Desert

• 1. Introduction

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  A desert is a landscape where so little rain falls that some weather stations in the Atacama have never recorded a single drop. Evidence suggests the Atacama may have gone without significant rainfall from 1570 to 1971. Yet about one-third of Earth's land surface is arid or semi-arid, and that share includes places most people would never picture. Antarctica counts as the world's largest cold desert, a continent that is roughly 98% ice and 2% barren rock. The word itself comes from the Latin deserere, meaning to abandon, and once meant simply an unpopulated place. So what makes a landscape a desert, if not sand and heat? How does a region so hostile to water still hold springs, oases, and even fields of crops? And how have people, plants, and animals learned to survive where streams dry up unless fed from somewhere far away?

• 2. Defining Aridity

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  Most deserts receive less than 250 mm of precipitation each year, and that single threshold hides a great deal of complexity. Tucson, Arizona offers a clear illustration. It gets about 300 mm of rain a year, but about 2500 mm of water could evaporate over the same period. About eight times more water could leave the region than ever falls on it, which is why scientists weigh potential evapotranspiration alongside rainfall. The water budget of an area can be calculated with the formula P minus PE plus or minus S, where P is precipitation, PE is potential evapotranspiration, and S is surface storage. Rates differ sharply by climate. In cold places like Alaska, evaporation is far lower because there is little heat to drive it. In 1961, Peveril Meigs sorted desert regions into three categories by rainfall. Extremely arid lands have at least twelve consecutive months with no precipitation. Arid lands receive less than 250 mm a year, and semiarid lands between 250 and 500 mm. Both extremely arid and arid lands count as deserts, while semiarid grasslands are usually called steppes. Among the driest, hyperarid deserts take in under 25 mm a year, with no seasonal cycle and twelve-month stretches of total drought.

• 3. The Shapes Of Drought

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  Trade wind deserts straddle the horse latitudes at 30 to 35 degrees north and south, where the subtropical anticyclone forces dry air to descend. The Sahara is the great example of this type. Mid-latitude deserts form farther toward the poles, between 30 and 50 degrees, in places remote from the sea where prevailing winds have already shed their moisture; the Tengger and Sonoran fall here. The Thar Desert near the India and Pakistan border is a monsoon desert, formed where moist air rises over land, drops its water, and circles back to sea. A rain shadow makes others. As air masses climb a mountain range they cool and lose their moisture on the windward slope, then warm and dry as they descend the far side. The Taklamakan lies in the rain shadow of the Himalayas and receives less than 38 mm of precipitation a year. Coastal deserts hug the western edges of continents where cold currents chill the passing winds, leaving fog and dew as the main moisture. Their temperature swings are gentle, only about 11 degrees daily and 5 degrees yearly in the Atacama. They appear in Namibia, Chile, southern California, and Baja California. Montane deserts sit at extreme height, like those north of the Himalayas in the Kunlun Mountains and on the Tibetan Plateau, many above 3000 m and often with less than 40 mm of rain. The northeastern slopes of Mount Kilimanjaro can be scorching by day and bitterly cold by night.

• 4. How Deserts Are Built

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  Large temperature swings do the slow work of breaking a desert apart. The diurnal range may reach 20 to 30 degrees, and the rock surface endures even greater differentials. In hot deserts daytime heat can pass 45 degrees in summer and drop below freezing on winter nights. The repeated strain cracks the flanks of mountains, sending fragmented strata sliding into the valleys to splinter further. Exfoliation peels the outer skin of rocks off in flat flakes, fracturing parallel to the surface as the stone expands and contracts. Chemical weathering plays a larger part than once thought, with dew, mist, and salt crystals prying particles loose. There is a critical size, about 0.5 mm, below which temperature-driven weathering stops, and this sets the minimum size for a grain of sand. Wind takes over from there. At high speeds, grains lift off the surface and bounce along in a process called saltation, sandblasting any solid object in their path. Around the world, only about 20% of desert is sand, ranging from just 2% in North America to 30% in Australia and over 45% in Central Asia. Where it gathers, the wind sculpts it. Barchan dunes form crescents under a steady wind, seif dunes stretch in long lines, and transverse dunes run at right angles to the prevailing breeze. Star dunes, raised by variable winds, can climb to 500 m, the tallest type of all.

• 5. Where The Water Hides

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  The River Nile, the Colorado, and the Yellow River cross deserts as exotic rivers, fed by distant mountains and bleeding much of their flow to evaporation as they pass. Below ground lies far more. The Nubian Sandstone Aquifer System beneath the Sahara is the largest known accumulation of fossil water, and Libya's Muammar Gaddafi launched the Great Man-Made River to tap it and supply coastal cities. Kharga Oasis in Egypt stretches 150 km, the largest oasis in the Libyan Desert, where wells reach down into porous sandstone laid by a lake that filled the depression in ancient times. When rain does come it often arrives with violence, tearing down dry channels called arroyos or wadis as flash floods after a storm that may be many kilometers away. Lakes form in basins from rain or glacial meltwater, usually shallow and saline, and when they dry they leave a hardpan known as a playa. The deserts of North America hold more than a hundred playas, many of them relics of Lake Bonneville, which covered parts of Utah, Nevada, and Idaho during the last ice age. Their flat surfaces have served strange purposes. Vehicle speed records have been attempted at Black Rock Desert and Bonneville Speedway, and the United States Air Force uses Rogers Dry Lake in the Mojave as runways for aircraft and the Space Shuttle.

• 6. Life At The Edge

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  Cacti carry a post-Gondwana origin and have abandoned their leaves entirely, shifting chlorophyll into trunks rebuilt to store water. The giant saguaro of the Sonoran Desert grows slowly, may live up to two hundred years, and a large specimen can hold eight tons of water after a downpour, its concertina-folded surface expanding to take it in. Other plants reach the same answers by convergent evolution, shrinking their stomata, waxing their leaves, or storing water in succulent stems like the aloes. The mesquite tree of the Americas has seed so hard it fails to sprout even when planted carefully, yet after passing through the gut of a pronghorn it germinates readily in a moist pile of dung well clear of the parent. Animals adapted to deserts are called xerocoles, and many never drink at all. Oxidising a gram of fat yields 1.07 grams of water, so the kangaroo rat lives on the water of metabolism, staying underground by day and producing little waste. The camel can lose 40% of its body weight to dehydration without dying, making concentrated urine and dry dung. The addax antelope, dik-dik, Grant's gazelle, and oryx draw enough moisture from plants that they apparently never need to drink. Reptiles, being ectotherms, thrive in heat but cannot live in cold deserts; the horned viper of Africa and the sidewinder of North America independently evolved a sideways crawl to climb dunes. The long-legged darkling beetle in Namibia stands on its front legs and raises its shell to catch the morning mist, funnelling condensed water into its mouth.

• 7. People And The Sand

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  The Tuareg moved goods across the Sahara by camel, with slaves, ivory, and gold going northward and salt heading south, while Berbers who knew the land guided the caravans between oases and wells. Several million slaves may have been taken north across the Sahara between the 8th and 18th centuries. Motor vehicles, shipping, and air freight ended most of that traffic, yet caravans still carry salt along routes between Agadez and Bilma and between Timbuktu and Taoudenni. Where rims of the desert caught more rain, herders turned to crops, often at bare subsistence and at the mercy of the weather. That fragility turned catastrophic in the American Dust Bowl of the 1930s, when ploughed grasses and dry years let dust storms strip the topsoil and forced half a million Americans off their land. About twelve million hectares now turn to desert each year. Even so, people keep wresting harvests from the sand. The Hohokam built over 500 miles of large canals and maintained them for centuries, raising maize, beans, squash, and peppers. The Imperial Valley in California, with average rainfall of just 3 inches a year, draws water from the Colorado River through the All-American Canal and has become one of the most productive farming regions in the state. That bounty has a cost, since below the extraction sites the river often has no above-ground flow for most of the year.

• 8. Deserts Imagined And Beyond

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  Marco Polo, who lived from about 1254 to 1324, crossed several deserts on a twenty-four year trek through Central Asia to China, leaving one of the earliest historical accounts. Charles Montagu Doughty set down vivid desert conditions in Travels in Arabia Deserta in 1888, and Antoine de Saint-Exupery wrote of flying over sand in Wind, Sand and Stars. Gertrude Bell became an expert on the Arabian desert and advised the British government, while Freya Stark travelled alone through Turkey, Arabia, Yemen, Syria, Persia, and Afghanistan, writing over twenty books. The American poet Robert Frost turned the landscape inward in Desert Places in 1933, closing with the line about scaring himself with his own desert places. Saints kept to the sands too, among them Anthony the Great, called Anthony of the Desert, and Pope Benedict XVI spoke of internal deserts growing as the external ones spread. The idea reaches past Earth. Despite a surface pressure only one-hundredth of Earth's, Mars holds a sea of circumpolar sand larger than most deserts here, more than 5 million square kilometers of half-moon dunes near its northern ice caps. Titan, a moon of Saturn, carries its own dune seas. The Mars Exploration Rover even found a surface film on rocks that resembles the desert varnish bacteria leave on stones back home.