(Advances in agronomy 77) donald l sparks (eds ) advances in agronomy elsevier, academic press (2002)

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 yearswith a below-aver

D V A N C E S I N

VO L U M E 7 7

Texas A&M University

Prepared in cooperation with the

American Society of Agronomy Monographs Committee

Diane E Stott, Chairman

Diane H RickerlWayne F RobargeRichard ShiblesJeffrey VolenecRichard E Zartman

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C ontents

CONTRIBUTORS ix

PREFACE xi

DESERTIFICATION ANDITSRELATION TOCLIMATE VARIABILITY ANDCHANGE Daniel Hillel and Cynthia Rosenzweig I Introduction 2

II Concepts and Definitions 3

III Processes 5

IV Case Study: The Sahel 16

V Monitoring Desertification 20

VI Future Climatic Variability and Change 21

VII Prospects 31

References 35

FATE ANDTRANSPORT OFVIRUSES INPOROUSMEDIA Yan Jin and Markus Flury I Introduction 40

II Characteristics of Viruses Relevant for Subsurface Fate and Transport 43

III Virus Sorption 45

IV Protein Sorption and Denaturation 57

V Virus Survival 64

VI The Role of the Gas–Liquid Interface in Protein/ Virus Inactivation 67

VII Transport of Viruses in Porous Media 70

VIII Indicators for Human Enteroviruses 86

IX Concluding Remarks 88

References 91

v

vi CONTENTS

Jan W Hopmans and Keith L Bristow

I Introduction 104

II Water Transport in Plants 109

III Linking Plant Transpiration with Assimilation 115

IV Transport of Water and Nutrients in the Plant Root 120

V Nutrient Uptake Mechanisms 126

VI Flow and Transport Modeling in Soils 132

VII Root Water Uptake 135

VIII Nutrient Uptake 145

IX Coupled Root Water and Nutrient Uptake 152

X Comprehensive Example 162

XI Prognosis 169

References 175

MICRONUTRIENTS INCROPPRODUCTION N K Fageria, V C Baligar, and R B Clark I Introduction 186

II Status in World Soils 188

III Soil Factors Affecting Availability 195

IV Factors Associated with Supply and Acquisition 206

V Improving Supply and Acquisition 227

VI Conclusion 246

References 247

SOILSCIENCE INTROPICAL ANDTEMPERATEREGIONS—SOME DIFFERENCES ANDSIMILARITIES Alfred E Hartemink I Introduction 270

II Soil Science in Temperate Regions 271

III Soil Science in Tropical Regions 274

IV Diametrically Opposite Interests 282

V Impact of Soil Science 285

VI Concluding Remarks 286

References 287

CONTENTS vii

B A Kimball, K Kobayashi, and M Bindi

I Introduction 294

II Methodology 295

III Results and Discussion of Crop Responses to Elevated CO2 326

IV Compendium and Conclusions 350

V Summary 359

References 360

THEAGRONOMIC ANDECONOMICPOTENTIAL OF BREAK CROPS FORLEY/ARABLEROTATIONS INTEMPERATE ORGANICAGRICULTURE M C Robson, S M Fowler, N H Lampkin, C Leifert, M Leitch, D Robinson, C A Watson, and A M Litterick I Introduction 370

II Crop Rotations as the Central Management Tool in Organic Farming 371

III Break Crops for Nutrient Management 391

IV Break Crops for Improving Soil Structure 403

V Break Crops for Weed Management 409

VI Break Crops for Pest and Disease Management 411

VII Conclusions 416

References 417

INDEX 429

This Page Intentionally Left Blank

C ontributors

Numbers in parentheses indicate the pages on which the authors’ contributions begin.

V C BALIGAR (185), Alternate Crops and Systems Research Laboratory,

Beltsville Agricultural Research Center, USDA-ARS, Beltsville, Maryland 20705

M BINDI (293), Department of Agronomy and Land Management, University of

Florence, 50144 Florence, Italy

K L BRISTOW (103), CSIRO Land and Water/CRC Sugar, Townsville Qld

4814, Australia

R B CLARK (185), Appalachian Farming Systems Research Center, USDA-ARS,

Beaver, West Virginia 25813

N K FAGERIA (185), National Rice and Bean Research Center of EMBRAPA,

Santo Antˆonio de Goi´as-GO, 75375-000, Brazil

M FLURY (39), Department of Crop and Soil Sciences, Washington State

Uni-versity, Pullman, Washington 99164

S M FOWLER (369), Welsh Institute of Rural Studies, University of Wales,

Aberystwyth, SY23 3AL, United Kingdom

A E HARTEMINK (269), International Soil Reference and Information Center

(ISRIC), 6700 AJ Wageningen, The Netherlands

D HILLEL (1), Columbia University Center for Climate Systems Research and

NASA Goddard Institute for Space Studies, New York, New York 10025

J W HOPMANS (103), Hydrology Program, Department of Land, Air and Water

Resources, University of California, Davis, California 95616

Y JIN (39), Department of Plant and Soil Sciences, University of Delaware,

Newark, Delaware 19717

B A KIMBALL (293), U.S Water Conservation Laboratory, USDA, Agricultural

Research Service, Phoenix, Arizona 85040

K KOBAYASHI (293), National Institute of Agro-Environmental Sciences,

Tsukuba, Ibaraki 305-8604, Japan

N H LAMPKIN (369), Welsh Institute of Rural Studies, University of Wales,

Aberystwyth, SY23 3AL, United Kingdom

C LEIFERT (369), Tesco Centre for Organic Agriculture, University of Newcastle,

Newcastle upon Tyne, NE1 7RU, United Kingdom

M LEITCH (369), Welsh Institute of Rural Studies, University of Wales,

Aberystwyth, SY23 3AL, United Kingdom

A M LITTERICK (369), Land Management Department, SAC, Craibstone

Estate, Bucksburn, Aberdeen AB21 9YA United Kingdom

ix

x CONTRIBUTORS

D ROBINSON (369), Department of Plant and Soil Science, Aberdeen University,

Aberdeen, AB24 5UA, United Kingdom

M C ROBSON (369), Department of Plant and Soil Science, Aberdeen University,

Aberdeen, AB24 5UA, United Kingdom

C E ROSENZWEIG (1), Columbia University Center for Climate Systems

Research and NASA Goddard Institute for Space Studies, New York, New York 10025

C A WATSON (369), Land Management Department, SAC, Craibstone Estate,

Bucksburn, Aberdeen AB21 9YA, United Kingdom

P reface

Volume 77 contains seven excellent reviews that should be of great interest tocrop, soil, and environmental scientists Chapter 1 is a timely review on desertifi-cation and its relation to climate variability and change that includes discussions

on processes, use of the Sahel as a case study, maintaining desertification, andfuture climatic variability and change Chapter 2 is a comprehensive review on avery timely topic—fate and transport of viruses in porous media Topics that arecovered include characteristics of viruses, virus sorption, protein sorption and de-naturation, survival of viruses, inactivation of viruses, and their transport Chapter 3discusses the current capabilities and future needs of root water and nutrient uptakemodeling including water transport and uptake in plants, nutrient uptake mecha-nisms, and flow and transport modeling in soils Chapter 4 reviews past and presentdevelopments in understanding the chemistry and fertility of micronutrients andtheir role in crop production Topics that are covered include status of micronu-trients in world soils, and factors affecting and ways to improve micronutrientsupply and availability Chapter 5 is an interesting review on the comparisons andcontrasts between tropical and temperate region soils Chapter 6 is an informativereview on the response of agricultural crops to free-air CO2enrichment Compre-hensive discussions are included on methodologies and plant responses to elevated

CO2along with effects on soil processes Chapter 7 provides a thorough treatment

on the agronomic and economic potential of break crops for ley/arable rotations

in temperate organic agriculture The use of break crops in nutrient management,soil structure improvement, weed management, and pest and disease management

is discussed

Many thanks to the authors for their superb contributions

DONALDL SPARKS

xi

This Page Intentionally Left Blank

D ESERTIFICATION IN R ELATION TO

Daniel Hillel and Cynthia Rosenzweig

Columbia University Center for Climate Systems Research and

NASA Goddard Institute for Space Studies

New York, New York 10025

1

Advances in Agronomy, Volume 77

Copyright 2002, Elsevier Science (USA) All rights reserved.

2 HILLEL AND ROSENZWEIG

I INTRODUCTION

Ecosystems in semiarid and arid regions around the world appear to be going various processes of degradation commonly described as desertification.According to UNEP (1992), all regions in which the ratio of total annual precip-itation to potential evapotranspiration (P/ET) ranges from 0.05 to 0.65 should beconsidered vulnerable to desertification Such regions constitute some 40% of theglobal terrestrial area, which totals about 130 million km2(13 billion ha) Dregne(1983) calculated that the arid, semiarid, and dry subhumid regions of the worldoccupy 12.1, 17.1, and 9.9% of the world’s total land area Relatively dry areascover much of northern Africa, southwestern Africa, southwestern Asia, centralAsia, northwestern India and Pakistan, southwestern United States and Mexico,western South America, and much of Australia (Fig 1, see color insert)

under-Arid and semiarid regions cover over a fourth of the world’s land area, andare home to nearly one-sixth of the world’s population (WRI, 2000) The totalpopulation of the world has doubled in the last four decades, resulting in thecurrent total of about 6 billion As of 1998, some 80% of humanity resided in theso-called developing countries, which contain only 58% of the total land area and54% of the total cropped area Moreover, many of the developing countries arelocated 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 Inthe same period, the world’s population has more than doubled, so the area ofcropland per capita has been reduced by nearly a half

At present, the annual growth rate of cropland (0.2%) is only one-seventh thegrowth in population (Lal, 1997), so the decline in arable land per capita is contin-uing 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 ofincreased 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 largepart, marginal lands of relatively low productivity and high vulnerability.Desertification is an emotive term, conjuring up the specter of a tide of sandswallowing fertile farmland and pastures The United Nations Environmental Pro-gramme (UNEP) sponsored projects in the early 1980s to plant trees along theedge of the Sahara, with the aim of warding off the invading sands While there areplaces where the edge of the desert can be seen encroaching on fertile land, the morepressing problem is the deterioration of the land due to human abuse in regionswell outside the desert The latter problem emanates not only from the desert butalso from the centers of population; not only from the spread of the sand dunesbut 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

DESERTIFICATION: CLIMATE VARIABILITY AND CHANGE 3

behind it The true challenge is not so much to stop the desert at the edge of asemiarid region as to protect the entire region from internal abuse of its vegetationand of its soil and water resources

A vicious cycle is already operating in many areas: as the land degrades, it isworked ever more intensively so its degradation accelerates; and as the returnsfrom “old” land diminish, “new” land is brought under cultivation or grazed byencroachment onto marginal or submarginal areas But attempts to encapsulatethese complex problems in the catchall term “desertification” may have obscuredits true character and confused the search for its amelioration

In this paper, we review the concepts, definitions, and processes pertinent todesertification, and offer an alternative, more inclusive term, namely, “semi-aridecosystem degradation.” We use the long-term drought in the Sahelian region ofAfrica as a case study for analyzing the complex set of climatic, biophysical,and social factors that interweave to create the process of semiarid ecosystemdegradation, and we evaluate current monitoring techniques, including remotesensing We next consider the potentialities and hazards of irrigation development

as a possible means to improve agricultural production in semiarid regions We thenask the question, “How might global climate change affect the Sahelian region ofAfrica?” and analyze a set of recent projections derived from global climate changescenarios, in light of the region’s vulnerabilities Finally, we offer our views onprospects 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 ing the actual and potential biological productivity of ecosystems in semiarid andarid regions The term desertification (or desertization) was apparently coined bythe French ecologist LeHouerou (1977) to characterize what was perceived to

involv-be a northward advance of the Sahara in Tunisia and Algeria It gained currencyfollowing the severe drought that afflicted the Sud region of Africa in the early1970s, and again in the 1980s, during which the Sahara was reported to be ad-vancing southward into the Sahelian zone as well For example, Lamprey (1975)estimated that during the period from 1958 to 1975, while mean annual rainfalldiminished by nearly 50%, the boundary between the Sahara and the Sahel hadshifted southward by nearly 100 km

As defined in recent dictionaries, desertification is the process by which an areabecomes (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 tosupport 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

4 HILLEL AND ROSENZWEIG

provide adequate subsistence to humans This is a very qualitative definition, sincenot 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, themodes of human subsistence and levels of consumption differ greatly from place

to place

The United Nations Conference on Desertification (UNCOD) was held inNairobi in 1977 It was convened in response to the severe drought that had befallenthe Sahel from the late 1960s through most of the 1970s Its report defined deser-tification as “the diminution or destruction of the biological potential of land thatcan lead ultimately to desert-like conditions under the combined pressure ofadverse and fluctuating climate and excessive exploitation.” That statement leavesopen 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 landdegradation 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 ofthose various processes An alternative approach would be to define the processesthemselves and characterize the degree of degradation at which their separate orcombined effects may be considered to have become irreversible

In recent years, the very term desertification has been called into question asbeing too vague, and the processes it purports to describe too ill-defined Somecritics 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 itcan 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 asfollows: “Land degradation in arid, semiarid, and dry subhumid areas resultingfrom various factors, including climate variations and human activities” (Warren,1996) That definition still does not either clarify the relative importance of thetwo potential causes or imply the possibility that they may be interactive It merelyshifts 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 onthe 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 withrespect to one function and not necessarily with respect to another For exam-ple, a tract of land may continue to function hydrologically—to regulate infiltra-tion, runoff generation, and groundwater recharge—even if its vegetative cover is

DESERTIFICATION: CLIMATE VARIABILITY AND CHANGE 5

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 dation.” A semiarid ecosystem encompasses the diverse biotic community living

degra-in this given domadegra-in Included degra-in this community is the host of plants, animals,and microorganisms that share the habitat and that interact with one anotherthrough such modes as competition or symbiosis, predation, and parasitism Italso 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 anartificially managed one, such as an agroecosystem

Each ecosystem performs a multiplicity of ecological functions Included amongthese are photosynthesis, absorption of atmospheric carbon and its incorporationinto biomass and the soil, emission of oxygen, regulation of temperature and thewater cycle, as well as the decomposition of waste products and their transmutationinto nutrients for the perpetuation of diverse interdependent forms of life Integratedecosystems may thus play a vital role in controlling global warming and in absorb-ing and neutralizing pollutants that might otherwise accumulate to toxic levels

An agroecosystem is a portion of the landscape that is managed for the economicpurpose of agricultural production The transformation of a natural ecosystem into

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

sus-to maintain biodiversity and sus-to perform vital ecological functions

In too many cases, however, the requirements of sustainability fail, especiallywhere agricultural systems expand progressively at the expense of the remainingmore or less natural ecosystems The appropriation of ever-greater sections of theremaining native habitats, impelled by the increase of population as well as bythe 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 system can be masked by increasing the inputs of fertilizers, pesticides, water, andtillage Sooner or later, however, if such destructive effects as organic matter loss,erosion, leaching of nutrients and salination continue, the degradation is likely toreach a point at which its effects are difficult to overcome either ecologically oreconomically

agroeco-III PROCESSES

Key processes related to desertification include drought, primary production andcarrying capacity, soil degradation, and water resources The role of social factors

is also important

6 HILLEL AND ROSENZWEIG

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 yearswith 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 wellabove realistic expectations for rainfall in most years More than 90% of the totalvariation in annual rainfall can generally be encompassed within a range betweenone-half and twice the mean

The variability in biologically effective rainfall is yet more pronounced, asyears with less rain are usually characterized by greater evaporative demand, sothe 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 evenabove-average rainfall may fail to produce adequate yields if the rain occurs asjust a few large storms with long dry periods between them

In semiarid agricultural regions, “drought,” like desertification, is a broad, what subjective term that designates years in which cultivation becomes an un-productive activity, crops fail, and the productivity of pastures is significantlydiminished Drought is a constant menace, a fact of life with which rural dwellers

some-in arid regions must cope if they are to survive The occurrence of drought is acertainty, 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 andsoil erosion occurs at an accelerated pace

Any management system that ignores the certainty of drought and fails to providefor it ahead of time is doomed to fail in the long run That provision may take theform 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 playedout, or of the controlled migration of people and animals to other regions able toaccommodate them for the period of the drought

There has been a prolonged period of drought in the Sahelian region of Africasince 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).

DESERTIFICATION: CLIMATE VARIABILITY AND CHANGE 7

anthropogenic factors have been advanced to explain the persistence of thisdrought

1 Atmospheric Dust

One hypothesis is that the recent droughts are due to a cooling of the landmasses of the Northern Hemisphere by about 0.3◦C between 1945 and the early1970s, owing to an increase in atmospheric dust from drylands, as well as fromair 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 the1950s when the Northern Hemisphere cooled, and by the severe Sahel droughtthat 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–atmospheredynamics, specifically changes in sea–surface temperatures (SSTs) in the world’soceans Such changes might tend to reduce the northward penetration of theIntertropical Convergence Zone (ITCZ)—the great band of equatorial cloudswhose shifting pattern brings monsoonal rain to the humid tropics as well as

to the Sahel (Nicholson, 1986) Many studies have linked interannual variation ofSSTs 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 andthe Indian Ocean, and negative SSTs in the Northern Hemisphere oceans, espe-cially 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 otheroceans (Lamb and Peppler, 1991, 1992) The interhemispheric SST gradient in theAtlantic Ocean appears to be a key mechanism for precipitation in the Sahelianlatitudes (Fontaine and Janicot, 1996; Ward, 1998)

Warmer than normal SSTs in the tropical Pacific related to the El Ni˜no/SouthernOscillation (ENSO) phenomenon have similarly been linked with droughts inAustralasia, India, South America, and Southern Africa, though these droughtstypically 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 correlatedwith Pacific SSTs, so the persistence of the Sahelian drought sets it apart fromdroughts in other parts of the world There does appear to be some ENSO-driventeleconnection 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 amechanism by which tropical Pacific SSTs influence Sahel rainfall by modulating

8 HILLEL AND ROSENZWEIG

the North Atlantic Oscillation (NAO) via the Pacific–North America tion Druyan and Hall (1996) suggest that extreme Pacific Ocean SST anoma-lies influence climate variability in the Sahel through wave disturbances of thetropical easterly jet, with associated effects on convergence, humidity, and precip-itation These and other ocean–atmosphere relationships are being used to forecastseasonal rainfall in the region (Nnaji, 2001; Ward, 1998)

oscilla-3 Land–Surface Change

Still another hypothesis is that droughts can be caused or worsened by

large-scale 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 processmay thus have started whereby the drought can become self-reinforcing According

to the theory of “biophysical feedback,” losses of vegetative cover resulting fromthe drought as well as from overcultivation, overgrazing, and deforestation, alongwith the consequent increase of the dust content of the air, combine to enhancethe area’s reflectivity to incoming sunlight That reflectivity, called “albedo,” mayrise 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 reflectedskyward rather than absorbed, the surface becomes cooler, and so the air in contactwith the surface has less tendency to rise and condense its moisture so as to yieldrainfall

An additional effect of denudation is to decrease interception of rainfall byvegetation and infiltration, while increasing surface runoff, thereby reducing theamount of soil moisture available for evapotranspiration Crops and grasses, whichhave shallower roots than trees and in any case transpire less than the natural mixedvegetation of the savanna, transpire even less when deprived of moisture during adrought The meteorological consequences of such changes have been explored inmodeling studies (Xue and Shukla, 1993) The hypothesis is that such changes mayhave some effect on regional precipitation, since in many continental areas rainfall

is derived in significant part from water evaporated regionally It proposes that thebiophysical and physical processes interact, as lower rainfall leads in turn to moreovergrazing, less regrowth of biomass, and further reduction in reevaporated rainowing to the decline in soil moisture Thus, the feedback hypothesis offers its ownexplanation 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 increasedsurface albedo (the reflectance of solar radiation) and increased generation ofdust, both of which are consequences of the exposure of bare, dry ground followingremoval of the original vegetative cover

DESERTIFICATION: CLIMATE VARIABILITY AND CHANGE 9

The albedo of a bare soil depends on the organic matter content and the mineralcomposition of the topsoil It also depends on the moisture content of the soilsurface 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 ofthe Sahel, where the mean annual rainfall was only 200 mm, the surface albedowas about 43% Albedo is also affected, to some degree, by the smoothness orroughness of the surface Above all, however, it is affected by the vegetative coverand 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 tween land use and climate change Specifically, they raised the possibility that anincrease in albedo resulting from anthropogenic denudation of the land can in turncause a diminution of rainfall The mechanistic reasoning underlying this hypothe-sis is that an increase in surface reflectivity implies a reduction in the absorption ofsolar energy, which entails a reduction in soil surface temperature and a consequentreduction in sensible heating of the atmospheric layer in contact with the soil.Proponents of the Charney hypothesis speculated that because a more highlyreflective surface should tend to be cooler, it should enhance the subsidence ofwarm dry air and hence exacerbate the area’s aridity This, in turn, reduces theupward convective rise of warm air that normally results in condensation of vaporand the formation of clouds If the rise in albedo occurs over a large enough area,

be-it might thus reduce the regionally generated rainfall Hence, so the reasoninggoes, surface denudation—which is the common effect of humans attempting tosurvive with their livestock during a drought—is a self-reinforcing process thatexacerbates the very drought that initially induced it Lare and Nicholson (1994)imply that if desertification (i.e., denudation) is extreme, it could indeed evoke thesort of feedback originally postulated by Charney

A striking example of the albedo difference between grazed and ungrazed landcan be seen along the border between the western Negev of Israel and northeasternSinai 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 Israelrestricted grazing on its own side of the border Consequently, the area withinIsrael developed a relatively dense vegetative cover that appears much darker onaerial 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 correspondingvalues 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 theshrubs and grasses growing in the area but also to a biological crust (consisting ofalgae, fungi, and cyanobacteria) that developed on the surface of the sandy soil

10 HILLEL AND ROSENZWEIG

A vegetated area, though it appears darker in aerial photographs, may not bewarmer 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 coolthe foliage During the dry season, however, many of the indigenous plants curtailtranspiration so that they, along with the area as a whole, may indeed becomewarmer 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 consistentlyshown 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 ofthe Mexican border were 2 to 4◦cooler than on the overgrazed and lighter-coloredMexican side The latter measurements may well have been made during a periodwhen the vegetation was actively transpiring, and hence produced a cooling effectdespite 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 fromthe Earth In some cases, a turbid atmosphere may actually warm the air near theground, 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 other feedback hypothesis Denudation of an area’s vegetation is usually associatedwith biomass burning, which releases smoke into the air In addition, denudationalso results in deflation of the soil surface by wind erosion, which in turn creates a

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

Rosenfeld et al (2001) have presented evidence that concentrations of such

aero-sols in the troposphere can suppress rainfall significantly

The postulated mechanism is that moisture condensed on the dust particles formssmall droplets that do no coalesce sufficiently to generate rainfall The detrimentalimpact 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 Thereduction of rainfall affected by desert dust can cause drier soil, which raises stillmore dust, thus creating a feedback loop to further reduce rainfall

B PRIMARYPRODUCTION ANDCARRYINGCAPACITY

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

DESERTIFICATION: CLIMATE VARIABILITY AND CHANGE 11

able to create living matter from inorganic raw materials They do so by combiningatmospheric carbon dioxide with soil-derived water, thus converting radiant energyfrom the Sun into chemical energy in the process of photosynthesis Green plantsalso respire, which is the reverse of photosynthesis, and in so doing they utilizepart of the energy to power their own growth The net primary production thenbecomes 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 balanceover an extended period of time

When humans enter into an ecosystem and appropriate some of its productsfor 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) plantand animal products, leaving the remainder more or less intact As their populationincreased, 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 thoseproducts At a still later stage, humans tended to take over sections of the ecosystementirely, aiming to eradicate all species that did not serve them directly, and toplant (and harvest) only the plants and animals they chose to domesticate In theprocess, the ecosystem’s biodiversity and natural productivity were profoundlyaffected (Hillel, 1992)

As long as the tracts dominated by humans consist of small enclaves within alarge and continuous ecological domain, the ecosystem as a whole is not seriouslyaffected However, as population grows progressively and human managementbecomes both more extensive and more intensive, the ecological integrity of entireregions is threatened Especially affected are areas within the semiarid and aridregions, which, because of the paucity of water and the fragility of the soil (typicallydeficient in organic matter, structurally unstable, and highly erodible) are mostvulnerable 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 obtainablefrom 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 thehunter-gatherer mode of subsistence, an area may be able to carry only, say, 1 personper square kilometer, whereas under shifting cultivation it may carry 10, and underintensive agriculture perhaps 100 The more intensive forms of utilization alsoinvolve 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 usableproductivity 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

12 HILLEL AND ROSENZWEIG

(as well as variability) is difficult to determine, especially given the prospect ofclimate change It is therefore doubtful that any given regions can be assigned anintrinsic and objectively quantifiable “carrying capacity.”

Human pressure on the meager resources of arid ecosystems arises primarilybecause of increasing population and the trend toward sedentarization of formerlynomadic people What typically follows includes the cutting down of woodedplants for fuel, overcultivation, and overgrazing by livestock (especially in theimmediate peripheries of water supply centers such as wells, cisterns, or surface-water impoundments) The denuded and pulverized soil surface then falls prey toerosion 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 intothe soil to produce deep gullies During fallow periods, rainfall may also leachaway soluble nutrients The net result can be an overall reduction in biologicalproductivity

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

interven-to a few decades, especially if humans continue interven-to overgraze and/or overcultivatethe land, soil erosion may be, in effect, irreversible One problem is to measure theproductivity 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 neededfor its completion (Dregne, 1994)

Desertification from anthropogenic and climatic factors in Senegal caused a fall

in standing wood biomass of 26 kg C ha− 1y−1in the period 1956–1993, releasingcarbon at the rate of 60 kg C cap− 1y− 1(Gonzalez, 1997) The significance of thesequantities in the global balance may be small, but perhaps important nonetheless(Bouwman, 1992; Lal, 2001)

C SOILDEGRADATION

An important criterion of soil degradation (itself a major component of land andecosystem degradation) is the loss of soil organic matter Compared to soils in morehumid regions, those in arid regions tend to be inherently poor in organic mattercontent, owing to the relatively sparse natural vegetative cover and to the rapidrate of decomposition The organic matter present is, however, vitally important

to soil productivity Plant residues over the surface protect the soil from the directerosive impact of raindrops and from deflation by wind and help to conserve soilmoisture by minimizing evaporation Plant and animal residues that are partiallydecomposed and that are naturally incorporated into the topsoil help to stabilize its

DESERTIFICATION: CLIMATE VARIABILITY AND CHANGE 13

structural aggregates, which in turn enhance infiltrability, minimize water loss byrunoff, and enable seed germination and root growth The organic matter presentalso contributes to soil fertility by releasing nutrients

When the natural vegetative cover is removed, and especially when the soil istilled repeatedly, there follows a rapid process of organic matter decomposition anddepletion Accelerated erosion also removes the layer of topsoil that is richest inorganic matter Consequently, the destabilized soil tends to form a surface crust thatfurther inhibits infiltration Water losses by both runoff and evaporation increase.Moreover, the soil loses an important source of nutrients

These destructive processes can be countered or ameliorated by methods ofconservation management, including minimum or zero tillage, maintenance ofcrop residues, the periodic inclusion of green manures in the crop rotation (ASA,1983; USDA, 1991), and agroforestry (Nair, 1993)

The destructive processes induced by soil mismanagement, and—in contrast—the constructive processes induced by conservation management, though seem-ingly local, may have—when practiced on a regional scale—an impact on climate.Soils subject to accelerated decomposition of organic matter tend to release carbondioxide and thus contribute to the enhanced greenhouse effect Conversely, soilsthat are being enriched with organic matter can absorb and sequester quantities ofcarbon that are extracted from the atmosphere in photosynthesis (Bouman, 1992;Lal, 2001)

be poorly distributed during the year and variable from year to year Wherevertraditional rain-fed farming is a high-risk enterprise owing to scarce or uncertainprecipitation, irrigation can help to ensure stable production

Irrigation has long played a key role in feeding expanding populations and isexpected to play a still greater role in the future It not only raises the yields ofspecific crops but also prolongs the effective crop-growing period in areas with dryseasons, thus permitting multiple cropping (two, three, or even four, crops per year)where only a single crop could be grown otherwise With the security provided byirrigation, additional inputs needed to intensify production further (pest control,fertilizers, improved varieties, and better tillage) become economically feasible.Irrigation reduces the risk of these expensive inputs being wasted by crop failureresulting from lack of water

14 HILLEL AND ROSENZWEIG

Although irrigated land amounts to only some 17% of the world’s cropland,

it contributes well over 30% of the total agricultural production That vitalcontribution is even greater in arid regions, where the supply of water by rainfall

is least, even as the demand for water imposed by the bright Sun and the dry air isgreatest

The practice of irrigation consists of applying water to the part of the soil profilethat serves as the root zone, for the immediate and subsequent use of the crop.Inevitably, however, irrigation also entails the addition of water-borne salts Manyarid-zone soils contain natural reserves of salts, which are also mobilized by irriga-tion Underlying groundwater in such zones may further contribute salts to the rootzone by capillary rise Finally, the roots of crop plants typically extract water fromthe soil while leaving most of the salts behind, thus causing them to accumulate.The problem is age-old From its earliest inception in the Fertile Crescent, somesix or more millennia ago, irrigated agriculture, especially in ill-drained rivervalleys, has induced processes of degradation that have threatened its sustainability

The artificial application of water to the land has ipso facto caused the water table

to rise, which in turn induced the self-destructive twin phenomena of waterloggingand salination (Fig 3)

Some investigators include the degradation of irrigated lands, generally by terlogging and salination, in the category of desertification (Dregne and Chou,1993) Though the processes taking place differ fundamentally from those in

wa-Figure 3 Waterlogging and salination The rising water-table in poorly drained land saturates the soil, impedes aeration, and infuses the root zone with salts (Source: Hillel, 1998).

DESERTIFICATION: CLIMATE VARIABILITY AND CHANGE 15

rainfed lands, the damage done to injudiciously irrigated lands is indeed in thecategory of ecosystem degradation (Hillel, 2000)

Processes occurring off-site (upstream as well as downstream of the irrigatedarea) strongly affect the sustainability of irrigation For example, denudation ofupland watersheds by forest clearing, cultivation, and overgrazing induces ero-sion and the consequent silting of reservoirs and canals, thereby reducing thewater supply The construction of reservoirs often causes the submergence of nat-ural habitats as well as of valuable scenic and cultural sites Concurrently, thedownstream disposal of drainage from irrigated land tends to pollute aquifers,streams, estuaries, and lakes with salts, nutrients, and pesticides Finally, the irri-gation system itself may harbor and spread water-borne diseases, thus endangeringpublic health So the very future of irrigation is threatened by land degradation aswell as by dwindling water supplies and deteriorating water quality

In the last few decades, even as great investments have been made in the ment of new irrigation projects, the total area under irrigation has hardly expanded.That is because large tracts of irrigated land have degenerated to the point of be-ing rendered uneconomic to cultivate, or—in extreme cases—have become totallysterile The dilemma of land deterioration is not exclusive to the less developednations, where it has caused repeated occurrences of famine It applies to an equalextent to such technologically advanced countries as Australia, the United States,and the central Asian regions of the former Soviet Union So pervasive and inherentare the problems that some critics doubt whether irrigation can be sustained in anyone area for very long—and they have much evidence to support their pessimism.Irrigated agriculture can be sustained, albeit at a cost The primary cost is effec-tive salinity control, along with the prevention of upstream, on-site, and down-stream environmental damage Although there will be cases where the costs ofcontinued irrigation (especially if severe damage has already occurred) may beprohibitive in practice, in most instances the cost is indeed well worth bearing.Investing in the maintenance of irrigation can result in improved economic andsocial well-being as well as in a healthier environment

develop-Developing and implementing an effective salinity control program require anunderstanding of complex interrelationships with multiple causes, effects, andfeedbacks, operating at different scales of space and time Except in the mostproblematic locations, irrigation can be maintained, provided that water supplies

of adequate quality can be assured, the salt balance and hence the productivity ofthe land can be maintained, the drainage effluent can be disposed of safely, andthe economic returns can justify the costs

The sine qua non of ensuring the sustainability of irrigation is the timely

instal-lation and continuous operation of a drainage system to dispose safely of excesssalts All too often, drainage creates an off-site problem, beyond the on-site cost

of installation and maintenance, since the discharge of briny effluent can degrade

16 HILLEL AND ROSENZWEIG

the quality of water along its downstream route Where access to the open sea isfeasible, solving the problem is likely to be easier than in closed basins or in areasfar from the sea In those cases, the disposal terminus (whether a lake or an aquifer)may eventually become unfit for future use, hence the importance of reducing thevolume and salinity of effluents

Much can be achieved by improving the efficiency of water use Modernirrigation technology offers the opportunity to conserve water through reducedtransport and application losses coupled with increased efficiency of utilization(Hillel, 1997)

E SOCIALFACTORS

Social factors are necessarily involved in both semiarid ecosystem conservationand its inverse degradation Farmers who do not have tenure to the land are not

likely to invest in its conservation or improvement (Syers et al., 1996) Neither are

communities that lack stable institutional structure likely to establish and maintainessential infrastructure and services to enable, encourage, and coordinate farmers’efforts to implement land improvement and conservation measures (especially oncommunal lands) And no effective action at all may be possible in the absence

of a proactive governmental policy, including the provision of credit or subsidies,professional guidance and training, as well as the preparation and implementation

of national and regional drought contingency plans for both farmers and herders(Jolly and Torrey, 1993) The conservation of land resources is a collective societalconcern, not merely a private concern of the people utilizing the land directly(Sen, 1981)

Finally, there is the most difficult, yet inescapable issue of population numbers

No system of management, however efficient, can be sustained if the populationcontinues to grow without limit A crucial aspect of population control is theempowerment of women, through education and equal rights (social, political,and economic), as full participants in the management of their societies’ physical,

biological, and human resources (Arizpe et al., 1994).

IV CASE STUDY: THE SAHEL

The debate over desertification has tended to focus upon a particular region

of Africa south of the Sahara called the Sahel The word sahil in Arabic means

a plain, a coast, or a border Used geographically, the term refers to a band ofterritory approximately 200–400 km wide, centered on latitude 15◦N, lying justsouth of the Sahara and stretching across most of the width of Africa The Sahel

DESERTIFICATION: CLIMATE VARIABILITY AND CHANGE 17

covers well over 2 million km2and constitutes significant portions of Senegal,Gambia, Mauritania, Mali, Burkina Faso, Niger, Chad, and the Sudan By somedefinitions, the Sahel covers a wider latitudinal belt that extends roughly between

10 and 20◦N into parts of the Ivory Coast, Ghana, Benin, Togo, Nigeria, Cameroon,and Ethiopia For our climate change analysis, we utilize the broader designation.The mean annual temperature of the more broadly defined Sahel region rangesfrom 15 to 30◦C, while rainfall varies from about 100 mm in the north to about

1000 mm in the south (Fig 4, see color insert) The climatic regime depends on theexcursions of the Intertropical Convergence Zone (ITCZ) and the African jetstreamand is highly variable The rainstorms are erratic and occasionally violent, and theirvariability increases from south to north The rainy season, lasting 3 to 5 months,alternates with an extended, unrelieved, dry season The periodic occurrence ofdrought is an inherent feature of this harsh climate and successive years of droughtmay be followed by years with torrential rains

The soils of the Sahel are generally of low fertility, particularly poor in phates and nitrogen, structurally unstable, with low humus content and low waterretention Hardened layers, laterization, and vulnerability to wind and watererosion are common features Water for irrigation is available in some placesfrom streams and rivers (Senegal, Niger, and Chari-Logone), and possibly fromgroundwater aquifers, but the area under irrigation is rather small and the irrigationpotential has not been fully developed The vegetation is a mixed stand of trees,shrubs, and perennial and annual grasses, typical of savannas and steppes

phos-In the African Sahel, and similarly in other regions, the establishment and solidation of European colonial rule in the 19th century brought about fundamentalchanges that subsequently were to modify the relation of indigenous societies totheir environment After an initial period of warfare, the area was stabilized andsecurity conditions improved So did medical and veterinary facilities includingvaccination services These interventions allowed human and animal populations

con-to increase rapidly during favorable periods At the same time, traditional patterns

of land utilization and tenure, and of migration and transhumance, were disrupted

by arbitrary boundaries and by imposed political and economic structures.Although the available historical records are rather meager, they suggest thatsimilar major droughts, lasting 12–15 years, evidently occurred in the 1680s, themid-1700s, the 1820s and 1830s, and the 1910s In the first half of the 19th century,the level of Lake Chad apparently declined for 2 or 3 decades, to about where itwas during the drought of the mid-1980s The geological record shows several

similar falls of the lake level in the past 600 years (Rind et al., 1989) On the other

hand, we know that the Sahel has also gone through much wetter periods in the 9ththrough the 13th centuries, and 16th through the 18th centuries; also from 1870 to

1895, and during the 1950s The area near Timbuktu, which now has only 100 mm

of annual rainfall, was humid enough in the late 19th century for wheat to begrown

18 HILLEL AND ROSENZWEIG

The fortuitous occurrence of favorable weather conditions during most of the20th century, and particularly during the abnormally wet period of 1950–1965following the attainment of independence by the region’s states, obscured theeffects of the changes imposed earlier Given good rains, freshly cleared landsproduced good harvests even in areas that normally would have been consideredill-suited for cultivation Instead of deliberately keeping areas underpopulated andproviding for eventual drought, the authorities in some cases encouraged farmers

to move into marginally arable lands Pastoral tribes were then pushed furtherinto even more marginal grazing lands, where they were provided with water bymeans of mechanically powered tubewells Inevitably, drought struck As access

to the wells was free to all, traditional control over management of pastures waseliminated The overall result has been an increase in herd numbers, a decrease

in pasture through more widespread cropping, and an abandonment of traditionalrange management mechanisms (Hillel, 1992)

The Sahel region seems to have undergone a general decline of rainfall since thelate 1960s (see Fig 2) There have been several unusually prolonged and severedroughts since then, in marked contrast with the preceding 20 relatively wet years

(Rind et al., 1989) At each drought, people may remain on the land in the hope that

the rains might soon return, and while waiting, they do what they can to save theirherds of goats, sheep, cattle, and camels When the grass plays out, they may try

to increase their animals’ intake of browse by lopping trees already weakened bylack of soil moisture They also continue to collect firewood from the sparse shrubsand trees When many months elapse without rain, the vegetation dies out, whilethe soil—desiccated, pulverized, and trampled—begins to blow away in the wind.And when a sudden rainstorm visits the area, it scours and gullies the erodibletopsoil Finally, the people are left with no choice but to abandon their traditionalhomes and villages and migrate to the cities, where they seek employment or reliefassistance

The drought of 1968–1973 highlighted the basic problems that had been too longignored Family and tribal structures and their autonomous traditional practices

of resource management and land tenure had been broken down, so the localpopulation was now unable to cope with the drought on its own The plight ofthe Sahel was exacerbated by the drought’s recurrence, in even more severe form,during the early 1980s Consequently, sections of the region were almost emptied

of inhabitants, as thousands of people migrated from their villages to refugeecamps and overcrowded cities Semiarid ecosystem degradation has been linked

to migrations that may have displaced 3% of the population of Africa since the1960s (Westing, 1994)

Some of the Sahel’s problems have been compounded by ill-conceived velopment efforts Some planners seem to have misunderstood the logic of tra-ditional production systems, and have underestimated the difficulty of improvingthem They also failed to foresee the potentially negative consequences of intended

de-DESERTIFICATION: CLIMATE VARIABILITY AND CHANGE 19

improvements brought in under the imprimatur of “technology transfer.” In manycases, they seem to have neglected the fundamental significance of rainfall vari-ability the probability of drought, and the principle of risk avoidance Some ofthe traditional production systems were based on probable outcomes and weretherefore better able to contend with droughts (though, of course, no productionsystem can cope with a severe drought prolonged over several successive years).The population of the western African regions of the Sahel and the regionslying south of it, called the Sahelo-Sudanian and Sudanian zones, was estimated

at 31 million in 1980 Though the population density is still fairly low throughout,varying from fewer than 2 per square kilometer in Mauritania to nearly 60 inGambia, it has been increasing steadily In recent decades, population growth rateshave been close to 3% per annum The area has reached 54 million inhabitants bythe year 2000 (75% more than in 1980, and almost three times as many as in 1961).The urban population, incidentally, has been swelling at rates exceeding 5% peryear, in large part from the influx of people driven off the land because of drought.Gonzalez (2001) has measured declines in forest species richness and tree den-sity in the West African Sahel in the last half of the 20th century Such changeshave apparently shifted vegetation zones in Northwest Senegal towards areas ofhigher rainfall at an average annual rate of 500–600 m Xerophytic Sahel specieshave expanded in the north, while mesic Sudan and Guinean species have re-tracted to the south Rainfall and temperature are identified as the most significantfactors explaining tree and shrub distribution The changes have also decreasedhuman carrying capacity below actual population densities The rural population

of 45 people per square kilometer exceeded the 1993 carrying capacity of firewoodfrom shrubs of 13 people per square kilometer Gonzalez advocates the traditionalpractice of regeneration of local species over the planting of exotic species Inthe practice of native regeneration, farmers select small trees in their field, protectthem, and prune them to promote rapid growth of the apical meristem

The continued destruction of the rural environment is likely to result in furtherurbanization As the demand for food, other agricultural products, and firewoodcontinues to mount, it is likely to generate greater exploitation of the region’smeager resources Policy options include social and educational programs thatfoster reduced population growth rates and improvements in rural productivity.The latter can be achieved by intensifying the use and conservation of favorablelands, developing the irrigation potential, improving management of range lands,reforesting marginal lands, and raising the efficiency of household energy use so

as to curtail the burning of firewood Above all, adequate provision must be madefor the possible occurrence of drought in the future

Fortunately, the land itself exhibits a remarkable resilience It had suffered manydroughts in the past, and when the rains subsequently returned, so eventually didmuch of the vegetation In large measure, the recent damage was temporary, and theland can recover if it is rehabilitated, or at least left undisturbed for a sufficient time

20 HILLEL AND ROSENZWEIG

V MONITORING DESERTIFICATION

The techniques of remote sensing have made possible the monitoring of changes

to ecosystems on a regional scale (Fig 5, see color insert) (e.g., Justice, 1986) ies based on the remote sensing of the African Sahel were reported by Nicholson

Stud-et al (1998) The authors state that there has been no progressive change of either

the Saharan boundary or of vegetation cover in the Sahel during the 16-year period

of the study, nor has there been a systematic reduction of productivity as assessed

by the water-use efficiency of the vegetation cover

In principle, statistical criteria designed to test the probability levels of ferences (between sites or between successive measurements on the same site)should not be used to “prove” the opposite, namely that there are no differences

dif-In this case, absence of evidence of change by one criterion or another is not initself evidence of absence of any change Measurements (partly indirect) made atvarious times on large areas may have obscured subtle local changes that may haveoccurred in specific sites Generality may tend to ignore specificity The authorsthemselves report that while their data “showed little change in surface albedoduring the years analyzed, a change in albedo of up to 0.10% since the 1950s isconceivable.” (The figure 0.10% is apparently a misprint of what may have been

a 1% or a 10% change in albedo)

NDVI is the ratio between the red and near-red infrared reflectance bands,obtained from advanced high-resolution radiometer data from the polar-orbiting

satellite of the National Oceanic and Atmospheric Administration (Tucker et al.,

1991) In arid and semiarid regions, NDVI evidently correlates with the density ofthe vegetative cover and its biomass, as well as with its “leaf area index” (Nicholson

et al., 1998) and photosynthetic activity (Prince, 1991).

Another criticism is in order regarding the use of NDVI (the Normalized ference Vegetation Index) as a measure of net primary production That index mayindeed indicate the activity of the vegetative cover at the time of measurement,but it is oblivious to the amount of vegetation harvested by humans and/or theiranimals prior to the time of measurement

Dif-Taken to be a general indicator of the “greenness” of an area, NDVI has alsobeen conjectured to correlate with biological productivity, but that correlation maynot necessarily hold In principle, the amount of vegetation present per unit of area

should depend on the amount produced in situ, minus the amount removed from it.

Therefore, the relation between an area’s productivity and its vegetative biomass atany time must depend on whether the vegetation has been or is being “harvested”(e.g., grazed by livestock, or cut and carried away by humans) An area could bequite productive yet relatively bare, if it had been harvested just prior to the NDVImeasurement

DESERTIFICATION: CLIMATE VARIABILITY AND CHANGE 21

A more serious caveat is in order: even if there is no discernible change in thedensity of an area’s overall vegetative cover, there might well be a considerablechange in the composition of the vegetation (i.e., in its biodiversity, ecologicalfunction, and feed value) For instance, an overgrazed area may exhibit a prolif-eration of less nutritious plants at the same time that it loses the most palatablespecies of grasses and legumes that had contributed to the area’s original carryingcapacity Evidence of this effect was demonstrated by Gonzalez (2001)

Nicholson et al (1998) noted that the interannual fluctuations of the desert

boundary, as assessed from NDVI, were indeed considerable, with a displacement

as great as 3◦latitude (roughly 300 km) back and forth These fluctuations sponded to the variations of the region’s rainfall However, the investigators coulddiscern no progressive “march” of the desert over West Africa during the period

corre-of their study (1980 to 1995) Furthermore, they reported that the ratio corre-of NDVI torainfall, which they took to represent the rain-use efficiency of the vegetation, in-dicated little interannual variability and no discernible decline during the 13 years

of their analysis

A criterion used by Tucker et al (1991) to delineate the boundary between the

Sahara and the Sahel is the mean annual rainfall contour (isohyet) of 200 mm Maloand Nicholson (1990) found that this boundary corresponds approximately to anannually integrated NDVI of 1 However, the density of the vegetative cover mustdepend not only on rainfall but also on whether and to what extent that vegetation

is being utilized

As seen in Fig 2, the annual precipitation in the Sahel has fluctuated widely,but the amounts for the last 3 decades of the 20th century are generally lower thanthose of the preceding decades And although the trend in recent years appears

to be an upward one, the annual amounts of rainfall are still low relative to thecentury’s earlier decades Clearly, an analysis based on any particular short periodmay be misleading

VI FUTURE CLIMATIC VARIABILITY AND CHANGE

Climate in arid and semiarid regions is likely to be even more influenced in thefuture by human activity due to the phenomenon known as global climate change.Emissions of greenhouse gases, among them carbon dioxide (CO2), methane(CH4), and nitrous oxide (N2O), and aerosols due to human activities are alteringthe atmosphere in ways that are expected to warm the climate The warming trend,

or enhanced greenhouse effect, is attributed to the release into the atmosphere ofradiatively active trace gases, which have the property of trapping a growing pro-portion of the heat emitted by the earth’s surface The atmospheric concentration

22 HILLEL AND ROSENZWEIG

Table I Observed and Projected Changes in Extreme Weather and Climate Events Related

to Temperature and Precipitationa

and more hot days over nearly all land areas

fewer cold days and frost days over nearly all land areas

range over most land areas

Likely, over many Northern More intense precipitation events Very likely, over many areas Hemisphere mid- to

high-latitude land areas

Likely, in a few areas Increased summer continental drying Likely, over most mid-latitude

and associated risk of drought continental interiors (lack of

consistent projections in other areas)

a

Source: IPCC WGI (2001).

of CO2has increased by∼30% since 1750, mostly due to fossil fuel burning andpartially due to land-use change, especially deforestation The present CO2con-centration has not been exceeded during the past 420,000 years, and the rate ofincrease is unprecedented during the past 20,000 years (IPCC, 2001)

One of the more insidious manifestations of global climate change may be anincrease of climate instability (Rosenzweig and Hillel, 1998) In a warmer world,climatic phenomena are likely to intensify Thus, episodes or seasons of anoma-lously wet conditions (violent rainstorms of great erosive power) may alternatewith severe droughts, in an irregular and unpredictable pattern Table I presentsthe IPCC assessment of confidence in observed changes in extremes of weather andclimate during the latter half of the 20th century and projected changes during the21st century Nearly all land areas are very likely to experience higher maximumand higher minimum temperatures and more intense precipitation events

A more unstable climatic regime will make it harder to devise and more pensive to implement optimal land use and agricultural production practices,including drought-contingency provisions Failure to prepare for such contin-gencies may exacerbate the consequences of such extreme events as floods and

ex-DESERTIFICATION: CLIMATE VARIABILITY AND CHANGE 23

droughts, to the effect of worsening land degradation and periods of severe foodshortages

Working Group I of the Intergovernmental Panel on Climate Change (IPCC) hasfound that an increasing body of observations reveals that warming at the globalscale is already underway (IPCC, 2001) The global average surface temperaturehas increased over the 20th century by 0.6◦C+/−0.2◦C Most of the warminghas occurred during two periods: 1910–1945 and 1976–2000 Since 1975, theSahelian region has experienced warming of up to 1.5◦C (Fig 6, see color insert).The IPCC further finds that the frequency and the intensity of droughts in parts ofAfrica have increased in recent decades; in particular, there has been a decrease inrainfall over large portions of the Sahel (IPCC, 2001)

Working Group II of the Intergovernmental Panel on Climate Change on pacts, Adaptation, and Vulnerability finds that Africa is highly vulnerable to climatechange (IPCC WGII, 2001) Sectors of concern include water resources, food secu-rity, natural resources and biodiversity, human health, and desertification (Table II).Global climate models (GCMs) are mathematical formulations of the processesthat comprise the climate system, including radiation, energy transfer by winds,cloud formation, evaporation and precipitation of water, and transport of heat

Im-by ocean currents (Fig 7) GCMs are used to simulate climate Im-by solving thefundamental equations for conservation of mass, momentum, energy, and water.For boundary conditions relevant to the Earth’s geographic features and with therelevant parameters, the equations of the GCMs are solved for the atmosphere, landsurface, and oceans over the entire globe GCMs project global climate responses

at relatively coarse-scaled resolutions (2.5 to 3.75◦latitude by∼3.75◦longitude).

Table II Sectors Vulnerable to Climate Change in Africaa

Water resources Dominant impact is predicted to be a reduction in soil moisture in

the subhumid zones and a reduction in runoff.

Food security There is wide consensus that climate change, through increased

extremes, will worsen food security in Africa.

Natural resources

and biodiversity

Climate change is projected to exacerbate risks to already threatened plant and animal species, and fuelwood.

Human health Vector-borne and water-borne diseases are likely to increase,

especially in areas with inadequate health infrastructure.

Desertification Changes in rainfall, increased evaporation, and intensified land use

may put additional stresses on arid, semiarid, and dry ubhumid ecosystems.

aSource: IPCC WG II (2001).

24 HILLEL AND ROSENZWEIG

Figure 7 The climate system (Source: WMO, 1985).

The models are used to simulate the climate system’s future responses to tional greenhouse gases and sulfate aerosols emitted into the atmosphere by humanactivities

addi-Temperature and precipitation changes for the Sahel region of Africa in the2050s projected by two global climate models (GCMs) are shown in Figs 8 and 9(see color inserts) The global climate models are the United Kingdom HadleyCentre (HC) and the Canadian Centre for Climate Modeling and Analysis (CC)

(Flato et al., 1997; Johns et al., 1997).

There are two types of scenarios for each GCM: the first accounts for the effects

of greenhouse gases on the climate (GG), and the second accounts for the effect

of greenhouse gases and sulfate aerosols (GS) The GCM simulations for the21st century are forced with a 1% per year increase of equivalent carbon dioxide(CO2) concentration in the atmosphere These simulations are based on “business-as-usual” greenhouse gas emission scenarios of the Intergovernmental Panel onClimate Change and account for changes in other greenhouse gases besides CO2(IPCC, 1996) Sulfate aerosols are emitted as by-products of industrial activitiesand create a cooling effect as they reflect and scatter solar radiation Thus, thescenarios that incorporate both greenhouse gases and sulfate aerosols tend to besomewhat cooler than those with greenhouse gas forcing alone Simulated annualtemperature and precipitation were linearly interpolated across the GCM gridboxes

in the Sahel region

DESERTIFICATION: CLIMATE VARIABILITY AND CHANGE 25

The scenarios vary in the magnitude of the projected temperature changes, butthey all project a warming trend for the Sahel region The GCM models projecttemperature changes ranging from 2 to 7◦C in the 2050s The Canadian Centrefor Climate Modeling and Analysis (CC) scenario consistently projects highertemperatures for the region than the United Kingdom Hadley Centre (HC), whilethe scenarios that combine greenhouse gases and sulfate aerosols (GS) are con-sistently cooler than those with the greenhouse gases alone (GG) Precipitationprojections of the two global climate models show different patterns for the 2050s,indicating uncertainty regarding future hydrological conditions Changes in pre-cipitation range from−40% to +40% in the 2050s

At three sites across the Sahel (Fig 10), an analysis was done to project thepotential for future drought in the region Mean monthly temperature and precipi-tation for Bamako, Mali; Kano, Nigeria; and Kosti, Sudan for the period of recordare shown in Fig 11 (Data were available from 1945 to 1988 in Bamako, Mali;from 1947 to 1965 in Kano, Nigeria; and from 1943 to 1979 in Kosti, Sudan).Mean annual temperature is 28.2, 26.3, and 27.3◦C for Bamako, Kano, and Kosti,respectively Mean annual precipitation is low at the Mali (1014 mm y−1) andNigeria (859 mm y−1) sites, and extremely low at the Sudan site (400 mm y−1),with highest rainfall occurring in August

Figure 10 Study sites for analysis of future droughts in the Sahel.

26 HILLEL AND ROSENZWEIG

Figure 11 Monthly mean temperature and precipitation for Sahel study sites (Bamako, Mali 1945–1988; Kano, Nigeria 1947–1965; and Kosti, Sudan 1943–1979) (Source: NASA GISS).

For the coming decades, both GCMs project significant warming at all threesites (between∼4 and 8◦C by the 2080s) (Fig 12) Precipitation projections, onthe other hand, are mixed, with the Hadley Centre GCM simulating declines up

to 30% in Bamako, Mali, in the 2080s, and increases of more than 20% in Kano,Nigeria, in the 2050s (Fig 13)

We explored the potential for drought in the Sahelian region further by lating potential evaporation (PET) with the Penman–Monteith (Monteith, 1980)equation and the Thornthwaite (1948) equation and then using these formulas tocalculate the Palmer Drought Stress Index (Palmer, 1965) The PDSI comparesanomalous dry and wet years to normal years and is used to identify relativedroughts and floods at particular places (Table III) It uses a water balance approach

calcu-to calculate infiltration, runoff, and potential and actual evaporation Inputs aremonthly mean temperature and precipitation, soil water capacities, and Thorn-thwaite (1948) parameters, which are a function of the mean temperature andlatitude

DESERTIFICATION: CLIMATE VARIABILITY AND CHANGE 27

Figure 12 Projected annual change in temperature for the Sahel study sites for the Hadley Centre (HC) and Canadian Centre (CC) climate change scenarios with greenhouse gases alone (GG) and with greenhouse gases and sulfate aerosols (GS).