Deserts

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Deserts

Introduction

A desert is a biome where precipitation is rare and unpredictable. Many deserts lie in the continental interior, such as the Sahara of northern Africa, and the Mojave, which covers parts of California, Nevada, Arizona, and Utah. Few deserts are completely barren. Most have at least some vegetation, although it is adapted to dry conditions. For instance, desert cacti have spines rather than leaves to reduce water loss. Desert animals include invertebrates and small mammals, like kangaroo mice.

Deserts are always dry but they are not necessarily hot. For instance, Antarctica is a cold desert, while coastal deserts have cool winters and long, warm summers. A desert is a vulnerable environment whose vegetation is readily damaged, causing the soil to become drier still. Many formerly productive lands are turning into deserts, especially in Africa and China.

Historical Background and Scientific Foundations

Deserts cover one fifth of Earth’s surface and are characterized by their dryness. Precipitation, more usually in the form of rain, is rare and unpredictable. In the desert biome, the overall average amount of water falling is fewer than 12 in (30 cm) in a year. More rain than this may fall, but some of it is balanced out by the amount of water lost from the soil through evaporation. In some years, there may be no rain at all. On the other hand, there may sometimes be sudden, heavy rainfall which can even cause flooding.

Deserts are favored by warm, dry climates with high atmospheric pressure, which is why many are found in continental interiors at latitudes of around 30 degrees, North and South. However, coastal deserts also exist, such as the Atacama Desert in Chile, which is one of the driest places on Earth. Although the presence of the ocean normally provides plenty of precipitation through evaporation of surface waters, the Atacama Desert lies within the rain shadow of the Andes mountains.

There are four types of desert, classified according to climate. Hot, dry deserts are warm all year round, with very hot summers. Examples include the Mojave, Sahara, and Australian deserts. The atmosphere of a hot, dry desert contains very little moisture to filter the sun’s rays, which means the ground surface absorbs a lot of heat during the day and loses a lot of heat at night. This leads to daily temperature extremes with temperatures that can be as high as 115°F (46°C) during the day, while plunging to around 17°F (-8°C) at night. Rain tends to occur in short, intense, bursts and sometimes evaporating before it even reaches the ground. Soil is coarse, shallow, or rocky with good drainage.

The semi-arid desert, such as those of Utah and Montana, has moderately long, dry summers. Daily temperatures are less extreme than in hot, dry deserts. Soil texture varies from coarse to fine and sandy. Coastal deserts are found in moderately cool to warm areas, and are characterized by their cool winters and long warm summers. The coastal desert enjoys more rainfall than the hot, dry, and semi-arid deserts, amounting to a few inches or more a year. The soil of a coastal desert is fine-textured with a significant salt content. It is fairly porous and drains well. The cold desert, occurring in the Antarctic, Arctic, and Greenland regions, has snowfall and high rainfall in the winter. Summers are short and moderately warm. The soil of a cold desert is heavy, salty, and full of silt. Mean winter temperature will be between 35–39°F (2–4°C) and mean summer temperature is 77°F (25°C).

Deserts are home to a diverse range of plant and animal life. Species tend to be specially adapted to the dry environment with special features to conserve water. For

WORDS TO KNOW

BIOME: A well-defined terrestrial environment (e.g., desert, tundra, or tropical forest) and the complex of living organisms found in that region.

PRECIPITATION: Moisture that falls from clouds as a result of condensation in the atmosphere.

SEMIARID: Receiving very little annual rainfall, roughly 10-20 in (25-50 cm), and characterized by short grasses and shrubs.

instance, desert plants, like cacti, have tough leathery skin and spines, rather than leaves. Many are salt-tolerant. Some desert plants are also capable of blooming and setting seed very quickly, to make the most of sudden, intense rainfall.

Desert animals include reptiles and small mammals, such as kangaroo rats and pocket mice. They tend to pursue a nocturnal lifestyle, sheltering from the desert heat during the day. Desert rodents can get most of their moisture from their food and convert its fat con-

tent into water in their bodies. They also have highly concentrated urine and dry feces, which also helps reduce water loss. Large mammals are rare in the desert for they cannot store or conserve enough water to survive such dry conditions. The camel, whose hump stores fat, is an exception.

Impacts and Issues

The desert environment is easily damaged. For instance, tracks left by army tanks during World War II (1939–1945) are still visible in the California deserts. Vegetation has an important protective role in a desert, retaining moisture in the soil. Overgrazing causes the soil to drift and become even drier. Another important issue is desertification, which is the creation of new desert from previously productive land, with the accompanying risk of drought, crop failure, and hunger. The United Nations says that 80% of the world’s grassland is suffering from overgrazing and soil degradation. Three quarters of this area is now showing some signs of desertification.

See Also Desertification; Drought