Part 4: Impact of climate change on Zambezi Basin hydrology and hydropower

The African continent is highly vulnerable to climate change, and the Zambezi River Basin is particularly at risk. Part 2 characterized the climate cycles and natural hydrological variations in the Zambezi Basin, including long-term cycles of wet and dry periods over the past century. Zambezi runoff is highly sensitive to these variations in climate, as small changes in rainfall produce large changes in runoff. Over the next century, climate change is expected to increase this variability, and the vulnerability of the basin – and its hydropower dams – to these changes. Concerns about the impact of climate change on water resources development in the Zambezi River Basin are given prominent treatment in the recent "investment opportunity assessment" commissioned by the World Bank (2010) and the "dam synchronization and reoperation study" commissioned by SADC/GTZ (SWRSD 2010).

The details of climate change trends and forecasts for Southern Africa can be difficult to discern from the high level of natural variability in temperature, rainfall, and runoff; and confounded by the relatively low density of long-term monitoring stations across the continent. Most climate change assessments for Africa rely on large-scale General Climate Models (GCMs), developed for a range of different emission scenarios (which, in turn, are based on different assumptions about economic growth, population expansion, and technological change). A few Regional Climate Models (RCMs) downscaled from global models recently have been constructed for Africa (Christensen et al. 2007), but further modeling efforts (now underway) are needed to improve the accuracy of climate forecasts specific to the Zambezi Basin and its sub-basins. River basin managers often site this "uncertainty" as a justification for ignoring or downplaying climate change.

The general climate picture for Southern Africa is increasingly clear, however, based both on observed trends over the past century and increasing confidence in the range of climate change scenarios already developed. The following sections describe the current state-of-the-art predictions for climate change in Southern Africa, including temperature, evapotranspiration, rainfall, and runoff, based on the IPCC and other peer-reviewed technical reports.