Deserts are land areas that receive less than 25.4 cm (10 inches) of rain, unevenly distributed throughout the year. A number of climatic processes produce desert areas. The most influential are the six cells of cold air currents that descend at the poles and near the Tropic of Cancer and Tropic of Capricorn. These air currents, driven by the sun and rotation of the earth, create areas of relatively warm, dry conditions. Many of the world’s deserts live in a “rain shadow,” an area to the leeward side of a mountain range that prevents the small amount of moisture that is present in the air to move over the mountains. As the air rises, the moisture cools and precipitates in the higher elevations. Therefore the area in the “shadow” of the mountain range receives little moisture. The air that does descend is quite dry and adds to the evaporative effect. The Atlas Mountains shadow the Sahara, the Andes the Patagonian, the Great Dividing Range the Australian, and the Sierra Nevada and Cascades the Great Basin deserts. The amount of rainfall is not an absolute indicator of “dryness” because the rate of evaporation and timing of the rainfall must also be taken into consideration. The amount and type of vegetation, soil composition, altitude, average temperature, wind speed, and solar radiation all contribute to “dryness” and desert formation. Antarctica would be the world’s largest desert by the definition of less than 25.4 cm (10 inches) of rainfall annually, some areas of that continent having had no recorded rain in 30 years. There is a large amount of water present in the form of ice, but it is not available for use by plants. Antarctica has its own special survival problems not associated with precipitation and, for the purposes of desert survival, will not be considered here. In contrast to Antarctica stands the northern coast of Alaska, which receives less than 10.2 cm (4 inches) of rain annually yet is quite wet because evaporation is so low.
Deserts are one type of environment on Earth that is increasing in total area, likely because of human as well as geologic factors. Overgrazing, destruction of forests, global warming, and other aspects of increased human population contribute to desertification. Currently about 15% of the land area of the earth is desert (30% if Antarctica is included) (Figure 44-1; Table 44-1). Most of the earth’s deserts can be found between 30 degrees south and 30 degrees north latitude, making them hot as well as dry. These deserts include the Sahara, Arabian, Kalahari, Australian, Atacama, Thar, Namib, and southwest United States. About 50% of Africa is desert; the Sahara by itself is almost as large as the United States. About 8% of the United States, or 776,996 square kilometers (300,000 square miles), is desert. Most of the U.S. desert areas are adjacent to national parks and forests and are frequently visited, for example, the Grand Canyon, Big Bend, Arches, Zion, Organ Pipe, Joshua Tree, Great Basin, Saguaro, and Capital Reef. Beyond 40 degrees south and north latitude and at elevations over 3048 m (10,000 feet) are the “cold” deserts, which have wide swings in temperature, for example, the Patagonian, Turkestan, Gobi, and Taklamakan. The large temperature variations in desert regions are greater at higher elevations and latitudes but are present in all deserts. Lack of vegetation, cloud cover, and ground-water surface allows 90% of solar radiation to reach the desert surface. By contrast, a forest may reflect 50% to 60% of the solar radiation, and its vegetation disperses the rest. At night, lack of cloud cover and vegetation allows almost 100% of the accumulated heat to escape, as opposed to only 50% from a humid climate. This explains why the desert temperature may reach 48.9° C (120° F) during the day and drop to 4.4° C (40° F) at night. Tropical rain forests may only reach 35° C (95° F) during the day, but at night the temperature only drops to 29.4° C (85° F).
It might seem that the extreme desert climate would only allow for sparse life, but that is not the case (Figure 44-2). Death Valley, one of the harshest environments in North America, where air temperatures have been recorded at 56.7° C (134° F), has 600 species of plants, 30 species of mammals, 25 species of reptiles, and 2 species of fishes. Oases are found in most deserts. They are isolated depressions usually fed by a constant source of water. Underground springs and wells supply moisture for plants and animals. Often one must dig to find water at the lowest point of the depression. Many named oases have supported camel caravans, allowing them to move from oasis to oasis and thus cross an otherwise impenetrable desert. Desert way stations and ancient cities have sprung up along these routes. Many ancient oases have wells hundreds of feet deep and because of overuse are gradually drying up. When the water is used up, the oasis disappears, along with its desert life.
All desert flora and fauna have one guiding principle for survival, which is to conserve water. The ground surface of the desert has the highest temperature because of the direct effect of solar heat and wind. Therefore, during the hottest times of the day, most animals are either below the surface, in underground burrows, or above the surface in available vegetation, cacti, trees, or shrubs. Most animals forage from dusk until dawn because temperatures are cooler. Some mammals, such as kangaroo rats, never drink but obtain necessary water through plant seeds. Plants have evolved a number of survival skills to maintain water, including stomata that are closed during the day and crassulacean acid metabolism (CAM) photosynthesis. The latter allows for accumulation of carbon compounds at night through the dark reaction. These compounds are converted to carbon dioxide during the day when the stomata are closed. Other adaptations include stem photosynthesis in plants without leaves, thick cuticles, water storage tissues, and widespread shallow root systems. Desert plants also have evolved a variety of defense mechanisms, such as production of toxic compounds that act as herbicides to other plants, and the formation of needles, spines, and thorns that dissuade browsing animals. Obviously humans are not able to evolve these physiologic changes but must rely on behavior, technology, and other adaptations to mimic the methods used by indigenous desert dwellers.