Deserts cover more than one fifth of the Earth's land, and they are found on every continent. A place that receives less than 10 inches (25 centimeters) of rain per year is considered a desert. Deserts are part of a wider classification of regions called "drylands." These areas exist under a moisture deficit, which means they can frequently lose more moisture through evaporation than they receive from annual precipitatiom.
And despite the common conceptions of deserts as dry and hot, there are cold deserts as well. The largest hot desert in the world, northern Africa's Sahara, reaches temperatures of up to 122 degrees Fahrenheit (50 degrees Celsius) during the day. But some deserts are always cold, like the Gobi desert in Asia and the desert on the continent of Antarctica. Others are mountainous. Only about 10 percent of deserts are covered by sand dunes. The driest deserts get less than half an inch (one centimeter) of precipitation each year, and that is from condensed fog not rain.
Desert animals have adapted ways to help them keep cool and use less water. Camels, for example, can go for days without food and water. Many desert animals are nocturnal, coming out only when the brutal sun has descended to hunt. Some animals, like the desert tortoise in the southwestern United States, spend much of their time underground. Most desert birds are nomadic, crisscrossing the skies in search of food. Because of their very special adaptations, desert animals are extremely vulnerable to introduced predators and changes to their habitat.
Desert plants may have to go without fresh water for years at a time. Some plants have adapted to the arid climate by growing long roots that tap water from deep underground. Other plants, such as cacti, have special means of storing and conserving water. Many desert plants can live to be hundreds of years old.
Some of the world's semi-arid regions are turning into desert at an alarming rate. This process, known as "desertification," is not caused by drought, but usually arises from the demands of human populations that settle on the semi-arid lands to grow crops and graze animals. The pounding of the soil by the hooves of livestock may degrade the soil and encourage erosion by wind and water.
Global warming also threatens to change the ecology of desert. Higher temperatures may produce an increasing number of wildfires that alter desert landscapes by eliminating slow-growing trees and shrubs and replacing them with fast-growing grasses.
