African Great Lakes are sensitive to climate variation on time scales of decades to millennia (Kendall, 1969; Livingstone, 1975; Haberyan and Hecky, 1987). Lake Victoria (the world's second-largest freshwater lake by area), Lake Tanganyika (the world's second-deepest lake), and Lake Malawi were closed basins for extended periods in the Pleistocene and Holocene epochs (Owen et al., 1990). Lakes Malawi and Tanganyika were hundreds of meters below their current levels; Victoria dried out completely. Today these lakes are in delicate hydrological balance and are nearly closed. Only 6% of the water input to Tanganyika leaves at its riverine outflow (which was totally blocked when the lake was explored by Europeans) (Bootsma and Hecky, 1993).
Higher temperatures would increase evaporative losses, especially if rainfall also declined. Minor declines in mean annual rainfall (10-20%) for extended periods would lead to the closure of these basins even if temperatures were unchanged (Bootsma and Hecky, 1993). Tropical temperatures are increasing; temperatures in the 1980s were 0.5°C warmer than a century earlier and 0.3°C warmer than during the period 1951-1980. Concurrently, Lake Victoria's epilimnion was warmer by 0.5°C in the early 1990s than in the 1960s (Hecky et al., 1994). Although current climate scenarios project only small increases in tropical temperatures, small changes in temperature and water balance can dramatically alter water levels, as well as mixing regimes and productivity.
Recent temperature and rainfall data and GCM simulations indicate increasing aridity in the tropics (Rind, 1995). Increases of 1-2°C in air temperatures could substantially increase the stability of stratification in permanently stratified Tanganyika and Malawi. Their deep waters are continuously warm, but the <1°C difference between surface and deep water in warm seasons maintains a density difference that prevents full circulation. Lake Tanganyika's deep water has been characterized as a "relict" hypolimnion that formed under a cooler climate within the past 1,000 years (Hecky et al., 1994). Since then, warming has created a barrier to vertical circulation. Additional warming could strengthen this barrier and reduce the mixing of deep, nutrient-rich hypolimnetic water and nutrient-depleted surface layers; that mixing sustains one of the most productive freshwater fisheries in the world (Hecky et al., 1981).
Source: IPCC 1996, WG II, Box 10-1.
Water supply undoubtedly is a most important resource for Africa's social,
economic, and environmental well-being. Currently, about two-thirds of the rural
population and one-quarter of the urban population are without safe drinking
water, and even higher proportions lack proper sanitation. Climate change will
likely make the situation more adverse. The greatest impact will continue to
be felt by the poor, who have the most limited access to water resources. This
section focuses mainly on sub-Saharan Africa (SSA).
|Figure 2-10: Water scarcity and people in Africa (Sharma et al., 1996).|
|Figure 2-11: Water availability at river-basin level (1995) (Sharma et al., 1996).|
Availability of water in SSA is highly variable. Only the humid tropical zones in central and west Africa have abundant water. Availability of water varies considerably within countries, too, influenced by physical characteristics and seasonal patterns of rainfall. According to Sharma et al. (1996), eight countries were suffering from water stress or scarcity in 1990; this situation is getting worse as a consequence of rapid population growth, expanding urbanization, and increased economic development. By 2000, about 300 million Africans risk living in a water-scarce environment. Moreover, by 2025, the number of countries experiencing water stress will rise to 18-affecting 600 million people (World Bank, 1995b). Figure 2-10 shows how countries will shift from water surplus to water scarcity as a result of population changes alone between 1990 and 2025, using a per capita water-scarcity limit of 1,000 m3/yr. The scarcity statistics also can be associated with challenges to international water resources: Eight river basins already face water stress, and four face scarcity (Figure 2-11); Figure 2-12 illustrates water availability in the year 2025 (taking account of population increase alone) (Sharma et al., 1996).
|Figure 2-12: Water availability at river-basin level (2025) for projected population levels (Sharma et al., 1996).|
During the 1980s and 1990s, drought affected urban areas and industry very severely. Most water-dependent industries in southern Africa were forced to reduce their activities after water reservoirs fell to critical levels. Beverage companies, which use a lot of water to wash bottles, had to change to nonreturnable aluminum cans (which require less water). Botswana's construction and textile industries had to retrench workers after operations were scaled down because of a severe shortage of water. Similar problems hit Bulawayo, the heart of Zimbabwe's industrial sector; companies were almost forced to pull out and relocate elsewhere because of a lack of water, and half of the small businesses crumbled. In South Africa, Swaziland, and Zimbabwe, sugarcane industries almost ground to a halt because there was no water for irrigation. Power rationing in Kenya in 1996-97 as a consequence of drought severely disrupted the country's manufacturing and engineering industries (UNEP, 1997).
Unfortunately, there are few assessments of how climate change or responses to it could affect local wetland biodiversity. The climate change scenarios of Greco et al. (1994) project that there could be less water in most of the large rivers in the Sahel over the next 30-60 years, with the possible exception of the major rivers flowing into Lake Chad. This shift would mean less available water in the large wetlands along these rivers, unless there are changes to the management of outflow from dams. Changes to the hydrology of smaller wetlands will depend not only on climate change but also on whether they are supplied with surface water or groundwater, as well as the extent of cropping in their catchment areas. The loss of small wetlands may lead to a significant risk of extinction for local populations of turtles and small birds (Gibbs, 1993), although taxa that are easily transported as adults, eggs, cysts, larvae, and so forth would be subjected to less risk (Magadza, 1991). If wetlands in the eastern Sahel become drier, relatively mobile birds that are dependent on wetland habitat could move into wetlands further east (i.e., in Niger, northern Nigeria, northern Cameroon, Chad).
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