Impact of warmer climate periods on flood hazard in the European Alps

Flooding is a pervasive natural hazard-costly in both human and economic terms-and climate change will probably exacerbate risks around the world. Mountainous areas, such as the densely populated European Alps, are of particular concern as topography and atmospheric conditions can result in large and sudden floods. In addition, the Alps are experiencing a high warming rate, which is probably leading to more heavy rainfall events. Here, we compile palaeoflood records to test the still uncertain impact these climatic trends might have on flood frequency and magnitude in the European Alps. We demonstrate that a warming of 0.5-1.2 degrees C, whether naturally or anthropogenically forced, led to a 25-50% decrease in the frequency of large (>= 10 yr return period) floods. This decreasing trend is not conclusive in records covering less than 200 years but persistent in those ranging from 200 to 9,000 years. By contrast, extreme (>100 yr) floods may increase with a similar degree of warming in certain small alpine catchments impacted by local intensification of extreme rainfall. Our results show how long, continuous palaeoflood records can be used to disentangle complex climate-flooding relationships and assist in improving risk assessment and management at a regional scale. Moderate flooding in the European Alps declined during past warmer periods, whereas extreme floods both increased and decreased, according to an analysis of palaeoflood records.

Automated summary

Flooding is a pervasive natural hazard that can cause significant human and economic losses, and climate change is expected to worsen the risks worldwide. This study focuses on the European Alps, a densely populated mountainous region that is particularly vulnerable to floods due to its topography and atmospheric conditions. By examining palaeoflood records, the researchers find that a warming of 0.5-1.2 degrees C, whether naturally or anthropogenically induced, leads to a decrease in the frequency of large floods. However, extreme floods may increase in certain small alpine catchments impacted by intense rainfall. These findings highlight the importance of long-term palaeoflood records in understanding the complex relationship between climate and flooding, and improving risk assessment and management at a regional scale.