11.3.3 Climate Projections
11.3.3.1 Mean Temperature
The observed evolution of European temperatures in the 20th century, characterised by a warming trend modulated by multi-decadal variability, was well within the envelope of the MMD simulations (Figure 11.4).
In this century, the warming is projected to continue at a rate somewhat greater than its global mean, with the increase in 20-year mean temperatures (from its values in 1980 to 1999) becoming clearly discernible (as defined in Section 11.1.2) within a few decades. Under the A1B scenario, the simulated area and annual mean warming from 1980 to 1999 to 2080 to 2099 varies from 2.3°C to 5.3°C in NEU and from 2.2°C to 5.1°C in SEM. The warming in northern Europe is likely to be largest in winter and that in the Mediterranean area largest in summer (Figure 11.5). Seasonal mean temperature changes typically vary by a factor of three among the MMD models (Table 11.1); however, the upper end of the range in NEU in DJF is reduced from 8.1°C to 6.7°C when one model with an extreme cold bias in present-day winter climate is excluded. Further details are given in Table 11.1 and Supplementary Material Figures S11.2 to S11.4.
Although changes in atmospheric circulation have a significant potential to affect temperature in Europe (e.g., Dorn et al., 2003), they are not the main cause of the projected warming (e.g., Rauthe and Paeth, 2004; Stephenson et al., 2006; van Ulden et al., 2007). A regression-based study using five of the MMD models (van Ulden and van Oldenborgh, 2006) indicated that in a region comprising mainly Germany, circulation changes enhanced the warming in most models in winter (due to an increase in westerly flow) and late summer (due to a decrease in westerly flow), but reduced the warming slightly in May and June. However, the circulation contribution to the simulated temperature changes (typically –1°C to 1.5°C depending on model and month) was generally much smaller than the total simulated warming in the late 21st century.
Despite a decrease in the North Atlantic MOC in most models (see Section 10.3), all the MMD simulations show warming in Great Britain and continental Europe, as other climatic effects of increased greenhouse gases dominate over the changes in ocean circulation. The same holds for earlier simulations with increased greenhouse gas concentrations, except for a very few (Russell and Rind, 1999; Schaeffer et al., 2004) with slight cooling along the north-western coastlines of Europe but warming over the rest of the continent. The impact of MOC changes depends on the regional details of the change, being largest if ocean convection is suppressed at high latitudes where the sea ice feedback may amplify atmospheric cooling (Schaeffer et al., 2004). Sensitivity studies using AOGCMs with an artificial shutdown of the MOC and no changes in greenhouse gas concentrations typically show a 2°C to 4°C annual mean cooling in most of Europe, with larger cooling in the extreme north-western parts (e.g., Stouffer et al., 2006).
Statistical downscaling (SD) studies tend to show a large-scale warming similar to that of dynamical models but with finer-scale regional details affected by factors such as distance from the coast and altitude (e.g., Benestad, 2005; Hanssen-Bauer et al., 2005). Comparing RCM and SD projections for Norway downscaled from the same GCM, Hanssen-Bauer et al. (2003) found the largest differences between the two approaches in winter and/or spring at locations with frequent temperature inversions in the present climate. A larger warming at these locations in the SD projections was found, consistent with increased winter wind speed in the driving GCM and reduced snow cover, both of which suppress formation of ground inversions.