When someone says the word "desert," it's almost certain that you immediately picture the stereotype depicted in movies and other forms of popular culture: Sand as far as the eye can see in all directions, no plants with the possible exception of cactus or two, a total absence of water and an abundance of searing sunlight. Deserts appear, in a word, inhospitable. Yet few people in North America have any first-hand experience with deserts.
While in general the above impressions are reasonably accurate, a desert is not merely a patch of arid land; rather, a desert constitutes a biome, or community of living things connected to a particular type of geography. In addition, deserts are anything but scarce. Deserts, in fact, account for one-fifth of Earth's land area, and come in four distinct varieties.
What Is a Desert?
Deserts are characterized by extreme environmental conditions. They get at most 50 centimeters (cm), or 20 inches, a year of precipitation; more commonly, they're lucky to get half of that. Most of them are found at low latitudes, that is, closer to the equator than to the poles. The massive Sahara, probably the most famous desert on Earth and its third largest, lies just north of the equator in Africa. While they are far less densely populated than other biomes owing to how dry they are and being poorly hospitable overall, most deserts do feature a range of vegetation as well as both vertebrate and invertebrate animal life.
Large mammals are uncommon in deserts because most of them cannot store sufficient water and tolerate the heat (camels are a notable exception). While smaller animals might be able to find patches of shade sufficient to cover their bodies, deserts typically offer little protection from the sun for larger animals. The dominant animals of the warm deserts are non-mammalian vertebrates, mainly reptiles. Whatever mammals have managed to thrive in these biomes tend to be small, such as the kangaroo mice that inhabit some deserts in North America.
A few sentences ago, you read that the Sahara is the third-largest desert in the world. Did this perhaps surprise you? Have you heard elsewhere that the Sahara is far and away the world's biggest desert? The explanation for this is surprising and powerful.
How Many Types of Deserts Are There in the World?
While ecologists agree that there are four fundamental types of deserts, the nomenclature of these four desert biomes varies slightly from source to source. The four basic desert types are the hot-and-dry (or subtropical) desert, the semiarid (or cold-winter) desert, the coastal desert and the cold (or polar) desert. These are described individually in detail later, but a brief overview is helpful to start.
Hot-and-dry deserts are, well, hot and dry. Various kinds of deserts experience very hot weather, but this type gets it all year. Cold winter deserts have long, arid summers and a small amount of rainfall in the winter. Coastal deserts have cool winters, but warm summers. Polar deserts are cold year-round.
To continue the intrigue from the preceding section, the two largest deserts in the world are polar deserts. One is the Antarctic Polar Desert, and the other is the Arctic Polar Desert. How can vast areas covered mainly or entirely in snow and ice, which is clearly a form of moisture, qualify as deserts?
What Are the Four Different Types of Desert?
As for animals, insects and jack rabbits are seen during the day, staying in shadow as much as possible. Many animals seek protection in burrows underground, where they are insulated from the hot, dry air. These include:
- kangaroo rats
- some insects
What Are the Major Types of Desert Biomes?
Some sources list more than four desert types to better account for the variability in geographic and ecological factors from place to place. For example, the U.S. Geological Survey lists eight types of deserts: trade wind, mid-latitude, rain shadow, coastal, monsoon, polar deserts, paleodeserts and extraterrestrial deserts. The last two are not found on Earth; paleodeserts are areas that show evidence of having been deserts in the recent geological past, whereas extraterrestrial deserts are found on other planets, such as Mars.
Trade wind deserts are analogous to hot-and-dry (subtropical) deserts. Mid-latitude deserts overlap with the cold-winter deserts in the four-desert-type scheme. Rain shadow deserts, which are also cold-winter-style deserts, form on the sides of tall mountain ranges blocked from receiving much moisture. Monsoon deserts are seen in India and Pakistan. Coastal and polar deserts retain the same basic definitions as before.
What Are the Five Largest Deserts in the World?
The two largest deserts in the world are the Antarctic Polar Desert, which is 5.5 square million miles in area, and its northern counterpart, the Arctic Polar Desert, which includes 5.4 million square miles. The United States, by way of comparison, is about 3.5 million square miles in size. The Antarctic Polar Desert is more easily visualized because it is confined to a single, large, somewhat circular land mass.
The Sahara Desert in northern Africa covers about 3.5 million square miles and is recognized by some sources as being the world's largest desert because the polar deserts are not traditional deserts. The fourth-largest at 1 million square miles is the Arabian Desert that takes up the Arabian Peninsula in the Middle East, while the fifth-largest is the Gobi Desert of China and Mongolia, which covers 500,000 square miles.
About the Author
Kevin Beck holds a bachelor's degree in physics with minors in math and chemistry from the University of Vermont. Formerly with ScienceBlogs.com and the editor of "Run Strong," he has written for Runner's World, Men's Fitness, Competitor, and a variety of other publications. More about Kevin and links to his professional work can be found at www.kemibe.com.