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 °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.
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Historical information sheds new light on the intensification of flooding in the Central Mediterranean
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The authors declare that the palaeoflood data supporting the findings of this study (Extended Data Table 1) are available in the NOAA database at the following address: https://www.ncei.noaa.gov/access/paleo-search/study/34712. The temperature data from Extended Data Table 4 are all available in the NOAA or PANGEA repositories.
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The data collection and the study design have been facilitated by the PAGES Floods Working Group that fosters collaborations. Sediment coring on Lake Bourget was performed using the French national sediment coring facility C2FN, in the framework of the excellence equipment project Equipe CLIMCOR (11-EQPX-0009, W.R., F.A., P.S. and B.W.) funded by the French National Agency for Research, ANR. The study of Lake Bourget sediment cores was performed in the framework of the CRIT-LAKES project funded by the Université Savoie Mont Blanc and the national CNRS programme EC2CO BIOHEFECT. The data analysis was performed in R using the supporting package trend. The authors acknowledge comments on preliminary publication versions from J.D. Creutin, G. Durand, C. Obled and M. Ménégoz as well as further colleagues for informal discussion during our Friday’s beer.
The authors declare no competing interests.
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Extended Data Fig. 1 Data acquired to extend the 350-year paleoflood record of the Rhône River to the the last millennium.
Data acquired to extend the 350-year paleoflood record of the Rhône River to the the last millennium. The method used is identical to Jenny et al. (2014)56. (A) Bathymetry of Lake Bourget and coring sites from Jenny et al. (2014)56 and from the 2017 campaign (this study). (B) Stratigraphical correlation of the two core datasets along transect A and B, with indications of historical flood dates56 and radiocarbon samples (Extended Data Table 4). (C) Age-depth model based on historical flood events and radiocarbon ages, using the software-package ‘clam’58.
Evaluation of the newly reconstructed Lake Bourget palaeoflood record over the last 350 years (a) and the last two millennia (b). Over the last 350 years, the new record (b.) is compared to the record from Evin et al. (2019)23(a.), which is an update of the dataset from Jenny et al. (2014)56 that combined reconstructed palaeoflood discharges from 1650 to 1852 (black squares) and annual flood discharges from 2010 to 1852 (black and red dots). Red dots denote the floods recorded in the sedimentary sequence among the gauged, annual floods (black dots). Our new record includes 27 on these 32 floods and the five extreme floods correspond well to the largest flood discharges. The lacking floods in our dataset can be related to the lower number of sediment cores compared to Jenny et al. (2014)56 and Evin et al. (2019)23. Over the last 2000 years, the new record (d) is compared to the record from Arnaud et al. (2016)60, which correspond to detrital inputs brought by the Rhône River floods to the deepest part of Lake Bourget. These detrital inputs were very low in the oldest part of the record and largely increased during the Little Ice Age. This period of increased detrital inputs corresponds well to higher occurrence of flood events. Therefore, these comparisons support the robustness of our extended Rhône River palaeoflood record.
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Wilhelm, B., Rapuc, W., Amann, B. et al. Impact of warmer climate periods on flood hazard in the European Alps. Nat. Geosci. 15, 118–123 (2022). https://doi.org/10.1038/s41561-021-00878-y
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