TURN DO WN THE H E AT: C L IM AT E E X T RE ME S , R EGION A L IMPA C TS, A N D TH E C A SE FOR R ESILIENCE
increased drought risk over southern Africa is consistent across other drought indicators, but added West Africa as an area where projections consistently show an increased drought risk. However, Figure 3.6 shows that precipitation changes are highly uncertain in the latter region, which Taylor et al (2012) might not have been taken into account fully. According to Giannini, Biasutti, Held, and Sobel (2008a), the uncertainties in western tropical Africa are mainly because of competing mechanisms affecting rainfall. On the one hand, the onset of convection and subsequent rainfall is mainly affected by temperature at the surface and higher levels in the atmosphere. On the other hand, the amount of moisture supply is primarily affected by changes in atmospheric circulation, which can be induced by the temperature contrast between land and ocean. The effect of El Niño events mainly act via the first mechanism, with warming of the whole tropical troposphere stabilizing the atmospheric column and thereby inhibiting strong convection (Giannini, Biasutti, Held, and Sobel 2008a). Sillmann and Kharin (2013a) studied precipitation extremes for 2081–2100 in the CMIP5 climate model ensemble under the low emission high emission scenario. Under the high-emission scenario, the total amount of annual precipitation on days with at least 1 mm of precipitation (total wet-day precipitation) increases in tropical eastern Africa by 5 to 75 percent, with the highest increase in the Horn of Africa, although the latter represents a strong relative change over a very dry area. In contrast to global models, regional climate models project no change, or even a drying for East Africa, especially during the long rains. Consistently, one recent regional climate model study projects an increase in the number of dry days over East Africa (Vizy and Cook 2012b). Changes in extreme wet rainfall intensity were found to be highly regional and projected to increase over the Ethiopian highlands. Sillmann and Kharin (2013a) further projected changes of +5 to –15 percent in total wet-day precipitation for tropical western Africa with large uncertainties, especially at the monsoon-dependent Guinea coast. Very wet days (that is, the top 5 percent) show even stronger increases: by 50 to 100 percent in eastern tropical Africa and by 30 to 70 percent in western tropical Africa. Finally in southern Africa, total wet day precipitation is projected to decrease by 15 to 45 percent, and very-wet day precipitation to increase by around 20 to 30 percent over parts of the region. However, some localized areas along the west coast of southern Africa are expected to see decreases in very wet days (up to 30 percent). Here, increases in consecutive dry days coincide with decreases in heavy precipitation days and maximum consecutive five-day precipitation, indicating an intensification of dry conditions. The percentile changes in total wet-day precipitation, as well as in very wet days, are much less pronounced in the low emission scenario RCP2.6. 30
Aridity The availability of water for ecosystems and society is a function of both demand and supply. The long-term balance between demand and supply is a fundamental determinant of the ecosystems and agricultural systems able to thrive in a certain area. This section assesses projected changes in Aridity Index (AI), an indicator designed for identifying “arid” regions, that is regions with a structural precipitation deficit (UNEP 1997; Zomer 2008). AI is defined as total annual precipitation divided by potential evapotranspiration; the latter is a standardized measure of water demand representing the amount of water a representative crop type would need over a year to grow (see Appendix 2). Potential evapotranspiration is to a large extent governed by (changes in) temperature, although other meteorological variables play a role as well. A smaller AI value indicates a larger water deficit (i.e., more arid condition), with areas classified as hyper-arid, arid, semiarid, and sub-humid as specified in Table 3.2. In the absence of an increase in rainfall, an increase in potential evapotranspiration translates into a lower AI value and a shift toward more structurally arid conditions. Analysis by the authors shows that, in general, the annual mean of monthly potential evapotranspiration increases under global warming (see Appendix 2). This is observed over all of Sub-Saharan Africa with strong model agreement, except for regions projected to see a strong increase in precipitation. In Eastern Africa and the Sahel region, the multi-model mean shows a small reduction in potential evapotranspiration—but the models disagree. Thus regions that are getting wetter in terms of increased rainfall see either only a limited increase or even a decrease in potential evapotranspiration. By contrast, a more unambiguous signal emerges for regions projected to get less rainfall (notably southern Africa), where the projections show an enhanced increase in potential evapotranspiration. This is likely because of the feedback between precipitation and evaporation via temperature. In regions receiving more rainfall there is enough water available for evaporative cooling; this limits the warming of the surface. In regions where the soil dries out because of a lack of precipitation, however, no more heat can be converted into latent heat and all heat results in increased surface temperatures. Table 3.2: Climatic classification of regions according to Aridity Index (AI) Minimum AI Value Hyper-arid Arid
0
Maximum AI Value 0.05
0.05
0.2
Semi-arid
0.2
0.5
Sub-humid
0.5
0.65