and reduces water loss. Sand lizards, native to the deserts of Europe and Asia, are nicknamed &#;dancing lizards&#; because of the way they quickly lift one leg at a time off the hot desert sand . A jackrabbit&#;s long ears contain blood vessels that release heat. Some desert vultures urinate on their own legs, cooling them by e vaporation .
Many desert animals have developed ingenious ways of getting the water they need. The thorny devil, a lizard that lives in the Australian Outback, has a system of tiny grooves and channels on its body that lead to its mouth. The lizard catches rain and dew in these grooves and sucks them into its mouth by gulping.
Camels are very efficient water users. The animals do not store water in their humps, as people once believed. The humps store fat. Hydrogen molecules in the fat combine with inhaled oxygen to form water. During a shortage of food or water, camels draw upon this fat for nutrition and moisture. Dromedary camels, native to the Arabian and Sahara deserts , can lose up to 30 percent of their body weight without harm. Camels, nicknamed &#;ships of the desert ,&#; are widely used for transportation, meat, and milk in the Maghreb (a region in Northwest Africa), the Middle East , and the Indian Sub continent .
People and the Desert
About 1 billion people live in deserts . Many of these people rely on centuries-old customs to make their lives as comfortable as possible
Civilizations throughout the Middle East and Maghreb have adapted their clothing to the hot, dry conditions of the Sahara and Arabian deserts . Clothing is versatile and based on robes made of rectangles of fabric. Long-sleeved, full-length, and often white, these robes shield all but the head and hands from the wind , sand , heat, and cold. White reflects sunlight, and the loose fit allows cooling air to flow across the skin.
These robes of loose cloth can be adjusted (folded) for length, sleeves, and pockets, depending on the wearer and the climate . A thobe is a full-length, long-sleeved white robe. An abaya is a sleeveless cloak that protects the wearer from dust and heat. A djebba is a short, square pullover shirt worn by men. A kaffiyeh is a rectangular piece of cloth folded loosely around the head to protect the wearer from sun exposure, dust , and sand . It can be folded and unfolded to cover the mouth, nose, and eyes. Kaffiyehs are secured around the head with a cord called an agal. A turban is similar to a kaffiyeh , but wrapped around the head instead of being secured with an agal . Turbans are also much longer&#;up to six meters (20 feet)!
Desert dwellers have also adapted their shelters for the unique climate . The ancient Anasazi peoples of the southwestern United States and northern Mexico constructed huge apartment complexes in the rocky cliffs of the Sonoran Desert . These cliff dwellings, sometimes dozens of meters off the ground, were constructed with thick, earthen walls that provided insulation . Although temperatures outside varied greatly from day to night, temperatures inside did not. Tiny, high windows let in only a little light and helped keep out dust and sand .
The need to find food and water has led many desert civilizations to become nomadic. Nomadic cultures are those that do not have permanent settlements. In the deserts of the Middle East and Asia, nomadic tent communities continue to flourish. Tent walls are made of thick, sturdy cloth that can keep out sand and dust , but also allow cool breezes to blow through. Tents can be rolled up and transported on pack animals (usually horses, donkeys, or camels). Nomads move frequently so their flocks of sheep and goats will have water and grazing land.
Besides animals like camels and goats, a variety of desert vegetation is found in oases and along the shores of rivers and lakes. Figs, olives, and oranges thrive in desert oases and have been harvested for centuries.
Some desert areas rely on resources brought from more fertile areas&#;food trucked in from distant farmlands or, more frequently, water piped from wetter regions. Large areas of desert soil are irrigated by water pumped from underground sources or brought by canal from distant rivers or lakes. The booming Inland Empire of southeastern California is made up of deserts (the Mojave and the Sonoran) that rely on water for agriculture , in dustry , and residential development. Canals and aqueducts supply the Inland Empire with water from the Colorado River , to the east, and the Sierra Nevada snowmelt to the north.
A variety of crops can thrive in these irrigated oases. Sugar cane is a very water-intensive crop mostly harvested in tropical regions. However, sugar cane is also harvested in the deserts of Pakistan and Australia. Water for irrigation is transported from hundreds of kilometers away, or drilled from hundreds of meters underground.
Oases in desert climates have been popular spots for tourists for centuries. Spas ring the Dead Sea, a saline lake in the Judean Desert of Israel and Jordan. The Dead Sea has had flourishing spas since the time of King David.
Air transportation and the development of air conditioning have made the sunny climate of deserts even more accessible and attractive to people from colder regions. Populations at resorts like Palm Springs , California, and Las Vegas, Nevada, have boomed. Desert parks, such as Death Valley National Park, California, attract thou sands of visitors every year. People who migrate to the warm, dry desert for the winter and return to more temperate climates in the spring are sometimes called &#;snowbirds.&#;
In rural areas, hot days turn into cool nights, providing welcome relief from the scorching sun. But in cities, structures like buildings, roads, and parking lots hold on to daytime heat long after the sun sets. The temperature stays high even at night, making the city an &#;island&#; of heat in the middle of the desert . This is called the urban heat island effect. It is less pronounced in desert cities than cities built in heavily forested areas. Cities like New York City, New York, and Atlanta, Georgia, can be 5 degrees warmer than the surrounding area. New York was built on wetland habitat , and Atlanta was built in a wooded area. Cities like Phoenix, Arizona, or Kuwait City, Kuwait, have a much smaller urban heat island effect. They may be only slightly warmer than the surrounding desert .
Deserts can hold economically valuable resources that drive civilizations and economies. The most notable desert resource in the world is the massive oil reserves in the Arabian Desert of the Middle East . More than half of the proven oil reserves in the world lie beneath the sands of the Arabian Desert , mostly in Saudi Arabia. The oil in dustry draws companies, migrant workers, engineers, geologists, and biologists to the Middle East .
Desertification
Desertification is the process of productive cropland turning into non-productive, desert -like environments. Desertification usually happens in semi- arid areas that border deserts .
Human activities are a primary cause of desertification . These activities include overgrazing of livestock, deforestation, overcultivation of farmland, and poor irrigation practices. Overgrazing and de forestation remove plants that anchor the soil. As a result, wind and water erode the nutrient-rich topsoil. Hooves from grazing livestock compact the soil, preventing it from absorbing water and fertilizers. Agricultural production is devastated, and the economy of a region suffers.
The deserts of Patagonia, the largest in South America, are expanding due to desertification . Patagonia is a major agricultural region where non-native species such as cattle and sheep graze on grassland. Sheep and cattle have reduced the native vegetation in Patagonia , causing loss of valuable topsoil . More than 30 percent of the grasslands of Argentina, Chile, and Bolivia are faced with desertification .
People often overuse natural resources to survive and profit in the short term, while neglecting long-term sustainability. Madagascar, for instance, is a tropical island in the Indian Ocean. Seeking greater economic opportunities, farmers in Madagascar engaged in slash-and-burn agriculture . This method relies on cutting and burning forests to create fields for crops . Unfortunately, at the time farmers were investing in slash-and-burn agriculture , Madagascar experienced long-term droughts . With little vegetation to anchor it, the thin topsoil quickly eroded . The island&#;s central plateau is now a barren desert .
Rapid population growth also can lead to overuse of resources , killing plant life and depleting nutrients from the soil. Lake Chad is a source of freshwater for four countries on the edge of the Sahara Desert : Chad, Cameroon, Niger, and Nigeria. These developing countries use Lake Chad&#;s shallow waters for agriculture , in dustry , and hygiene . Since the s, Lake Chad has shrunk to half its size. Desertification has severely reduced the wetland habitats surrounding the lake, as well as its fishery and grazing lands.
Desertification is not new. In the s, parts of the Great Plains of North America became the &#;Dust Bowl&#; through a combination of drought and poor farming practices. Millions of people had to leave their farms and seek a living in other parts of the country.
Desertification is an increasing problem. Every year, about 6 million square kilometers (2.3 million square miles) of land become useless for cultivation due to desertification . The Sahara Desert crept 100 kilometers (39 miles) south between and . South Africa is losing 300-400 million metric tons (330-441 short tons) of topsoil each year.
Many countries are working to reduce the rates of desertification . Trees and other vegetation are being planted to break the force of the wind and to hold the soil. Windbreaks made of trees have been planted throughout the Sahel, the southern border region of the Sahara Desert . These windbreaks anchor the soil and prevent sand from invading populated areas.
In China&#;s Tengger Desert , researchers have developed another way to control wandering dunes . They anchor the drifting sand with a gridlike network of straw fences. Straw is poked partway into the sand , forming a pattern of small squares along the contours of the dunes . The resulting fences break the force of the wind at ground level, stopping dune movement by confining the sand within the squares of the grid.
New technologies are also being developed to combat desertification . &#;Nanoclay&#; is a substance sprayed on desert sands that acts as a binding agent. Nanoclay keeps the sand moist, clumping it together and preventing it from blowing away.
Deserts Get Hotter
Rising temperatures can have huge effects on fragile desert ecosystems . Global warming is the most current instance of climate change . Human activities such as burning fossil fuels contribute to global warming .
In deserts , temperatures are rising even faster than the global average. This warming has effects beyond simply making hot deserts hotter. For example, increasing temperatures lead to the loss of nitrogen, an important nutrient , from the soil. Heat prevents microbes from converting nutrients to nitrates, which are necessary for almost all living things. This can reduce the already limited plant life in deserts .
Climate change also affects rainfall patterns. Climate scientists predict that global warming will lead to more rainfall in some regions, but less rainfall in other places. Areas facing reduced precipitation include areas with some of the largest deserts in the world: North Africa (Sahara), the American Southwest (Sonoran and Chihuahuan), the southern Andes ( Patagonia ), and western Australia (Great Victoria).
In literature and in legend, deserts are often described as hostile places to avoid. Today, people value desert resources and biodiversity. Communities, governments , and organizations are working to preserve desert habitats and increase desert productivity.
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