All deserts have two things in common: they are dry, and they support little plant and animal life. If a region receives an average of less than 10 inches (25 centimeters) of rain each year, scientists classify it as a desert. Contrary to what most people believe, all deserts are not hot. Some ice deserts near the North and South Poles are so cold that all moisture is frozen. This chapter will discuss deserts in tropical and temperate areas. (Tropical areas are those near the equator. Temperate areas are between the equator and the North and South Poles).
True deserts cover about 14 percent of the world's land area, or about 8,000,000 square miles (20,800,000 square kilometers). Another 15 percent of the Earth's land surface possesses some desertlike characteristics. Most deserts lie near the tropic of Cancer and the tropic of Capricorn, two lines of latitude lying about 25 degrees from the equator (see map). The area between these two lines is called the Torrid Zone. ("Torrid" means very hot.)
In general, deserts are caused by the presence of dry air. The average humidity (moisture in the air) is between 10 and 20 percent. In some cases, mountain ranges prevent moisture-laden clouds from reaching the area. Mountains can also cause heavy, moisture-filled clouds to rise into the colder atmosphere. There, the moisture condenses and falls in the form of rain, leaving the air devoid of moisture as it crosses the range. In other cases, certain wind patterns along the equator bring air in from dry regions. In another scenario, cold-water ocean currents can cause moist air to drop its moisture over the ocean. The resulting dry air quickly evaporates (dries up) ground moisture along the coastal regions as it moves inland.
Deserts have always existed, even when glaciers covered large portions of the Earth during the great Ice Ages. Although geological evidence is scarce, scientists tend to agree that some desert areas have always been present, although they were probably smaller than those of today. Fossils, the ancient remains of living organisms that have turned to stone, can reveal the climatic history of a region. For example, because scientists have found fossils of a small species of hippopotamus there, they believe that the Arabian Desert, which covers most of the Arabian Peninsula to the east of North Africa, once included wetlands. In the Sahara Desert of North Africa, rock paintings done 5,000 years ago show pictures of elephants, giraffes, and herds of antelope that could no longer survive there.
Today, "desertification" (dee-zur-tih-fih-KAY-shun; desert formation), occurs continuously, primarily on the edges of existing deserts. Desertification is caused by a combination of droughts (rainless periods) and human activity such as deforestation (cutting down forests) or overgrazing of herd animals. When every blade of grass is used and rain is scarce, plants don't grow back. Without plants to hold the soil in place, the wind blows away the smaller and finer particles of soil exposing the less compacted layer of sand, leaving a barren, unprotected surface. Eventually, even groundwater disappears. In 1882, the percentage of land classified as desert was only around 9 percent. By 1950, arid (very dry) and semiarid lands combined had grown, at a rate of about 30 miles (48 kilometers) a year, to more than 23 percent. Today, because of desertification, the Sahara in North Africa, the world's largest desert, has advanced southward another 600 miles (1,000 kilometers).
Source: "Desert." U·X·L Encyclopedia of Biomes, Vol. 1. U·X·L, Detroit: 2000.