Tropical climates - The Global Climate System: Patterns, Processes, and Teleconnections - page 76

When examining climate over a given time period, it is important to recognize
that different terms need to be used. Climate variability refers to the natural
variations that appear in the atmospheric statistics for a designated period of
time, usually on the order of months to years, and can occur in any climate type.
However, in dealing with desertification it should be noted that during the annual
dry season, characteristic of semiarid regions, the climate is desertlike. Improper
use of the land during this period leads to land degradation that has little to do with
climate variability. Thus, both seasonal dry periods and short-term variations in
climate when combined with improper land-use practices, can give the appearance
of the impact of longer term regional climate changes when none may have
occurred at all. Climate fluctuations are variations in climate conditions that
occur on the order of decades. Such may have occurred in the West African
Sahel when drought conditions extended over multi-year growing seasons.
The question of relative responsibility for the desertification, whether it be
climate variability or human activity, has been approached by Kelly and Hulme
( 1993 ), who related a satellite-derived index of active vegetation to rainfall
variations. Figure 3.9 shows how the extent of the Sahara relates to the rainfall
deficit. When removing the rainfall effects from the relationship, variations in
the Sahara still exist; this may be due to the cumulative effect of dry years
delaying the recovery of vegetation or directly related to human degradation of
the environment.
The records of rainfall in the Sahel indicate that the drought of the 1970s and
1980s was severe and long lasting. Only recently has it eased. By 2003 a wet
period appears to have returned, for in 2003 Burkina Faso recorded record
rainfall amounts. However, how well the data represent the actual conditions
Figure 3.9 A rainfall
deficit index (bars) is
compared with a satellited
index (circles) to obtain the
residual trend showing the
extent of the Sahara when
rainfall effects are removed.
This is shown by the squares.
(After Tiempo Climate
Cyberlibrary, Tiempo
Issue 9)