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
Figure 3.9: Multi-model mean (thick line) and individual models (thin lines) of the percentage of Sub-Saharan African land area under sub-humid, semi-arid, arid, and hyper-arid conditions for scenarios RCP2.6 (left) and RCP8.5 (right)
the evapotranspiration-driven shift in AI is amplified by a decline in rainfall (see Figure 3.6). By contrast, the improved (higher) aridity index in East Africa is correlated with higher rainfall projected by global climate models, a characteristic that is uncertain and not reproduced by higher-resolution regional climate models (see Chapter 3 on “Precipitation Projections”). In addition, note that for Somalia and eastern Ethiopia the shift implies a large relative shift imposed on a very low aridity index value, which results in AI values still classified as arid or semi-arid. The shift in AI in Figure 3.8 translates into a shift of categorization of areas into aridity classes. Figure 3.9 shows that although there is little change in net dry areas in a 2°C world, a 4°C world leads to a shift of total area classification toward arid and hyperarid. The overall area of hyper-arid and arid regions is projected to grow by 10 percent in a 4°C world (from about 20 percent to 23 percent of the total sub-Saharan land area), and by 3 percent in a 2°C world by 2080–2100 relative to 1986–2005. As semi-arid area shrinks, total arid area increases by 5 percent in a 4°C world and 1 percent in a 2°C world. The results for a 4°C world are consistent with Fischer et al. (2007), who used a previous generation of GCMs and a more sophisticated classification method based on growing period length to estimate a 5–8 percent increase in arid area in Africa by 2070–2100.
Regional Sea-level Rise The difference in regional sea-level rise in Sub-Saharan Africa between a 2°C and a 4°C world is about 35 cm by 2100 using the semi-empirical model employed in this report. As explained in Chapter 2, current sea levels and projections of future sea-level rise are not uniform across the world. Sub-Saharan Africa as defined 32
in this report stretches from 15° north to 35° south. Closer to the equator, but not necessarily symmetrically north and south, projections of local sea-level rise show a stronger increase compared to mid-latitudes. Sub-Saharan Africa experienced sea-level rise of 21 cm by 2010 (Church and White 2011). For the African coastlines, sea-level rise projected by the end of the 21st century relative to 1986–2005 is generally around 10-percent higher than the global mean, but higher than this for southern Africa (for example, Maputo) and lower for West Africa (for example, Lomé). Figure 3.10 shows the regional sea-level rise projections under the high emission scenario RCP8.5 for 2081–2100. Note that these projections include only the effects of human-induced global climate change, not those of local land subsidence resulting from natural or human influences.
Figure 3.10: Regional sea-level rise in 2081–2100 (relative to 1986–2005) for the Sub-Saharan coastline under RCP8.5