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Home , Climate change, Desertification, Overexploitation

Desertification and Author(s): M. M. Verstraete and S. A. Schwartz Source: Vegetatio, Vol. 91, No. 1/2, Vegetation and Interactions in Semi- Regions (Jan. 31, 1991), pp. 3-13 Published by: Springer Stable URL: http://www.jstor.org/stable/20038708 Accessed: 28-01-2017 15:43 UTC

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This content downloaded from 192.206.10.53 on Sat, 28 Jan 2017 15:43:35 UTC All use subject to http://about.jstor.org/terms Vegetation: 3-13, 1991. A. Henderson-Sellers and A. J. Pitman (eds). 3 Vegetation and climate interactions in semi-arid regions. ? 1991 Kluwer Academic Publishers. Printed in Belgium.

D?sertification and global change

M. M. Verstraete1 & S. A. Schwartz2 1 Institute for Applications, CEC Joint Research Centre, Ispra Establishment, TP 440, 1-21020 Ispra (V?rese), Italy; 2 Department of Atmospheric, Oceanic and Space , The University of Michigan, Ann Arbor, MI 48109-2143, USA

Accepted 24.8.1990

Abstract

Arid and semiarid regions cover one third of the continental areas on . These regions are very sensitive to a variety of physical, chemical and biological degradation processes collectively called . Although interest in desertification has varied widely in time, there is a renewed concern about the of dryland because (1) a significant fraction of existing already suffers from miscellaneous degradation processes, (2) increasing will inevitably result in further over-utilization of the remaining productive areas, (3) climatic changes expected from the greenhouse warming might result in drier continental interiors, and (4) some of the desertification processes themselves may amplify local or regional climatic changes. This reviews some of the many aspects of this issue in the context of the Global Change research program.

Introduction This degradation now affects many or most ecosystems on this planet, and there is a progres Humanity has had along association with arid and sive awareness by the public and the decision semiarid regions: the first great (in makers of the many aspects of this abuse and Mesopotamia) developed at the end of (destruction of tropical , various forms the climatic optimum some 3000 years B.C., at a of , including toxic and nuclear , time when the appears to have been vege , acid deposition, etc., the so-called tated as parts of the are today (Butzer 1966; '-hole', oils spills and chemical or nuclear Lamb 1977). By the time the Great Pyramids were accidents, to name but a few). The release into the erected in Egypt (around 2700 B.C.), the climate of large quantities of , of Northeast and the Middle East was in and other pollutants by industrial and a drying phase that resulted in the arid agricultural activities is now so large that the com we have known for much of the last 5000 years position of the atmosphere is affected, and that in (El-Baz 1983). Superimposed on this long term turn is expected to affect the climate of the Earth climatic evolution, however, is the increasingly (Henderson-Sellers & Blong 1989; Schneider large impact of growing populations. The 1989). expansion into new territories, and exploitation of In response to this increased awareness, the the natural of these drylands beyond scientific has started to design and their has resulted in rapidly implement a coordinated research effort geared at deteriorating environmental conditions. documenting the current state and probable evo

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lution of the global system. The International The concept of desertification Geosphere Program (IGBP) is an interdisciplinary research effort 'to describe Muchand time and effort has been spent trying to understand the interactive physical, chemical, define the concept of desertification. Such a task and biological processes that regulate the total is difficult, if not impossible, because of the num Earth system, the unique environment that it berpro and complexity of the issues involved, the vides for , the changes that are occurring interdisciplinary in of the problem, and the this system, and the manner in which they rangeare of spatial and temporal scales over which influenced by human actions' (NAS 1988, p. this2). concept is applied (Verstraete 1983, 1986). This ambitious project is coordinated by the Inter For the present purpose, we shall define desertifi national Council of Scientific Unions (ICSU). cation as the set of all environmental degradation The expression 'Global Change' is often used processes as in hot drylands (hyperarid, arid, semi a synonym to IGBP research program, it also arid and subhumid regions), as a result of either designates the US component of the IGBP. climatic In stress or human mismanagement, or this paper, we review the nature, extent and sever both. Desertification will also include the causes ity of desertification, and discuss inter (to the extent these can be identified) and the actions between this form of impactand of degradation on natural and managed the expected climate and environmental changes. ecosystems. Clearly, environmental degradation The specific contributions of in situ observations can occur in all , but the fragile nature, and remote sensing systems in the overall harsh climate and expanding area of these dry strategy to monitor global environmental delands have made it a primary focus of attention. gradation are described in a companion paper Desertification usually has severe long term con (Verstraete & Pinty 1990). sequences for the productivity of the land, and therefore for the populations that inhabit these regions. The word 'desertification' was first introduced by the French forester Aubr?ville in his book Table 1. Extent of drylands by geographical region.

[km2, %] Total Hyperarid Arid Semiarid Subhumid Dryland

Africa 30,321,130 6,094,094 6,169,507 5,129,749 4,051,032 21,444,382 20.10 20.35 16.92 13.36 70.72

America 42,567,895 164,582 2,099,881 4,679,068 4,327,944 11,271,475 0.39 4.93 10.99 10.17 26.48

Middle East 6,139,098 1,125,997 3,052,996 985,490 769,062 5,933,545 18.34 49.73 16.05 12.53 96.65

Asia 38,120,322 382,439 4,008,463 5,312,779 3,925,704 13,629,385 1.00 10.52 13.94 10.30 35.75

Australia 7,686,884 3,766,572 1,537,377 1,229,900 6,533,850 49.00 20.00 16.00 85.00

Europe 10,507,630 10,096 239,179 267,503 516,778 0.10 2.28 2.55 4.92

Grand total 135,897,300 7,767,112 19,107,518 17,883,642 14,571,146 59,329,417 5.72 14.06 13.16 10.72 43.66

Sources of raw data: (Rogers 1981) and Hammond (1985).

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Climats, For?ts, et D?sertification de l'Afrique oases (Meckelein 1980), much of the arid, semi Tropicale (Aubr?ville 1949). He witnessed the arid and subhumid areas are generally considered degradation and disappearance of tropical forests to be a risk of desertification. in many humid and sub-humid parts of Africa, Table 2 shows the distribution of productive and attributed it to a large extent to the slash and drylands, broken down by type of economic activ burn agricultural practices of the local popu ity, as well as the percentage thought to be deserti lations. This destruction of the lead to fied as of the early eighties. These data are not with scattered trees, and then further to directly comparable to those of Table 1, but it is and a general tendency towards more interesting to note that are by far the xeric environmental conditions (Dregne 1986, largest areas affected by desertification. p. 6). Aubr?ville had identified as In the late seventies, the esti a potential factor, but could not estimate its mated the rate of desertification at between 54000 importance for lack of adequate data. It is only and 58000 km2 (UNCOD 1977, p. 9; United later on that the concept became commonly asso Nations, 1978, p. 2). By 1984, Norman Myers ciated with arid and semiarid regions. estimated that some 120000 km2 of agricultural Hot drylands are characterized by high solar and pastoral land were deteriorating beyond use radiation, potential ?vapotranspiration rates, and ful economic use per year (Myers 1984, p. 46). diurnal ranges of temperature, and low precipi These numbers should be compared to estimates tation and atmospheric . The location of worldwide, which range from and extent of these regions depends strongly on 100000 to 113 000 km2 per year, with the bulk of how they are defined, and many definitions have the destruction occurring in the tropics (Myers been proposed in the literature. Table 1 shows the 1984, p. 42; WRI 1986, p. 72). extent of hyperarid, arid, semiarid and subhumid Arid are dynamic regions, they have been areas for six regions of the . The data were evolving over thousands of years, mostly in compiled from information given by Rogers response to climatic changes (Warren 1984). (1981), who digitized UNESCO's map showing Throughout most of its history, humanity has the World Distribution of Arid Regions (UNESCO been able to cope with such an evolution (and 1977). This map was constructed on the basis of with its own degradation of the environment) by hydrological data, using a balance colonizing new and hitherto unaffected areas. approach; it arguably represents the best map of Only during the last century has it become appar aridity currently available. ent that the land was finite; and indi It can be seen that almost 33% of the conti viduals and societies are only now becoming nents are hyperarid, arid or semiarid, and that aware of the consequences of unlimited growth almost 44% of all continental areas (excluding and careless abuse of finite natural resources. ) can be classified as drylands. While Recurrent severe in the Sahel and more degradation in hyperarid regions is limited to recently in Ethiopia, have helped to periodically focus attention on this problem, although the degradation occurs in many other regions. Table 2. Distribution of desertif?ed areas by economic Drylands often experience significant inter activity. seasonal and inter-annual climate variability, and Economic use Area [km2] % desertif?ed in particular periods of reduced water availability. therefore constitutes a 'normal', ex 25,560,000 62 pected (if largely unpredictable) component of Rainfed 5,700,000 60 Irrigated agriculture 1,310,000 30 their climate: it is a temporary situation where the amount of available water is smaller than what Total productive drylands 32,570,000 61 would be needed for the intented economic use, or Source: WRI, 1986, p. 278. for the normal growth and development of the

This content downloaded from 192.206.10.53 on Sat, 28 Jan 2017 15:43:35 UTC All use subject to http://about.jstor.org/terms 6 cover (Gibbs 1975, p. 11). Droughts have degradation of plant and soil resources (Dregne been identified both on the basis of meteorological 1986). Desertification threatens many of the arid or hydrological parameters alone (see Landsberg and semiarid regions of the world, and involves a 1975, for a review), or by reference to economic large number of complex interactions between impacts, or even perceptions of risk (Heathcote physical, topographical, edaphic, and biological 1969). Many authors view drought as a short parameters, as well as human components (land lived phenomenon responsible for recurrent but use and , social structures, economic not permanent stress to the environment (e.g., Le development, and health status) (Spooner 1982). Hou?rou 1977). This approach, although under Different processes and interactions take place in standable a posteriori, may not be operational different ecosystems, but the general tendency because it is impossible to know how long a dry towards degradation that results from the inter spell is going to last. The Sahel is a case in point, play of these processes and interactions is com where the relatively dry period that started around mon to all situations. In that sense, desertifi 1968 was long identified as a drought, even cation, which has been called the cancer of dry though is has not ended yet, more than 20 years lands, can affect any area which has been made later (Nicholson 1989). How long does a drought vulnerable enough by climatic stress or human need to be before it is called a climatic shift or , or both. change? It is difficult to estimate the number of people Desertification is generally conceived as a affected by desertification because geographical, much wider concept than drought, involving not political and economic differences whithin each only water availability issues, but also various country result in some classes being forms of soil degradation, loss of biological pro more affected or more at risk than others. Never ductivity, and a host of human impacts. It is theless, it is probable that around 100 million deemed to occur on 'long' time scales, causing a people are directly affected by desertification slow but cumulative decline in biological produc today, and that another 900 million may be at risk tivity and in the capability of the land to support now or in the near future (UNCOD 1977, p. 8; its natural vegetation or agricultural exploitations. United Nations 1978, p. 2; Paylore & Greenwell Desertification must be distinguished from 1980, p. 14-18; Paylore 1984, p. 18-19; Al drought, but since the end of a drought even can Sudeary 1988, p. 13). not be predicted, its duration can only be known after the fact, and the distinction between desertification and drought in terms of their The processes of land degradation respective duration cannot be applied opera tionally. The combination of progressive desertifi Desertification often results from the degradation cation and droughts can be severely crippling to of the vegetation cover by , over the environment, as the stress created by human trampling, collection, repeated burning, or overexploitation of the land becomes especially inappropriate agricultural practices. It leads to a visible during severe droughts. Drought and general decrease in productivity of the land and in desertification can amplify each other's impacts, the accelerated degradation of the soil resource and the resulting degradation of the environment due to (both by wind and water), can further affect adjacent areas, either directly , salinization and alkalinization of irri (invading sand , siltation of down gated lands, or dry salting. The excessive loss of stream), or indirectly (migration of populations, soil, nutrients, and sometimes even seeds from the increased international tensions over increasingly affects the capability of the vegetation scarce resources). to recover and constitutes the principal mecha The intensity of desertification processes can nism of irreversible damage to the environment. range from slight to very severe in terms of the The impact of on pastoral rangelands

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depends largely on its intensity and timing. Light on lower parts of slopes, presenting similar physi grazing may increase the productivity of the range cal obstacles to farming operations as they do to by stimulating new growth, while moderate to rangeland practices. In addition, runoff from the heavy grazing often results in the preferential slopes causes sedimentation downhill; waterways removal of the more palatable species (Warren & and dams are filled, and flooding is intensified in Maizels 1977, p. 203-210). This facilitates the low-lying areas (UNCOD 1977). The degradation invasion of less palatable or inedible species (in processes act synergistically in both rainfed farm vaders) by modifying the ecological niches. Over ing and pastoral systems. grazing during the dry further reduces the Soil erosion by wind and water may take place vegetation cover and increases the risk of soil in both arid and semiarid regions, and at various erosion. Perennial species are particularly impor times during the year (Verstraete & van Ypersele tant in stabilizing the soil, especially during the 1986). The impact of wind erosion clearly domi dry season, while ephemeral and annual species nates in the drier regions, while water erosion will may help control soil erosion during the wet affect mostly the wetter areas (Marshall 1973, season. p. 58). Furthermore, it is the extreme events which The deterioration of the soil surface, which are the most destructive, especially if they occur results from the removal of the vegetation cover, in the dry season, a period of maximum sensitivity strongly affects the health, vigor and reproductive to perturbation. The risk of soil erosion depends capacity of the remaining through the ondis both the susceptibility of the soil to erosion, ruption of plant-water relations. Subsequently, and on the probability of a meteorological event there is an increase in runoff, sheet and gully (wind gust or rainfall) of sufficient magnitude to erosion on sloping ground; the top soil is lost, displace the soil. For example, the erodibility of together with its store of water, nutrients, and the soil may be highest during the dry season, seeds. Compounding the problem is the higher when the protective vegetation cover is minimal, bulk density of the soil caused by severe tram but the probability of intense is highest dur pling, which decreases infiltration by water and ing the rainy season. The highest risk of water also increases runoff. Ultimately, if erosion is erosionnot therefore occurs at the end of the dry stopped, it will completely destroy the productive season, when the plant cover has not grown back value of the land by soil stripping and gully exten yet, but the likelihood of an intense rainfall sion. increases. Rainfed farming practices have their own dis Wind erosion affects the soil structure and tinctive signatures of desertification. The problem composition in different ways. First, the flow of often originates on land cleared for cultivation or particles is proportional to the cube of the friction left fallow. As soon as the natural vegetation is wind speed above a threshold value (Bagnold cleared, the soil is made vulnerable to accelerated 1943; Gillette etal. 1982; Skidmore 1986). wind and water erosion. The introduction of agri Second, the coarser sandy materials drift over cultural equipment, designed for the deep of short distances (a few hundreds of meters to a few mid-latitude regions, into tropical regions with kilometers), while the finer materials are carried thin soils can further aggravate the soil loss (soil away in the wind, sometimes over continental pulverization and removal of humus and distances: from the Sahara has been nutrients). The removal of the plant cover during observed to cross the Atlantic , and 'sand results in bare soils and promotes the rains' have been reported from as far as Northern formation of a hard crust at the surface. This and the (Carlson & Prospero crusting of the soil reduces infiltration and in 1972; Rapp 1974; MacLeod*/al. 1911 \ Morales creases runoff. Thus, as in pastoral rangelands, 1979). the fertile surface soil is stripped away, leaving The fine top soil which is lost to wind erosion behind infertile subsoils. Gullies may then form is the most fertile portion of the soil complex and

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only sterile soil is left behind (UNCOD 1977). soils. The drying of the top soil impedes water The lack of water in these shallow soils also movement and may therefore reduce the total prevents seedlings from surviving prolonged evaporation. Deep soils have a better chance of droughts, making permanent plant life even more conserving moisture than shallow soils. difficult. When perennial seedlings do manage to Since water availability is the primary limiting emerge, they may easily be uprooted and blown factor for plant productivity in dry regions, irri away. With little protection left on the ground, the gation has often been seen as the best way to blasting impact of moving sand can destroy young boost production. This technique, however, pres crops (Le Hou?rou 1977). ents its own set of problems and can also lead to Water erosion is intimately linked to the hydro the permanent degradation of the land. The pro logical cycle of arid and semiarid lands. The pri ductivity of the vegetation is directly linked to its mary physical agent of erosion is the transfer of transpiration rate; therefore the goal of kinetic from the raindrops to the soil par is to provide plants with just enough water in the ticles. Once these particles have been dislodged, soil. Excess water should be drained. When this they can be transported away by surface runoff. is not done, irrigation water infiltrates the soil and The presence of vegetation greatly decreases the picks up soluble salts. The drying of the top soil impact of raindrops because the leaves absorb creates a gradient of moisture and generates a much of their momentum: the drops break up and slow but continuous upward flow of mildly salty slow down considerably. The sparser the vege water. These salts therefore accumulate at the tation cover, the more rainfall reaches the ground, surface as the water evaporates. Most plants can where it must either percolate into the ground or not tolerate very high salt concentrations in the runoff. upper soil, as it affects their capability to absorb Soils are extremely variable in their spatial dis water in the root layer. Ultimately, crops must be tribution. The most important characteristics of switched to halophytes (salt tolerant plants) or the soils are their texture, structure and chemical field must be abandoned. On the other hand, composition, as well as their depth. Soil texture is proper irrigation and drainage may be designed to determined on the basis of the relative fractions of flush the salts in the upper soil layers, but this sand, silt, and . Runoff from a specific soil requires high capital investments and constant depends primarily on the intensity of precipi maintenance of the drainage system. tation, the infiltration rate and the local slope. The Last but not least, fire has long been recognized coarser soils (i.e., with an important sand as a potentially destructive agent. It is well known fraction), have a high infiltration rate and can that a light fire may actually stimulate new plant therefore absorb intense showers. Soils with a growth, especially in grasslands, and accelerate major clay fraction have a much reduced infiltra the turnover of nutrients in the environment. tion rate, and tend to generate more runoff, even Burning may also help control some bush or tree under lower intensities (Hillel 1982). invasion, but there are many possible negative The breakup of soil aggregates by large raindrops effects. Hot fires can sterilize the ground by may lead to the dispersal of the fine elements and destroying the -fixing bacteria, thereby the sealing of the surface when the soil dries up. reducing the productivity of the land, especially if This crusting contributes to the formation of an they are ill-timed. Hot fires can lead to the impermeable surface (Le Hou?rou 1977). Soil destruction of humus and a loss of fertility, a depth plays a crucial role because it directly affects destabilization of the water relations in the the water storage capacity, and the amount of , or even the loss of the seedstore nutrients (both and organic) available for (UNCOD 1977). Even light fires can take their plant growth and development. The hydraulic toll by causing distillates of organic matter to coat conductivity of soils is a non-linear function of the particles of the soil surface, decreasing infiltration water content, with much lower values for drier and increasing runoff or erosion

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interplay between economic constraints and the

^/cloudiness") availability of natural resources, as well as by the expectations of individuals and other cultural fac tors. in medicine and the provision of medical services has improved the quality of life and decreased mortality rates, at least for those segments of the population that have access to these services. On the other hand, environmental degradation and the breakdown in the production Boundary layer wind shear systems from desertification is often accompa nied by a lowering of health standards, a decrease in nutritional intake, and an increase in morbidity. The health status of a population and the location and accessibility of natural resources, in turn, Wind J son Water directly influence the rates of migration of popu erosion ^Htexture, structure^ erosion lations. Hence, a region may be indirectly affected Fig. 1. Soil, vegetation and atmosphere interactions within by desertification in a neighboring area. On a the context of desertification. larger political scale, these problems can result in severe internal problems between social classes, or even in massive environmental migrations, (Warren & Maizels 1977). Fires can drastically possibly causing international tensions. There is modify the geophysical environment near the ansur extensive literature on this subject. The inter face: the may be affected, at least specested reader should refer to Copans (1975), trally, the roughness of the surface may UNCOD be (1977), Spooner (1982) and Glantz changed in either direction, depending on the type(1987) for introductions to this vast subject. and density of the cover, and hydrological rela tions at the surface may be modified. The interactions between the components Climate of and environmental change the environment (soil, plants, climate) are shown shematically in Figure 1. This diagram is notClimate variability, and in particular extreme con intended to be exhaustive, or to describe the ditions,par must be expected in drylands. What is ticular situation at a specific site; but it doesmore worrisome, however, is the possibility that attempt to show how various aspects of a dryland human activities may be responsible for signifi ecosystem depend on each other and are affected, cant further climatic changes in the foreseeable directly or indirectly, by biogeophysical processes future. The primary cause for concern lies with the that take place in this environment. One of observed the change in the chemical composition of major difficulties of modeling these interactions the atmosphere. Agricultural, industrial and results from the fact that they take place at many urban activities, and specifically the heavy con different spatial and temporal scales (Avissar sumption & of fossil , result in the injection of Verstraete 1990). large quantities of carbon dioxide and many other This description of desertification would not pollutants be in the lower atmosphere. Some of these complete without a discussion of human aspects, compounds find their way into the or may both as causes of degradation and as victims of be its re-absorbed by the surface, but a large fraction impact. Societies directly affect the environment remains airborne and contributes to the trapping through such activities as , agriculture, of thermal radiation emitted by the sur transportation, and industrializa face. This so-called results in a tion. These activities are largely controlled by thesubstantial heating of the air near the ground.

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It is currently estimated that the concentration cal cycles provide a new convenient framework to of C02 in the atmosphere could reach twice its understand the invasion of shrubs and the pre-industrial value in the first half of the next progressive degradation of the environment, and century. Many atmospheric scientists have to try to predict the likely of these attempted to evaluate the likely temperature areas as well as the interactions between arid change that would result, using general circulation lands and other ecosystems (Schlesinger etal. models of the atmosphere. Different models pre 1990). dict somewhat different distributions of heating It is important to appreciate the significance of because of internal differences in their represen the climatic changes described above. First of all, tation of the physics and dynamics of the atmos local and regional temperature or moisture phere (Schlesinger 1989), but results from these changes may be much higher (or lower) than these numerical sensitivity studies show that the glob global values. Some regions will likely be more ally-averaged annual mean surface temperature affected than others. Second, a slight change in could increase by 2 to 6 ? C by 2050. Furthermore, mean values also results in more frequent extreme these expected changes in temperature will likely conditions, even without a change in the statisti be associated with major perturbations of the cal distribution of the climatic parameters. Third, hydrological cycle. The actual impact on the water a change of global mean temperature of this mag cycle is difficult to assess, but it is quite possible nitude in such a short amount of time is essentially that the interior of large may ex unprecedented: even the warming responsible perience further drying (Kellogg & Zhao 1988). for the d?glaciation 15 000 years ago was ten times Furthermore, the impact of changes at the sur slower. As a result, there is some concern about face on the climate itself must also be considered. the capability of the biosphere to cope with such Focusing on land areas, the climate is affected by a change. the underlying surface in three major ways : first, the optical properties of the surface, in particular Scientific issues albedo, control the rate of absorption by the . Second, the vegetation The bottom line is therefore relatively clear: the cover affects the rates of transfer of energy, water human population is growing at the rate of and various chemical compounds between the 95 million individuals per year (or about 11000 surface and the atmosphere. And third, the per hour) (Ehrlich and Ehrlich, 1990, p. 9), and roughness of the surface controls the exchange of desertification alone accounts for the loss of momentum between the atmosphere and the 120000 km2 of productive land per year (about planet, and strongly affects the divergence of the 1400 ha per hour). Since we also loose at least wind fields in the atmospheric boundary layer 120000 km2 of forests and 24 to 26 billion tonnes (Verstraete & Dickinson 1986; Nicholson 1988). of top soil per year through erosion (Ehrlich & The concern for global change goes far beyond Ehrlich 1990, p. 28), it is not difficult to see that just climatic change, however. The study of the this situation cannot be sustained for much longer. cycling of major biogeochemicals among the var The time is for action, not panic, and the scientific ious components of the climate system has pro community has a definite role to play. vided a new paradigm for global investigations. First of all, it is urgent to clearly assess the Arid and semiarid regions have long been con situation and establish the probable future evolu sidered wastelands, but they are now recognized tion. In fact, the lack of precise information about as integral parts of the Earth system. Detailed the extent and severity of the population-environ investigations in the southwest United States ment-climate problem has consistently been showed that grazing can increase heterogeneities recognized over the last 40 years. Here are a few in the spatial and temporal distribution of water, selected quotes from the literature on this subject: nitrogen, and other soil resources. Biogeochemi Aubr?ville (1949), p. 330:

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A few years ago, we studied this problem of decision making in such tasks as prioritiz desertification of along the Saharan ing the target areas for relief and conducting cost border without reaching definite conclusions. We benefit analyses of various remedial actions or were not able to demonstrate a progressive drying feasibility studies, as well as support field activ of the climate because meteorological observa ities geared towards minimizing further degrada tions and statistics were not reliable enough and tion or reclaiming affected areas, and (6) evaluate did not cover long enough periods. [Translated the effectiveness of these , plans and from French by M. Verstraete] remedial actions. McGinnies et ai (1968), p. 3: It is unfortunately not clear where desertifi To the extent that we have experienced wide cation is the most severe, or at what rate it is spread desert encroachment within historic times, occurring. Preventive measures or remedial we are far from unanimous as to its causes. [...] actions should be based on a accurate assessment For the immediate future lack of basic data about of the situation, and that crucial element is still desert environments remains our largest single missing. While there are countless reports of category of arid-lands problem.' impending at the local scale, there is no Warren and Maizels (1977), p. 177 and 186: coherent way to obtain a synthetic accurate and This chapter briefly reviews the evidence for quantitative view of the global situation. Satellite desertification. It will soon become apparent that remote sensing techniques appear to be the most this is not an easy task. Statistics are seldom in appropriate for this task, but a lot more work the right form, and are hard to come by, and even must be done in order to retrieve quantitative harder to believe, let alone interpret. [...] The information from these data, as opposed to evidence for desertification is diffuse and almost qualititative estimates. A discussion of the poten impossible to quantify.' tial contribution of space platforms, along with Bie (1989), p. 2: traditional in situ measurement techniques, can 'Although the occurrence, if not the severity, of be found in a companion paper (Verstraete & is well documented, there is a scarcity of Pinty 1990). data linking general dryland degradation to In parallel to this effort, the second contribution . There is little doubt, however, that local of the scientific community must be a better land degradation has taken place. There is there understanding of the processes involved and an fore a need to develop better methods whereby analysis of the implications of the remedial land degradation can be assessed. Inaccurate actions that may be envisaged. The global models methodologies appear in the past to have had used to investigate the scenarios described above major impact on donor attitudes on the occur and their impacts represent the state of the art in rence, frequency and severity of dryland degrada climate modeling, even though they still suffer tion.' from significant shortcomings. For example, in Monitoring the environment (in drylands as most cases, the representation of the oceans and well as other ecosystems) plays a number of cru of air- interactions are rather crude, and the cial roles and must be pursued to (1) establish a parameterization of and surface processes baseline against which future observations can be cannot do justice to the number and complexity compared, (2) document the spatial and temporal of the processes that actually take place. These variability of the relevant environmental parame models must be expanded to include the most ters, (3) identify the regions at risk of further significant biogeochemical cycles. In fact, a degradation, and the nature of the processes at hierarchical set of models at various scales must work, (4) provide the data needed to build and be designed to study and understand better the validate the mathematical models of the environ interactions between physical, chemical, biologi ment that are needed to understand and predict cal, and human processes. This will require fur the evolution of these ecosystems, (5) support ther emphasis on interdisciplinary investigations,

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as well as a renewed effort to train generations of ment of Saharan air outbreaks over the northern Equa torial Atlantic. Journal of Applied 11: young scientists to deal effectively with this com 283-297. plex system. The Global Change Research Pro Copans, J. 1975. S?cheresses et Famines du Sahel, Volumes I gram is a major step forward in that direction. and II. F. Masp?ro, Paris, 150 pp. and 144 pp. Dregne, H. 1986. Desertification of arid lands. In: Physics of Desertification, Edited by F. El-Baz and M. Hassan, Martinus NijhofT Publishers, Dordrecht, 4-34. Acknowledgments Ehrlich, P. R. & Ehrlich, A. E. 1990. The Population Explo sion. Simon and Schuster, , 320 pp. El-Baz, F. 1983. A geological perspective of the desert. In: S. The Department of Atmospheric, Oceanic and Wells and D. Harag?n (Eds.), Origin and Evolution of Space Sciences of the University of Michigan and . University of New Mexico Press, Albuquerque, the School of Earth Sciences of Macquarie 163-183. University financed the participation of the first Gibbs, W. J. 1975. Drought: Its definition, delineation and author to the 'Conference on Degradation of effects, in Drought: Lectures presented at the 26th session of the WMO Executive Committee, Special Environmental Vegetation in Semi-Arid Regions: Climate Impact Report No. 5, WMO No. 403, Geneva, 1-39. and Implications', Macquarie University, Janu Gillette, D. A., Adams, J., Muhs, D. & Kihl, R. 1982. Thresh ary 29-31, 1990. The support and encourage old friction velocities and rupture moduli for crusted desert ments of Prof. W. Kuhn and A. Henderson soils for the input of soil particles into the air. J. Geophys. Res. 87: 9003-9015. Sellers are gratefully acknowledged. Serena Glantz, M. H. (Editor) 1987. Drought and in Africa: Schwartz is grateful to the Population-Environ Denying Famine a Future. Cambridge University Press, ment Dynamics Program administred by the Cambridge, 457 pp. School of Public Health of the University of Hammond 1985. Ambassador World Atlas. Hammond Inc., Michigan for financial support of her interdis 484 pp. ciplinary studies. Heathcote, R. L. 1969. Drought in : A problem of perception. The Geographical Review 59: 175-194. Henderson-Sellers, A. & Blong, R. 1989. The Greenhouse Effect: Living in a Warmer Australia. New South Wales University Press, Kensington, 211 pp. References Hillel, D. 1982. Introduction to Soil Physics. Academic Press, New York, 365 pp. Al-Sudeary, A. M. 1988. Alleviation of rural in arid Kellogg, W. W. & Zhao, Z. 1988. Sensitivity of lands. In: Arid Lands: Today and Tomorrow. Edited by E. to doubling of carbon dioxide in experi Whitehead, C. Hutchinson, B. Timmermann and R. ments. Part 1: , Journal of Climate 1: Varady, Westview Press, Boulder, 13-20. 348-366. Aubreville, A. 1949. Climats, For?ts, et D?sertification de Lamb, H. H. 1977. Climate: Present, Past and Future, Vol. 2, l'Afrique Tropicale. Soci?t? d'Editions G?ographiques, Climatic History and the Future. Methuen and Co., Maritimes et Tropicales, Paris, 351 pp. London, 835 pp. Avissar, R. & Verstraete, M. M. 1990. The representation of Landsberg, H. E. 1975. Drought: A recurrent element of continental surface processes in mesoscale atmospheric climate. In: Drought: Lectures presented at the 26th models. Reviews of 28: 35-52. session of the WMO Executive Committee, Special Bagnold, R. A. 1943. The Physics of Blown Sand and Desert Environmental Report No. 5. WMO No. 403, Geneva, Dunes. William Morrow & Co., New York. 41-90. Bie, S. W. 1989. Dryland degradation assessment tech Le Hou?rou, H. N. 1977. The nature and causes of deserti niques. Outline of a presentation for Session 1 of the zation. In: Desertification. Edited by M. Glantz, Westview Professional Development Workshop on Dryland Man Press, Boulder, 16-38. agement, The , Washington DC, May 10-11, MacLeod, N. H., Schubert, J. S. & Ananejionu, P. 1977. 1989. Report on the Sahel 4 African Drought and Arid Lands Butzer, K. W. 1966. Climatic changes in the arid zones of Experiment. In: Skylab Explores the Earth, NASA, Wash Africa during early to mid-Holocene times, in World Cli ington DC, 263-286. mate from 8000 to 0 BC. Proceedings of the International Marshall, J. K. 1973. Drought, land-use and soil erosion. In: Symposium held at Imperial College, London, 18 and The Environmental, Economic and Social Significance of 19 April 1966. Royal Meteorological Society, 72-83. Drought. Edited by J. V. Lovett, Angus and Robertson Carlson, T. N. & Prospero, J. M. 1972. The large-scale move Publishers, , 55-77.

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