Historic floods and droughts - Climatic Change in Deserts - page 467

Table 23.4. Assumptions, testable conclusions and model results of the
biogeophysical feedback models of the Sahel drought proposed by Charney ( 1975 )
and Charney et al. ( 1975 , 1977 ). (After Williams and Balling 1996 ,p.33.)
1. Overgrazing reduces vegetation cover
2. Reduced plant cover increases albedo
3. Increased albedo decreases surface net radiation
4. Decreased surface net radiation results in surface cooling
5. Surface cooling promotes subsidence of air aloft
6. Subsidence decreases convection and cloud formation
7. Reduced convectional instability leads to less precipitation
8. Additional drying in the Sahel region leads to regional climatic desertification, which
positively feeds back to 1
9. Atmospheric general circulation models show that an albedo increase from 14% to 35%
north of the Intertropical Convergence Zone (ITCZ) results in a southward shift of a few
degrees in the ITCZ
10. Rainfall in the Sahel region is thus decreased in the model by 40% during the rainy
season
grazing pressure, resulting in a greater plant cover, lower albedo, warmer soil surface,
greater convection and, hence, more convectional rainfall ( Figure 23.9 ; Table 23.4 ).
The albedo model explains neither the synchronous nature of droughts in both
hemispheres nor why they begin and end at about the same time. Nor does it account
for even more severe historic droughts in Africa that occurred well before population
increase and overgrazing were evident. There is now overwhelming evidence that
the dominant cause of drought in the Sahel is a change in heat transport within the
oceans, reflected in sea surface temperature (SST) anomalies in the Pacific, Indian and
Atlantic oceans (Lamb, 1978a ;Lamb, 1978b ; Newell and Hsiung, 1987 ; Rasmusson,
1987 ; Street-Perrott and Perrott, 1990 ; Lamb and Peppler, 1991 ; Lamb and Peppler,
1992 ). For example, when the SSTs in the equatorial Atlantic are warmer than average
and the Atlantic SSTs north of the equator and to the west of West Africa are lower
than average, rainfall is reduced across the Sahel (Lamb and Peppler, 1991 ;Lamb
and Peppler, 1992 ). It now appears that the drought in the Sahel probably involves a
combination of land use change and dust and carbon aerosols interacting with more
regional atmospheric and oceanic circulation processes (Menon et al., 2002 ; Giannini
et al., 2003 ; Prospero and Lamb, 2003 ; Zeng, 2003 ).
While it is certainly true that local changes in surface albedo can cause changes in
diurnal wind regime, it thus seems unlikely that they have more than a local influence.
Although changes in albedo are no longer thought to be responsible for causing
regional droughts, a local effect at the scale of tens of square kilometres has been
documented within semi-arid Australia, and appears to be able to influence both the
strength and direction of local sand-moving winds (Tapper, 1991 ).