The Regional Impacts of Climate Change

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There were striking responses in runoff for the Nile. Riebsame et al. (1995) conclude that despite potential adjustments, Nile flows throughout the basin are extremely sensitive to temperature and precipitation changes. GCM scenarios provide widely diverging pictures of possible future river flows, from a 30% increase to a 78% decrease. There are formal agreements between Egypt and Sudan on the allocation of flows from the Nile, now and under any future enhancements. However, any reductions over 20% would exceed the management capability of the agreements and would result in major social and economic impacts. Adjustments in response to climate change would either involve changes in water allocation or structural adjustments in the upper and lower basin. The large uncertainty in climate-change projections makes it very hard for basin managers to adopt any response policy. There is need for a regional climate modeling effort over the Nile to help reduce this uncertainty. It remains prudent to make capital investments in decreasing water demand via more efficient irrigation management as a very wise adaptation to climate change.

Figure 2-9: The Nile and Zambezi River basins. For the Zambesi River basin, climate change impacts were projected for the basin above Lake Kariba with the existing Kariba Dam and with a proposed new dam and reservoir at Batoka Gorge (Riebsame et al., 1995).

The seasonal runoff pattern for the Zambezi remained relatively unchanged; the river was sensitive, however, to temporal shifts of the rainy season. There was a net deficit in river flows due to higher surface temperatures, which increase the rate of evapotranspiration. Hydropower production at Kariba decreased slightly under the GISS and GFDL scenarios, while the cooler scenarios of UKMO and GISS led to small increases in power generation. Seasonality of flow had more marked effects on production, a function of storage capacity of the dams in relation to ability to store excess and regulate water flows. Under climate change, there would be less water entering Kariba, and this would likely lead to reduction in fish populations. Adaptation to climate change for the Zambezi basin was suggested to depend on better planning of water projects that consider hydrological inter-relationships of the basin as a whole, crossing many national boundaries. This requirement for countries to look beyond their own needs is a major factor in implementing adaptation options.

The Niger River runs over 4,000 km across west Africa, and its basin covers about one third of the subregion including Guinea, Cote d'Ivoire, Mali, Burkina Faso, Niger, Benin, Nigeria, Cameroon, and Chad. The pressure on this river basin is intense. For example, the Sahelian drought of the 1970s seriously affected hydropower generation from Nigeria's Kaiji Dam on the Niger River during the 1973-77 period. This caused a severe shortfall in power generation to consumers in Nigeria, Mali, Benin, and Chad.

There is some concern that the negative impacts of climate change on water supply could be larger (and the gains smaller) than those reported in current assessments. Many GCMs have not explicitly incorporated the influence of persistent drought in evaluating the impact of global warming. In particular, equilibrium models begin each year with no model memory of groundwater depletion in a preceding year. Yet the successive accumulation of back-to-back drought years often can have devastating effects on groundwater, runoff, reservoir storage, marginal agricultural activities, and water quality (Cline, 1992).

Despite relatively small climatic changes projected for the tropics, tropical lakes also may be quite sensitive to climate change (see Box 2-5). The level of Lake Victoria (in eastern Africa) rose rapidly in the early 1960s following only a few seasons with above-average rainfall and has remained high since (Sene and Pinston, 1994-as cited in IPCC 1996, WG II, Section 10.5.2).