Environmental Engineering Reference
In-Depth Information
blamed on the intensification of the tropical
Hadley cell. The descending arm of that cell is
responsible for the general aridity in the region,
and any increase in its strength or extent is
accompanied by further suppression of
precipitation, particularly in areas peripheral to
the desert. The synchronicity of wet and dry
years, north and south of the Sahara, supports
this explanation (Nicholson 1981).
It has also been suggested that human activities
have caused 'degradation induced drought' by
contributing to the process by which the Hadley
cell is intensified. Charney (1975) proposed that
overgrazing and wood cutting, in the Sahel,
increased the surface albedo, and disrupted the
regional radiation balance. Surface heating
declined as more solar radiation was reflected,
and this in turn caused some cooling of the
atmosphere. This cooling encouraged subsidence,
or augmented existing subsidence, and helped to
reduce the likelihood of precipitation by retarding
convective activity. With less precipitation,
vegetation cover decreased, and the albedo of the
surface was further enhanced. This process was
described as a biogeophysical feedback
mechanism. It received considerable attention in
the 1980s because it seemed to fit the observation
that drought in the Sahel was more persistent
than elsewhere in Africa. While such persistence
may appear to be a function of the positive
feedback mechanism central to the theory, it
cannot be taken as confirmation of it (Hulme
1989).
As with many of the efforts to explain drought
in Africa, it was difficult to develop the theory
of degradation-induced drought further, because
of the lack of empirical evidence. Following a
study of long-term changes in African rainfall,
Nicholson (1989) concluded that the human
cause of drought, as espoused by Charney, did
not seem feasible, but allowed that surface
changes—whatever the cause—might feed back
into the system to reinforce the drought.
Establishing the nature and extent of these
surface changes has not been easy. The most
obvious approach would be the use of satellite
remote sensing to estimate both vegetation
destruction and changes in the surface albedo.
However, the controversy over the interpretation
of satellite data in assessing the extent of
desertification has yet to be resolved, and
problems still remain in the use of satellite derived
data in this type of modelling (Thomas and
Henderson-Sellers 1987). The existing data
record is also short. Hulme (1989), in his
assessment of the theory, estimated that changes
of the magnitude proposed by Charney—a 14 to
35 per cent increase in albedo—would require
vegetation destruction on at least a sub-
continental scale over a period of as much as 20
years. As yet there is no evidence of this, but that
may be in part a function of the inadequacy of
the data, and he concluded that the theory is at
best 'not proven'. Since then, Balling (1991) has
calculated that desertification may actually have
caused surface air temperatures to increase
(rather than decrease as Charney's theory
requires), although Hulme and Kelly (1993) have
suggested that Balling's estimates of warming are
too high.
While such studies may ultimately provide a
better understanding of the problems of drought,
and the mechanisms involved, they are
insufficient to provide a direct forecasting
mechanism. Researchers have re-examined
certain relationships in the earth/atmosphere
system in search of something more suitable.
Their approach is based on the observation that
the various units in the system are interconnected
in such a way that changes in one unit will
automatically set in motion changes in others,
through autovariation. Since many of the changes
are time-lagged, it should be possible to predict
subsequent developments if the original change
can be recognized. This forms the basis of the
concept of teleconnection, or the linking of
environmental events in time and place.
In recent years the search for a drought
forecasting mechanism involving teleconnection
has centred on changing conditions in the world's
oceans. Sea-surface temperatures (SSTs) have
been examined as possible precursors of the
circulation patterns which cause drought in the
Sahel, and a correlation between global SSTs and
