Geoscience Reference
In-Depth Information
Table 24.3.
Extent of soil degradation in susceptible drylands (in Millions of
Hectares) (From UNEP,
1997
)
Type of soil degradation
Wa t er
erosion
Wind
erosion
Chemical
deterioration
Physical
deterioration
Region
Total
Africa
119.1
159.9
26.5
13.9
319.4
Asia
157.5
153.2
50.2
9.6
370.5
Australia
69.6
16.0
0.6
1.2
87.4
Europe
48.1
38.6
4.1
8.6
99.4
North America
38.4
37.8
2.2
1.0
79.4
South America
34.7
26.9
17.0
0.4
79.0
Total
467.4
432.4
100.7
34.7
1035.2
(ISRIC) in the Netherlands, and in 1987 both partners began the Global Assessment
of Soil Degradation (GLASOD) project. Data from this project were used in both edi-
tions of the
World Atlas of Desertification
(UNEP,
1992a
; UNEP,
1997
).
Table 24.2
shows the global extent and severity of soil degradation obtained from the GLASOD
data (UNEP,
1997
). Nevertheless, we should only regard these estimates as order
of magnitude values, since in most instances we still lack adequate ground control.
For example, it seems highly likely that the extent of salt-affected soils in Australia
is seriously underestimated in
Table 24.3
. There is also a methodological problem
in defining areas by drawing polygons around what are often only point sources of
soil degradation, leading to overestimates (David Thomas, personal communication,
2000). This is particularly the case in regard to overgrazing around wells, boreholes
and other watering points in the rangelands of Africa and Australia (Pickup,
1996
;
Pickup,
1998
). Despite these methodological teething problems, it seems reasonable
to accept that the degradation of dryland soils and ecosystems is a major problem
on every inhabited continent in terms of both the total areas affected and the relative
proportions of arable and pastoral lands.
24.12 Mitigation and prevention of desertification
Desertification is caused, directly and indirectly, by a variety of biophysical, social and
economic factors and therefore needs to be considered at a variety of scales ranging
from local through regional and national to international. We start with the interna-
tional efforts to curb or prevent desertification. One potentially important initiative
to emerge from the 1992 Earth Summit was a request to the UN General Assembly
to establish an intergovernmental committee for negotiating a convention on deser-
tification. At its forty-seventh session in 1992, the General Assembly acted swiftly
and set up the International Negotiating Committee, which met five times to debate
