Runoff changes will depend on changes in temperatures and precipitation, among other variables. Based on the median values of 12 climate models using A1B scenario described in the IPCC Special Report on Emission Scenarios (SRES, 2000) (Figure 1), runoff is projected to increase by 10 to 40% by mid-century at higher latitudes and in some wet tropical areas, including populous areas in East and South-East Asia, and decrease by 10 to 30% over some dry regions at mid-latitudes and dry.  There is also high confidence that many semi-arid areas (e.g. the Mediterranean Basin, western United States, southern Africa and north-eastern Brazil) will suffer a decrease in water resources due to climate change (IPCC AR4).

Figure 1.  Percentage changes in average annual runoff projected by four climate models for the period 2090-2099, relative to 1980-1999  Source: IPCC. 2007. Climate Change 2007: Synthesis Report. Intergovernmental Panel on Climate Change. Figure 3.5, p. 49.