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Desertification Atlas (Global) Maps 1-20 Entry ID: NBId0288_101 |
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Summary
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Abstract:
INTRODUCTION Desertification/Land Degradation - The Background More than 6.1 billion hectares, over one third of the Earth's land area, is dryland. Nearly one billion hectares of this area are naturally hvperarid deserts, with very low biological productivity. The remaining 5.1 billion hectares are made up of arid, semiarid and dry subhumid areas, part of which have become desert since the dawn of civilization while other parts of these areas are still being degraded by human action today. These lands are the habitat and the source of livelihood for one quarter of the world's population. They are areas characterized by the persistent natural menace of recurrent drought, a natural hazard accentuated by imbalanced management of natural resources. Particularly acute drought years in the Sahelian region of Africa from 1968 to 1973, and their tragic effects on the peoples of the region, drew worldwide attention to the problems of human survival and development in drylands, particularly on desert margins. These problems have been addressed by the United Nations (UN) General Assembly, in conformity with the Charter of the United Nations. The UN General Assembly's Resolution 3202 (vi) of 1 May 1974 recommended that the international community undertake concrete and speedy measures to arrest desertification and assist the economic development of affected areas. The Economic and Social Council's Resolution 1878 (LVII) of 16 July 1974 requested all the concerned organizations of the UN system to pursue a broad attack on the drought problem. Decisions of the Governing Councils of the UN Development Programme (UNDP) and the UN Environment Programme (UNEP) emphasized the need for undertaking measures to check the spread of desert conditions. The General Assembly then decided, by Resolution 3337 (xxix) of 17 December 1974, to initiate concerted international action to combat desertification and, in order to provide an impetus to this action, to convene a UN Conference on Desertification (UNCOD), between 29 August and 9 September 1977 in Nairobi, Kenya, which would produce an effective, comprehensive and coordinated programme for solving the problem. For the purposes of this atlas, desertification/land degradation is defined as: Land degradation in arid, semiarid and dry subhumid areas resulting mainly from adverse human impact. Sustainable Land Use The concept of degradation is inseparable from that of sustainability. Expressed simply, a sustainable land use is one that is able to continue without degrading the land it is using. In this case the sustainability of a particular land use depends both on the properties of the resource and the way it is managed. The feature of a resource that determines its sustainability under a particular use is its resilience, and it is important to note that the resilience of a resource system will according to different land uses and indeed may vary from time to time, depending largely on seasonal and interannual variability and management practices and technologies. A good way to measure the resilience of a particular unit land is to look at its ability to recover after a disturbance. Such a disturbance may be climatic, for example a drought, or human induced, such as vegetation clearance or soil tillage. The greater the disturbance the area can recover from, the greater its resilience. In essence, land degradation is the weakening of an area's resilience. One measure of land degradation is the cost of rehabilitation. Variability in Drylands One serious difficulty of examining land degradation in dryland regions is their inherent variability. A definitive characteristic of drylands is their aridity which, put simply, means their lack of available moisture. An area can be said to be arid when its moisture inputs (precipitation) are exceeded by the moisture losses (evapotranspiration) plus any changes in storage (in rivers, groundwater, lakes and soil moisture). Various climatic and biological indexes have been used to measure the aridity of drylands and these can be used to delimit dryland areas (see the section on climatic data pages 2-5). However, it is important to note that although drylands are usually deficient in moisture on an annual basis, the moisture inputs as precipitation are notoriously variable in both time and space. It is not unheard of, for example, for the more extreme dryland areas to receive all their "average" annual precipitation in just one rainfall event, and commonly such regions receive their average annual totals in just a few days. The meteorological systems that bring rainfall to dryland regions are typically convective cells which means that spatially rainfall often falls in small specific zones. These characteristics of dryland climate, to which indigenous plants and animals have adapted, mean that these regions are highly dynamic on a timescale of weeks and months. Added to this is the variability of precipitation over longer timescales of years and decades. Droughts, in essence the absence of expected precipitation, are also characteristic of the dryland environment. A year, two years or several years may pass in which precipitation is well below "average". Again, indigenous plants and animals have adapted to cope with this inherent variability of the dryland ecosystem. Implications for Desertification Study The implications of this environmental variability for the study of desertification are manifold. Firstly it has implications for the location of drylands themselves. Although for the purposes of the assessment carried out in the pages of this atlas dryland areas are highlighted and given specific boundaries, it is important to note that these boundaries are simply based upon average conditions and that actual ground conditions vary greatly through variable timescales. There are also implications for the human inhabitants of dryland regions. The use to which land is put must be as dynamic as the environment and its resources if those resources are to be used sustainably. Fields that produce a good crop of millet for example during near-average rainfall years may have to be left fallow during a drought year, or put to another use, the land is not to be degraded. It is in areas where this flexible land use response does not occur that desertification/land degradation takes place. Perhaps the most serious implication of dryland variability in the present context is for the identification, monitoring and combating of desertification itself. Satellite imagery of vegetation greenness on the fringes of drylands indicate that the natural variability in climate is reflected in a green vegetation dryland boundary that can fluctuate by up to 200 km from a dry year to a following wet year. With such great natural fluctuations in the ground state of drylands, it is clearly necessary to monitor potential areas of desertification/land degradation over a timescale of decades before it is possible to safely state that a particular region has suffered from land degradation in the form of desertification. The Need for Data Reliable identification of the locations and situations in which such land degradation takes place is essential if viable remedies to the problem are to be reached. The actual reasons for unsustainable land use taking place in a particular area may well have their roots in social and economic conditions. But before these underlying causes can be tackled it is necessary to locate and quantify the nature of the problem. Unfortunately, accurate and reliable data on the extent of desertification and the rate of its progress based on actual ground surveys are very scarce. The existing data are often controversial and open to doubts and criticisms. Early attempts to assess the extent of desertification on the global scale, such as the world map prepared for UNCOD, represent useful first steps towards the goal of solving the problem. But these first efforts had their problems. Perhaps most importantly was the perception that desertification threatened all the world's drylands. However, when the prime motivation for studying desertification is to help relieve the problems faced by inhabitants in using dryland resources, it is clear that areas of hyperarid desert which by definition have very sparse biological resources should not be included in the areas of investigation. Very few people use these regions because of their lack of resources, and these areas can hardly become more desert-like. Hence, for the purposes of this atlas, the regions deemed to be susceptible to desertification are those in the arid, semiarid and dry subhumid zones and these regions are referred to as the ""susceptible drylands"". "There can be grounds for criticizing a global approach to the problem in itself. The complex nature of desertification means that adequate assessment and consequent plans to counteract the problem can only be usefully carried out on the local scale. Nonetheless, it is useful to attain a worldwide appreciation of the phenomenon in order to estimate its magnitude worldwide and to identify more specific problem areas at the national and local scale. For this reason this atlas is organized specifically to start from a global perspective and to zoom in to the local scale. Hence the atlas is divided into three sections - global, continental and case studies - and within each section the general picture is given before more detailed analysis. A Thematic Approach The scarcity of data on desertification and the many forms it can take has necessitated a fresh approach to assessing the problem. It is not realistic to produce a single map of world desertification. A more viable approach is to map the many indicators of desertification and the factors that affect those indicators. Global data sets for all these variables are not currently available however, so that the contents of this atlas have been constrained by data availability. The basic indicator chosen for this atlas is human-induced soil degradation, and information is available on types, severity, causes and extent of human-induced soil degradation for the global land surface. These data are supplemented by various other data sets, principally on climate and vegetation. The constraints of data availability mean that this atlas is by no means exhaustive. Human-induced soil degradation is an important indicator of desertification, but it is certainly not the only one. The degradation of vegetation is another important aspect of desertification. The loss of grassland resource potential due to overgrazing of pastures for example may be as important on the world scale as soil degradation, but unfortunately, no adequate global database for vegetation degradation exists. There may be some overlap between vegetation degradation and soil degradation, such as in areas where vegetation cover has been lost, hence exposing soils to erosion, but there will also be areas where a degraded vegetation resource has no visible impact on the soil resource, or at least not immediately. These deficiencies of the atlas are not highlighted in order to denigrate the contents of this volume. On the contrary, this atlas represents a significant step forward in our appreciation of desertification as a phenomenon and in our approach to its resolution. It uses new data and employs a fresh technical approach. The difficulties outlined above simply illustrate some of the problems involved in studying such a complex environmental issue. Map Projections All of the maps shown in the global and continental sections of this atlas use the Van der Grinten projection. Whilst at the global scale some projections achieve minimal area distortions and others minimal distortion of the shape of continents, Van der Grinten is amongst those which achieve a good compromise between these two variables. The benefits of this projection, however, decline towards the poles, hence the maps are cut off at 72N and 57S. This does not detract from the utility of the atlas since dryland areas susceptible to desertification are largely found within thirty degrees of latitude either side of the Equator. Tables in the text showing data for continents refer to the regions defined by the Times Atlas of The World and shown in the map below. Since the Van der Grinten projection is not a true equal area projection, it could not be used for area calculations. using the computer, all maps were transformed into Mollweide, an equal area projection, before calculating data on areas. Maps used in the national section use various different projections as appropriate to the representation of areas at a larger scale. Geographic Information Systems The data used in the compilation of the maps, tables and diagrams in the global and continental sections of this atlas are stored on computer in the UNEP Global Resources Information Database (GRID). GRID is developing a global network of centres which use computer technology to process environmental data and analyse the interactions of environmental variables, thus forming a bridge between monitoring and assessment, and environmental management. GRID uses geographic information systems, or GIS, and remote sensing technologies for environmental monitoring and assessment. A GIS stores both spatial or geo-referenced and non-spatial data, and allows the user to manipulate, retrieve and analyse these data to produce information which can be used for environmental assessment and management. Results can be produced as maps, in tabular form, or as statistics. Hence annual rainfall data for a global network of meteorological stations can be spatially referenced, processed by the computer, and then output as a world map showing "surface" rainfall totals. If monthly rainfall totals are available, they can be used to output a series of monthly rainfall distribution maps. When rainfall data for the next year become available, these data can be fed into the GIS and another annual map produced which can be compared to the previous year's distribution for changes in time as well as space analysis. The system enables the investigator to have almost infinite flexibility to generate maps according to the particular research interest. One map might show annual rainfall distribution by 100 mm classes for example. A different query of the database could be prepared to show all areas that have received less than 250 mm in a year. A GIS allows the investigator to superimpose or overlay different data sets to produce a new map, enabling the user to visualize, model and quantify the interaction between many different parameters. Hence the annual rainfall map could be superimposed on a world map of soil degradation in order to identify those areas where water erosion is most prevalent. If the rainfall were combined with a map of population distribution, it would highlight those areas where large human populations are affected. The overlay maps can be produced at global, continental or regional level, depending on the nature and scale of the data provided and the need of the researcher. For the decision-making process, GISs provide the opportunity of fast updating of databases and speedy data analyses. Therefore, scientists can propose a number of alternative scenarios, analyse results and modify parameters according to the desired objectives. This approach to problem-solving could be much slower, cumbersome and prone to error with conventional methodologies. Data sets Several data sets have been used in the compilation of this atlas. Specific databases are discussed as they are introduced, but there are two which are central to the global and continental sections. These are a global soils degradation database and a global climatic database. The development of climatic data sets to produce a bioclimatic database used to delineate drylands is explained in detail on pages 2-5. The data on soil degradation are taken from the Global Assessment of Soil Degradation (GLASOD). GLASOD is the result of a collaboration between UNEP and the International Soil Reference and Information Centre (ISRIC) in the Netherlands. There are in fact two soil degradation databases prepared by ISRIC, one of global extent; the other, which is more detailed, is specific to Africa. The data contained in these databases are a compilation of existing information and of expert knowledge made available by more than 250 soil and environmental experts worldwide on the status of human-induced soil degradation in their specialist geographical regions. They contain information on the type of soil degradation, the degree, the area affected, and the major causes. More detailed appraisals of these data sets and their characteristics are given on pages 11 and 29. |
| N: 90.0 | S: -90.0 | E: 180.0 | W: -180.0 |
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Temporal Coverage
| Start Date: 1990-01-01 |
| Stop Date: 1992-12-30 |
Location Keywords
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GEOGRAPHIC REGION
> GLOBAL
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Science Keywords
| BIOSPHERE >TERRESTRIAL ECOSYSTEMS >DESERTS [Definition] |
| HUMAN DIMENSIONS >HABITAT CONVERSION/FRAGMENTATION >DESERTIFICATION >GLOBAL DESERTIFICATION [Definition] |
| HUMAN DIMENSIONS >POPULATION >POPULATION DISTRIBUTION [Definition] |
ISO Topic Category
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BIOTA
ENVIRONMENT SOCIETY |
Project
| GTOS >Global Terrestrial Observing System [Information] |
Access Constraints
| Public |
Keywords
| DESERT |
| DESERTIFICATION |
| DRYLANDS |
| GIS |
| LAND USE |
| SAHEL |
| Terrestrial Ecosystems > Arid Land Ecosystem > Desertification |
Originating Center
| GRID/UNEP |
Data Center
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Global Resource Information Database - Nairobi, Division of Early Warning and Assessment, United Nations Environment Programme
Data Center URL: http://gridnairobi.unep.org/
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Personnel
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JOHANNES
AKIWUMI Role: TECHNICAL CONTACT Phone: (+254-20) 7624214 Fax: (+254-20) 7624315 Email: Johannes.Akiwumi at unep.org Contact Address: Head, Data and Information Management Section Division of Early Warning and Assessment (DEWA) United Nations Environment Programme P. O. Box 30552 City: Nairobi Postal Code: 00100 Country: Kenya |
Creation and Review Dates
Last DIF Revision Date:
2012-05-31
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