so i l conserva t ion – l and management – f lood fo recas t ing – food secur i ty

World Meteorological Organization

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climate information – resource conservation – sustainable management of land

Climate and Land Degradation

WMO-No. 989

2005

climate information – resource conservation – sustainable management of land

Climate and Land Degradation

WMO-No. 989

2

WMO-No. 989© 2005, World Meteorological OrganizationISBN 92-63-10989-3

NOTEThe designations employed and the presentation ofmaterial in this publication do not imply the expression ofany opinion whatsoever on the part of the Secretariat of theWorld Meteorological Organization concerning the legalstatus of any country, territory, city or area, or of itsauthorities, or concerning the delimitations of its frontiersor boundaries.

Foreword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Extent and rate of land degradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Land degradation - causes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Climatic consequences of land degradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Climatic factors in land degradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Rainfall . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Floods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Droughts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Solar radiation, temperature and evaporation . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Wind . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Causes of wind erosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

Climatic implications of duststorms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Wildfires, land degradation and atmospheric emissions . . . . . . . . . . . . . . . . . . . . . 24

Climate change and land degradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Carbon sequestration to mitigate climate change and combat land degradation . 27

Understanding the interactions between climate and land degradation—

role of WMO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Future perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

3

CONTENTS

The United Nations Convention to CombatDesertification (UNCCD) entered into forceon 26 December 1996 and over 179 coun-tries were Parties as at March 2002. TheConvention defines desertification as “landdegradation in the arid, semi-arid and drysub-humid areas resulting from variousfactors, including climatic variations andhuman activities”.

Over 250 million people are directly affectedby desertification. In addition, some onebillion people in over 100 countries are atrisk. These people include many of theworld's poorest, most marginalized, andpolitically weak citizens. Hence combatingdesertification is an urgent priority in globalefforts to ensure food security and the liveli-hoods of millions of people who inhabit thedrylands of the world.

Sustainable development of countriesaffected by drought and desertification canonly come about through concerted effortsbased on a sound understanding of thedifferent factors that contribute to landdegradation around the world. Climatic vari-ations are recognized as one of the majorfactors contributing to land degradation, asdefined in the Convention. It is more impor-tant to address climate, an underlying driverof land degradation, than try to address onlythe consequenses of land degradation. Forexample, development and adoption ofsustainable land management practices isone of the major solutions to combat theproblem over the vast drylands around theworld, but to accurately assess sustainableland management practices, the climateresources and the risk of climate-related orinduced natural disasters in a region mustbe known.

Six sessions of the Conference of Parties(COP) have been held to date, at whichseveral important issues related to the prob-lems of drought and desertification havebeen addressed. Article 5 of the Conventioncalls on the affected country parties toaddress the underlying causes of desertifi-cation and it is timely that more efforts bedevoted to better understand the role ofclimatic factors in land degradation. It isalso important to note that Article 16 onInformation Collection, Analysis andExchange emphasizes the importance ofintegrating and coordinating the collection,analysis and exchange of relevant short-term and long-term data and information toensure systematic observation of landdegradation in affected areas and to under-stand better and assess the processes andeffects of drought and desertification.Research into the causes and effects ofclimate variations and long-term climatepredictions with a view to providing earlywarning is essential. These issues requirethe attention of the Committee on Scienceand Technology (CST) of the COP.

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FOREWORD

The World Meteorological Organization(WMO), as a specialized agency of theUnited Nations, furthers the applications ofmeteorology and hydrology to severalsectors, including agriculture and otherhuman activities. In this respect, WMO willpromote systematic observation, collection,analysis and exchange of meteorological,climatological and hydrological data andinformation; drought planning, prepared-ness and management; research on climaticvariations and climate predictions; andcapacity building and transfer of knowledgeand technology. WMO's programmes, inparticular the Agricultural MeteorologyProgramme and the Hydrology and WaterResources Programme, will support theseefforts.

Given the importance of the interactionsbetween climate and desertification, WMOaccorded a major priority to this area and itsaction plan to combat desertification wasfirst adopted in 1978 at the thirteenthsession of the Executive Council of WMOand has gone through several revisions.

WMO will continue to encourage theincreased involvement of the NationalMeteorological and Hydrological Services(NMHSs) and regional and subregionalmeteorological and hydrological centres inaddressing the issues of relevance to theCCD, especially those stipulated in Articles10, and 16 to 19, of the Convention.

On the occasion of the Seventh session ofthe COP, WMO has prepared this brochurewhich explains the role of different climaticfactors in land degradation and WMO'scontribution in addressing this importantsubject. We hope that this document willhelp enhance the understanding of theParties to some of the issues involved sothat they can be addressed knowledgeably.

(M. Jarraud)Secretary-General

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Introduction

Desertification is now defined in the UNCCDas “land degradation in the arid, semi-arid anddry sub-humid areas resulting from variousfactors, including climatic variations andhuman activities” (this definition excludes thehyper-arid lands). Furthermore, UNCCDdefines land degradation as a "reduction orloss, in arid, semi-arid, and dry sub-humidareas, of the biological or economic produc-tivity and complexity of rain-fed cropland,irrigated cropland, or range, pasture, forest,and woodlands resulting from land uses orfrom a process or combination of processes,including processes arising from human activ-ities and habitation patterns, such as: (i) soilerosion caused by wind and/or water; (ii) dete-rioration of the physical, chemical, andbiological or economic properties of soil; and(iii) long-term loss of natural vegetation."

According to UNCCD, over 250 millionpeople are directly affected by land degrada-tion. In addition, some one billion people inover 100 countries are at risk. These peopleinclude many of the world's poorest, mostmarginalized, and politically weak citizens.

The land degradation issue for world foodsecurity and the quality of the environmentassumes a major significance when oneconsiders that only about 11 per cent of theglobal land surface can be considered asprime or Class I land, and this must feed the6.3 billion people today and the 8.2 billionexpected by the year 2020. Hence landdegradation will remain high on the interna-tional agenda in the 21st century.

Sustainable land management practicesare needed to avoid land degradation.Land degradation typically occurs becauseof land management practices or humandevelopment that is not sustainable over aperiod of time. To accurately assesssustainable land management practices,the climate resources and the risk ofclimate-related or induced natural disas-

ters in a region must be known. Only whenclimate resources are paired with potentialmanagement or development practices canthe land degradation potential be assessedand appropriate mitigation technologyconsidered. The use of climate informationmust be applied in developing sustainablepractices as climatic variation is one of themajor factors contributing to or even a trig-ger to land degradation and there is a clearneed to consider carefully how climateinduces and influences land degradation.

Extent and rate of land degradation

Global assessment of land degradation isnot an easy task, and a wide range of meth-ods are used, including expert judgement,remote sensing and modeling. Because ofdifferent definitions and terminology, therealso exists a large variation in the availablestatistics on the extent and rate of landdegradation. Further, most statistics refer tothe risks of degradation or desertification(based on climatic factors and land use)rather than to the current state of the land.

Different processes of land degradationalso confound the available statistics onsoil and/or land degradation. Principalprocesses of land degradation includeerosion by water and wind, chemicaldegradation (comprising acidification,salinization, fertility depletion, anddecrease in cation retention capacity),physical degradation (comprising crusting,compaction, hard-setting, etc.) and biologi-cal degradation (reduction in total andbiomass carbon, and decline in land biodi-versity). The latter comprises importantconcerns related to eutrophication ofsurface water, contamination of groundwater, and emissions of trace gases (CO2,CH4, N2O, NOx) from terrestrial/aquaticecosystems to the atmosphere. Soil struc-ture is the important property that affectsall degradative processes. Factors thatdetermine the kind of degradative

6

processes include land quality as affectedby the intrinsic properties of climate,terrain and landscape position, climaxvegetation and biodiversity, especially soilbiodiversity.

In an assessment of population levels in theworld's drylands, the Office to CombatDesertification and Drought (UNSO) of theUnited Nations Development Programme(UNDP) showed that globally 54 million km2

or 40 per cent of the land area is occupiedby drylands. About 29.7 per cent of this areafalls in the arid region, 44.3 per cent in thesemi-arid region and 26 per cent in the drysub-humid region. A large majority of thedrylands are in Asia (34.4 per cent) andAfrica (24.1 per cent), followed by theAmericas (24 per cent), Australia (15 percent) and Europe (2.5 per cent).

Figure 1 indicates that the areas of the worldvulnerable to land degradation cover about33 per cent of the global land surface. At theglobal level, it is estimated that the annualincome foregone in the areas immediately

affected by desertification amounts toapproximately US$ 42 billion each year.

The semi-arid to weakly aridic areas ofAfrica are particularly vulnerable, as theyhave fragile soils, localized high populationdensities, and generally a low-input form ofagriculture. About 25 per cent of land inAsian countries is vulnerable.

Long-term food productivity is threatenedby soil degradation, which is now severeenough to reduce yields on approximately16 per cent of the agricultural land, espe-cially cropland in Africa, Central Americaand pastures in Africa. Sub-Saharan Africahas the highest rate of land degradation. It isestimated that losses in productivity of crop-ping land in sub-Saharan Africa are in theorder of 0.5-1 per cent annually, suggestingproductivity loss of at least 20 per cent overthe last 40 years.

Africa is particularly threatened because theland degradation processes affect about 46per cent of the continent. The significance of

7

Figure 1. Areasvulnerable todesertification indifferent parts of theworld (Source: U.S.Department ofAgriculture, NaturalResourcesConservation Service).

Vulnerability Other regionsDry

Cold

Humid/not vulnerable

Ice/Glacier

Low

Moderate

High

Very high

this large area becomes evident when oneconsiders that about 43 per cent of Africa ischaracterized as extreme desert (the desertmargins represent the areas with very highvulnerability). There is only about 11 percent of the land mass which is humid andwhich by definition is excluded from deserti-fication processes. There is about 2.5 millionkm2 of land under low risk, 3.6 million km2

under moderate risk, 4.6 million km2 underhigh risk, and 2.9 million km2 under veryhigh risk. The region with the highestpropensity is located along the desertmargins and occupies about 5 per cent of thelandmass. It is estimated that about 22million people (2.9 per cent of the total popu-lation) live in this area. The low, moderateand high vulnerability classes occupy 14, 16,and 11 per cent respectively and togetherimpact about 485 million people.

Land degradation is also a serious problemin Australia with over 68 per cent of the landestimated to have been degraded (Table 1).

According to UNCCD, the consequences ofland degradation include undermining offood production, famine, increased socialcosts, decline in the quantity and quality offresh water supplies, increased poverty andpolitical instability, reduction in the land'sresilience to natural climate variability anddecreased soil productivity.

Land degradation - causes

Land degradation involves two interlocking,complex systems: the natural ecosystem andthe human social system. Natural forces,through periodic stresses of extreme andpersistent climatic events, and human useand abuse of sensitive and vulnerable dryland ecosystems, often act in unison,creating feedback processes, which are notfully understood. Interactions between thetwo systems determine the success or failureof resource management programmes.Causes of land degradation are not onlybiophysical, but also socioeconomic (e.g.land tenure, marketing, institutional support,income and human health) and political (e.g.incentives, political stability).

High population density is not necessarilyrelated to land degradation. Rather, it is whata population does to the land that deter-mines the extent of degradation. People canbe a major asset in reversing a trendtowards degradation. Indeed, mitigation ofland degradation can only succeed if landusers have control and commitment tomaintain the quality of the resources.However, they need to be healthy and politi-cally and economically motivated to care forthe land, as subsistence agriculture, povertyand illiteracy can be important causes ofland and environmental degradation.

8

Type Area (000 km2)

Total 443

Not degraded 142

Degraded 301

i) Water erosion 206

ii) Wind erosion 52

iii) Combined water and wind erosion 42

iv) Salinity and water erosion 0.9

v) Others 0.5

Table 1. Land degradationon cropland in Australia(Source: Woods, 1983;Mabbutt, 1992).

There are many, usually confounding,reasons why land users permit their land todegrade. Many of these reasons are relatedto societal perceptions of land and thevalues placed on it. The absence of landtenure and the resulting lack of stewardshipis a major constraint to adequate care forthe land in some countries. Degradation isalso a slow, imperceptible process, meaningthat many people are not aware that theirland is degrading.

Loss of vegetation can propagate furtherland degradation via land surface-atmos-phere feedback. This occurs when adecrease in vegetation reduces evaporationand increases the radiation reflected backto the atmosphere (albedo), consequentlyreducing cloud formation. Large-scaleexperiments in which numerical models ofthe general circulation have been run withartificially high albedo over drylands, havesuggested that large increases in thealbedo of subtropical areas could reducerainfall.

Climatic consequences of landdegradation

Land surface is an important part of theclimate system (Figure 2). The interactionbetween land surface and the atmosphereinvolves multiple processes and feedbacks,all of which may vary simultaneously. It isfrequently stressed that the changes ofvegetation type can modify the characteris-tics of the regional atmospheric circulationand the large-scale external moisture fluxes.Changes in surface energy budgets result-ing from land surface change can have aprofound influence on the Earth's climate.

Following deforestation, surface evapotran-spiration and sensible heat flux are related tothe dynamic structure of the low-levelatmosphere. These changes in fluxes withinthe atmospheric column could influence theregional, and potentially, global-scale

atmospheric circulation. For example,changes in forest cover in the Amazon basinaffect the flux of moisture to the atmos-phere, regional convection, and henceregional rainfall. More recent work showsthat these changes in forest cover haveconsequences far beyond the Amazon basin.

Fragmentation of landscape can affectconvective flow regimes and rainfallpatterns locally and globally. El Niñoevents and land surface change simula-tions with climate models suggest that inequatorial regions where towering thun-derstorms are frequent, disturbing areashundreds of kilometres wide may yieldglobal impacts.

Use of a numerical simulation model tostudy the interactions between convectiveclouds, the convective boundary layer and aforested surface showed that surfaceparameters such as soil moisture, forestcoverage, and transpiration and surfaceroughness may affect the formation ofconvective clouds and rainfall through theireffect on boundary-layer growth.

An atmospheric general circulation modelwith realistic land-surface properties was

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Figure 2. Land surfaceis an important part ofthe climate system andland surface parame-ters could affectrainfall.

employed to investigate the climatic effectof doubling the extent of the Earth's desert-sand. It showed a notable correlationbetween decreases in evapotranspirationand resulting precipitation. It was shownthat Northern Africa suffers a strong year-round drought while Southern Africa has asomewhat weaker year-round drought.Some regions, particularly the Sahel,showed an increase in surface temperaturecaused by decreased soil moisture andlatent-heat flux.

Land use and land cover changes influencecarbon fluxes and greenhouse gas (GHG)emissions which directly alter atmosphericcomposition and radiative forcing proper-ties. They also change land-surfacecharacteristics and, indirectly, climaticprocesses. Observations during the HAPEX-Sahel project suggested that a large-scaletransformation of fallow savannah intoarable crops like millet, may lead to adecrease in evaporation. Land use and landcover change is an important factor indetermining the vulnerability of ecosys-tems (Figure 3) and landscapes toenvironmental change.

Since the industrial revolution, global emis-sions of carbon (C) are estimated at 270±30gigatons (Gt) due to fossil fuel combustionand 136±5 Gt due to land use change andsoil cultivation. Emissions due to land usechange include those by deforestation,biomass burning, conversion of natural toagricultural ecosystems, drainage ofwetlands and soil cultivation. Depletion ofthe soil organic C (SOC) pool hascontributed 78±12 Gt of C to the atmos-phere, of which about one-third is attributedto soil degradation and accelerated erosionand two-thirds to mineralization.

Land degradation aggravates CO2-inducedclimate change through the release of CO2from cleared and dead vegetation andthrough the reduction of the carbon seques-tration potential of degraded land.

Climatic factors in land degradation

Climate exerts a strong influence overdryland vegetation type, biomass and diver-sity. Precipitation and temperaturedetermine the potential distribution ofterrestrial vegetation and constitute theprincipal factors in the genesis and evolu-tion of soil. Precipitation also influencesvegetation production, which in turncontrols the spatial and temporal occur-rence of grazing and favours nomadiclifestyle. Vegetation cover becomesprogressively thinner and less continuouswith decreasing annual rainfall. Drylandplants and animals display a variety of phys-iological, anatomical and behaviouraladaptations to moisture and temperaturestresses brought about by large diurnal andseasonal variations in temperature, rainfalland soil moisture.

The generally high temperatures and lowprecipitation in the drylands lead to poororganic matter production and rapid oxida-tion. Low organic matter leads to pooraggregation and low aggregate stabilityleading to a high potential for wind andwater erosion. For example, wind andwater erosion is extensive in many parts ofAfrica. Excluding the current deserts, whichoccupy about 46 per cent of the landmass,about 25 per cent of the land is prone towater erosion and about 22 per cent, towind erosion.

Structural crusts/seals are formed by rain-drop impact which could decreaseinfiltration, increase runoff and generateoverland flow and erosion. The severity,frequency, and extent of erosion are likely tobe altered by changes in rainfall amountand intensity and changes in wind.

Land management will continue to be theprincipal determinant of the soil organicmatter (SOM) content and susceptibility toerosion during the next few decades, butchanges in vegetation cover resulting from

10

Figure 3. Land use isan important factor indetermining thevulnerability ofecosystems.

short-term changes in weather and near-term changes in climate are likely to affectSOM dynamics and erosion, especially insemi-arid regions.

From the assessment of the land resourcestresses and desertification in Africa, whichwas carried out by the Natural ResourcesConservation Service of the United States

Department of Agriculture, utilizing infor-mation from the soil and climate resourcesof Africa, it can be concluded (Table 2) thatclimatic stresses account for 62.5 per centof all the stresses on land degradation inAfrica. These climatic stresses include highsoil temperature, seasonal excess water,short duration low temperatures, seasonalmoisture stress and extended moisture

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Land stresses Inherent land qualityStress

Kinds of stress Area Class Area Areaclass(1,000 km2) (1,000 km2) (%)

1 Few constraints 118.1 I 118.1 0.4

2 High shrink/swell 107.6 II

3 Low organic matter 310.9 II

4 High soil temperatures 901.0 II 1,319.6 4.5

5 Seasonal excess water 198.9 III

6 Minor root restrictions 566.5 III

7 Short duration low temperatures .014 III 765.4 2.6

8 Low structural stability 333.7 IV

9 High anion exchange capacity 43.8 IV

10 Impeded drainage 520.5 IV 898.0 3.1

11 Seasonal moisture stress 3,814.9 V

12 High aluminium 1,573.2 V

13 Calcareous, gypseous 434.2 V

14 Nutrient leaching 109.9 V 5,932.3 20.2

15 Low nutrient holding capacity 2,141.0 VI

16 High P, N retention 932.2 VI

17 Acid sulphate 16.6 VI

18 Low moisture and nutrient status 0 VI

19 Low water holding capacity 2,219.5 VI 5,309.3 18.1

20 High organic matter 17.0 VII

21 Salinity/alkalinity 360.7 VII

22 Shallow soils 1,016.9 VII 1,394.7 4.8

23 Steep lands 20.3 VIII

24 Extended low temperatures 0 VIII 20.3 0.1

25 Extended moisture stress 13,551.4 IX 13,551.4 46.2

Land Area 29,309.1

Water bodies 216.7

Total area 29,525.8

Table 2. Major landresources stressesand land qualityassessment of Africa(Source: Reich, P.F.,S.T. Numben, R.A.Almaraz, and H.Eswaran, 2001. Landresource stresses anddesertification inAfrica. In: Eds. Bridges,E.M., I.D. Hannam,F.W.T. Penning deVries, S.J. Scherr, andS. Sombatpanit. 2001.Response to LandDegradation. Sci.Publishers, Enfield,USA. 101-114)

stress, and affect 18.5 million km2 of theland in Africa. This study clearly exempli-fies the importance of the need to givemore careful consideration to climaticfactors in land degradation.

According to the database of the BelgianCentre for Research on the Epidemiology ofDisasters (CRED), weather, climate andwater-related hazards that occurredbetween 1993-2002 were responsible for 63per cent of the US$ 654 billion damagecaused by all natural disasters. These natu-ral hazards are therefore the most frequentand extensively observed ones (Figure 4)and they all have a major impact on landdegradation.

Rainfall

Rainfall is the most important climatic factorin determining areas at risk of land degra-dation and potential desertification. Rainfallplays a vital role in the development anddistribution of plant life, but the variabilityand extremes of rainfall can lead to soilerosion and land degradation (Figure 5). Ifunchecked for a period of time, this landdegradation can lead to desertification. Theinteraction of human activity on the distri-bution of vegetation through landmanagement practices and seeminglybenign rainfall events can make land more

vulnerable to degradation. These vulnerabil-ities become more acute when the prospectof climate change is introduced.

Rainfall and temperature are the primefactors in determining the world's climateand therefore the distribution of vegetationtypes. There is a strong correlation betweenrainfall and biomass since water is one ofprimary inputs to photosynthesis.Climatologists use an “aridity index” (theratio of annual precipitation to potentialevaporation) to help classify desert (arid) orsemi-arid areas. Drylands exist because theannual water loss (evaporation) exceeds theannual rainfall; therefore these regions havea continual water deficit. Deserts are theultimate example of a climate where annualevaporation far exceeds the annual rainfall.In cases where the annual water deficits arenot so large, some plant life can take holdusually in the form of grasslands or steppes.However, it is these drylands on the marginsof the world's deserts that are most suscep-tible to desertification, and the mostextreme example of land degradation.Examples of these regions include thePampas of South America, the GreatRussian Steppes, the Great Plains of NorthAmerica, and the Savannas of SouthernAfrica and Sahel region of west Africa. Withnormal climatic variability, in some yearsthe water deficits can be greater than othersbut sometimes there can be a severalconsecutive years of water deficit or long-term drought. During this period, one cansee examples of land degradation as in theDust Bowl years of the 1930s in the GreatPlains or the nearly two-decade longdrought in the Sahel in the 1970s and 1980s.It was this period of drought in the Sahelthat created the current concern aboutdesertification.

For over a century, soil erosion data havebeen collected and analysed from soil scien-tists, agronomists, geologists, hydrologists,and engineers. From these investigations,scientists have developed a simple soil

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Avalanches andlandslides 6% Droughts and famines 9%

Earthquakes 8%

ExtremeTemperatures 5%

Floods 37%

Windstorms 28%

Volcaniceruptions 2%

Forest/scrubfires 5%

Figure 4. Globaldistribution of naturaldisasters (1993-2002)

erosion relationship that incorporates themajor soil erosion factors. The UniversalSoil Loss Equation (USLE) was developed inthe mid-1960s for understanding soilerosion for agricultural applications. In1985, it was updated and renamed theRevised Universal Soil Loss Equation(RUSLE) to incorporate the large amount ofinformation that had accumulated since theoriginal equation was developed and toaddress land use applications besides agri-culture, such as soil loss from mined lands,construction sites, and reclaimed lands. TheRUSLE is derived from the theory of soilerosion and from more than 10,000 plot-years of data from natural rainfall plots andnumerous rainfall simulations.

THE RUSLE IS DEFINED AS:

A = R K L S C P

where A is the soil loss per year (t/ha/year);R represents the rainfall-runoff erosivityfactor; K is the soil erodibilty factor; L repre-sents the slope length; S is the slopesteepness; C represents the cover manage-ment, and P denotes the supportingpractices factor. These factors illustrate theinteraction of various climatic, geological,and human factors, and that smart landmanagement practices can minimize soilerosion and even land degradation.

The extremes of either too much or too littlerainfall can produce soil erosion that canlead to land degradation (Figure 6).However, soil scientists consider rainfall themost important erosion factor among themany factors that cause soil erosion.Rainfall can erode soil by the force ofraindrops, surface and subsurface runoff,and river flooding. The velocity of rainhitting the soil surface produces a largeamount of kinetic energy, which candislodge soil particles. Erosion at this micro-scale can also be caused by easilydissoluble soil material made watersoluble

by weak acids in the rainwater. The breakingapart and splashing of soil particles due toraindrops is only the first stage of theprocess, being followed by the washingaway of soil particles and further erosioncaused by flowing water. However, withoutsurface runoff, the amount of soil erosioncaused by rainfall is relatively small.

Once the soil particles have been dislodgedthey become susceptible to runoff. Ingeneral, the higher the intensity of therainfall, the greater the quantity of soilavailable in runoff water. In the case of lightrain for a long duration, most of the soildislodgement takes place in the underwaterenvironment and the soil particles aremostly fine. The greater the intensity ofrainfall and subsequent surface runoff, thelarger the soil particles that are carriedaway. A critical factor that determines soilerosion by rainfall is the permeability of thesoil, which indirectly influences the totalamount of soil loss and the pattern oferosion on slopes. One unfortunate by-product of runoff is the correspondingtransport of agricultural chemicals and theleaching of these chemicals into thegroundwater.

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Pollution

Runoff/floodwater

Sediment

Surface runoff

Rain

Flood

Land

River SeaSea

Figure 5. Schematicdiagram of rainfall-induced processesinvolved in land degradation.

Rainfall intensity is the most importantfactor governing soil erosion caused byrain. Dryland precipitation is inherently vari-able in amounts and intensities and so is thesubsequent runoff. Surface runoff is oftenhigher in drylands than in more humidregions due to the tendency of dry land soilsto form impermeable crusts under theimpact of intense thunderstorms and in theabsence of significant plant cover or litter. Inthese cases, soil transport may be an orderof magnitude greater per unit momentum offalling raindrops than when the soil surfaceis well vegetated. The sparser the plantcover, the more vulnerable the topsoil is todislodgement and removal by raindropimpact and surface runoff. Also, the timingof the rainfall can play a crucial role in soilerosion leading to land degradation. Anerratic start to the rainy season along withheavy rain will have a greater impact sincethe seasonal vegetation will not be availableto intercept the rainfall or stabilize the soilwith its root structure.

An ongoing effort of scientists is to try tointegrate all these factors into models thatcan be used to predict soil erosion. TheWater Erosion Prediction Project (WEPP)model is a process-based, distributedparameter, continuous simulation, erosionprediction model for use on personalcomputers and can be applied at the fieldscale to simulate hillslope erosion or morecomplex watershed scale erosion. It mimicsthe natural processes that are important insoil erosion. It updates the everyday soiland crop conditions that affect soil erosion.When rainfall occurs, the plant and soilcharacteristics are used to determine ifsurface runoff will occur. The WEPP modelincludes a number of conceptual compo-nents that include: climate and weather(rainfall, temperature, solar radiation, wind,freeze - thaw, snow accumulation and melt-ing), irrigation (stationary sprinkler, furrow),hydrology - (infiltration, depressional storage, runoff), water balance (evapo-transpiration, percolation, drainage), soils(types and properties), crop growth - (crop-land, rangeland, forestland), residuemanagement and decomposition, tillageimpacts on infiltration and erodibility,erosion - (interrill, rill, channel), deposition(rills, channels, and impoundments), sedi-ment delivery, particle sorting andenrichment.

Of special note is the impact of other formsof precipitation on soil erosion. Hail has asevere effect on the soil surface because itskinetic energy is several times that of rain,resulting in much more soil surface beingdestroyed and a greater amount of materialbeing washed away. And if hailstorms areaccompanied by heavy rain, as is the casewith some thunderstorms, large amounts ofsoil can be eroded, especially on agricul-tural land before the crops can stabilize thesoil surface. Snow-thaw erosion occurswhen the soil freezes during the cold periodand the freezing process dislodges the soil,so that when the spring thaw occurs, finesoil particles are released in the runoff. This

14

Figure 6. The extremesof either too much ortoo little rainfall canproduce soil erosionthat can lead to land

degradation.

kind of erosion can often produce greatererosion losses than rain. Also, when the soilfreezes, the infiltration rate is greatlyreduced so that when the thaw arrives, rela-tively intense soil erosion can take placeeven though the amount of snow-thaw issmall. In this situation, the erosiveprocesses can be multiplied by a combina-tion of a heavy rain event and sudden influxof warm air. Leeward portions of mountain-ous areas are susceptible to this since theyare typically drier and have less vegetationand are prone to katabatic winds (rapidlydescending air from a mountain rangewarms very quickly).

Floods

Dryland rivers have extremely variableflows and river discharge, and the amountof suspended sediments are highly sensitiveto fluctuations in rainfall as well as anychanges in the vegetation cover in thebasins. The loss of vegetation in the head-waters of dryland rivers can increasesediment load and can lead to dramaticchange in the character of the river to a lessstable, more seasonal river characterized bya rapidly shifting series of channels.However, rainfall can lead to land degrada-tion in other climates, including sub-humid

ones. Excessive rainfall events eitherproduced by thunderstorms, hurricanes andtyphoons, or mid-latitude low-pressuresystems, can produce a large amount ofwater in a short period of time across localareas. This excess of water overwhelms thelocal watershed and produces river flooding(Figure 7). Of course, this is a naturalphenomenon that has occurred for millionsof years and continuously shapes the Earth.River flooding occurs in all climates, but it isin dryland areas where the problem is mostacute.

Droughts

Drought is a natural hazard originating froma deficiency of precipitation that results in awater shortage for some activities orgroups. It is the consequence of a reductionin the amount of precipitation over anextended period of time, usually a season ormore in length, often associated with otherclimatic factors - such as high temperatures,high winds and low relative humidity - thatcan aggravate the severity of the event. Forexample, the 2002-03 El Niño-relatedAustralian drought, which lasted fromMarch 2002 to January 2003, was arguablyone of, if not the worst short-term droughtin Australia's recorded meteorological

15

Figure 7. Flooding ofcrop fields due to highintensity of rains iscommon in semi-aridregions.

16

Flooding occurs when rainwater orsnowmelt accumulates faster than soilscan absorb it or rivers carry it away.There are various types of floods fromlocalized flash floods to widespreadriver flooding and they can be triggeredby severe thunderstorms, tropicalcyclones, monsoons, ice jams or melt-ing snow. In coastal areas, storm surgescaused by tropical cyclones, tsunamis,or rivers swollen by exceptionally hightides can cause flooding, while largelakes can flood when the rivers feedingthem are carrying a lot of snowmelt.Therefore flooding can contribute toland degradation in almost any climatebut dryland climates are especiallyvulnerable due to the limited amount ofvegetation the roots of which hold thesoil together.

Flood forecasting is a complex processthat must take into account many differ-ent factors at the same time, dependingon the type and nature of the phenome-non that triggers the flooding. Forexample, widespread flash floods areoften started off by heavy rain falling inone area within a larger area of lighterrain, a confusing situation that makes itdifficult to forecast where the worstflood will occur. Forecasting floodscaused by the heavy rain or stormsurges that can sweep inland as part ofa tropical cyclone can also be acomplex task, as predictions have toinclude where they will land, the stageof their evolution and the physical char-acteristics of the coast.

To make predictions as accurately aspossible, National Meteorological andHydrological Services (NMHSs), under

the auspices of the WMO, undertakeflood forecasting based on quantitativeprecipitation forecasts (QPFs), whichhave become more accurate in recentyears, especially for light and moderateamounts of precipitation, although highamounts and rare events are still diffi-cult to predict. Setting up forecastingsystems that integrate predictions forweather with those for water-relatedevents is becoming more of a possibil-ity every day, paving the way for a trulyintegrated approach.

Forecasting also needs to be a coopera-tive and multidisciplinary effort. Withthe many issues and the complexity offactors surrounding floods, floodmanagers have to join forces with mete-orologists, hydrologists, town planners,and civil defence authorities using avail-able integrated models. Determining thesocio-economic impacts of floods willmean taking a close look at constructionor other activities in and around riverchannels. Up-to-date and accurate infor-mation is essential, through all theavailable channels: surface observation,remote sensing and satellite technology,as well as computer models.

Flood forecasting

17

history. Analysis of rainfall records for this11-month period showed that 90 per cent ofthe country received rainfall below that ofthe long-term median, with 56 per cent ofthe country receiving rainfall in the lowest10 per cent (i.e. decile-1) of recorded totals(Australia-wide rainfall records commencedin 1900). During the 2002-03 droughtAustralia experienced widespread bush-fires, severe dust storms and agriculturalimpacts that resulted in a drop in Australia'sGross Domestic Product of over 1 per cent.The first five months of 2005 were excep-tionally dry for much of Australia (Figure 8),leading many to label this period a trulyexceptional drought.

Extended droughts in certain arid landshave initiated or exacerbated land degrada-tion. Records show that extensive droughtshave afflicted Africa, with serious episodes,in 1965-1966, 1972-1974, 1981-1984, 1986-1987, 1991-1992, and 1994-1995. Theaggregate impact of drought on theeconomies of Africa can be large: 8-9 percent of GDP in Zimbabwe and Zambia in1992, and 4-6 per cent of GDP in Nigeria andNiger in 1984. In the past 25 years, the Sahelhas experienced the most substantial andsustained decline in rainfall recordedanywhere in the world within the period ofinstrumental measurements. The Saheliandroughts in the 1970s were unique in theirseverity and were characterized as “thequintessence of a major environmentalemergency” and their long-term impactsare now becoming clearer (Figure 9).

Sea surface temperature (SST) anomalies,often related to the El Niño SouthernOscillation (ENSO) or North AtlanticOscillation (NAO), contribute to rainfallvariability in the Sahel. Droughts in WestAfrica correlate with warm SST in thetropical South Atlantic. Examination of theoceanographic and meteorological datafrom the period 1901-1985 showed thatpersistent wet and dry periods in the Sahelwere related to contrasting patterns of SST

anomalies on a near-global scale. From1982 to 1990, ENSO-cycle SST anomaliesand vegetative production in Africa werefound to be correlated. Warmer easternequatorial Pacific waters during ENSOepisodes correlated with rainfall of <1,000mm yr-1 over certain African regions.

A coupled surface-atmosphere model indi-cates that — whether anthropogenicfactors or changes in SST initiated the

Figure 9. Types andimpacts of droughts

Figure 8. Rainfalldeciles for January toMay 2005 in Australia

Natural climate variability

Precipitation deficiency(amount, intensity, timing)

Environmental impactsEconomic impacts Social impacts

Met

eoro

logi

cal

drou

ght

Agr

icul

tura

ldr

ough

tH

ydro

logi

cal

drou

ght

Tim

e (d

urat

ion)

Soil water deficiency

Reduced infiltration, runoff,deep percolation, andgroundwater recharge

Increased evaporationand transpiration

Plant water stress, reducedbiomass and yield

High temperature, high winds,low relative humidity, greater

sunshine, less cloud cover

Reduced streamflow, inflow toreservoirs, lakes and ponds;

reduced wetlands, wildlife habitat

18

Drought can be characterized as anormal, recurring feature of climate andit occurs in almost all climaticregimes,in areas of high as well as lowrainfall. It is a temporary anomaly, incontrast to aridity, which is a permanentfeature of the climate and is restrictedto areas of low rainfall. Drought is theconsequence of a natural reduction inthe amount of precipitation receivedover an extended period of time,usually a season or more in length.Drought is also related to the timing(i.e.main season of occurrence, delaysin the start of the rainy season,occurrence of rains in relation toprimary crop growth stages) and to theintensity and number of rainfall events.Thus, each drought is unique in itsclimatic characteristics and impacts.

The absence of a precise anduniversally accepted definition ofdrought contributes to the confusionabout whether or not a drought existsand, if it does, its degree of severity.Realistically, definitions of drought mustbe made on a regional and impact-specific basis (agricultural, waterresources, etc.). Drought impacts arespread over a larger geographical areathan damages that result from othernatural hazards such as the impacts offloods and hurricanes. For thesereasons, the quantification of impactsand the provision of disaster relief arefar more difficult tasks for drought thanthey are for other natural hazards.

Early warning systems can reduceimpacts by providing timely informationabout the onset of drought.

Conventional surface observationstations within National MeteorologicalServices are one link in the chain,providing essential benchmark data andtime series necessary for improvedmonitoring of the climate andhydrologic system. Tracking certainindicators such as stream flow or soilmoisture can help in formulatingdrought index values - typically singlenumbers, far more useful than raw datafor decision-making.

Drought plans should contain threebasic components: monitoring and earlywarning, risk assessment, andmitigation and response. Because of theslow onset characteristics of droughts,monitoring and early warning systemsprovide the foundation for an effectivedrought mitigation plan. A plan mustrely on accurate and timelyassessments to trigger mitigation andemergency response programmes.

Various WMO programmes monitorextreme climate events associated withdrought, while four monitoring centres -two in Africa, one in China and theGlobal Information and Early WarningSystem - provide weather advisoriesand one and three-month climatesummaries. Among other African earlywarning systems, the Southern AfricaDevelopment Community (SADC)monitors the crop and food situation inthe region and issues alerts duringperiods of impending crisis. Suchnetworks can be the backbone ofdrought contingency planning andcoordinated plans for dealing withdrought when it comes.

Drought preparedness and management

Sahel drought of 1968-1973 — permanentloss of Sahel savannah vegetation wouldpermit drought conditions to persist. Theeffect of drought, reducing soil moistureand thus evaporation and cloud cover, andincreasing surface albedo as plant cover isdestroyed, is generally to increase groundand near-surface air temperatures whilereducing the surface radiation balance andexacerbating the deficit in the radiationbalance of the local surface-atmospheresystem. This entails increased atmosphericsubsidence and consequently furtherreduced precipitation.

Solar radiation, temperature andevaporation

The only source of energy for the Earth isthe Sun but our world intercepts only a tinyamount of this energy (less than a tenth ofone per cent) needed for the various biolog-ical (photosynthesis) and geophysical(weather and climate) processes that lifedepends on. The Earth system, based onfundamental rules of physics, must emit thesame amount of radiation it receives.Therefore, the complex transfer of energy tosatisfy this requirement is the basis for ourweather and climate. Solar radiation ishighly correlated with cloudiness, and inmost dryland climates where there are littleor no clouds, solar radiation can be quiteintense. In fact, some of the highest knownvalues of solar radiation can be found inplaces like the Sahara desert. Solar heatingof the land surface is the main contributionto the air temperature.

Along with rainfall, temperature is the mainfactor determining climate and therefore thedistribution of vegetation and soil forma-tion. Soil formation is the product of manyfactors that include: the parent material(rock), topography, climate, biological activ-ity, and time. Temperature and rainfall causedifferent patterns of weathering and leach-ing in soils. Seasonal and daily changes in

temperature can affect the soil moisture,biological activity, rates of chemical reac-tions, and the types of vegetation. Importantchemical reactions in the soil include thenitrogen and carbon cycles.

During the summer in the tropics, surfacesoil temperatures can exceed 55°C and thisintense heat contributes to the cracking ofhighly-clay soils that expose not only thesoil surface but also the soil subsurface towater or wind erosion. Of course, these hightemperatures will also increase soil evapo-ration (Figure 10) and further reduceavailable soil moisture for plant growth.

In temperate dry lands, the freeze-thawcycle can have a direct effect on thecomposition of the soil by the movementof rocks and stones from various depths tothe surface. In high elevations, the freeze-thaw is one factor degrading rockstructures, causing cracks and fissureswhich could lead to landslides and rockavalanches.

Evaporation is the conversion of water fromthe liquid or solid state into vapour, and itsdiffusion into the atmosphere. A vapourpressure gradient between the evaporatingsurface and the atmosphere and a source ofenergy are necessary for evaporation. Solarradiation is the dominant source of energy

19

Figure 10. High soiltemperatures on sandysoils can enhance soilevaporation.

and sets the broad limits of evaporation.Solar radiation values in the tropics arehigh, modified by the cloud cover, whichlead to a high evaporative demand of theatmosphere. In the arid and semi-aridregions, considerable energy may beadvected from the surrounding dry areasover irrigated zones. Transfer of energydownwards to the evaporating surface isoften termed “the oasis effect” and in thecotton-growing areas of Gezira in Sudan,marked oasis effects have been shown tocreate water losses of up to twice thosecalculated using standard meteorologicalformulae.

Climatic factors induce an evaporativedemand on the atmosphere, but the actualresulting evaporation will be influenced bythe nature of the evaporating surfaces aswell as by the availability of water. Ondegraded land, the land surface itself influ-ences the evaporative demand by thealbedo and surface roughness, the latteraffecting turbulence. In the arid and semi-arid regions, the high evaporation whichgreatly exceeds precipitation leads to accu-mulation of salts on soil surface. Soils withnatric horizon are easily dispersed and thelow moisture levels lead to limited biologi-cal activity.

Wind

The drylands of the world are affected bymoderate to severe land degradation fromwind erosion and there is evidence that thefrequency of sand storms/dust storms inincreasing. It has been estimated that in thearid and semi-arid zones of the world, 24 percent of the cultivated land and 41 per cent ofthe pasture land are affected by moderate tosevere land degradation from wind erosion.

The worldwide total annual production ofdust by deflation of soils and sediments wasestimated to be 61 to 366 million tonnes.Losses of desert soil due to wind erosion are

globally significant. The upper limit forglobal estimates of the long-range transportof desert dust is approximately 1 × 1016 gyear-1.

For Africa, it is estimated that more than 100million tonnes of dust per annum is blownwestward over the Atlantic. The amount ofdust arising from the Sahel zone has beenreported to be around or above 270 milliontonnes per year (Figure 11) which corre-sponds to a loss of 30 mm per m2 per yearor a layer of 20 mm over the entire area.

Every year desert encroachment caused bywind erosion buries 210,000 hectares ofproductive land in China. It was shown thatthe annual changes in the frequency ofstrong and extremely strong sandstorms inChina are as follows: five times in the 1950s,eight times in the 1960s, 13 times in the1970s, 14 times in the 1980s, and 20 times inthe 1990s.

20

Figure 11. The amount of dust arising from the Sahel zoneis reported to be around 270 million tonnes per year.

Sand and dust storms are hazardousweather and cause major agricultural andenvironmental problems in many parts ofthe world. There is a high on-site as well asoff-site cost due to the sand and duststorms. They can move forward like anoverwhelming tide and strong winds takealong drifting sands to bury farmlands, blowout top soil, denude steppe, hurt animals,attack human settlements, reduce thetemperature, fill up irrigation canals androad ditches with sediments, cover the rail-roads and roads, cause household dustdamage, affect the quality of water in riversand streams, affect air quality, pollute theatmosphere and destroy mining andcommunication facilities. They acceleratethe process of land degradation and causeserious environment pollution and hugedestruction to ecology and the living envi-ronment. Atmospheric loading of dustcaused by wind erosion also affects humanhealth and environmental air quality.

Wind erosion-induced damage includesdirect damage to crops through the loss ofplant tissue and reduced photosyntheticactivity as a result of sandblasting, burial ofseedlings under sand deposits, and loss of

topsoil. The last process is particularlyworrying since it potentially affects the soilresource base and hence crop productivityon a long-term basis, by removing the layerof soil that is inherently rich in nutrients andorganic matter. Wind erosion on light sandysoils can provoke severe land degradation(Figure 12 left) and sand deposits on youngseedlings can affect crop establishment(Figure 12 right).

Calculations based on visibility and windspeed records for 100 km-wide dust plumes,centred on eight climate stations aroundSouth Australia, indicated that dust trans-port mass was as high as ten million tonnes.Thus dust entrainment during dust eventsleads to long-term soil degradation, which isessentially irreversible. The cost to produc-tivity is difficult to measure but is likely to bequite substantial.

Causes of wind erosion

The occurrence of wind erosion at any placeis a function of weather events interactingwith soil and land management through itseffects on soil structure, tilth and vegetation

21

Figure 12. Wind erosionon light sandy soils canprovoke severe landdegradation (left) and sand deposits on young seedlings can affect crop establishment (right)

cover. In regions where long dry periodsassociated with strong seasonal windsoccur regularly, wind erosion usually is aserious problem, as the vegetative cover ofthe land does not sufficiently protect thesoil, and the soil surface is disturbed due toinappropriate management practices.

At the southern fringe of the Sahara Desert,a special dry and hot wind, locally termedHarmattan, occurs. These Northeasterly orEasterly winds normally blow in the winterseason under a high atmospheric pressuresystem. When the wind force of Harmattanis beyond the threshold value, sand parti-cles and dust particles will be blown awayfrom the land surface and transported forseveral hundreds kilometres to the AtlanticOcean.

In Northwest India, the convection sand-dust storm that occurs in the seasonpreceding the monsoon is named Andhi. Itis called Haboob in Africa and Arabic coun-tries. It is titled “phantom” or “devil” insome regions.

In general, two indicators, wind velocity andvisibility, are adopted to classify the gradeof intensity of sand-dust storms. Forinstance, the sand-dust storms occurring inNorthwest India are classified into threegrades. Firstly, the feeble sand-dust stormdevelops when wind velocity is at force 6(Beaufort) and visibility varies between 500-1,000 m. The secondary strong sand-duststorm will occur when wind velocity is atforce 8 and visibility varies between 200-500m. Strong sand-dust storms will take placewhen wind velocity is at force 9 and visibil-ity is <200 metres.

In China, a sand-dust storm is similarlydefined . The only difference is that the cate-gory of strong sand-dust storms is definedagain into two grades, namely strong sand-dust storms and serious sand-dust storms.When wind velocity is 50 metres per second(m/s) and visibility is <200 metres, the sand-

storm is called a strong sand-dust storm.When wind velocity is 25 m/s and visibility is0-50 metres, the sandstorm is termed a seri-ous sand-dust storm (some regions name itBlack windstorm or Black Devil).

Wind erosivity is the main factor control-ling the broad pattern of wind erosion. Ithas been defined as “that property of thewind which determines its ability to entrainand move bare, dry soil in fine tilth”. It canbe estimated from daily or hourly recordsof wind speed above a threshold related tothe lowest speed at which soil particles areentrained. The Chepil-Woodruff index ofwind erosion capacity (C) had been definedas:

where V = wind speed at standard observ-ing levels (~ 10 m), m s-1; P = precipitation(mm); and Ep is potential evapotranspiration(mm). Table 3 gives a classification of thewind erosion capacity as per the differentvalues of the index of wind erosion capacity.

When soil movement is sustained, thequantity of soil that can be transported bythe wind varies as the cube of the velocity.Models demonstrate that wind erosionincreases sharply above a threshold windspeed. In the US corn belt, a 20 per centincrease in mean wind speed greatlyincreases the frequency with which the

€

CV

P E p

=−( )3

2 9.

22

0-20 Insignificant or zero

20-50 Moderate

50-100 High

> 150 Very high

Index value Wind erosion capacity

Table 3. Wind erosion capacity (Source: W.S. Chepil andN.P. Woodruff. 1963. Physics of wind erosion and its control.Advances in Agronomy, 15)

threshold is exceeded and thus thefrequency of erosion events.

There have been several efforts to integrateall these wind erosion factors into acomputer model. One such effort is theWind Erosion Prediction System (WEPS)which is a process-based, daily time-stepmodel that predicts soil erosion by simula-tion of the fundamental processescontrolling wind erosion. WEPS can calcu-late soil movement, estimate plant damage,and predict PM-10 emissions when windspeeds exceed the erosion threshold. It alsoprovides users with spatial informationregarding soil flux, deposition, and lossfrom specific regions of a field over time.The structure of WEPS is modular andconsists of seven submodels and four data-

bases. Most of the WEPS submodels usedaily weather as the natural driving forcefor the physical processes that change fieldconditions. The other submodels focus onhydrology including the changes in temper-ature and water status of the soil; soilproperties; growth of crop plants; cropplant decomposition; typical managementpractices such as tillage, planting, harvest-ing, and irrigation; and finally, the power ofthe wind on a subhourly basis.

Climatic implications of dust storms

The very fine fraction of soil-derived dust(Figure 13) has significant forcing effects onthe radiative budget. Dust particles arethought to exert a radiative influence onclimate directly through reflection andabsorption of solar radiation and indirectlythrough modifying the optical propertiesand longevity of clouds. Depending on theirproperties and in what part of the atmos-phere they are found, dust particles canreflect sunlight back into space and causecooling in two ways. Directly, they reflectsunlight back into space, thus reducing theamount of energy reaching the surface.

23

Four definitions of the dust phenomenonare used by the Australian Bureau ofMeteorology, which conforms to theworldwide standards of the WMO. SYNOPpresent weather [WW] codes are included:

1. Dust storms (SYNOP WW code: 09) arethe result of turbulent winds raisinglarge quantities of dust into the air andreducing visibility to less than 1,000 m.

2. Blowing dust (SYNOP WW code: 07) israised by winds to moderate heightsabove the ground reducing visibility ateye level (1.8 m), but not to less than1,000 m.

3. Dust haze (SYNOP WW code: 06) isproduced by dust particles insuspended transport which have beenraised from the ground by a dust stormprior to the time of observation.

4. Dust swirls (or dust devils) (SYNOP WWcode: 08) are whirling columns of dustmoving with the wind and usually lessthan 30 m high (but may extend to 300m or more). They usually dissipate aftertravelling a short distance.

Figure 13. Soil deriveddust can have signifi-cant forcing effects onthe radiative budget.

24

Wind erosion is possible when the windvelocity at the soil surface exceeds thethreshold velocity required to move theleast stable soil particle. The detachedparticle may move a few millimetresbefore finding a more protected site onthe landscape. The wind velocity requiredto move this least stable particle is calledthe static threshold. If the wind velocityincreases, soil movement begins and ifthe velocity is sufficient, soil movement issustained. This velocity is called thedynamic threshold.

When the wind force reaches thethreshold value a number of particles willbegin to vibrate, Increasing the windspeed still further, and a number ofparticles will be ejected from the surfaceinto the airflow. When these injectedparticles impact back on the surface,more particles are ejected, thus starting achain reaction. Once ejected, theseparticles move in one of three modes oftransport depending on particle size,shape and density of the particle. Thesethree modes are designated suspension,saltation and creep. Their size anddensity determine movement pattern ofsand-dust particles.

The suspension mode involves dustparticles of less than 0.1 mm in diameterand clay particles of 0.002 mm indiameter i.e. small in size and light indensity. These fine dust particles may betransported at altitudes of up to 6 km andmove over distances of up to 6,000 km.

Saltating particles (i.e. those between0.01-0.5 mm in diameter) leave thesurface, but are too large to besuspended. The remaining particles (i.e.

above 0.5 mm) are transported in thecreep mode. These particles are too largeto be ejected from the surface and aretherefore rolled along by the wind andimpacting particles. Due to the nature ofthis mode, the heights carried are rarelymore than 30 cm and the distancetravelled rarely exceeds a few metres.

During a storm, creep can move particlesover distances from a few centimetres toseveral metres, saltating particles travelfrom a few metres to a few hundredmetres, and suspension transport rangesfrom several tens of metres to thousandsof kilometres. In the sand-dust wall, theuplifting force produced by the rising aircurrent is powerful. The sand grains atthe lower stratum of the sand-dust wallare coarse particles, the sand particles inthe middle stratum are the next in sizeand those in the upper stratum aremainly suspension dusts.

Sand particles, transported by saltationand by creep will accumulate to form newsand dunes when they are blown out,graded and transported for a distance.

MECHANICS OF SAND AND DUST STORMS

25

Indirectly, they act as condensation nuclei,resulting in cloud formation. Clouds act asan “atmospheric blanket,” trapping long-wave radiation within the atmosphere thatis emitted from the Earth. Thus, dust stormshave local, national and international impli-cations concerning global warming.Climatic changes in turn can modify thelocation and strength of dust sources.

Wildfires, land degradation andatmospheric emissions

Uncontrolled wildfires occur in all vegeta-tion zones of the world. It is estimated thatfires annually affect 1,015 million hectares(m ha) of boreal and temperate forest andother lands, 2,040 m ha of tropical rainforests due to forest conversion activitiesand escaped agricultural fires, and up to 500m ha of tropical and subtropical savannas,woodlands, and open forests. The extent ofthe soil organic carbon pool doubles thatpresent in the atmosphere and is about twoto three times greater than that accumu-lated in living organisms in all the Earth'sterrestrial ecosystems. In such a scenario,one of the several ecological and environ-mental impacts of fires is that they are asignificant source of greenhouse gasesresponsible for global warming.

Globally, biomass burning (Figure 14), whichincludes wildfires, is estimated to produce40 per cent of the carbon dioxide, 32 per centof the carbon monoxide, 20 per cent of theparticulates, and 50 per cent of the highlycarcinogenic poly-aromatic hydrocarbonsproduced by all sources. Current approachesfor estimating global emissions are limitedby accurate information on area burned andfuel available for burning.

Emissions from fires are considerable andcontribute significantly to gross globalemissions of trace gases and particulatesfrom all sources to the atmosphere. Naturalemissions are responsible for a major

portion of the compounds, including non-methane volatile organic compounds(NMVOC), carbon monoxide (CO) and nitricoxide (NO), which determine troposphericoxidant concentrations. The total NMVOCflux is estimated to be about 84 × 1012 g ofcarbon (Tg C) which is comprised primarilyof isoprene (35 per cent), 19 other terpenoidcompounds (25 per cent) and 17 non-terpenoid compounds (40 per cent).

The influence of fire on soil characteristics(soil-water content, soil compaction, soiltemperature, infiltration ability, soil proper-ties especially organic matter, pH,exchangeable Ca, Mg, K, Na and extractableP) of a semi-arid southern African rangelandwas quantified over two growing seasons(2000/01-2001/02) following an accidentalfire. The decrease in basal cover due to fire(head fires) exposed the soil more to thenatural elements and therefore to highersoil temperatures and soil compaction inturn leading to lower soil-water content anda decline in soil infiltrability.

Climate change and land degradation

Human activities — primarily burning offossil fuels and changes in land cover — aremodifying the concentration of atmosphericconstituents or properties of the Earth'ssurface that absorb or scatter radiant energy.

Figure 14. Fires are a significant source of greenhousegasses

26

In particular, increases in the concentrationsof greenhouse gases (GHGs) and aerosolsare strongly implicated as contributors toclimatic changes observed during the 20thcentury (Figure 15), and are expected tocontribute to further changes in climate inthe 21st century and beyond. These changesin atmospheric composition are likely toalter temperatures precipitation patterns,sea level, extreme events, and other aspectsof climate on which the natural environmentand human systems depend.

According to the Intergovernmental Panelon Climate Change (IPCC), established byWMO and UNEP, ecosystems are subject tomany pressures (e.g. land-use change,resource demands, population changes);their extent and pattern of distribution ischanging, and landscapes are becomingmore fragmented. Climate change consti-tutes an additional pressure that couldchange or endanger ecosystems and themany goods and services they provide. Soilproperties and processes — includingorganic matter decomposition, leaching,and soil water regimes — will be influencedby temperature increase. Soil erosion anddegradation are likely to aggravate thedetrimental effects of a rise in air tempera-ture on crop yields. Climate change mayincrease erosion in some regions, throughheavy rainfall and through increased windspeed.

CO2-induced climate change and landdegradation remain inextricably linkedbecause of feedbacks between land degra-

dation and precipitation. Climate changemight exacerbate land degradation throughalteration of spatial and temporal patternsin temperature, rainfall, solar radiation, andwinds. Several climate models suggest thatfuture global warming may reduce soilmoisture over large areas of semi-aridgrassland in North America and Asia. Thisclimate change is likely to exacerbate thedegradation of semi-arid lands that will becaused by rapidly expanding human popu-lations during the next decade. It ispredicted that there will be a 17 per centincrease in the world area of desert land dueto the climate change expected, with adoubling of atmospheric CO2 content.

Water resources are inextricably linked toclimate, so the prospect of global climatechange has serious implications for waterresources and regional development.Climate change — especially changes inclimate variability through droughts andflooding — will make addressing theseproblems more complex. The greatestimpact will continue to be felt by the poor,who have the most limited access to waterresources. The impact of changes in precip-itation and enhanced evaporation couldhave profound effects in some lakes andreservoirs. Studies show that, in the paleo-climate of Africa and in the present climate,lakes and reservoirs respond to climate vari-ability via pronounced changes in storage,leading to complete drying up in manycases. Furthermore, these studies also showthat under the present climate regimeseveral large lakes and wetlands show a

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delicate balance between inflow andoutflow, such that evaporative increases of40 per cent, for example, could result inmuch reduced outflow.

The frequency of episodic transport by windand water from arid lands is also likely toincrease in response to anticipated changesin global climate. Lower soil moisture andsparser vegetative cover would leave soilmore susceptible to wind erosion.Reduction of organic matter inputs andincreased oxidation of SOM could reducethe long-term water-retention capacity ofsoil, exacerbating desertification. Moreover,increased wind erosion increases wind-blown mineral dust, which may increaseabsorption of radiation in the atmosphere.

Carbon sequestration to mitigate climatechange and combat land degradation

The soil organic carbon (SOC) pool to 1mdepth ranges from 30 tons ha-1 in the aridclimates to 800 tons ha-1 in organic soils incold regions. Conversion of natural to agri-cultural ecosystems causes depletion of theSOC pool by as much as 60 per cent in soilsof temperate regions and 75 per cent ormore in the cultivated soils of the tropics.The depletion is exacerbated when theoutput of carbon (C) exceeds the input andwhen soil degradation is severe.

Carbon sequestration implies transferringatmospheric CO2 into long-lived pools andstoring it securely so it is not immediatelyre-emitted. Thus, soil C sequestrationmeans increasing SOC and soil inorganiccarbon stocks through judicious land useand recommended management practices.Some of these practices include mulchfarming, conservation tillage, agroforestryand diverse cropping systems, cover cropsand integrated nutrient management,including the use of manure, compost,biosolids, improved grazing, and forestmanagement.

The potential carbon sink capacity ofmanaged ecosystems approximately equalsthe cumulative historic C loss estimated at55 to 78 gigatons (Gt). Offsetting fossil-fuelemissions by achievable SOC potentialprovides multiple biophysical and societalbenefits. An increase of 1 ton of soil carbonof degraded cropland soils may increasecrop yield by 20 to 40 kg ha-1 for wheat, 10to 20 kg ha-1 for maize, and 0.5 to 1 kg ha-1

for cowpeas, and could enhance world foodsecurity.

Understanding the interactions betweenclimate and land degradation - role ofWMO

WMO is the United Nations specializedagency responsible for meteorology andoperational hydrology. WMO providessupport to the National Meteorological andHydrological Services (NMHSs) of its 187Member States and Territories in theirrespective missions of observing andunderstanding weather and climate andproviding meteorological and related serv-ices in support of national needs. Theseneeds especially relate to protection of lifeand property, safeguarding the environ-ment and contributing to sustainabledevelopment.

The scientific programmes of WMO havebeen vital in expanding knowledge of theclimate system. The systematic observa-tions (Figure 16) carried out usingstandardized methods have provided world-wide data for analysis, research andmodeling of the atmosphere and its chang-ing patterns of weather systems. WMOcoordinates a global network for the acqui-sition and exchange of observational dataunder the Global Observing System of itsWorld Weather Watch Programme(Figure 17). The system comprises some10,000 stations on land, 1,000 upper-airstations, 7,000 ships, some 3,000 aircraftproviding over 150,000 observations daily

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and a constellation of 16 meteorological,environmental, operational and researchsatellites (Figure 18). WMO also coordinatesa network of three World MeteorologicalCentres, 35 Regional SpecializedMeteorological Centres and 187 NationalMeteorological Centres. Specializedprogrammes of observations, includingthose for chemical constituents of theatmosphere and characteristics of theoceans and their circulations, have led to abetter understanding of interactionsbetween the domains of the climate system(the atmosphere, the oceans, the land

surface and the cryosphere) and of climatevariability and change.

Specifically, WMO contributes to under-standing the interactions between climateand land degradation through dedicatedobservations of the climate system;improvements in the application of agrome-teorological methods and the properassessment and management of waterresources; advances in climate science andprediction; and promotion of capacity build-ing in the application of meteorological andhydrological data and information indrought preparedness and management. Inthis context, WMO will continue to addressthe issue of land degradation through itsAgricultural Meteorology Programme,Hydrology and Water ResourcesProgramme, and other scientific and techni-cal programmes by:

(a) Advocating for enhanced observingsystems at national, regional and inter-national levels. WMO is committed towork with the Parties to the UNCCD toimprove the observing systems forweather, climate and water resources inorder to meet the needs of theConvention, and to assist developingcountries to strengthen their participa-tion in the collection and use of theseobservations to meet their commit-ments to the Convention. In this regard,it is quite relevant to examine the deci-sions of the Conference of Parties of theUnited Nations Framework Conventionon Climate Change (UNFCCC) whichaddress the issue of climate observingsystems, and the regional workshopprogramme that has been developedand is being implemented in differentparts of the world by the Global ClimateObserving System (GCOS) Secretariatco-sponsored by WMO.

(b) Promoting effective early warningsystems, which also serve as an essen-tial and important alert mechanism for

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combating land degradation. As mete-orological and hydrological hazards arelinked with climate variability, regularassessments and authoritative state-ments on the interpretation andapplicability of observational data areneeded for the study of climate variabil-ity and the implementation of a climatealert system to allow NMHSs to makeearly warnings on pending significantclimate anomalies. Warnings of climate-related disasters are becoming feasiblefrom weeks to seasons in advance.WMO's World Climate Programme willcontinue to issue routine statements onthe state of El Niño or La Niña, which,through the NMHSs, can alertGovernments to ensure preparednessagainst the impacts of El Niño-relatedanomalies, which can trigger variousdisasters. WMO played an active role inthe activities of the ad hoc Panel onearly warning systems established bythe Committee on Science andTechnology (CST) of the UNCCD. Highon the list of recommendations of thePanel is the need to undertake a criticalanalysis of the performance of earlywarning, monitoring and assessment

systems; the improvement of methodsfor and approaches to the prediction ofdrought and monitoring of desertifica-tion; and the development ofmechanisms to facilitate an exchange ofinformation focusing in particular onnational and subregional networks.WMO's new major programme onNatural Disaster Prevention andMitigation will provide the focus for the

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Figure 17. Typicalcoverage of surfaceobservations.

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consolidation of its efforts in the area ofearly warnings and for taking newinitiatives in this area in collaborationwith other organizations.

(c) Further enhancing climate predictioncapability through the Climate Variability(CLIVAR) project of the World ClimateResearch Programme (WCRP). Theprediction of El Niño and the associatedimpacts are becoming possible, withreasonable skill, up to a few seasons inadvance. Related to this, WMO is broad-ening the implementation of the WMOClimate Information and PredictionServices (CLIPS) project, which isdesigned to promote the use of climateinformation and prediction services,capacity building, multi-disciplinaryresearch and the development of newapplications. Consensus long-range fore-casts on droughts, which were issued atseveral Regional Climate Outlook Fora,organized in different parts of the worldwith active support from WMO, providegood early warning information tonational authorities (Figure 19).

(d) Assessing vulnerability and analysinghazards by employing the knowledge ofvulnerability at the local, national and

regional levels, which is an importantfactor in evaluating the adequacy ofearly warnings. A good tool to assessthese different vulnerabilities is the link-age between weather, climate anddisaster databases to the different typeof meteorological or hydrological disas-ters. In this regard, a pilot project isongoing in Chile linking climate withflood disaster databases with thesupport of WMO through the WorldClimate Programme, as part of theactivities of the Inter-Agency Task Forcefor Disaster Reduction Working Groupson Climate and Disasters and on RiskVulnerability and Impact Assessment.This is an important tool for riskcommunication among policy makersand communities. WMO will continueto assist in developing and managingthe relevant climate databases throughdata rescue and climate databasemanagement projects.

(e) Implementing risk management appli-cations to combat droughts andmitigate floods. In this context, hazardmapping, suitable agroclimatic zoningand the establishment of partnershipsare essential tools for land use andpreparedness planning. Several expertteams established by the Commissionfor Agricultural Meteorology (CAgM) ofWMO are examining these issues criti-cally and are issuing guidance reportsfor the users. In the area of flood fore-casting and management, WMO'sHydrology and Water ResourcesProgramme is implementing theAssociated Programme for FloodManagement (APFM) in collaborationwith the Global Water Partnership, inthe context of integrated waterresources management. Several relatedprojects are being developed in differ-ent parts of the world in order toprovide guidance on the developmentof support systems for sustainable landmanagement and agroclimatic zoning.

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Figure 19. A typicalclimate outlook

produced by theWMO-supported IGADClimate Prediction and

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In order to make quality assessmentsand decisions on the interactionsbetween climate and such topics suchas drought and desertification, naturaldisasters, deforestation, extreme eventsand for the assessment of their socio-economic impacts, scientists need toanalyse and integrate very differentkinds of spatial and temporal data setsfrom many different disciplines. Thesedisciplines range from the plant, crop,and soil sciences to meteorology with abroad range of knowledge andtechniques that have each developedover the past few decades. The datasets used can range from digitized arealmaps of soil types, land use, andvarious remotely sensed data fromsatellites to data sets of point data ofrainfall, temperature, and elevation.

Agroclimatic mapping andagroecological zoning are somemethodologies that can be used toanalyse these diverse data sets. Overthe past decade, GeographicInformation Systems (GIS) have beendeveloped to aid in these kinds ofanalyses. GIS is a computer systemdesigned to allow users to collect,manage, archive, analyse, andmanipulate large volumes of spatiallyreferenced and associated attributedata. The data is referenced accordingto the geographic coordinates of itselements (latitude and longitude). It isuseful to view GIS as a process ratherthan an object. GISs are frequentlyused in analysing remotely-sensed datafrom satellites.

Efforts in adopting state-of-the-art toolsand technologies such as remotesensing, GIS, ground measurementsand modeling applications ingenerating combined agroclimaticmaps already contribute to thedevelopment of sustainable and moreefficient crop production systems inrelation to local crops and humanresources. Forecasts of vulnerability toclimate variability and risk assessmentare included in such analyses asimportant tools for farm managementand decision taking. Research isunderway in applying thesemethodologies for sustainable landmanagement, biodiversity conservationand evaluation of the specific potentialsof different agricultural realities.

Analytical tools for agroclimatic mapping

Environmentally sensitive areas to desertification inSardinia (Montroni and Canu, 2005)

(f) Contributing actively to the implemen-tation of the UN system's InternationalStrategy for Disaster Reduction (ISDR).It is to be noted that society's ability tocope with and adapt to, climate changewill depend heavily on its ability toassess how and where weather andclimate patterns are likely to change, topredict the continuous fluctuations inrisk and vulnerability to communities,and to develop adaptive strategies thatwill increase the community's resiliencewhen the next potential disaster strikes.WMO leads the ISDR Working Group onClimate and Disasters.

(g) Supporting the strengthening of thecapabilities of the Parties and regionalinstitutions with drought-relatedprogrammes and promoting collabora-tion with other institutions in droughtand desertification-prone regions, withemphasis on Africa, Asia, Latin Americaand the Caribbean, and the northernMediterranean region, which are allreferred to in the Regional Annexes tothe Convention. Examples of such insti-tutions in Africa are the AGRHYMETCentre and the African Centre ofMeteorological Applications forDevelopment (ACMAD), both located inNiamey, Niger, and the WMO-supported IGAD Climate Prediction andApplications Centre (ICPAC) and theSACD Drought Monitoring Centres(DMC) for Eastern and Southern Africalocated in Nairobi, Kenya and Harare,Zimbabwe, respectively. In order toenhance capacity building in the devel-opment of National Action Plans withinthe framework of the Convention, WMOorganized Roving Seminars on theApplication of Climatic Data forDesertification Control, DroughtPreparedness and Management ofSustainable Agriculture in Beijing,China in May 2001, and in Antigua andBarbuda in April 2004.

Future perspectives

The definition of land degradation adoptedby UNCCD assigns a major importance tocontributing climatic factors, but there is noconcerted effort at the global level tosystematically monitor the impacts of differ-ent climatic factors on land degradation indifferent regions and for different classes ofland degradation. Hence there is an urgentneed to monitor the interactions betweenclimate and land degradation. To betterunderstand these interactions, it is alsoimportant to identify the sources and sinksof dryland carbon, aerosols and trace gasesin drylands. This can be effectively donethrough regional climate monitoringnetworks. Such networks could also helpenhance the application of seasonal climateforecasting for more effective drylandmanagement.

There are serious gaps in the basic meteor-ological network and observational facilitiesin many areas, some of them in regions withsevere land degradation problems. Themost serious single and geographicallywidespread shortcoming is the lack of infor-mation on rainfall intensity. WMO is takingsteps to facilitate the development of earlywarning systems by organizing the develop-ment of suitable instruments and statisticalprocessing. Furthermore, WMO is coordi-nating efforts on the part of its Members tofurther investigations of using data frommeteorological satellites to supplementknowledge of meteorological conditionsinfluencing land degradation, especiallyover areas inadequately covered by ground-level observations. WMO through its 187Members is pleased to be part of the effortto better understand the role of climate inland degradation and work with variousnational, regional and international organi-zations and civil society in combating andarresting land degradation.

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so i l conserva t ion – l and management – f lood fo recas t ing – food secur i ty

World Meteorological Organization

For information about WMO, please contact:Communications and Public Affairs Office

World Meteorological Organization7bis, avenue de la Paix - P.O. Box 2300 - CH 1211 Geneva 2 - Switzerland

Tel.: (+41-22) 730 83 14 - 730 83 15 - Fax: (+41-22) 730 80 27

E-mail: cpa@wmo.int - Website: www.wmo.int

For more information, please contact:World Climate Programme Department

World Meteorological Organization7bis, avenue de la Paix - P.O. Box 2300 - CH 1211 Geneva 2 - Switzerland

Tel.: (+41-22) 730 83 80 - Fax: (+41-22) 730 80 42

E-mail: agm@wmo.int - Website: http://www.wmo.int/web/wcp/agm/agmp.html

climate information – resource conservation – sustainable management of land

Climate and Land Degradation

WMO-No. 989

2005