Events of extreme precipitation have a huge influence on society, and it is commonly expected that precipitation extremes will increase as the climate warms. The basic physical cause for this is that warmer air can hold more moisture.

Geert Lenderink and Erik van Meijgaard have recently published a new study on the relation between temperature, atmospheric moisture and summertime precipitation extremes in observations and model results. They elaborate on their earlier findings reported in Nature Geoscience from measurements taken at De Bilt that extreme hourly precipitation rates increase faster with temperature than can be expected from basic physics: the famous Clausius-Clapeyron relation. They found a 14% increase per degree, which is twice the rate expected from Clausius-Clapeyron.

In their new study Lenderink and van Meijgaard find the same 14% increase in four different precipitation data sets in western Europe. Using the dew point temperature - a measure of atmospheric moisture - reveals an even more robust and slightly stronger scaling behaviour. Their hypothesis is that these relations between hourly precipitation extremes and (dew point) temperature can be used as a predictor of the change in these extremes caused by climate change. Modelling results of the KNMI regional climate model RACMO2 appear to largely support this hypothesis, and changes in hourly precipitation extremes by the end of this century even amount up to 70% in one integration. However, reductions in relative humidity that are expected to occur in the future climate may limit the increase in precipitation extremes, and much lower increases in (hourly) precipitation extremes are therefore found in southern Europe. Also a different regional climate model, CLM, predicts much smaller increases for the future climate, but this model also appears to be much to sensitive to decreases in relative humidity for the present day climate.