Simulations consistently predict that summer precipitation will be reduced in many mid-latitude regions, whereas at higher latitudes there will be little change or possibly an increase. Although OAGCMs indicate an intensified global hydrological cycle in a warmer climate, evidence of changes in extreme precipitation at the regional scale remains unconvincing. The low spatial resolution of OAGCMs precludes a realistic simulation of regional circulation and therefore of extreme precipitation.

Relative change in the mean five-day precipitation for July–September that exceeds the 99th percentiles in scenario A2 with respect to the control (present day): even when a reduction in total mean precipitation is simulated, the amounts of precipitation in the intensive events are much less reduced, and even increase in many places. The higher the percentile considered, the larger are the areas that show a positive change. A standard Mann–Whitney test on a series of events that exceed different thresholds indicates that there is a significant change at the 95% level for events above the 95th percentile, but the small sample size precludes a general, unequivocal detection of change for the 99th percentile (or higher).

This, however, does not necessarily imply the absence of change. These changes are seen for a wide range of percentiles and for most European land points, with exceptions being mostly located on the Iberian Peninsula and over the Balkans.

Similar analysis, averaging the precipitation data over several European river catchments: With few exceptions (involving rivers situated in a very dry environment), an increase in the amount of precipitation during extreme precipitation episodes is simulated in both scenarios (most prominent in A2).

Using the combined information from both simulations, we find that CO2-induced warming can lead to a shift towards heavier intensive summertime precipitation over large parts of Europe; the warmer scenario (A2) gives the largest shift. This finding may be explained by the fact that the atmosphere will contain more water in a warmer climate (according to the Clausius-Clapeyron equation), which will provide further potential for latent-heat release during the build-up of low-pressure systems, thereby possibly both intensifying the systems and making more water available for precipitation.

In the European Union project PRUDENCE, the high-resolution (50-km grid) regional climate model HIRHAM4 created by the anish Meteorological Institute has been applied to two of the emission scenarios, A2 and B2, drawn up by the intergovernmental Panel on Climate Change (IPCC). Three 30-year time-slice experiments were carried out for periods representing roughly the present (1961–90) and two future scenarios (2071–2100), respectively. The large-scale controlling conditions originated from transient climate-change simulations using the coupled ocean–atmosphere global climate model (OAGCM) ECHAM4/OPYC (300-km grid).

(4) - Remarques générales