Geoscience Reference
In-Depth Information
1968 as 'among the most undisputed and largest regional climate changes experienced
on the earth during the last half-century'. They noted that initial drought in 1968 was
followed by pulses of severe drought in 1971-1973, 1977, 1982-1984 and 1987,
with only modest rainfall recovery thereafter, followed by severe drought again in
2005. Most of the rainfall in this region occurs during June-September and comes
from westward-propagating mesoscale convective systems, or 'disturbance lines'.
The marked decline in seasonal rainfall totals from the early 1950s to the mid-1980s
(
Figure 23.8
) was due to a decrease in the size, organisation and intensity of these
disturbance lines (Bell and Lamb,
2006
). Another issue was whether or not rainfall in
this region varied with the latitude reached by the Intertropical Front (ITF) during its
seasonal northward migration. Lele and Lamb (
2010
) found that peak rainfall occurred
about 400 km south of the ITF. They also noted that meningitis epidemics occur
in the dry conditions north of the ITF, while the incidence of malaria was confined to
the warm, wet conditions south of the ITF. Sporadic locust outbreaks accompanied
the southward retreat of the ITF, devastating ripening crops.
In the Horn of Africa, which includes Ethiopia and Somalia, rainfall during the
June-September wet season reflects the intensity of the lower tropospheric south-
westerly air masses from the equatorial Atlantic (Segele et al.,
2009a
). Wetter-than-
average years coincide with lower-than-average mean sea level pressure over penin-
sular Arabia and higher-than-average pressure over the Gulf of Guinea, and vice versa
(Segele et al.,
2009b
).
23.7 The albedo model of drought in the Sahel region of West Africa
The prolonged drought that afflicted the Sahel region of Africa as well as Ethiopia
from 1968 onwards (
Figure 23.8
) also led to renewed debate about the role of humans
in accentuating normal climatic variability and helping prolong severe drought. A
question that is frequently asked is whether or not human activities such as overgrazing
in savanna areas can cause or aggravate regional droughts. Charney proposed what he
termed a 'biogeophysical' model for the contemporary drought in the Sahel region of
West Africa (Charney,
1975
; Charney et al.,
1975
; Charney et al.,
1977
). An alternative
name is the albedo model of drought.
Albedo is the proportion of incoming solar radiation reflected from the earth's
surface in the form of outgoing, long-wave terrestrial radiation, and it is lower on dark,
moist and well-vegetated surfaces and higher on bare, sandy or sparsely vegetated
surfaces. If albedo increases, the surface will become cooler. In essence, overgrazing
reduces plant cover and increases the ground surface albedo, or reflectivity. A cooler
surface will mean less convection and hence less instability in the lower atmosphere.
The result will be a reduction in convectional rain. Less rain will mean less plant
growth, and thus a vicious spiral of degradation or desertification will ensue along
the desert margins (
Figure 23.9
;
Table 23.4
). The opposite occurs when there is less
