S ub -S aharan A frica : F ood P roduction at R isk
Box 3.5: Tree Mortality in the Sahel At a regional scale, Gonzalez, Tucker, and Sy (2012) observe a 20-percent decline in tree density in the western Sahel and a significant decline in species richness across the Sahel in the last half of the 20th century. Based on an econometric model and field observations, they attribute the observed trend to changes in temperature and rainfall variability, which in turn are attributable to climate change. Furthermore, available data on tree density at Njóobéen Mbataar (Senegal) and precipitation data suggests a threshold of resilience to drought stress for Sudan and Guinean tree species at approximately one standard deviation below the long-term average five out of six years (Gonzalez et al. 2012).
can lead to elevated respiration at the expense of stored carbon, again posing the risk of carbon starvation (McDowell et al. 2008). These mechanisms and their interdependencies are likely to be amplified because of climate change (McDowell et al. 2011). Despite persistent uncertainties pertaining to these mechanisms and thresholds marking tree mortality, C. A. Allen et al. (2010) conclude that increases in extreme droughts and temperatures pose risks of broad-scale climate-induced tree mortality. According to Allen et al. (2010), the potential for abrupt responses at the local level, once climate exceeds physiological thresholds, qualifies this as a tipping point of non-linear behavior (Lenton et al. 2008). In light of the opposing trends described above, William J. Bond & Parr (2010) conclude that “it is hard to predict what the future holds for forests vs. grassy biomes given these contrasting threats.” Thus, whether drought-related tree feedback may prevail over CO2- stimulated woody encroachment, remains unclear.
Aquatic Ecosystems Climate change is expected to adversely affect freshwater as well as marine systems (Ndebele-Murisa, Musil, and Raitt2010; Cheung et al. 2010including declines in key protein sources and reduced income generation because of decreasing fish catches (Badjeck, Allison, Halls, and Dulvy 2010). Non-climatic environmental problems already place stress on ecosystem services. For example, overfishing, industrial pollution, and sedimentation have degraded water resources, such as Lake Victoria (Hecky, Mugidde, Ramlal, Talbot, and Kling 2010), reducing fish catches.
Freshwater Ecosystems Reviewing the literature on changes in productivity in African lakes, Mzime R. Ndebele-Murisa et al. (2010) note that while these lakes are under stress from human usage, much of the
changes observed are attributable to years of drought. Associated reductions in river inflow can contribute to a decrease in nutrient concentrations. Increasing water temperatures and higher evaporation further lead to stronger thermal stratification, further inhibiting primary productivity as waters do not mix and nutrients in the surface layers are depleted. Similarly, Mzime R. Ndebele-Murisa, Mashonjowa, and Hill (2011) state that temperature is an important driver of fish productivity in Lake Kariba, Zimbabwe, and best explains observed declines in Kapenta fishery yields. Inland freshwater wetlands are another freshwater ecosystem likely to be affected by climate change. One such wetland is the Sudd in Sahelian South Sudan, which provides a rich fishery, flood recession agriculture, grazing for livestock, handcrafts, and building materials, and plant and animal products (including for medicinal purposes). The Sudd, which is fed by the White Nile originating in the Great Lakes region in East Africa, could be depleted by reduced flows resulting from changes in precipitation patterns (Mitchell 2013). Furthermore, increasing freshwater demand in urban areas of large river basins may lead to reducing river flows, which may become insufficient to maintain ecological production; this means that freshwater fish populations may be impacted (McDonald et al. 2011).
Ocean Ecosystems Climate-change related changes in ocean conditions can have significant effects on ocean ecosystems. Factors influencing ocean conditions include increases in water temperature, precipitation, levels of salinity, wind velocity, wave action, sea-level rise, and extreme weather events. Ocean acidification, which is associated with rising atmospheric CO2 concentrations, is another factor and is discussed in Chapter 4 under “Projected Impacts on Coral Reefs” in the context of coral reef degradation. Ocean ecosystems are expected to respond to altered ocean conditions with changes in primary productivity, species distribution, and food web structure (Cheung et al. 2010). Theory and empirical studies suggest a typical shift of ocean ecosystems toward higher latitudes and deeper waters in response to such changes (Cheung et al. 2010a). However, there is also an associated risk that some species and even whole ecosystems will be placed at risk of extinction (Drinkwater et al. 2010). Taking into account changes in sea-surface temperatures, primary production, salinity, and coastal upwelling zones, Cheung et al. (2010) project changes in fish species distribution and regional patterns of maximum catch potential by 2055 in a scenario leading to warming of approximately 2°C in 2050 (and 4°C by 2100). The results are compared to a scenario in which conditions stabilize at year 2000 values. Comparing both scenarios shows potential yield 51