Background
The authors note that it has been suggested that global warming may increase the frequency of extreme precipitation events and, therefore, that it may enhance the occurrence of natural mass-movement processes such as debris flows in mountainous regions. In fact, they state in regard to this particular phenomenon that there is a "widely accepted assumption that climatic changes will univocally lead to an increase in event frequency." But is this strident climate-alarmist claim correct?

What was done
In a study designed to explore this question, Bollschweiler and Stoffel developed a history of debris-flow frequencies for eight different areas in the Zermatt Valley -- a dry inner-alpine valley of the Valais Alps (Switzerland, with central coordinates of 46°10'N, 47°7'E) -- based on data obtained from "tree-ring series of affected conifers and complemented, where available, with data from local archives," which work entailed the sampling of 2467 individual trees that had been impacted by debris-flow activity in order to obtain 4491 pertinent increment cores.

What was learned
The two Swiss scientists found there were peaks in debris-flow activity "toward the end of the Little Ice Age and in the early twentieth century when warm-wet conditions prevailed during summers in the Swiss Alps," but they say they also observed "a considerable decrease in frequency over the past decades which results from a decrease in the frequency of triggering precipitation events." Most importantly, they report that when longer-term changes were sought, they could not identify "any significant trends in the debris-flow series between 1850 and 2009."

What it means
In discussing their real-world debris-flow results, Bollschweiler and Stoffel say they "contradict the widely accepted assumption that climatic changes will univocally lead to an increase in event frequency." But they add that their findings "are in concert with data from Jomelli et al. (2007), indicating that the most recent past (2000-2009) represents the period with the lowest frequency of debris-flow events since AD 1900," which latter decade is touted by climate alarmists as having been the warmest such period of the past millennium or more. Hence, the world's global warming gurus once again appear to be close to one hundred and eighty degrees out of phase with reality on this significant subject, insofar as it has been empirically examined to date.

Reference
Jomelli, V., Brunstein, D., Grancher, D. and Pech, P. 2007. Is the response of hill slope debris flows to recent climate change univocal? A case study in the Massif des Ecrins (French Alps). Climatic Change 85: 119-137.