12.3.1.2 Extreme events

The yearly maximum temperature is expected to increase much more in southern and central Europe than in northern Europe (Räisänen et al., 2004; Kjellström et al., 2007). Kjellström (2004) shows that, in summer, the warming of large parts of central, southern and eastern Europe may be more closely connected to higher temperatures on warm days than to a general warming. A large increase is also expected for yearly minimum temperature across most of Europe, which at many locations exceeds the average winter warming by a factor of two to three. Much of the warming in winter is connected to higher temperatures on cold days, which indicates a decrease in winter temperature variability. An increase in the lowest winter temperatures, although large, would primarily mean that current cold extremes would decrease. In contrast, a large increase in the highest summer temperatures would expose Europeans to unprecedented high temperatures.

Christensen and Christensen (2003), Giorgi et al. (2004) and Kjellström (2004) all found a substantial increase in the intensity of daily precipitation events. This holds even for areas with a decrease in mean precipitation, such as central Europe and the Mediterranean. Impact over the Mediterranean region during summer is not clear due to the strong convective rainfall component and its great spatial variability (Llasat, 2001). Palmer and Räisänen (2002) estimate that the probability of extreme winter precipitation exceeding two standard deviations above normal would increase by a factor of five over parts of the UK and northern Europe, while Ekström et al. (2005) have found a 10% increase in short duration (1 to 2 days) precipitation events across the UK. Lapin and Hlavcova (2003) found an increase in short duration (1 to 5 days) summer rainfall events in Slovakia of up to 40% for a 3.5°C summer warming.

The combined effects of warmer temperatures and reduced mean summer precipitation would enhance the occurrence of heatwaves and droughts. Schär et al. (2004) conclude that the future European summer climate would experience a pronounced increase in year-to-year variability and thus a higher incidence of heatwaves and droughts. Beniston et al. (2007) estimated that countries in central Europe would experience the same number of hot days as currently occur in southern Europe and that Mediterranean droughts would start earlier in the year and last longer. The regions most affected could be the southern Iberian Peninsula, the Alps, the eastern Adriatic seaboard, and southern Greece. The Mediterranean and even much of eastern Europe may experience an increase in dry periods by the late 21st century (Polemio and Casarano, 2004). According to Good et al. (2006), the longest yearly dry spell could increase by as much as 50%, especially over France and central Europe. However, there is some recent evidence (Lenderink et al., 2007) that these projections for droughts and heatwaves may be slightly over-estimated due to the parameterisation of soil moisture (too small soil storage capacity resulting in soil drying out too easily) in regional climate models.

Regarding extreme winds, Rockel and Woth (2007) and Leckebusch and Ulbrich (2004) found an increase in extreme wind speeds for western and central Europe, although the changes were not statistically significant for all months of the year. Beniston et al. (2007) found that extreme wind speeds increased for the area between 45°N and 55°N, except over and south of the Alps. Woth et al. (2005) and Beniston et al. (2007) conclude that this could generate more North Sea storms leading to increases in storm surges along the North Sea coast, especially in the Netherlands, Germany and Denmark.