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2 The road to desertification.

1. How does a desert form?

1.1 The scourge of desertification

Human ability to favor the spread of deserts has increased greatly during the twentieth century. Destruction of the vegetation and topsoil that in the past would have taken 10 years' labor by a peasant can now be performed in a single day by a worker with a bulldozer. Vegetation is cleared (leading to the intensification and spread of deserts) not only for agriculture but also for mining, for military objectives, and even for tourist development. Modern technology can also create small-scale deserts, even in sites where the rainfall and other climatic factors are suitable for plant growth. Tens of square kilometers of land may be sterilized by the poisonous emissions and discharges from industrial centers. Desertification due to human activity is especially intense in northern Africa. In the classical period, Carthage, within the frontiers of today's Tunisia, was the grain-basket of the Roman empire, but now the countries of North Africa do not produce enough food to meet their own needs.

Desertization or desertification?

Desertization refers to the set of processes that turn an initially nondesert area into a desert. The term desertification, however, has been used with many different meanings. It is almost a synonym of desertization when it is used to designate the reduction of the potential production of the land in arid, semiarid, and subhumid areas that may lead to conditions similar to those of a desert (conditions that are difficult to imagine in subhumid or even semiarid climatic conditions, with average annual rainfall of 16-31 in [400-800 mm]). It is, however, often used as a synonym of abandonment of the land and rural depopulation (desertion), that is to say, the situation created by people migrating to the large villages and cities, with the consequent abandonment of agricultural land that is left untended or simply abandoned. This is the commonly understood meaning among European Union circles, where economists, sociologists, geographers, administrators, journalists, and politicians routinely talk about the desertification of the countryside in Western Europe. For other authors, desertification means the degradation and the apparently irreversible change of arid land into areas with desert conditions (bare soils, sand dunes), or to put it another way, deserts formed as a result of human activity. At the other end of the scale, people also use desertification to refer to the natural process of desert formation. The term desertification has thus become a considerable source of confusion.

It is often not very clear if the degradation of arid ecosystems into anthropic deserts is irreversible or not. From a geological perspective, it cannot be said to be totally irreversible because the climate has changed continuously over Earth's history, although at different rates in different periods. During the Pleistocene, the climate changed drastically more than 10 times within three million years. Even in the recorded past there have been two well-documented major climatic fluctuations: the early Medieval Warm Period (or Little Climatic Optimum [800-1200]) and the Little Ice Age (1550-1850), when the temperature in Western Europe was calculated to have been, respectively, 1[degrees]C warmer and 2[degrees]C colder than current temperatures (see vol. 1, pp. 5672). Irreversible processes are those that are sustained for a period of at least 25-50 years, during which the climate can be shown to have remained stable within its normal variability without showing any long-term tendency.

In these terms, the irreversible transformation of arid ecosystems into anthropic deserts is well documented in many of the world's arid areas, and shown by the cities buried under sand dunes along the Old Silk Road in northwestern China, the steppe grasslands in northern Africa that have become desert pavements, and the new dune fields in former croplands in the Sahel. The spread of deserts is confirmed beyond all doubt by comparing ancient and recent maps with contemporary ones, and by comparing aerial photographs 40-50 years old and satellite images 20 years old with modern images.

From temporary drought to irreversible desertification

Desertification may be triggered by a drought, but this is neither essential nor sufficient on its own. Climatic variability is a permanent characteristic of the arid areas, and if they are protected and managed prudently, they are not affected by desertification, despite drought conditions.

Desertification can be, and in many cases has been, triggered in periods of moderate aridity, even in less arid conditions than normal. But this only happens if the naturally fragile and unstable arid ecosystems involved have been highly degraded by excessive human and livestock pressure. And, to the contrary, prolonged droughts up to 15 years long (1969-1984) did not lead to major changes in some areas of the Sahel because the moderate grazing pressure corresponded to the carrying capacity, while in the same period millions of hectares (1 hectare=2.5 acres) of overgrazed ground in the Sahel region suffered major desertification.

Overstocking and overgrazing are very common throughout the world's arid areas; the density of grazing animals is much greater than the carrying capacity and this reduces or totally eliminates the perennial vegetation, which in turn, causes erosion by wind and rain, and thus desertification. Gathering firewood also plays an important role in the desertification of many countries in Asia and Africa. Given that the need for firewood, often the only fuel available, is about 1.5 kg/day per person, a normal family consumes about 2.7 t dry matter per year, corresponding to the average woody biomass of 3-5 ha of virgin arid grassland. The collection of medicinal and other useful plants may play a similar role in some regions such as eastern China and western Africa.

The impact of drought on the natural ecosystems mainly affects the cover, biomass, and production of the annual plants, which are generally not xeromorphic and have a very short life cycle. They are said to evade drought. Some annual species have a very short life cycle, two weeks or less, and make up the acheb of the northern Sahara and ghizzu of the southern Sahara.

The adult plant may be absent for many years, surviving only as seed, reappearing for just a few days when climatic conditions are temporarily favorable. The cultivated annual species behave just like wild annuals: water stress (drought) reduces their size, number, and plant biomass and may even prevent them from reaching maturity, in which case the harvest fails or is at least reduced. In this case, however, the seed reserve is in the farmer's barn, not in the soil.

The perennial plants of arid zones are much less affected by drought due to their deeper root systems and other anatomical and physiological adaptations. They are adapted to the drought by their xeromorphy and by various different physiological mechanisms. Physiologists call these drought-tolerant species. Permanent crops normally survive dry years if they are well adapted to the environmental conditions but little or no crop is produced. On the other hand, if crops are planted in an environment to which they are not adapted, they may be destroyed by drought.

Naturally, desertification has much more persistent effects. The soil surface, exposed to erosion by water and/or wind, becomes more skeletal, less able to accumulate water and nutrients. Many or most of the perennial plants are eliminated by overgrazing, fuel wood gathering, and shifting agriculture. In the best of cases, the wild perennial plants are replaced by annual species or perennial weeds, of no use to humans or to livestock (though of great importance in protecting the soil surface from erosion by the harsh climate). In the worst case, the soil is left bare after every living thing has been eliminated and then the surface is subject to unrestricted erosion by the wind and rain.

The progress of erosive processes

The processes of water and wind erosion are the result of the elimination of the natural vegetation of the soil surface, which is left exposed to the forces of erosion: the rain drops that strike the soil surface, erosion by torrents whose speed is no longer reduced by the vegetation, the impact of the wind and of the particles it bears, etc. The index of rainfall interception in multilayered vegetation types such as forests, savannahs, and scrub varies between 10-50% over the course of the year, depending on the growth-form of the plants, the density of the canopy, and its vertical structure. Before reaching the soil, the raindrops are thus broken down into smaller droplets, whose speed and kinetic energy are much lower than that in unstratified vegetation types (grasslands) and far, far less than on bare ground.

In northern Africa, it has been shown that for a given catchment basin, the amount of water infiltrating into the soil is five times smaller in cereal crops than in forests, and the erosion is 50 times greater. Furthermore, as a result of the reduction in the perennial plant cover and plant biomass during the desertification process, less and less organic matter is produced and incorporated into the soil, and the soil structure becomes less stable. The loss of organic colloids caused by the declining organic matter content of the soil aggregates makes the soil fragile and vulnerable to destruction by several agents and phenomena such as the impact of raindrops. It has, for example, been shown that the structural stability of soils (in otherwise identical conditions) is two or three times greater under a permanent herbaceous cover than in the soils of cultivated fields.

The effect of raindrops falling on the bare surface of a soil with an unstable structure may also lead to the ejection of the smallest particles; they are thrown upward by the impact of the raindrop and then fall back to the soil surface, filling up the pores and forming a more or less continuous structural crust that acts as a seal. This seal greatly reduces the soil's permeability and the entry of water, thereby accelerating gullying and erosion.

When the vegetation is scarce or absent, there is nothing to slow down the flow of the water, which is not absorbed by the undergrowth or by topsoil rich in organic matter. Quite to the contrary, the runoff water gathers speed, joining up to form a network of channels and then gullies, excavating furrows, then streams and torrents. Erosion by water in arid areas may reach 50-200 t/ha (tons per hectare; 1 hectare=2.5 acres) per year (and even 300 t/ha per year) on some substrates that are liable to erosion, but the rate is normally 5-20 t/ha per year.

This is the beginning of a spiral of increasing soil aridity that shows positive feedback, a tendency that may or may not be reversed. When all the soft soil horizons have been lost, plant life becomes impossible (except during the short and infrequent rainfall episodes) because no water at all can accumulate in the soil. But as long as there are horizons of soft soil, this tendency can be reversed, though at great cost, by using a variety of conservation methods and techniques such as a total ban on felling forests, terracing, and loosening the soil by scarification. After intense rainfall, which is an infrequent but not exceptional event in arid areas, there may even be huge movements of soil and geological substrates made up of soft, fine materials (schist, loam, clay, chalk, anhydrite, gypsum). Intense rainfall in regions with little vegetation may cause catastrophic flooding and mudslides that greatly affect the entire landscape, even if they only occur relatively infrequently--for example, once a decade or once a century.

All the relevant studies show the vital role of the perennial plant cover in the dynamics of wind erosion. But the erosive effect of the wind also depends on the nature of the soil, especially its particle size and cohesion and its humidity. Wind erosion following salinization, solifluction, and deflation are greater for particles 0.1-0.5 mm in diameter, depending on the intensity of the energy source and its turbulence. The threshold of proneness to erosion for particles 0.1 mm in diameter is a wind with a velocity of 10 mph (4.4 m/s) at a height of 12 in (30 cm) above the soil surface. The presence of a perennial plant cover considerably reduces wind speed and thus wind erosion. So any factor reducing the perennial plant cover automatically increases wind erosion, all other conditions remaining equal.

It has been shown that a permanent plant cover of 25-30% is necessary to ensure a balance between erosion and deposition in arid sandy steppes with small shrubs. When all the perennial plant cover has been uprooted or destroyed, net erosion can increase from almost nothing to 150-300 t/ha per year within a short time, as has been shown by observation and experiments in northwestern China and southern Tunisia. Desertification becomes irreversible when all the soft shredded material has been washed away, leaving only bare rock, as there is no possibility of storing the water plants need to become established and survive. This process leads to the creation of desert pavements, the main cause of desertification.

1.2 Human intervention

In natural conditions, animals are not usually involved in the formation of deserts. The animal components of any ecosystem depend directly or indirectly on the plants for their food. Thus, the low plant biomass in the deserts can only support an even lower animal biomass. If the population of any species ever temporarily reaches plague proportions, the animals soon die of hunger; the plant populations are reduced but soon recover when the pressure to which they were subjected disappears.

Humans, livestock, and the desert

Human beings are an outstanding exception to the generalization that animals are not usually involved in desertification. Formerly, when humans lived as hunter-gatherers, they were subject, like other animals, to the limitations of the few plant resources in the desert. But when humans learned to use fire, hunter-gatherers began to have an impact on the desert. In the most extreme deserts there is not enough fuel to start a fire, but in the subdeserts, plant growth may be substantial in the seasons when rainfall occurs, and once dried, it burns very easily.

Human influence on deserts increased substantially after people learned to domesticate large herbivorous mammals and cultivate the soil. As a result of the great expansion of their herds, which often spread into neighboring regions with higher rainfall, the plant cover declined dramatically. Though they were nomads, the pressure exerted by the herders was not spread uniformly among all the vegetation and all the soils; rather, it was concentrated around the water holes. Human beings and their herds were more dependant on these water holes than the native fauna, and this led to the complete loss of vegetation in a large surrounding area, though farther away the vegetation may have survived or even recovered.

Herders also needed fire to cook and to warm themselves. If all the woody vegetation in a desert or subdesert area were used for these purposes, then the area's only plant cover would consist of short-lived herbaceous plants that could outcompete shrub seedlings for water. Even the livestock's dung was used as fuel, so this input of organic matter to the soil ceased to be available. The felling of trees and shrubs increased the effects of wind erosion and the loss of access to the subsoil water, which lay only within reach of shrubs with deep roots.

Human alteration of the plant cover and the river network

Human exploitation of deserts has not been restricted to stockraising. Natural vegetation in many places has been destroyed by the use of the plow, shovel, and hoe in order to replace the well-adapted native plants with exotic plants that are of greater use to humans, but often badly adapted to desert conditions. The crops introduced are usually annual species, and once the seeds or other usable parts are collected, the rest of the plant dies. In this case, the farmer may burn the harvest remains or plow the field up in order to reduce loss through transpiration of any water input to the soil as rainfall before the next crop. The soil surface is left bare and unstable and highly exposed to erosion by wind and rain should a storm occur, something the original native perennial vegetation would have prevented. In arid and hyperarid areas, farming without irrigation can only be sporadic, and its results are haphazard. In semiarid areas, however, commercial agriculture is often developed, with major food crops such as cereals, fruit trees, and olives, and industrial agriculture may even be successful. But limited water availability and regular droughts are the main problem facing agricultural, livestock, and forestry production.

Since the Neolithic period, humans have played a major role in destroying the plant cover and destabilizing the soil surface in desert and subdesert regions. They have also caused the hyperarid deserts to become more arid and to spread, turning the less arid surrounding areas into deserts. Here, the situation becomes more complicated, as this leads to the appearance of anthropic deserts (arid zones that have been turned into deserts by humans and their stockraising activities, and that have acquired the appearance of genuine hyperarid deserts, although their climatic characteristics are not the same). They are degraded arid zones where the vegetation has been destroyed by excessive exploitation, creating genuine desert landscapes.

These anthropic deserts have one essential difference from authentic climatic deserts: In principle, they can be restored and improved by appropriate technology and management without requiring irrigation. Authentic climatic deserts (hyperarid zones), cannot be transformed without intensive irrigation--the main tool used to transform the deserts.

After learning to cultivate the soil, humans learned how to divert water to increase its production and how to cultivate the plants they had domesticated in areas that would otherwise have been too arid. The large rivers crossing deserts from sources in wetter areas such as the Nile, Euphrates, Tigris, and Indus, have been used to irrigate crops for many thousands of years, making permanent human settlement possible and thus the development of advanced civilizations. Over time, crop yields began to decline, unprotected soil was eroded by the desert winds, irrigation structures silted up, and the desert advanced. These primitive civilizations disappeared under the dunes, leaving huge buildings buried under the shifting sands.

The effect of overpopulation

In the arid areas of many developing countries, human populations are increasing at a rate of 2.5-3.5% per year (and thus doubling in a period of 20-28 years). In most of these countries, the population will have increased eightfold, ninefold, or even tenfold in the twentieth century, doubling between 1900-1950, again between 19501975, and more than doubling between 1975-2000. All the studies by international organizations show that the population density in the arid areas of most developing countries is far higher than the soil's carrying capacity with only low and medium investment. The total population of the area of the world threatened with desertification is now about two billion while 300 million people now live in desert areas.

2. The struggle against desertification

2.1 The alarming advance of the deserts

The world's arid areas represent about 41 million [km.sup.2], about one-third of all dry land. Roughly 14 million [km.sup.2] are true climatic deserts, 13 million [km.sup.2] are semiarid, and another 14 million [km.sup.2] can be considered subdeserts threatened by desertification.

The scale of the tragedy

Independent monitoring of large areas over more than 10 years in the Middle Asian States of the CIS (the Commonwealth of Independent States), northwestern China, and northern Africa has yielded almost the same results: the annual advance of the anthropogenic deserts into the arid areas is 0.5-0.7%. At such a rate, the world would lose about 84,000 [km.sup.2] to deserts every year. The actual figure is in fact lower, as the large arid areas of North America and Australia are scarcely, if at all, affected. Leaving aside the arid areas in North America (1 million [km.sup.2]) and Australia (3.3 million [km.sup.2]), the area affected in the rest of the world would still represent a loss of about 60,000 [km.sup.2] to human-made deserts.

Yet the regional and national situation may be very different from the global one. As already pointed out, the main cause of desertification is the over-exploitation of natural resources. This is normally (but not necessarily) linked to excessive pressure on fragile ecosystems as a result of a high human population density and a high number of livestock (rarely of wild animals), but these do not necessarily have to be correlated. In most developing countries, high grazing pressure coincides with high human population density, but in countries like the United States, Australia, and Argentina, very low human population densities (for example, 1 person/[km.sup.2]) may be accompanied by high if not excessive livestock levels. In these cases, the human population bears no relation at all to the livestock population. This trend is becoming increasingly common as living standards rise and are more equitably distributed, and societies become less dependent on stockraising and farming activities.

Yet some activities causing desertification are linked to the prevailing model of society. Clearing dehesas (open woodlands) for high-risk cereal cultivation is typical of rural societies with low income and very high population growth rates of around 3% per year, as in North Africa, parts of the Near East and Middle East, the Sahel, eastern Africa, India, and northwestern China. The same is true of deforestation, charcoal manufacturing, the gathering of fuel wood, and other activities such as the collection of medicinal plants and the excessive browsing of the woody vegetation by livestock.

Other types of activities, however, are linked to societies with high incomes: the destabilization of dunes by four-wheel-drive vehicles; some types of tourist developments; urban and industrial air, plant, and soil pollution; and uncontrolled scrub fires among them. There are, for example, almost 10 times more fires in the industrialized countries on the Mediterranean's northern shoreline than in the developing countries on its southern shoreline. Large destructive fires are also much more frequent in southern California (United States) than in the chaparral over the frontier in Baja California (Mexico).

Some overgrazing and a moderate excess of livestock tend to cause the replacement of the fodder plants preferred by the livestock with other less palatable species. Taken to extremes, heavy and prolonged overgrazing may lead to the elimination of almost all the perennial plants in arid areas, thus leaving the soil surface bare and increasingly barren, except during the short annual rainy period. In the developing countries, the number of head of cattle has grown at half the rate of growth of the human population since World War II (1939-1945), at a rate of about 1-2%. During this period, the area of grazing land has declined at almost the same rate due to clearance for agriculture, so that livestock density has increased between one and four times in the last 40 years. The increased density of livestock has not, however, been compensated for by improved production techniques or practices. In some ways, modern technology has in fact made the problem worse. For example, drilling water holes in pasture areas of the Sahel without any planning criteria led to enormous concentrations of livestock, up to 20,000 or 30,000 head all year-round, and this led to the rapid destruction of all grazing within 12 mi (20 km) of these virtually unlimited and permanent water sources.

Clearing the natural vegetation in order to meet the food needs of a growing population is habitual in almost all developing countries. Several field studies in Africa and Asia show that the rate of clearing is increasing annually by 0.5-0.7% and locally by 2% or more. Thus, as the population grows, larger areas of land have to be cultivated to harvest a larger amount of food. In many cases there is no more land to clear, and the fallow period is reduced or totally eliminated, thus reducing the soil's fertility and therefore its yields. This leads to what is often called the "the spiral of degradation."

The geography of the desertification process

The relative importance of the many causes of land degradation and vegetation loss in the world's different arid areas depends on the area, the lifestyle, and the living standards of the human rural populations in question. The methods may differ, but the end result is always the same--the spread of the deserts. In northwestern China, it was shown that the desert was increasing at a rate of 0.6% per year (1,000 [km.sup.2]). The causes of this desertification, expressed as percentages of the affected land, were: 45% due to clearing vegetation to extend cultivation, 18% to fuel wood collection, another 18% to overgrazing, 3% to road building, construction of infrastructure, and urban development, 5.5% to expansion of dunes and areas under sand, and 1.5% to salinization.

In North Africa, the Near East, and the Middle East, where arid land occupies about three million [km.sup.2], desertification is increasing at an annual rate of at least 0.5% (more than 15,000 [km.sup.2]). Estimates of the different causes of desertification in these arid areas suggest that roughly 50% is due to the clearing of shrubby steppe areas for high-risk cultivation, 26% to overgrazing, 20% to gathering firewood, 2% to salinization, and 1% to urban development and tourism.

In the arid zone of the Sahel and eastern Africa, occupying more than 3.1 million [km.sup.2], the desert is advancing at a rate of about 0.5% per year (15,500 [km.sup.2]). Estimates of the relative importance of the different causes suggest 65% is due to overgrazing and over-browsing, 25% to agricultural clearing for cultivation, 10% to collection of wood for fuel and for fencing, and insignificant amounts to salinization, urban growth, and tourism.

In the arid zones of middle Asia (Turkmenistan, Uzbekistan, and southern Kazakhstan), an area of 1.07 million [km.sup.2] is desertified to some extent, representing about 60% of the total arid area of 1.8 million [km.sup.2]. But this area includes different degrees of degradation. Estimates of the different causes of desertification suggest that 62% is due to overgrazing, 10.6% to technological development, 10% to water exploitation, 9% to salinization, 5% to wind erosion, 1% to water erosion, and 0.4% to insufficient grazing.

In the United States, the importance of the different causes of desertification has not been quantified, although the size of the different land degradation processes has been measured, especially in the 17 western states, which cover an area of 5.6 million [km.sup.2]. About 51% of pastures are in poor (37%) or very poor (14%) condition; 37.1% of cultivated land suffers soil loss greater than 12 t/ha per year due to wind erosion, almost two-thirds of this area losing more than 25 t/ha per year; 13.1% of cultivated ground also suffers losses greater than 15 t/ha per year due to water erosion, one-third of it losing more than 25 t/ha per year; and almost 1% of irrigated land suffers moderate to serious salinization.

In Australia, where arid land represents 70% of the land surface, the main cause of the degradation of the vegetation and soil is, above all, overgrazing, whether by domesticates, wild animals (the local population of kangaroos), or naturalized animals such as rabbits, camels, and goats. Excessive stocking levels may be responsible for 75-80% of the damage. The second main cause of land degradation is the secondary salinization by filtration into dry ground, especially in the wheat belts of Western Australian and South Australia, where elimination of Eucalyptus forests and scrub has led to serious problems of secondary salinity. The destruction of the vegetation has also led to the formation and spread of scalds (saline wasteland with an impermeable crust) in Victoria and New South Wales. Uncontrolled fires are a major cause of the degradation of vegetation and soil, especially in the Mediterranean eucalyptus forest (where many fire-adapted species of eucalyptus grow) and in the eucalyptus savannahs of Northern Territory and Queensland. Urban and industrial growth are major causes of local desertification, mainly around the mining towns in the outback such as Kalgoorlie, Broken Hill, Mount Isa, Coober Pedy, and Cobar.

In general, in the less industrialized countries or scarcely inhabited ones, the degradation of the vegetation and the land are basically due to excess livestock, perhaps in combination with uncontrolled fires. Salinity may be greatly increased by incorrect irrigation or widespread deforestation.

The consequences of increasing desertification

The clear physical impact of the degradation of the vegetation and soil in arid areas leads to the advance of the desert or the desert landscape, the appearance of mobile dunes, the spread of seas of sand, and the formation of hardened crusts that leave the soil sterile. The result is the increase in the frequency of sand storms and dust storms (for example, the well-documented dust bowl in the United States in the 1930s), floods, and sedimentation. These are all consequences of the destruction of the vegetation.

The biological consequences are perhaps less spectacular, but equally dramatic, since the primary and secondary production of the ecosystem declines constantly with the elimination of the perennial plants that represent the best insurance against degradation. The productivity of degraded soils may decline to less than 10% of original levels. The efficiency of rainfall use may decline from 5-7 kg dry matter/ha and per mm rainfall in arid zone ecosystems in areas in relatively good condition to a mere 0.5-1 in areas that have become deserts. This reduction is a consequence of the changes in the natural plant community's botanical composition and structure and in the functioning of the ecosystems. All of this is the result of the drastic decline in soil organic matter, in turn provoked by the decline in the aboveground plant biomass. The vegetation tends towards communities of tiny ephemeral plants and annual eremophytes (desert plants) and small ephemeroid perennials.

Furthermore, the variability of primary production, and thus secondary production, greatly increases as the ecosystem becomes increasingly exhausted; the ratio of the coefficient of variation of annual primary production to the coefficient of variation of the rainfall may be 1.2-1.5 in vegetation in relatively good condition but as high as 3-5 in ecosystems that have become deserts. In other words, not only does productivity decline drastically in desertified ecosystems, it also becomes much more irregular.

One of the first consequences of the degradation of the vegetation and the soil is the elimination of the shrubs and trees that contribute so much to the functioning of nondegraded ecosystems. Woody species play an essential role in 1) the recycling of the nutrients and elements needed for life and the regulation of their uptake from deep in the soil and underlying materials, 2) the exploitation of deep aquifers, and 3) the symbiotic fixation of nitrogen (by leguminous trees and shrubs). Shrubs and trees reduce the impact of raindrops and the wind speed at ground level by shading the lower layers and producing organic matter.

Thus, trees and shrubs are essential for the maintenance of the ecosystem's general productivity. In the Sahel, for example, the primary production of the grass layer, and therefore its photosynthetic efficiency, is four to six times greater under trees than on bare ground. Likewise, millet production under the shade of the kad (Acacia [=Faidherbia] albida) is roughly 2.5 times greater than in open fields. Similar results have been obtained for millet production in Rajasthan under the shade of the jhand (Prosopis cineraria [=P. spicigera]).

The elimination of the tree and shrub layer in the arid zones of the Sahel and eastern Africa has dramatic consequences on stockraising. Trees and shrubs are the only source of fodder for the nine-to-ten-month-long dry season and thus the only source of proteins and beta-carotene (provitamin A) for the livestock during this period, as dry grass contains virtually no protein or vitamins. They also meet most of the mineral needs, especially for phosphorus, during the dry season. When the trees and shrubs have been destroyed, these huge areas can no longer support livestock except during the short rainy period. The local people's main means of subsistence is therefore threatened.

The consequences of degradation include the spread of the cultivation of staple cereals and the reduction or elimination of fallow periods in an attempt to meet the food needs of a constantly growing population. This leads to positive feedback as cutting down or cutting out fallow periods results in smaller harvests; thus more land has to be cleared and plowed. Other consequences include a reduction in food production per capita; increasing reliance on food aid; more frequent sandstorms, dust storms, and catastrophic flooding; the invasion of villages and towns and all types of infrastructure by the moving sands; the migration of destitute herders and farmers to the big cities; shortages of firewood; social and political destabilization such as that seen over the last few years in the Sahel and the Horn of Africa; food shortages, famine, and, ultimately, war--hundreds of thousands of deaths and immense suffering for millions of people.

2.2 The need to stop desertification

The key question is whether the process of desertification can be reversed. It would be excessively optimistic to say that human actions can always be undone. The surface layers of soil accumulate some organic matter and are enriched in nutrients brought up by roots from deep in the soil, but once the surface layers have broken down and been lost, it is very difficult for them to regenerate within a period acceptable on a human time scale, except by the use of a large quantity of water.

The original desert developed over many millennia. In a similar period of time and without human interference, the same result might be achieved. The same plant species, if still present in the area, might recolonize the bare soil, starting anew the same succession that gave rise to the original desert vegetation. Some soil would form, the microorganisms and microfauna dependent on plant remains would arrive, and over time the original desert would reform. But these processes are only possible if the region remains undisturbed for thousands of years.

The targets to meet

Most experimental research and pilot projects carried out in the world's arid areas have shown that, from a strictly technical perspective, even highly degraded soils and plant cover that have turned into deserts can be restored and made productive. If rehabilitation is possible, why it is not being performed? The reasons include the local population's lack of technical proficiency and the absence of cultural traditions for soil and water conservation. There are, however, many ethnic groups (with numerous examples in the African and Asian continents) that have the necessary technologies and traditions (water storage, terracing, building water channels, mulching, dry farming techniques) to accomplish this task.

Inappropriate land ownership systems are also very common in some areas; harvests and livestock are considered private property, while land and water are communal property. In this situation, each person seeks to obtain the greatest immediate profit from communal resources, regardless of what might happen in the long term. Governments have invariably shown little propensity to face up to this problem. There are some very rare cases of rational use of common resources, for example among the Boran herders in Kenya and Ethiopia, but they are the exception, not the rule.

Agrarian reform to transfer property to individuals or small family groups would help to improve the problem of land degradation and would make productive investment in agriculture possible. The development of agroforestry, for example, is almost impossible unless private landowners or long-term tenants adopt a responsible management strategy. Examples of such appropriate management are found in many rural societies in arid areas on all continents--for instance, the growing of millet under a cover of kad (Acacia albida) or jhand (Prosopis cineraria) in western and eastern Africa.

Over the last few years, the increasing use of government-subsidized cereals and prepared feed for sheep in northern Africa and the Near East has had a very negative impact, increasing the desertification of the arid steppes, as it has allowed a large increase in the number of animals with the consequent soil degradation and sharp decline in grazing. Stopping indiscriminate government subsidies for prepared feed and cereals would reduce the problem.

The constant expansion of high-risk cultivation of cereals in the arid steppes of northern Africa, the Near and Middle East, India, China, the Sahel, and eastern Africa is a major factor causing soil degradation. Laws have been passed in many countries prohibiting cultivation in areas with average annual rainfall below a certain level (normally 14-16 in [350-400 mm]) but despite the differences in these laws from one country to another, they all have one thing in common: they are never obeyed. Controlling population growth, though very unpopular in the affected countries, could be attempted, as long as there was some sort of compensation or other advantage for the smaller families. Little imagination has yet been shown in dealing with this question.

There are few conserved/protected areas in arid zones in developing countries, and they cover only small areas. Most were established before these countries became independent, when pressure on the land was much less intense. But it is very hard to protect sites where the population has nowhere else to go, as virtually the whole world is overpopulated. Botswana is an exception, with a population density of 1 person/[km.sup.2]. It is also one of the few arid areas with a biomass of large wild mammals of about 400 kg/[km.sup.2], as well as many bird reserves that attract a growing number of tourists.

Obviously there is no simple global solution, only a few agreements of greater or lesser suitability for any given case. Strategies have to be designed that involve the local population and use appropriate technology, but this alone may not be sufficient, as the local population may have erroneous or unrealistic views of the problems. If asked for their opinion, most local farmers or herders would seek greater subsidies on water, cereals, and feeds for their livestock, but it has been shown that misuse of water leads to ecological catastrophes, as do indiscriminate food subsidies.

A great many resources are now devoted to research and monitoring, but this is not much good if nothing is done on a large scale to improve agricultural practice. Most research and monitoring efforts are fruitless because they do not lead to real-life applications. One good example of this was an experimental grazing project in the Sahel from 1980-1985: each year the project provided a map of the grazing areas to the country's authorities at the end of the rainy season in October, but these maps were never used.

The best use of aid resources in developing countries comes without a doubt in the form of small projects by nongovernmental organizations (NGOs), as long as they have the necessary knowledge and government support but no direct governmental interference. Experience has shown that large bureaucratic structures like those of most bilateral or multilateral aid agencies get very poor results, considering the economic resources they use.

The failure of international initiatives

The United Nations Conference to Combat Desertification (UNCCD) published a world action plan in 1977 to combat and prevent desertification. This plan dealt with the many different aspects of the problem of controlling desertification: improving land use and management; developing corrective measures against desertification; providing insurance against drought; providing food security; promoting science and technology; introducing social and economic advancements, including birth control education; and fostering international cooperation to include programs against desertification in global development plans.

Some rather unsystematic actions have been undertaken in most countries, especially in the development of agroforestry. But their achievements are very limited in comparison with the scale of the destruction. In general terms, the efforts made have had no noticeable effect, and degradation is progressing at an alarming rate.

UNEP (United Nations Environmental Programme) calculated in 1984, seven years after the UNCCD conference, that the degradation of arid areas was continuing at an annual rate of 260,000 [km.sup.2], 60,000 of them turning irreversibly into desert and 200,000 reduced to zero economic productivity.

The failure of the conference recommendations resulted from 1) the inability to put desertification in its context of the process of social and economic development; 2) the inability of developing countries to take account of the problem in their rural development programs; 3) directing actions toward the correction of effects rather than the prevention of root causes; 4) the small number or complete absence of activities involving the local population; 5) the lack of viable remedial projects; 6) the unfavorable climatic conditions, especially in the African Sahara, which had suffered a near constant drought from 1970-1985; 7) the deterioration of economic conditions in the affected countries and the reduction of international aid; 8) the poor coordination between aid agencies and their low level of efficiency on a global scale; and 9) the unwillingness of most countries concerned (except India and China) to face up to the problem of controlling the birth rate.

Responsibilities to the future

Deserts will continue to exist because the major climate patterns will not change in the immediate future. Sequences of rainy years or dry years, which occur in apparent cycles, may slow down or accelerate the processes of degradation. But human intervention may change deserts greatly, for better or worse. If irrigation water is available, deserts can be cultivated, and, if well managed, they could be cultivated for much longer periods than in previous civilizations.

When irrigation is no available, attention should be paid to preserving the natural vegetation that has evolved to live in deserts despite the lack of water. Yet, without careful management, humans will be responsible for the degradation of arid land, and for the creation of new deserts lacking any plant cover.

12 The scarce rain falling in deserts causes intense erosion, because the lack of vegetation leaves the soils unprotected and the water is easily lost as runoff down the steep slopes. Sudden intense rains wash part of the soil away, creating gullying on clay materials in the form of badlands, a sculptured landscape consisting of a series of narrow deep erosion gullies separated by sharp ridges. Erosion exposes the underlying materials, which sometimes contain fossils, such as these fossilized tree trunks in the Painted Desert in Arizona (United States), proof that the region's climate was much wetter in the geological past.

[Photo: Ramon Torres]

13 Population growth in the arid areas of Africa from 1900-1990 has led to population densities far greater than Earth's carrying capacity, and this has accelerated desertification due to over-exploitation of resources. Population pressure has increased demand for foodstuffs, leading to larger herds and the clearing of ground for cultivation, often of exotic species that are poorly adapted to local conditions. This greatly reduces the original plant cover, destabilizing soils and favoring erosion while removing the food source of wild herbivores. Population growth has been especially fast in sub-Saharan Africa and has led to increasing demand for land for construction, roads, and other communication routes, which has also contributed to the loss of plant cover. These factors, together with excessive cutting for fuel wood, have been the main causes of desertification and the appearance of human-made deserts.

[Drawing: IDEM, based on data provided by the author]

14 In deserts, permanent rivers can cause intense erosion, as shown by these deeply eroded meanders on the San Juan River (Utah, United States). Rivers and other permanent bodies of water are scarce in deserts but play a very important role because they allow the growth of plants and animals. They also help other geological agents such as wind and rain to shape the landscape. [Photo: Francois Gohier / Ardea London]

15 The irrigated valley of the Nile is a green strip of fertility running through the Egyptian desert, as is shown in this Landsat 4 satellite image (1982) of Cairo and the surrounding area. Ancient Egypt's civilization was a product of the Nile and its regular freshets that flooded and fertilized the land. The river still brings life to the many cities and villages on its banks. About 95% of Egypt's inhabitants live in the Nile Delta and the Nile Valley. From 1971, with the completion of the Aswan High Dam, the river was completely tamed, and the cycles of flooding and drought were totally changed, as was the rhythm of agriculture.

[Photo: ESA / Eurimage, 1996]

16 Urban population growth in the countries of North Africa and the Middle East that contain desert biomes within their frontiers has been spectacular since the mid-twentieth century. One needs only compare the number of inhabitants and the percentage of the population that was urban in the 1960s with the latest available data. The latest figures are from censuses taken in 1981 in Syria, 1982 in Morocco, 1984 in Libya (the percentage of the urban population is an estimate), 1985 in Kuwait, 1986 in Yemen (the percentage of urban population is for 1981), 1987 in Iraq, 1990 in Algeria, 1991 in Iran (only full citizens), 1992 in Saudi Arabia (the percentage of urban population is an estimate), and 1993 in Sudan. Although city growth in arid areas took off in the 1950s, the countries of the Middle East and northern Africa grew especially fast between 1960 and 1980, when several cities in the area grew at rates of more than 4%. In Saudi Arabia, for example, the urban population accounted for 30% of the total in 1960, but by 1980 it had increased to 67%. These increases are continuing. The increase in Kuwait is especially remarkable, where almost 100% of the population was urban by 1985. From the point of view of environmental management, the growth of urban centers in arid areas has put greater and greater pressure on the existing infrastructure and the resources available.

[Drawing: IDEM, based on several sources]

17 The number of sheep and goats in northern Africa and the Near East increased from 1950-1980, at which time it started to decline slightly. The 1970s saw a decline in the number of goats, but this was compensated by a larger in-crease in the number of sheep and meant that the total number of sheep and goats continued to increase. This constant increase in the number of head of livestock has led to overgrazing and has contributed to desertification. Fortunately, in the last few years, the tendency has reversed, and though the problem is not yet solved, it is at least not getting any worse. The ideal solution is that the number of sheep and goats should not exceed the carrying capacity of the pastures available.

[Drawing: IDEM, based on data provided by the author]

18 The massive loss of plant cover in many African deserts is the result of the drought affecting the continent since 1991. A decline in rainfall has immediate effects and starts a series of disasters that get worse and worse, most notably in regions of low rainfall such as the Sahel, Somalia, and southern Africa. The exhaustion of the water reserves and loss of vegetation lead to a reduction in herds of livestock, which may end up in a sea of sand with nothing to graze, as illustrated by this small mixed herd of sheep and goats in the outskirts of Taghit in Algeria. The consequent shortage of food, the return of waterborne illnesses such as cholera, and the increased incidence of other disasters have caused a huge exodus of the rural population to the cities.

[Photo: Ernest Costa]

19 There are three different types of arid zone: hyperarid, arid, and semiarid (the values in brackets correspond to the moisture received as fog and dew, in addition to the figure for rainfall). The evergreen vegetation and the geomorphology are different in each type of arid area, and some form of agricultural exploitation is possible in each. Each zone has its own characteristic precipitation/ evapotranspiration ratio (P/PE), the ratio of the rainfall to the potential evapotranspiration (PE). The P/PE is an indicator of the degree of intensity of the drought, independent of how long it lasts (there are almost a hundred methods to determine the value of P/PE, calculated here using Pen-man's formula). This gives a broad classification of arid areas, but if other factors (such as seasonal distribution of rainfall and summer and winter temperatures) are taken into account, a more realistic classification results that reflects the world's great variety of deserts.

[Source: data provided by the author]

20 Deserts and arid zones occupy about 30% of all dry land. They cover much of Africa and Australia, almost all of central and southwestern Asia, the southwestern United States, and most of northern Mexico. There are no deserts in Europe and only small areas of South America have an arid climate, though these are extremely arid. The Sahara in Africa is the world's largest desert, covering more than 9 million [km.sup.2]. All these arid areas are surrounded by a strip of land of greater or lesser width that is at great risk of desertification, and if current climatic conditions do not change and their management does not improve, in a few decades they will become true deserts.

[Drawing: IDEM, based on several sources]

21 Land use has changed greatly in the arid areas of northern Africa since 1900. In the early twentieth century, most usable land was used for grazing and little ground was cultivated. Gradually, cultivation of crops gained ground. As agriculture spread, the woody plant cover was destroyed. This had a disastrous effect on stockraising because trees and shrubs are the only source of food for the livestock during the long dry season. While the area of cultivated ground increased, grazing declined. This trend accelerated in the 1960s, and by the early 1980s half of the land was destined for agricultural production and the other half for stockraising. This spectacular increase is still continuing, and crops now occupy far more land than pastures. It is, however, thought that this trend will stabilize over the next few years.

[Drawing: IDEM, based on data provided by the author]

22 Arable land is exposed to erosion by wind and rain as soon as it is plowed, as shown by this wheat field in southern Australia, which lost all its topsoil in the 1983 drought, blown away by the wind. Desertification of croplands is a serious problem in many of the world's arid zones. It can be prevented only by suitable treatment of agricultural lands to protect them during the six months after the harvest, when they have no plant cover.

[Photo: Jean-Paul Ferrero]

23 Images such as this dead tree in Hidden Vlei in the Namib National Park (Namibia) will be increasingly common if desertification is not prevented. The existence of arid ecosystems is due to climatic conditions, but human beings and their activities are responsible for their spread and what they look like. The over-exploitation of many dry regions such as the savannahs turns them into subdeserts, greatly reducing the diversity of the natural flora and fauna. Yet people have also managed to make some deserts bloom (the Nile Valley, for instance) by using irrigation systems, showing that good management can make sterile land productive.

[Photo: Francesc Muntada / Sincronia Audiovisuals]
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Publication:Encyclopedia of the Biosphere
Date:Apr 1, 2000
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