In Memory of Bruno Wilhelm: Impact of warmer climate periods on flood hazard in the European Alps

Flooding is a ubiquitous natural hazard, and climate change is likely to exacerbate the risks worldwide. Mountainous areas, such as the densely populated Alps, are of particular concern because topography and atmospheric conditions can lead to large, flash floods, and because they are experiencing a high rate of warming, which will likely result in more intense precipitation events.In his latest publication, Bruno Wilhelm and his colleagues have compiled 33 paleoflood records to test the impact that these climate trends might have on the frequency and magnitude of floods in the Alps. The paleoflood records are derived from lake sediments, the only archive that guarantees continuity of records over long periods of time. The dataset passed a selection procedure of hydrological, sedimentary and geochronological controls, resulting in a final selection of 27 records that continuously cover the last 150 to 10,000 years, documenting a total of 7,792 floods. We perform three analyses with this dataset:(i) analogous to modern projections that assess changes in climate variables between current and warmer future conditions, we determine changes in the occurrence of large (≥10 year) floods between cooler and warmer past subperiods.(ii) following trend analysis techniques using modern streamflow data, we analyze trends in the occurrence of large (≥10-year) floods during warmer or cooler periods.(iii) we replicate the first analysis by considering the occurrence of extreme floods (≥100 years) between colder and warmer past subperiods.From these analyses, we show that a warming of +0.5-1.2°C led to a 25-50% decrease in the frequency of large floods (return period ≥10 years). This downward trend is not observed in records spanning less than 200 years (i.e., the maximum period of the instrumental series), but it is persistent in those ranging from 200 to 9000 years. In contrast, extreme floods (return period > 100 years) may increase with a similar degree of warming in some small Alpine catchments. This may result from a local intensification of extreme precipitation with higher temperature. The latest results of Bruno and his colleagues show how long and continuous paleoflood records can be used to unravel the complex relationships between climate and flooding.