Climate change and drought occurrence in the Alpine region: How severe are becoming the extremes?

摘要

There are clear indications that in the future the Alpine region will increasingly suffer from droughts. In this paper, a simple method is devised for determining probability of drought occurrence and exceedance of severity thresholds. A reconstruction of drought occurrence during the past 100 yr indicates that the method is able to detect major events and also to correctly gauge their relative severity. The procedure is used in conjunction with climate simulations for the European region valid for 2071-2100 to study the impact of climate change on the likelihood and severity of droughts. The climate scenario used for the analysis refers to a SRES A2 emission pathway and specifies in particular a decrease in the frequency of wet days of about 20% with respect to the growing season of summer crops (April to September). Under these conditions the frequency of droughts is shown to increase from about 15% to more than 50%. In addition, the results indicate an overall shift in the distribution toward higher severity scores. The average severity increases by a factor of two, but also at the upper end of the spectrum severity increases by more than 20%. It is argued that this will affect the perception of extreme droughts, i.e. of those events rarer than the 10th percentile of the distribution. If this scenario comes true, by the end of the 21st century droughts comparable in severity to the 2003 event would represent the norm rather than the exception.