NOVEMBER 2007 – NO. 19
The desert causes ever more desperate measures by humans to survive, and those human activities made the desert worse
The Desert as a whole may or may not be expanding, but no one doubts that localized areas of desertification do exist, almost all human-induced, especially along the Sahara's southern fringes. In fact, the United Nations has recently encouraged changing the definition of desertification from "spreading of a desert" to "arid land degradation," to take account of new thinking. As vegetation is stripped from the land, the surface dries out and reflects more of the sun's heat. This condition in turn alters the thermal dynamics of the atmosphere in ways that suppress rainfall. Increased dust (itself a product of desertification) or other atmospheric pollutants are causing changes in the climate. Desertification results in declining water tables, and the salination of topsoil and the remaining water. Increased erosion and the extinction of vegetation, however caused, make the climate drier.
Population growth, the engine of all increased resource demands, is at the heart of the desertification problem. More means more animals, and more vegetation cut for fuel and construction. The additional herds needed by greater numbers of humans trample and compact the soil, reducing the infiltration of the little water that exists, causing erosion and damaged soil. In the Sahel, humans have come to use cattle dung as a source of fuel, but even so what sparse vegetation remains is being hacked back for kindling, a violation that leads to increased erosion and wind damage.
Even within natural deserts, human activities can magnify the effects. In southern Tunisia, for example, the native plant cover, such as it was, was replaced by olive groves, cultivated in a dry farming system. This meant repeated "cultivation," which in practice was the opening and destruction of the soil surface, which in turn provoked deflation and soil erosion. The paleontological record shows that this degradation happened in several steps, dating back to the 16th and 17th centuries, when newfound political stability encouraged nomads to stay put on the Jeffara plain.
The Tuareg rebellions in the Aïr mountain region of Niger in the 1980s and early 1990s are another example of soil degradation caused by politics. Because of the warfare, the Tuareg nomads could no longer easily wander off to find new forage for their camels, and since they were unwelcome in the cities, they were forced to rely on cultivation, based on irrigation from shallow wells. Some of the wells, inevitably, went dry. In other cases, the soil began to erode, and stone walls built to impede the erosion only made things worse by preventing the water from infiltrating the ground. Small dams accelerated evaporation instead of hoarding the water, increasing the salinity of the soil.
Other economic and social factors have also taken their toll. Diesel deep-water pumps at oases throughout the desert have enabled the tapping of ever-deeper aquifers, which have, in turn, encouraged herdsmen to give up their nomadic but sustainable way of life and remain near the wells, and made it possible for them to raise even more livestock. Overgrazing resulted, the animals tearing up plants by their roots, destroying their ability to reproduce. The insistence of Western donor countries on "modern" methods and commercial models meant that the most fertile lands were reserved for cash crops alien to the region, cotton and peanuts, for instance, for sale in Western markets. This led to the increasing cultivation of ever more marginal land. At the same time, in imitation of Western agricultural models (based on more temperate climates and better rainfall), farmers began working their lands with only one or two years; fallow time, whereas before they had left the land for 15 or even 20 years to recover on its own. It is the classic vicious circle:&bnsp; The desert causes ever more desperate measures by humans to survive, and those human activities made the desert worse.
However, skeptics about desertification do exist. A 1998 study reported in the journal Science disputed what it called "the popular belief" that the Sahara was growing southward. The study did acknowledge that "land degradation proves to be a problem in this area, reflecting the more localized effects of grazing and foraging for fuel," but it claimed to have established that, overall, the shifting desert boundaries were not, in fact, human-induced but were caused by the natural ups and downs of rainfall. The study also maintained that while the natural climate has shifted the desert's edge, there was no change in the total amount of vegetation. Desertification, in this view, was a small, and altogether localized, phenomenon. A Ph.D. dissertation by Ahmed Mokhtar Brere on the application of Landsat imagery to monitor dune movements in the Sahara found plenty of movement but little sign of spread.
Even the best evidence sometimes seems ambiguous — petrified wood, for example. Petrification is a curious thing. The botanists call it a "pseudomorph," where the natural wood fibers are replaced by silicon dioxide or by what is called "cryptocrystalline quartz," or chalcedony. The process involves the trees imbibing solutions containing silica, presumably from alkaline-saturated water; the mimicry is so accurate that the original cell structure can still be determined after the transformation is complete. Fossil wood is found everywhere in what the specialists call the "Inter-Calary Continental," a layer of sedimentary rocks that underlies the greater part of the Sahara, together with fossils of fish and reptiles, and has popularly been used to date the changed Saharan climate. Vintage geological texts assumed millions of years for the process, but more recent theories have reduced this to thousands of years, and the fossilized wood now found in Niger and Algeria is assumed to come from trees that were living ten thousand years ago. But tantalizing anecdotal evidence indicates that the process can, indeed, be much faster. The British writer Nigel Heseltine recounted how he had seen, "among the darkened ration tins at Uigh-es-Serir, a half orange petrified in exactly the same manner, which could not have been thrown there much earlier than fourteen years before."
From Sahara: The Extraordinary History of the World's Largest Desert. Copyright © 2007 by Jacobus Communications Corp. Reprinted by permission of Walker & Co.
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