Desertification Atlas (Global) Maps 1-20
Entry ID:
NBId0288_101
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Summary
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.
Geographic Coverage
Spatial coordinates
| N: 90.0 |
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S: -90.0 |
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E: 180.0 |
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W: -180.0 |
Temporal Coverage
Start Date:
1990-01-01
Stop Date:
1992-12-30
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Location Keywords
Science Keywords
ISO Topic Category
Project
Access Constraints
Public
Ancillary Keywords
Originating Center
Data Center
Personnel
MARIO
HERNANDEZ
Role:
TECHNICAL CONTACT
Phone:
(254)-2-62.12.34
Fax:
(254)-2-62.43.15
Email:
mario.hernandez at unep.org
Contact Address:
UNEP-GRID, Nairobi, Kenya
P.O. Box 30552
City:
Nairobi
Country:
Kenya
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Creation and Review Dates
Last DIF Revision Date:
2005-06-14
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