11.3.3.4 Precipitation Variability and Extremes
In northern Europe and in central Europe in winter, where time mean precipitation is simulated to increase, high extremes of precipitation are very likely to increase in magnitude and frequency. In the Mediterranean area and in central Europe in summer, where reduced mean precipitation is projected, extreme short-term precipitation may either increase (due to the increased water vapour content of a warmer atmosphere) or decrease (due to a decreased number of precipitation days, which if acting alone would also make heavy precipitation less common). These conclusions are based on several GCM (e.g., Semenov and Bengtsson, 2002; Voss et al., 2002; Hegerl et al., 2004; Wehner, 2004; Kharin and Zwiers, 2005; Tebaldi et al., 2006) and RCM (e.g., Jones and Reid, 2001; Räisänen and Joelsson, 2001; Booij, 2002; Christensen and Christensen, 2003, 2004; Pal et al., 2004; Räisänen et al., 2004; Sánchez et al., 2004; Ekström et al., 2005; Frei et al., 2006; Gao et al., 2006a; Shkolnik et al., 2006; Beniston et al., 2007) studies. However, there is still a lot of quantitative uncertainty in the changes in both mean and extreme precipitation.
Time scale also matters. Although there are some indications of increased interannual variability, particularly in summer precipitation (Räisänen, 2002; Giorgi and Bi, 2005; Rowell, 2005), changes in the magnitude of long-term (monthly to annual) extremes are expected to follow the changes in mean precipitation more closely than are those in short-term extremes (Räisänen, 2005). On the other hand, changes in the frequency of extremes tend to increase with increasing time scale even when this is not the case for the changes in the magnitude of extremes (Barnett et al., 2006).
Figure 11.7 illustrates the possible characteristics of precipitation change. The eight models in this PRUDENCE study (Frei et al., 2006) projected an increase in mean precipitation in winter in both southern Scandinavia and central Europe, due to both increased wet day frequency and increased mean precipitation for the wet days. In summer, a decrease in the number of wet days led to a decrease in mean precipitation, particularly in central Europe. Changes in extreme short-term precipitation were broadly similar to the change in average wet-day precipitation in winter. In summer, extreme daily precipitation increased in most models despite the decrease in mean precipitation, although the magnitude of the change was highly model-dependent. However, this study only covered the uncertainties associated with the choice of the RCM, not those associated with the driving GCM and the emissions scenario.
Much larger changes are expected in the recurrence frequency of precipitation extremes than in the magnitude of extremes (Huntingford et al., 2003; Barnett et al., 2006; Frei et al., 2006). For example, Frei et al. (2006) estimate that, in Scandinavia under the A2 scenario, the highest five-day winter precipitation totals occurring once in 5 years in 2071 to 2100 would be similar to those presently occurring once in 8 to 18 years (the range reflects variation between the PRUDENCE models). In the MMD simulations, large increases occur in the frequencies of both high winter precipitation in northern Europe and low summer precipitation in southern Europe and the Mediterranean area (Table 11.1).
The risk of drought is likely to increase in southern and central Europe. Several model studies have indicated a decrease in the number of precipitation days (e.g., Semenov and Bengtsson, 2002; Voss et al., 2002; Räisänen et al., 2003, 2004; Frei et al., 2006) and an increase in the length of the longest dry spells in this area (Voss et al., 2002; Pal et al., 2004; Beniston et al., 2007; Gao et al., 2006a; Tebaldi et al., 2006). By contrast, the same studies do not suggest major changes in dry-spell length in northern Europe.