IPCC Fourth Assessment Report: Climate Change 2007
Climate Change 2007: Working Group II: Impacts, Adaptation and Vulnerability

12.4.1 Water resources

It is likely that climate change will have a range of impacts on water resources. Projections based on various emissions scenarios and General Circulation Models (GCMs) show that annual runoff increases in Atlantic and northern Europe (Werritty, 2001; Andréasson et al., 2004), and decreases in central, Mediterranean and eastern Europe (Chang et al., 2002; Etchevers et al., 2002; Menzel and Bürger, 2002; Iglesias et al., 2005). Most of the hydrological impact studies reported here are based on global rather than regional climate models. Annual average runoff is projected to increase in northern Europe (north of 47°N) by approximately 5 to 15% up to the 2020s and 9 to 22% up to the 2070s, for the SRES A2 and B2 scenarios and climate scenarios from two different climate models (Alcamo et al., 2007) (Figure 12.1). Meanwhile, in southern Europe (south of 47°N), runoff decreases by 0 to 23% up to the 2020s and by 6 to 36% up to the 2070s (for the same set of assumptions). The projected changes in annual river basin discharge by the 2020s are likely to be affected as much by climate variability as by climate change. Groundwater recharge is likely to be reduced in central and eastern Europe (Eitzinger et al., 2003), with a larger reduction in valleys (Krüger et al., 2002) and lowlands (e.g., in the Hungarian steppes) (Somlyódy, 2002).

Figure 12.1

Figure 12.1. Change in annual river runoff between the 1961-1990 baseline period and two future time slices (2020s and 2070s) for the A2 scenarios (Alcamo et al., 2007).

Studies show an increase in winter flows and decrease in summer flows in the Rhine (Middelkoop and Kwadijk, 2001), Slovakian rivers (Szolgay et al., 2004), the Volga and central and eastern Europe (Oltchev et al., 2002). It is likely that glacier retreat will initially enhance summer flow in the rivers of the Alps; however, as glaciers shrink, summer flow is likely to be significantly reduced (Hock et al., 2005), by up to 50% (Zierl and Bugmann, 2005). Summer low flow may decrease by up to 50% in central Europe (Eckhardt and Ulbrich, 2003), and by up to 80% in some rivers in southern Europe (Santos et al., 2002).

Changes in the water cycle are likely to increase the risk of floods and droughts. Projections under the IPCC IS92a scenario (similar to SRES A1B; IPCC, 1992) and two GCMs (Lehner et al., 2006) indicate that the risk of floods increases in northern, central and eastern Europe, while the risk of drought increases mainly in southern Europe (Table 12.2). Increase in intense short-duration precipitation in most of Europe is likely to lead to increased risk of flash floods (EEA, 2004b). In the Mediterranean, however, historical trends supporting this are not extensive (Ludwig et al., 2003; Benito et al., 2005; Barrera et al., 2006).

Table 12.2. Impact of climate change on water availability, drought and flood occurrence in Europe for various time slices and under various scenarios based on the ECHAM4 and HadCM3 models.

Time slice Water availability and droughts Floods 
2020s Increase in annual runoff in northern Europe by up to 15% and decrease in the south by up to 23%.a Decrease in summer flow.b  Increasing risk of winter flood in northern Europe, of flash flooding across all of Europe. Risk of snowmelt flood shifts from spring to winter.c 
2050s Decrease in annual runoff by 20-30% in south-eastern Europe.d   
2070s Increase in annual runoff in the north by up to 30% and decrease by up to 36% in the south.a Decrease in summer low flow by up to 80%.d, b Decreasing drought risk in northern Europe, increasing drought risk in western and southern Europe. Today’s 100-year droughts return every 50 years (or less) in southern and south-eastern Europe (Portugal, all Mediterranean countries, Hungary, Romania, Bulgaria, Moldova, Ukraine, southern Russia).c Today’s 100-year floods occur more frequently in northern and north-eastern Europe (Sweden, Finland, northern Russia), in Ireland, in central and eastern Europe (Poland, Alpine rivers), in Atlantic parts of southern Europe (Spain, Portugal), and less frequently in large parts of southern Europe.c  

a Alcamo et al., 2007; b Santos et al., 2002; c Lehner et al., 2006; d Arnell, 2004

Increasing flood risk from climate change could be magnified by increases in impermeable surface due to urbanisation (de Roo et al., 2003) and modified by changes in vegetation cover (Robinson et al., 2003) in small catchments. The effects of land use on floods in large catchments are still being debated. The more frequent occurrence of high floods increases the risk to areas currently protected by dykes. The increasing volume of floods and peak discharge would make it more difficult for reservoirs to store high runoff and prevent floods.

Increasing drought risk for western Europe (e.g., Great Britain; Fowler and Kilsby, 2004) is primarily caused by climate change; for southern and eastern Europe increasing risk from climate change would be amplified by an increase in water withdrawals (Lehner et al., 2006). The regions most prone to an increase in drought risk are the Mediterranean (Portugal, Spain) and some parts of central and eastern Europe, where the highest increase in irrigation water demand is projected (Döll, 2002; Donevska and Dodeva, 2004). Irrigation requirements are likely to become substantial in countries (e.g., Ireland) where demand now hardly exists (Holden et al., 2003). It is likely that, due to both climate change and increasing water withdrawals, the river-basin area affected by severe water stress (withdrawal : availability >0.40) will increase and lead to increasing competition for available water resources (Alcamo et al., 2003; Schröter et al., 2005). Under the IS92a scenario, the percentage of river basin area in the severe water stress category increases from 19% today to 34-36% by the 2070s (Lehner et al., 2001). The number of additional people living in water-stressed watersheds in the EU15 plus Switzerland and Norway is likely to increase to between 16 million and 44 million, based on climate projected by the HadCM3 GCM under the A2 and B1 emissions scenarios, respectively (Schröter et al., 2005).