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Chapter 1. Desertification and It's Relation to Climate Variability and Change

Chapter 1. Desertification and It's Relation to Climate Variability and Change

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HILLEL AND ROSENZWEIG



I. INTRODUCTION

Ecosystems in semiarid and arid regions around the world appear to be undergoing various processes of degradation commonly described as desertification.

According to UNEP (1992), all regions in which the ratio of total annual precipitation to potential evapotranspiration (P/ET) ranges from 0.05 to 0.65 should be

considered vulnerable to desertification. Such regions constitute some 40% of the

global terrestrial area, which totals about 130 million km2 (13 billion ha). Dregne

(1983) calculated that the arid, semiarid, and dry subhumid regions of the world

occupy 12.1, 17.1, and 9.9% of the world’s total land area. Relatively dry areas

cover much of northern Africa, southwestern Africa, southwestern Asia, central

Asia, northwestern India and Pakistan, southwestern United States and Mexico,

western South America, and much of Australia (Fig. 1, see color insert).

Arid and semiarid regions cover over a fourth of the world’s land area, and

are home to nearly one-sixth of the world’s population (WRI, 2000). The total

population of the world has doubled in the last four decades, resulting in the

current total of about 6 billion. As of 1998, some 80% of humanity resided in the

so-called developing countries, which contain only 58% of the total land area and

54% of the total cropped area. Moreover, many of the developing countries are

located in semiarid regions that are most vulnerable to degradation.

According to a report published by the World Resources Institute (WRI, 1998),

the total area of land under cropping has increased by some 25% since 1950. In

the same period, the world’s population has more than doubled, so the area of

cropland per capita has been reduced by nearly a half.

At present, the annual growth rate of cropland (0.2%) is only one-seventh the

growth in population (Lal, 1997), so the decline in arable land per capita is continuing. That decline is most severe in the developing countries, which are expected

to increase their populations most rapidly and will therefore be most in need of

increased food production. In sub-Saharan Africa, for instance, the per capita area

of arable land, which was 1.6 ha in 1990, is projected to fall to 0.63 ha by 2025

(Scherr, 1999). The lands still available for the expansion of farming are, in large

part, marginal lands of relatively low productivity and high vulnerability.

Desertification is an emotive term, conjuring up the specter of a tide of sand

swallowing fertile farmland and pastures. The United Nations Environmental Programme (UNEP) sponsored projects in the early 1980s to plant trees along the

edge of the Sahara, with the aim of warding off the invading sands. While there are

places where the edge of the desert can be seen encroaching on fertile land, the more

pressing problem is the deterioration of the land due to human abuse in regions

well outside the desert. The latter problem emanates not only from the desert but

also from the centers of population; not only from the spread of the sand dunes

but also from the spread of people and their mismanagement of the land (Hillel,

1992). Therefore, protecting the front line may do nothing to halt the degradation



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behind it. The true challenge is not so much to stop the desert at the edge of a

semiarid region as to protect the entire region from internal abuse of its vegetation

and of its soil and water resources.

A vicious cycle is already operating in many areas: as the land degrades, it is

worked ever more intensively so its degradation accelerates; and as the returns

from “old” land diminish, “new” land is brought under cultivation or grazed by

encroachment onto marginal or submarginal areas. But attempts to encapsulate

these complex problems in the catchall term “desertification” may have obscured

its true character and confused the search for its amelioration.

In this paper, we review the concepts, definitions, and processes pertinent to

desertification, and offer an alternative, more inclusive term, namely, “semi-arid

ecosystem degradation.” We use the long-term drought in the Sahelian region of

Africa as a case study for analyzing the complex set of climatic, biophysical,

and social factors that interweave to create the process of semiarid ecosystem

degradation, and we evaluate current monitoring techniques, including remote

sensing. We next consider the potentialities and hazards of irrigation development

as a possible means to improve agricultural production in semiarid regions. We then

ask the question, “How might global climate change affect the Sahelian region of

Africa?” and analyze a set of recent projections derived from global climate change

scenarios, in light of the region’s vulnerabilities. Finally, we offer our views on

prospects for sustaining semiarid ecosystems and agroecosystems in the future.



II. CONCEPTS AND DEFINITIONS

Desertification is a single word used to cover a wide variety of effects involving the actual and potential biological productivity of ecosystems in semiarid and

arid regions. The term desertification (or desertization) was apparently coined by

the French ecologist LeHouerou (1977) to characterize what was perceived to

be a northward advance of the Sahara in Tunisia and Algeria. It gained currency

following the severe drought that afflicted the Sud region of Africa in the early

1970s, and again in the 1980s, during which the Sahara was reported to be advancing southward into the Sahelian zone as well. For example, Lamprey (1975)

estimated that during the period from 1958 to 1975, while mean annual rainfall

diminished by nearly 50%, the boundary between the Sahara and the Sahel had

shifted southward by nearly 100 km.

As defined in recent dictionaries, desertification is the process by which an area

becomes (or is made to become) desert-like. The word “desert” itself is derived

from the Latin desertus, being the past participle of deserere, meaning to desert, to

abandon. The clear implication is that a desert is an area too barren and desolate to

support human life. An area that was not originally desert may come to resemble

a desert if it loses so much of its formerly usable resources that it can no longer



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provide adequate subsistence to humans. This is a very qualitative definition, since

not all deserts are the same. An area’s resemblance to a desert does not make it

a permanent desert if it can recover from its damaged state, and, in any case, the

modes of human subsistence and levels of consumption differ greatly from place

to place.

The United Nations Conference on Desertification (UNCOD) was held in

Nairobi in 1977. It was convened in response to the severe drought that had befallen

the Sahel from the late 1960s through most of the 1970s. Its report defined desertification as “the diminution or destruction of the biological potential of land that

can lead ultimately to desert-like conditions . . . under the combined pressure of

adverse and fluctuating climate and excessive exploitation.” That statement leaves

open several questions, such as the definition of the land’s “biological potential,”

the type and degree of damage to the land that can be considered “destruction,”

and the exact meaning of “desert-like” conditions.

Mainguet (1994) characterized desertification as the “ultimate step of land

degradation to irreversible sterile land.” This definition ignores the complex set

of processes that progress gradually (and, for a time, reversibly) at different rates.

Rather, it confines the term to the final condition that is the extreme culmination of

those various processes. An alternative approach would be to define the processes

themselves and characterize the degree of degradation at which their separate or

combined effects may be considered to have become irreversible.

In recent years, the very term desertification has been called into question as

being too vague, and the processes it purports to describe too ill-defined. Some

critics have even suggested abandoning the term, in favor of what they consider

to be a more precisely definable term, namely, “land degradation” (e.g., Dregne,

1994). However, desertification has already entered into such common usage that it

can no longer be recalled or ignored (Glantz and Orlovsky, 1983). It must therefore

be clarified and qualified so that its usage may be less ambiguous.

The United Nations has since modified its definition of desertification as

follows: “Land degradation in arid, semiarid, and dry subhumid areas resulting

from various factors, including climate variations and human activities” (Warren,

1996). That definition still does not either clarify the relative importance of the

two potential causes or imply the possibility that they may be interactive. It merely

shifts the issue to the definition of “land degradation.” Does the latter pertain

to the soil, and, if so, to just what qualities or attributes of the soil (physical,

chemical, and/or biological)? Does it also pertain to the vegetation present on

the land, and, if so, to what attributes of the vegetation (biomass, photosynthesis,

respiration, transpiration, growth rate, ground coverage, species diversity, etc.)?

And what of the animal life associated with the land?

“Land degradation” itself is a vague term, since the land may be degraded with

respect to one function and not necessarily with respect to another. For example, a tract of land may continue to function hydrologically—to regulate infiltration, runoff generation, and groundwater recharge—even if its vegetative cover is



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changed artificially from a natural species-diverse community to a monoculture,

and its other ecological functions may be interrupted.

Rather than “land degradation,” we prefer the term “semiarid ecosystem degradation.” A semiarid ecosystem encompasses the diverse biotic community living

in this given domain. Included in this community is the host of plants, animals,

and microorganisms that share the habitat and that interact with one another

through such modes as competition or symbiosis, predation, and parasitism. It

also includes the complex physical and chemical factors that condition the lives

of those organisms and are in turn influenced by them. A semiarid ecosystem may

be a more or less natural one, relatively undisturbed by humans, or it may be an

artificially managed one, such as an agroecosystem.

Each ecosystem performs a multiplicity of ecological functions. Included among

these are photosynthesis, absorption of atmospheric carbon and its incorporation

into biomass and the soil, emission of oxygen, regulation of temperature and the

water cycle, as well as the decomposition of waste products and their transmutation

into nutrients for the perpetuation of diverse interdependent forms of life. Integrated

ecosystems may thus play a vital role in controlling global warming and in absorbing and neutralizing pollutants that might otherwise accumulate to toxic levels.

An agroecosystem is a portion of the landscape that is managed for the economic

purpose of agricultural production. The transformation of a natural ecosystem into

an agroecosystem is not necessarily destructive, if the latter is indeed managed sustainably and if it coexists harmoniously alongside natural ecosystems that continue

to maintain biodiversity and to perform vital ecological functions.

In too many cases, however, the requirements of sustainability fail, especially

where agricultural systems expand progressively at the expense of the remaining

more or less natural ecosystems. The appropriation of ever-greater sections of the

remaining native habitats, impelled by the increase of population as well as by

the degradation of farmed or grazed lands due to overcultivation or overgrazing,

decimates those habitats and imperils their ecological functions.

In the initial stages of degradation, the deteriorating productivity of an agroecosystem can be masked by increasing the inputs of fertilizers, pesticides, water, and

tillage. Sooner or later, however, if such destructive effects as organic matter loss,

erosion, leaching of nutrients and salination continue, the degradation is likely to

reach a point at which its effects are difficult to overcome either ecologically or

economically.



III. PROCESSES

Key processes related to desertification include drought, primary production and

carrying capacity, soil degradation, and water resources. The role of social factors

is also important.



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A. DROUGHT

A typical feature of arid regions is that the mode (the most probable) amount

of annual rainfall is generally less than the mean; i.e., there tend to be more years

with a below-average rainfall than years in which the rainfall is above average,

simply because a few unusually rainy years can skew the statistical average well

above realistic expectations for rainfall in most years. More than 90% of the total

variation in annual rainfall can generally be encompassed within a range between

one-half and twice the mean.

The variability in biologically effective rainfall is yet more pronounced, as

years with less rain are usually characterized by greater evaporative demand, so

the moisture deficit is greater than that indicated by the reduction of rainfall alone.

Timing and distribution of rainfall also play crucial roles. Below-average rainfall,

if well distributed, may produce adequate crop yields, whereas average or even

above-average rainfall may fail to produce adequate yields if the rain occurs as

just a few large storms with long dry periods between them.

In semiarid agricultural regions, “drought,” like desertification, is a broad, somewhat subjective term that designates years in which cultivation becomes an unproductive activity, crops fail, and the productivity of pastures is significantly

diminished. Drought is a constant menace, a fact of life with which rural dwellers

in arid regions must cope if they are to survive. The occurrence of drought is a

certainty, sooner or later; only its timing, duration, and severity are ever in doubt.

It is during a drought that ecosystem degradation in the form of devegetation and

soil erosion occurs at an accelerated pace.

Any management system that ignores the certainty of drought and fails to provide

for it ahead of time is doomed to fail in the long run. That provision may take the

form of grain or feed storage (as in the Biblical story of Joseph in Egypt), or

of pasture tracts kept in reserve for grazing when the regular pasture is played

out, or of the controlled migration of people and animals to other regions able to

accommodate them for the period of the drought.

There has been a prolonged period of drought in the Sahelian region of Africa

since the early 1970s (Fig. 2). Various hypotheses involving both natural and



Figure 2 Rainfall fluctuations 1901–1998, expressed as a regionally averaged standard deviation

(departure from the long-term mean divided by the standard deviation) for the Sahel. (Source: IPCC

WG II, 2001).



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anthropogenic factors have been advanced to explain the persistence of this

drought.

1. Atmospheric Dust

One hypothesis is that the recent droughts are due to a cooling of the land

masses of the Northern Hemisphere by about 0.3◦ C between 1945 and the early

1970s, owing to an increase in atmospheric dust from drylands, as well as from

air pollution and volcanic eruptions. The cooling may have changed the patterns

of air mass movement (Tegen et al., 1996). Evidence in support of this hypothesis

seems to be contradicted by the heavy rains that occurred in the Sahel during the

1950s when the Northern Hemisphere cooled, and by the severe Sahel drought

that occurred during the early 1980s when the Northern Hemisphere experienced

a warming.

2. Ocean–Atmosphere Dynamics

Another hypothesis links drought in the Sahel to changes in ocean–atmosphere

dynamics, specifically changes in sea–surface temperatures (SSTs) in the world’s

oceans. Such changes might tend to reduce the northward penetration of the

Intertropical Convergence Zone (ITCZ)—the great band of equatorial clouds

whose shifting pattern brings monsoonal rain to the humid tropics as well as

to the Sahel (Nicholson, 1986). Many studies have linked interannual variation of

SSTs and seasonal precipitation variability in the region (e.g., Druyan, 1987; 1989;

Folland et al., 1986; Lough, 1986; Rowell et al., 1995). Droughts in the Sahel tend

to be coincident with positive SST anomalies in Southern Hemisphere oceans and

the Indian Ocean, and negative SSTs in the Northern Hemisphere oceans, especially the subtropical North Atlantic Ocean. Abundant rain in the Sahel is often,

but not always, linked with SSTs of the opposite sign in the Atlantic and other

oceans (Lamb and Peppler, 1991, 1992). The interhemispheric SST gradient in the

Atlantic Ocean appears to be a key mechanism for precipitation in the Sahelian

latitudes (Fontaine and Janicot, 1996; Ward, 1998).

Warmer than normal SSTs in the tropical Pacific related to the El Ni˜no/Southern

Oscillation (ENSO) phenomenon have similarly been linked with droughts in

Australasia, India, South America, and Southern Africa, though these droughts

typically do not persist for more than one or two seasons. The Sahelian region

of Africa, on the other hand, has had many dry years that are not correlated

with Pacific SSTs, so the persistence of the Sahelian drought sets it apart from

droughts in other parts of the world. There does appear to be some ENSO-driven

teleconnection to drought in West Africa (e.g., Fontaine and Janicot, 1996), but

Janicot et al. (1996) show that the strength of the correlation of Sahel rainfall with

the Southern Oscillation Index (SOI) is quite variable. Hunt (2000) proposes a

mechanism by which tropical Pacific SSTs influence Sahel rainfall by modulating



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the North Atlantic Oscillation (NAO) via the Pacific–North America oscillation. Druyan and Hall (1996) suggest that extreme Pacific Ocean SST anomalies influence climate variability in the Sahel through wave disturbances of the

tropical easterly jet, with associated effects on convergence, humidity, and precipitation. These and other ocean–atmosphere relationships are being used to forecast

seasonal rainfall in the region (Nnaji, 2001; Ward, 1998).

3. Land–Surface Change

Still another hypothesis is that droughts can be caused or worsened by largescale changes in the land surface of Africa, and specifically by the deforestation and

overall denudation of the land (Charney, 1975; Sud and Molod, 1988). A process

may thus have started whereby the drought can become self-reinforcing. According

to the theory of “biophysical feedback,” losses of vegetative cover resulting from

the drought as well as from overcultivation, overgrazing, and deforestation, along

with the consequent increase of the dust content of the air, combine to enhance

the area’s reflectivity to incoming sunlight. That reflectivity, called “albedo,” may

rise from about 25% for a well-vegetated area to perhaps 35% or more for bare,

bright, sandy soil. As a larger proportion of the incoming sunlight is reflected

skyward rather than absorbed, the surface becomes cooler, and so the air in contact

with the surface has less tendency to rise and condense its moisture so as to yield

rainfall.

An additional effect of denudation is to decrease interception of rainfall by

vegetation and infiltration, while increasing surface runoff, thereby reducing the

amount of soil moisture available for evapotranspiration. Crops and grasses, which

have shallower roots than trees and in any case transpire less than the natural mixed

vegetation of the savanna, transpire even less when deprived of moisture during a

drought. The meteorological consequences of such changes have been explored in

modeling studies (Xue and Shukla, 1993). The hypothesis is that such changes may

have some effect on regional precipitation, since in many continental areas rainfall

is derived in significant part from water evaporated regionally. It proposes that the

biophysical and physical processes interact, as lower rainfall leads in turn to more

overgrazing, less regrowth of biomass, and further reduction in reevaporated rain

owing to the decline in soil moisture. Thus, the feedback hypothesis offers its own

explanation as to why the drought in the Sahel has tended to persist for so long.

There is still no conclusive evidence, however, that even large-scale changes in

land surface conditions do actually affect regional-scale climate (Nicholson et al.,

1998; Nicholson, 2000).

Key components in semiarid ecosystem degradation processes are increased

surface albedo (the reflectance of solar radiation) and increased generation of

dust, both of which are consequences of the exposure of bare, dry ground following

removal of the original vegetative cover.



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The albedo of a bare soil depends on the organic matter content and the mineral

composition of the topsoil. It also depends on the moisture content of the soil

surface. A moist soil is generally less reflective (i.e., “darker”) than a dry soil

(Hillel, 1998). Thus, Nicholson et al. (1998) found that near the southern edge

of the African Sahel (at a latitude of 15 degrees north), where the rainfall was

450 mm, the albedo was about 30%. However, near the northern boundary of

the Sahel, where the mean annual rainfall was only 200 mm, the surface albedo

was about 43%. Albedo is also affected, to some degree, by the smoothness or

roughness of the surface. Above all, however, it is affected by the vegetative cover

and its above-ground residues.

A widely cited hypothesis, promulgated by Charney (1975), Charney et al.

(1975), and Otterman (1974, 1977, 1981), suggested a feedback mechanism between land use and climate change. Specifically, they raised the possibility that an

increase in albedo resulting from anthropogenic denudation of the land can in turn

cause a diminution of rainfall. The mechanistic reasoning underlying this hypothesis is that an increase in surface reflectivity implies a reduction in the absorption of

solar energy, which entails a reduction in soil surface temperature and a consequent

reduction in sensible heating of the atmospheric layer in contact with the soil.

Proponents of the Charney hypothesis speculated that because a more highly

reflective surface should tend to be cooler, it should enhance the subsidence of

warm dry air and hence exacerbate the area’s aridity. This, in turn, reduces the

upward convective rise of warm air that normally results in condensation of vapor

and the formation of clouds. If the rise in albedo occurs over a large enough area,

it might thus reduce the regionally generated rainfall. Hence, so the reasoning

goes, surface denudation—which is the common effect of humans attempting to

survive with their livestock during a drought—is a self-reinforcing process that

exacerbates the very drought that initially induced it. Lare and Nicholson (1994)

imply that if desertification (i.e., denudation) is extreme, it could indeed evoke the

sort of feedback originally postulated by Charney.

A striking example of the albedo difference between grazed and ungrazed land

can be seen along the border between the western Negev of Israel and northeastern

Sinai of Egypt. The two contiguous areas of this arid region had been grazed

to the same degree until 1948, after which the newly established State of Israel

restricted grazing on its own side of the border. Consequently, the area within

Israel developed a relatively dense vegetative cover that appears much darker on

aerial and satellite photographs than the neighboring area on the Egyptian side.

According to Otterman (1977, 1981), the protected area of the Negev had an albedo

of 12% in the visible light and 24% in the infrared range, whereas the corresponding

values on the overgrazed Sinai side were as high as 40 and 53%.

Recent studies have shown, however, that the darkening is due not only to the

shrubs and grasses growing in the area but also to a biological crust (consisting of

algae, fungi, and cyanobacteria) that developed on the surface of the sandy soil.



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A vegetated area, though it appears darker in aerial photographs, may not be

warmer than a bare area, as long as the plants are actively transpiring. The process

of transpiration involves the absorption of latent heat and therefore tends to cool

the foliage. During the dry season, however, many of the indigenous plants curtail

transpiration so that they, along with the area as a whole, may indeed become

warmer than it would be if it were bare of vegetation. Otterman and Tucker (1985)

reported radiometric ground temperatures (evidently made in the summer season)

of about 40◦ C in Sinai and about 45◦ C in the Negev. More recently, Otterman et al.

(2001) reported that measurements made by NOAA satellites have consistently

shown the Negev to be warmer than Sinai by about 4.5◦ C during the generally dry

period of May to October. In contrast, Balling (1988) and Bryant et al. (1990) found

that the surface temperatures on the darker (more densely vegetated) U.S. side of

the Mexican border were 2 to 4◦ cooler than on the overgrazed and lighter-colored

Mexican side. The latter measurements may well have been made during a period

when the vegetation was actively transpiring, and hence produced a cooling effect

despite its lower albedo.

The persistent presence of dust in the atmosphere itself has an effect on an area’s

radiation balance (Fouquart et al., 1987). It tends to scatter and reflect a fraction of

the solar (shortwave) radiation, while absorbing longwave radiation emitted from

the Earth. In some cases, a turbid atmosphere may actually warm the air near the

ground, while in other cases it may do the opposite, depending on such variables

as its density as well as its reflective or absorptive properties.

Recent studies on the potential effects of aerosols on rainfall have advanced another feedback hypothesis. Denudation of an area’s vegetation is usually associated

with biomass burning, which releases smoke into the air. In addition, denudation

also results in deflation of the soil surface by wind erosion, which in turn creates a

“dust bowl” effect. Rosenfeld and Farbstein (1992), Rosenfeld (1999, 2000) and

Rosenfeld et al. (2001) have presented evidence that concentrations of such aerosols in the troposphere can suppress rainfall significantly.

The postulated mechanism is that moisture condensed on the dust particles forms

small droplets that do no coalesce sufficiently to generate rainfall. The detrimental

impact of dust on rainfall is less than that caused by smoke from biomass burning,

but the abundance of desert dust in the atmosphere renders it important. The

reduction of rainfall affected by desert dust can cause drier soil, which raises still

more dust, thus creating a feedback loop to further reduce rainfall.



B. PRIMARY PRODUCTION AND CARRYING CAPACITY

The biological productivity of any ecosystem is due to its primary producers

(known as autotrophs), which are the green plants growing in it. They alone are



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able to create living matter from inorganic raw materials. They do so by combining

atmospheric carbon dioxide with soil-derived water, thus converting radiant energy

from the Sun into chemical energy in the process of photosynthesis. Green plants

also respire, which is the reverse of photosynthesis, and in so doing they utilize

part of the energy to power their own growth. The net primary production then

becomes available for the myriad of heterotrophs, which subsist by consuming

(directly or indirectly) the products of photosynthesis. A stable ecosystem is one

in which production and consumption, synthesis and decomposition, are in balance

over an extended period of time.

When humans enter into an ecosystem and appropriate some of its products

for themselves, they normally do so in competition with, and at the expense of,

other potential consumers. Historically, in the hunter-gatherer phase of subsistence,

humans merely selected the most readily obtainable and useful (or desirable) plant

and animal products, leaving the remainder more or less intact. As their population

increased, humans began to manage the ecosystem so as to promote the production

of the goods they desired, and to suppress the species that competed for those

products. At a still later stage, humans tended to take over sections of the ecosystem

entirely, aiming to eradicate all species that did not serve them directly, and to

plant (and harvest) only the plants and animals they chose to domesticate. In the

process, the ecosystem’s biodiversity and natural productivity were profoundly

affected (Hillel, 1992).

As long as the tracts dominated by humans consist of small enclaves within a

large and continuous ecological domain, the ecosystem as a whole is not seriously

affected. However, as population grows progressively and human management

becomes both more extensive and more intensive, the ecological integrity of entire

regions is threatened. Especially affected are areas within the semiarid and arid

regions, which, because of the paucity of water and the fragility of the soil (typically

deficient in organic matter, structurally unstable, and highly erodible) are most

vulnerable and least resilient.

The term “carrying capacity” has been used to characterize an area’s productivity

in terms of the number of people or grazing animals it can support per unit area

on a sustainable basis (Cohen, 1995). However, the productive yield obtainable

from an area—and hence the number of people deriving their livelihood from it,

at whatever standard of life—depends on how the area is being used. Under the

hunter-gatherer mode of subsistence, an area may be able to carry only, say, 1 person

per square kilometer, whereas under shifting cultivation it may carry 10, and under

intensive agriculture perhaps 100. The more intensive forms of utilization also

involve inputs of capital, energy, and materials, such as fertilizers and pesticides,

that are brought in from the outside to enhance an area’s productivity. As the usable

productivity is affected by the availability of water (i.e., by seasonal rainfall),

it varies from year to year and from decade to decade, and a long-term average



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(as well as variability) is difficult to determine, especially given the prospect of

climate change. It is therefore doubtful that any given regions can be assigned an

intrinsic and objectively quantifiable “carrying capacity.”

Human pressure on the meager resources of arid ecosystems arises primarily

because of increasing population and the trend toward sedentarization of formerly

nomadic people. What typically follows includes the cutting down of wooded

plants for fuel, overcultivation, and overgrazing by livestock (especially in the

immediate peripheries of water supply centers such as wells, cisterns, or surfacewater impoundments). The denuded and pulverized soil surface then falls prey to

erosion by wind (during the dry season) and by water (during the rainy season).

Wind erosion blows away the fertile topsoil and greatly increases the content of dust

in the atmosphere. Water erosion also scours away the topsoil and often cuts into

the soil to produce deep gullies. During fallow periods, rainfall may also leach

away soluble nutrients. The net result can be an overall reduction in biological

productivity.

Over a long period of time (say, centuries), and in the absence of human intervention, even a severely eroded soil can recover. However, on the time scale of years

to a few decades, especially if humans continue to overgraze and/or overcultivate

the land, soil erosion may be, in effect, irreversible. One problem is to measure the

productivity of an area and its gradual change from year to year or from decade

to decade. Quite another problem is to assess the recoverability (or resiliency) of

an area following a partial loss of productivity, and the rate of potential recovery,

i.e., the time pattern of gradual restoration of productivity and the period needed

for its completion (Dregne, 1994).

Desertification from anthropogenic and climatic factors in Senegal caused a fall

in standing wood biomass of 26 kg C ha− 1 y−1 in the period 1956–1993, releasing

carbon at the rate of 60 kg C cap− 1 y− 1 (Gonzalez, 1997). The significance of these

quantities in the global balance may be small, but perhaps important nonetheless

(Bouwman, 1992; Lal, 2001).



C. SOIL DEGRADATION

An important criterion of soil degradation (itself a major component of land and

ecosystem degradation) is the loss of soil organic matter. Compared to soils in more

humid regions, those in arid regions tend to be inherently poor in organic matter

content, owing to the relatively sparse natural vegetative cover and to the rapid

rate of decomposition. The organic matter present is, however, vitally important

to soil productivity. Plant residues over the surface protect the soil from the direct

erosive impact of raindrops and from deflation by wind and help to conserve soil

moisture by minimizing evaporation. Plant and animal residues that are partially

decomposed and that are naturally incorporated into the topsoil help to stabilize its



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