11.3.1 Key Processes

In addition to global warming and its direct thermodynamic consequences such as increased water vapour transport from low to high latitudes (Section 11.1.3), several other factors may shape future climate changes in Europe and the Mediterranean area. Variations in the atmospheric circulation influence the European climate both on interannual and longer time scales. Recent examples include the central European heat wave in the summer 2003, characterised by a long period of anticyclonic weather (see Box 3.5), the severe cyclone-induced flooding in central Europe in August 2002 (see Box 3.6), and the strong warming of winters in northern Europe from the 1960s to 1990s that was affected by a trend toward a more positive phase of the NAO (Hurrell and van Loon, 1997; Räisänen and Alexandersson, 2003; Scaife et al., 2005). At fine geographical scales, the effects of atmospheric circulation are modified by topography, particularly in areas of complex terrain (Fernandez et al., 2003; Bojariu and Giorgi, 2005).

Europe, particularly its north-western parts, owes its relatively mild climate partly to the northward heat transport by the Atlantic MOC (e.g., Stouffer et al., 2006). Most models suggest increased greenhouse gas concentrations will lead to a weakening of the MOC (see Section 10.3), which will act to reduce the warming in Europe. However, in the light of present understanding, it is very unlikely to reverse the warming to cooling (see Section 11.3.3.1).

Local thermodynamic factors also affect the European climate and are potentially important for its future changes. In those parts of Europe that are presently snow-covered in winter, a decrease in snow cover is likely to induce a positive feedback, further amplifying the warming. In the Mediterranean region and at times in central Europe, feedbacks associated with the drying of the soil in summer are important even in the present climate. For example, they acted to exacerbate the heat wave of 2003 (Black et al., 2004; Fink et al., 2004).