With globally increasing temperatures, increases in global precipitation would be expected a priori, due to the greater rates of evaporation of sea surface water. Unfortunately, no reliable estimates of evaporation increase exist. One problem is the effect of varying wind speed on evaporation rates, which may or may not be related to increases in temperature.
Several large-scale analyses of precipitation changes in both hemispheres have been carried out (Bradley et al., 1987; Diaz et al., 1989; Vinnikov et al., 1990). These have demonstrated that during the last few decades precipitation has tended to increase in the mid-latitudes, but decrease in the Northern Hemisphere subtropics and generally increase throughout the Southern Hemisphere.
Two of the more striking precipitation changes have occurred in the African Sahel (Figure 6.10a) and the former Soviet Union nations (Figure 6.10b). The dramatic drying up of sub-Saharan Africa has been linked to changes in ocean circulation (Street-Perrott & Perrott, 1990) and tropical Atlantic sea surface temperatures (Folland et al., 1991). Nevertheless, the accuracy of other precipitation records should be treated with caution. Precipitation is more difficult to monitor than temperature due to its greater temporal and spatial variability (see section 18.104.22.168). Other uncertainties in the data set may be due to the collection efficiency of raingauges.