Geoscience Reference
In-Depth Information
activities put increased stress on the environment. The
driving forces are social, economic and political factors
such as population increase, unequal distribution of
resources, land tenure methods, government attitudes to
agriculture and the terms of trade. These factors may
limit the options open to the poorer strata of society, who
may have to degrade the soil resources in order to survive.
Tenant farmers may have a short-term view of the land's
value, trying to maximize yields rather than taking a long-
term view of soil improvement. Here we have the ethical
and practical question of who should pay: the individual
farmer or society as a whole? The United Nations has a
plan of action to combat desertification. This aims to
improve management in dryland areas, provide financial
incentives to eliminate overstocking of animals and
encourage rest periods from grazing that should allow the
natural vegetation to become re-established. It will be a
costly undertaking and needs careful management in parts
of the world that already suffer from economic and social
instability.
natural phenomenon associated with declining precipita-
tion has been superseded. However, we must not forget
the natural variability of precipitation in dry lands. It is
characterized by high variability in space and time. Much
of the annual precipitation falls during a few events in the
rainy season. A higher frequency of events in one year
would lead to a higher annual total and vice versa.
Superimposed on the high year-to-year variability,
short-term trends may occur towards wetter or drier
conditions. In practice it is then difficult to distinguish
between adverse effects generated by human action and
the dryland response to the natural climatic variability.
Areas of desertification
The area of the world affected by desertification is not
known with certainty. Despite the continuous monitoring
of Earth's surface by satellite, the values quoted are still
based on intelligent estimates rather than scientific data
(
Table 26.3
)
. What
Table 26.3
does show is that a large
proportion of the dry lands is affected by desertification.
What it does
not
mean is that large areas of the dry lands
are being engulfed by sand dunes blown in from the
desert. That image may be appropriate in a few areas, as
at Nouakchott, the capital of Mauritania, where dunes are
invading its suburbs, but the idea is oversimplified and is
not an accurate picture of the way desertification works.
The process of desertification is much more complex and
is more likely to involve vegetational degradation on the
desert fringe. Maps have been prepared to show the
location of areas affected by desertification, or the risk of
desertification, but they have been strongly criticized.
Desertification
We have already seen in
Chapter 25
that desertification can
be a problem in Mediterranean environments. When we
are dealing with even drier environments it is inevitable
that the problem becomes more severe. A UN conference
on desertification was convened in 1977, as, at the time,
desertification was seen as a threat affecting dry land
throughout the world. It was blamed partly on declining
precipitation levels, as dramatically demonstrated in the
Sahel (
Figure 9.7
),
and partly on overexploitation of a
limited natural resource by increasing populations. The
term is still used to denote the spread of desert-like
conditions into wetter areas, but what does it really mean?
A more recent (1990) UN definition of desertification
is 'land degradation in arid, semiarid and dry subhumid
areas resulting mainly from adverse human impact on the
environment'. The idea of desertification as purely a
Causes of desertification
The development of vegetation degradation through
inappropriate land use is rarely investigated by long-term
scientific monitoring; most comments appear to be based
on subjective judgement. We can identify three main
factors which, it is argued, are likely to give rise to desertifi-
cation: overgrazing, overcultivation and deforestation.
Table 26.3
UN Environment Programme estimates of types of dry land deemed susceptible to desertification,
proportion affected and actual extent
Measure
1977
1984
1992
Climatic zones susceptible to desertification
Arid, semi-arid
Arid, semi-arid
Arid, semi-arid
and subhumid
and subhumid
and dry subhumid
Total dryland area susceptible to desertification (million ha)
5281
4409
5172
Proportion of susceptible dry lands affected by desertification (%)
75
79
70
Total of susceptible dry lands affected by desertification (million ha)
3970
3475
3592
Source: After Thomas and Middleton (1994).
