11.3.3.3 Temperature Variability and Extremes

Based on both GCM (Giorgi and Bi, 2005; Rowell, 2005; Clark et al., 2006) and RCM simulations (Schär et al., 2004; Vidale et al., 2007), interannual temperature variability is likely to increase in summer in most areas. However, the magnitude of change is uncertain, even in central Europe where the evidence for increased variability is strongest. In some PRUDENCE simulations, interannual summer temperature variability in central Europe doubled between 1961 to 1990 and 2071 to 2100 under the A2 scenario, while other simulations showed almost no change (Vidale et al., 2007). Possible reasons for the increase in temperature variability are reduced soil moisture, which reduces the capability of evaporation to damp temperature variations, and increased land-sea contrast in average summer temperature (Rowell, 2005; Lenderink et al., 2007).

Simulated increases in summer temperature variability also extend to daily time scales. Kjellström et al. (2007) analyse the PRUDENCE simulations and find that the inter-model differences in the simulated temperature change increase towards the extreme ends of the distribution. However, a general increase in summer daily temperature variability is evident, especially in southern and central parts of Europe, with the highest maximum temperatures increasing more than the median daily maximum temperature (Supplementary Material Figure S11.23). Similarly, Shkolnik et al. (2006) report a simulated increase in summer daily time-scale temperature variability in mid-latitude western Russia. These RCM results are supported by GCM studies of Hegerl et al. (2004), Meehl and Tebaldi (2004) and Clark et al. (2006).

In contrast with summer, models project reduced temperature variability in most of Europe in winter, both on interannual (Räisänen, 2001; Räisänen et al., 2003; Giorgi et al., 2004; Giorgi and Bi, 2005; Rowell, 2005) and daily time scales (Hegerl et al., 2004; Kjellström et al., 2007). In the PRUDENCE simulations, the lowest winter minimum temperatures increased more than the median minimum temperature especially in eastern, central and northern Europe, although the magnitude of this change was strongly model-dependent (Supplementary Material Figure S11.23). The geographical patterns of the change indicate a feedback from reduced snow cover, with a large warming of the cold extremes where snow retreats but a more moderate warming in the mostly snow-free south-western Europe (Rowell, 2005; Kjellström et al., 2007).

Along with the overall warming and changes in variability, heat waves are very likely to increase in frequency, intensity and duration (Barnett et al., 2006; Clark et al., 2006; Tebaldi et al., 2006). Conversely, the number of frost days is very likely to decrease (Tebaldi et al., 2006).