Overview of the Desert
Welcome to the Desert
Photo by G. Morris Southward
The word desert brings to mind powerful images for many people. Some believe a desert to be a hot, barren place without surface water. Others think of a desert as a dry region covered with sand dunes and tall, branching cacti that resemble human forms. Actually, neither of these images is completely correct. There are deserts in Antarctica and on the icecaps of Greenland which are never hot. The Atacama Desert in Chile, one of the most arid places on earth, borders on the ocean. It is true that some deserts like the Sahara in Africa have sand dunes, but that's not what makes them deserts. In fact, only about 1/4 of all desert surfaces are made of sand - the rest are dirt, clay, rock, ice or some other mixture of organic and inorganic materials. And although deserts generally have little precipitation, many have at least one rainy season each year. In fact, the Chihuahuan Desert in New Mexico has two different annual rainy periods: a monsoon season in summer and mild rains in early winter.
As for barren, that is only a fable. Deserts actually support large, diverse populations of animals and plants, easily seen once you know how to look. Many of these desert species have made interesting adaptations to the dry conditions, and so live differently than animals who are from more humid regions. The species that inhabit each desert are part of a complete ecological community. For example, the lowland Sonoran Desert of Mexico and Arizona contains saguaro (sa HUA ro) cacti which provide homes for many cactus-nesting birds. Others deserts at higher altitudes, such as the Chihuahuan Desert, may be home to fewer cacti, but contain many shrubs like mesquite or creosote bushes with their own animal residents. Some deserts may be carpeted in green during rainy seasons, providing food for grazing animals. Or they may be flat expanses of tall grasses supporting large populations of kangaroo rats and other burrowing rodents. One thing is certain - no desert is completely empty of plant and animal life.
So What is a Desert?
Now that we know what a desert isn't, how can we tell if we are in one? According to the most common scientific method for classifying climates (the Köppen system) a desert is a place where more water would be lost through evaporation than is gained from precipitation. For example, in Las Vegas, Nevada, average yearly precipitation is only about 3.5 inches; but the average yearly loss of water due to evaporation would be 6 feet if that much water were available. That means that any living creature in the desert must have an efficient method of conserving or replacing body moisture, or it will dry out like a mummy. Therefore, many of the animals and plants that thrive in arid lands have developed interesting ways both to cool themselves and to gather, store, and use water.
There are several other common definitions that you may sometimes see for the word desert. Some people consider a desert to be a place where the average annual precipitation is less than 10 inches a year. Others look at the amount of moisture available to living creatures, rather than total moisture. But the moisture in a desert varies from year to year. Therefore, most scientists do not use these definitions.
In the North American West, there are four separate deserts that border on each other: the Chihuahuan, the Sonoran, the Mojave, and the Great Plains. Part of what sets one desert apart from another are climate differences caused by altitude (how high above sea level they are), latitude (the north/south location on the globe), the height and position of near-by mountain ranges, and the type of winds that blow through. For example, although the Sonoran and Chihuahuan deserts are in the same latitude, the Sonoran is hotter than the Chihuahuan because it is at a lower altitude. And the Great Basin, which extends all the way to the northern border of Idaho, is the coldest of all. This is partly due to its northern latitude, partly to the fact that it has the highest altitude of any North American desert, and partly to the fact that the wind currents blowing over the region during the winter carry bursts of Arctic air.
Water in the Desert
Although you may not always be able to see the water in the desert, it is often present. Water can be found in two forms: either surface water, or water below the surface called sub-surface.
Photo by G. Morris Southward
Photo by V. Espinoza
Surface water includes ephemeral streams, arroyos, lakes, ponds and puddles produced by heavy rains during rainy seasons; permanent rivers, such as the Rio Grande in the Chihuahuan Desert or the Nile in Egypt; or icecaps or other stored water. Most desert rivers start in mountain areas outside the desert, and flow through on their way to empty into the sea. They are important to human life, because they are often used for irrigation in agriculture, and as centers for the development of cities.
Photo by G. Morris Southward
Subsurface water comes from seepage that has collected, often for centuries, deep underneath the land. The water may spring up to the surface, which is how oasis are formed. In addition, wells may be drilled by people wanting to tap this ground water for human development. Scientists are currently trying to understand the effect on the desert of withdrawing large quantities of ground water, because as human societies continue to grow in arid lands, more water will be needed. Most of what will be drilled has formed over long time periods and cannot be quickly replenished.
How Do Deserts Form?
So now we know how to tell when we are in a desert. But where do deserts come from? There is evidence of deserts on earth as long ago as 125 million years, although their size, location, and exact form have changed many times over that period. After all, 125 million years is time enough for a lot of changes - look what happened to the dinosaurs! Geologists believe that there have been deserts in their current locations in North America for as much as ten million years. In fact, the fossils of two horned lizards have been found, dating from four or five million years ago. Since these horned lizards are desert species, scientists can be sure that arid conditions existed at that time.
However, that doesn't mean that the same Chihuahuan, Mojave, Sonoran and Great Plains deserts have been here for that long. Our current North American deserts are mere youngsters - only about 8000 - 10,000 years old. Glaciers were one important factor in the development of these deserts. During the last ice age, around 50,000 years ago, many of the basins in the North America were filled with water. About 8000 to 10,000 years ago, after the last glaciers retreated, the current deserts were formed with similar climate, size, and many of the same species of plants and animals that we have today.
With the exception of the frozen deserts in Greenland and Antarctica, most of the world's deserts are in two belts that lie within 25 degrees of the equator. One reason is that the high atmospheric pressure in this region can cause dry, cold air from the upper altitudes to compress and come down to earth. This dry air is so clear that it can be easily heated by the sun, causing high ground temperatures with very low humidity. The Sahara (the world's largest desert), the Atacama on the coast of Chile, and the Kalahari in Africa have been formed in this way.
Another type of desert has also developed in this region around the equator. A "rain shadow" desert often forms when there are two mountain ranges, one on the east and one on the west of a land expanse, which block moist ocean air from reaching the land. Instead, almost all the precipitation falls on the opposite side of each mountain range, leaving the region between the mountains dry. Very few deserts are formed solely by a rain shadow effect, because they are also influenced by the high atmospheric pressure. However, the North American deserts are usually called rain shadow deserts. Other famous ones are the Gobi in China (blocked by the Himalayas), and the eastern and central deserts in Australia.
The final type of desert to be mentioned is not near the equator, but instead in a climate that is so cold, that the air can only hold a small amount of moisture. There is very little precipitation, and all the surface water is locked in unusable blocks of ice. These conditions have created the Antarctican and Greenland deserts, which are nothing like the usual picture of arid lands. Deserts are certainly more diverse than most people suspect.
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