12.3 Assumptions about future trends
12.3.1 Climate projections
12.3.1.1 Mean climate
Results presented here and in the following sections are for the period 2070 to 2099 and are mostly based on the IPCC Special Report on Emissions Scenarios (SRES: Naki?enovi? and Swart, 2000; see also Section 12.3.2) using the climate normal period (1961 to 1990) as a baseline.
Europe undergoes a warming in all seasons in both the SRES A2 and B2 emissions scenarios (A2: 2.5 to 5.5°C, B2: 1 to 4°C; the range of change is due to different climate modelling results). The warming is greatest over eastern Europe in winter (December to February: DJF) and over western and southern Europe in summer (June to August: JJA) (Giorgi et al., 2004). Results using two regional climate models under the PRUDENCE project (Christensen and Christensen, 2007) showed a larger warming in winter than in summer in northern Europe and the reverse in southern and central Europe. A very large increase in summer temperatures occurs in the south-western parts of Europe, exceeding 6°C in parts of France and the Iberian Peninsula (Kjellström, 2004; Räisänen et al., 2004; Christensen and Christensen, 2006; Good et al., 2006).
Generally for all scenarios, mean annual precipitation increases in northern Europe and decreases further south, whilst the change in seasonal precipitation varies substantially from season to season and across regions in response to changes in large-scale circulation and water vapour loading. Räisänen et al. (2004) identified an increase in winter precipitation in northern and central Europe. Likewise, Giorgi et al. (2004) found that increased Atlantic cyclonic activity in DJF leads to enhanced precipitation (up to 15-30%) over much of western, northern and central Europe. Precipitation during this period decreases over Mediterranean Europe in response to increased anticyclonic circulation. Räisänen et al. (2004) found that summer precipitation decreases substantially (in some areas up to 70% in scenario A2) in southern and central Europe, and to a smaller degree in northern Europe up to central Scandinavia. Giorgi et al. (2004) identified enhanced anticyclonic circulation in JJA over the north-eastern Atlantic, which induces a ridge over western Europe and a trough over eastern Europe. This blocking structure deflects storms northward, causing a substantial and widespread decrease in precipitation (up to 30-45%) over the Mediterranean Basin as well as over western and central Europe. Both the winter and summer changes were found to be statistically significant (very high confidence) over large areas of the regional modelling domain. Relatively small precipitation changes were found for spring and autumn (Kjellström, 2004; Räisänen et al., 2004).
Change in mean wind speed is highly sensitive to the differences in large-scale circulation that can result between different global models (Räisänen et al., 2004). From regional simulations based on ECHAM4 and the A2 scenario, mean annual wind speed increases over northern Europe by about 8% and decreases over Mediterranean Europe (Räisänen et al., 2004; Pryor et al., 2005). The increase for northern Europe is largest in winter and early spring, when the increase in the average north-south pressure gradient is largest. Indeed, the simulation of DJF mean pressure indicates an increase in average westerly flow over northern Europe when the ECHAM4 global model is used, but a slight decrease when the HadAM3H model (Gordon et al., 2000) is used. For France and central Europe, all four of the simulations documented by Räisänen et al. (2004) indicate a slight increase in mean wind speeds in winter and some decrease in spring and autumn. None of the reported simulations show significant change during summer for northern Europe.