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

Abstract

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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Fig. 1: Spatiotemporal distribution of flood records from lake sediments available in the European Alps.
Fig. 2: Changes in large-flood occurrence from cooler to warmer climates.
Fig. 3: Trends in the occurrence of large floods during periods with warming or cooling trends.
Fig. 4: Changes in flood occurrence between the cool LIA and the warm MCA for the five records documenting flood magnitude.

Data availability

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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Acknowledgements

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.

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Authors and Affiliations

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Contributions

All authors designed the study and B.W. wrote the first draft of the paper. B.W. collated the database with the help of B.A., W.R., M.C., C.G.-C., L.G., R.I., P.S., T.S. and S.B.W. W.R. conducted the sedimentological and geochronological analyses of the Lake Bourget sequence. B.W., M.N. and J.B. conducted the statistical analyses. All authors contributed to interpreting the results. All authors contributed to framing and revising the paper.

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Correspondence to B. Wilhelm.

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Extended data

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.

Extended Data Fig. 2 Evaluation of the newly reconstructed Lake Bourget palaeoflood record.

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.

Extended Data Table 1 List of the 33 existing palaeoflood records reconstructed from lake sediments in the European Alps. List of the 33 existing palaeoflood records reconstructed from lake sediments in the European Alps and selection of the 27 used in this study in bold, with information about location/coordinates, lake catchment characteristics as well as time length and characteristics of records. Abbreviations as follows: Lat. = latitude, Long. = longitude, Flood season: Sp = Spring, S = Summer, A = Autumn, W = Winter and U = Unknown, D = number of dating points (v = varved record), F = number of floods events recorded, M = Magnitude of floods reconstructed, C = Calibrated record and Ref. = reference. * = explanation for the record rejection from the study
Extended Data Table 2 AMS radiocarbon dates for the new Lake Bourget core dataset. AMS radiocarbon dates for the new Lake Bourget core dataset, measured at the Poznań radiocarbon laboratory, calibrated using the IntCal13 calibration curve57. The radiocarbon age in bold was rejected given its old age compared to other (radiocarbon and historical) dates (Extended Data Fig. 3)
Extended Data Table 3 List of the 27 selected palaeoflood records used in this study for the different analyses of flood occurrence and magnitude. List of the 27 selected palaeoflood records used in this study for the different analyses of flood occurrence and magnitude. A record is used for flood occurrence analyses when it fully covers the time period of interest or for flood magnitude analyses when it includes information about magnitude. When a record is used for flood occurrence analyses, a number is marked that corresponds to the mean flood return period of the record for the given period, that is the number of recorded flood events divided by the considered time length. When a record is used for flood magnitude analysis, a number is marked that corresponds to the site number (Nr.) used in Extended Data Table 1 and Fig. 3. The last 150 years cover 1850–2000 CE; the Industrial Era 1800–2000 CE; the Last Millennium 950–1850 CE and the Holocene 9000–1000 years BP
Extended Data Table 4 List of collected temperature records from or close to the European Alps. List of collected temperature records from or close to the European Alps, sorted by time periods of interest, used in this study with information about site/location, archive type, proxy type, season and reference. Note that summer is the main flooding season (Table 1). References are the following: (1) Heiri O., Ilyashuk B., Millet L., Samartin S., Lotter A.F. (2015) Stacking of discontinuous regional palaeoclimate records: Chironomid-based summer temperatures from the Alpine region. The Holocene 25(1) 137–149; (2) Samartin S., Heiri O., Joos F., Renssen H., Franke J., Brönnimann S., Tinner W. (2017) Warm Mediterranean mid-Holocene summers inferred from fossil midge assemblages. Nature Geoscience. DOI: 10.1038/NGEO2891; (3) Büntgen U., Franck D.C., Nievergelt D., Esper J. (2006) Summer Temperature Variations in the European Alps, A.D. 755–2004, Journal of Climate, 19, 5606–5623; (4) Büntgen U., Tegel W., Nicolussi K., McCormick M., Frank D., Trouet V., Kaplan J.O., Herzig F., Heussner K.U., Wanner H., Luterbacher J., Esper J. (2011) 2500 Years of European Climate Variability and Human Susceptibility, Science 331, 578; (5) Corona C., Guiot J., Edouard J.L., Chalié F., Büntgen U., Nola P., Urbinati C. (2010) Millennium-long summer temperature variations in the European Alps as reconstructed from tree rings. Clim. Past, 6, 379–400; (6) Larocque-Tobler I., Heiri O., Wehrli M. (2010) Late Glacial and Holocene temperature changes at Egelsee, Switzerland, reconstructed using subfossil chironomids. J Palaeolimnology, 43:649–666; (7) Larocque-Tobler I., Stewart M.M., Quinlan R., Trachsel M., Kamenik C., Grosjean M. (2012) A last millennium temperature reconstruction using chironomids preserved in sediments of anoxic Seebergsee (Switzerland): consensus at local, regional and Central European scales. Quaternary Science Reviews 41, 49–56; (8) Mangini A., Spötl C., Verdes P. (2005) Reconstruction of temperature in the Central Alps during the past 2000 yr from a y18O stalagmite record. Earth and Planetary Science Letters 235, 741– 751; (9) Trachsel M., Kamenik C., Grosjean M., McCarroll S., Moberg A., Brázdil R., Büntgen U., Dobrovolný P., Esper J., Frank D.C., Friedrich M., Glaser R., Larocque-Tobler I., Nicolussi K., Riemann D. (2012) Multi-archive summer temperature reconstruction for the European Alps, AD 1053–1996. Quaternary Science Reviews 46 (2012) 66–79; (10) Auer I, Böhm R, Jurkovic A, Lipa W, Orlik A, Potzmann R, Schöner W, Ungersböck M, Matulla C, Briffa K, Jones PD, Efthymiadis D, Brunetti M, Nanni T, Maugeri M, Mercalli L, Mestre O, Moisselin J-M, Begert M, Müller-Westermeier G, Kveton V, Bochnicek O, Stastny P, Lapin M, Szalai S, Szentimrey T, Cegnar T, Dolinar M, Gajic-Capka M, Zaninovic K, Majstorovic Z, Nieplova E, 2007. HISTALP – Historical instrumental climatological surface time series of the greater Alpine region 1760–2003. International Journal of Climatology 27: 17–46
Extended Data Table 5 Changes in temperature by periods of interest from records of Extended Data Table 3. Changes in temperature by periods of interest from records of Extended Data Table 3. The Neoglacial Period (NP) covers 5000–1000 years BP; the Holocene Thermal Maximum (HTM) 9000–5000 cal. years BP; the Little Ice Age (LIA) 1450–1850 CE and the Medieval Climate Anomaly (MCA) 950–1250 CE. 19th c. = nineteenth century, 20th c. = twentieth century, Diff. = Differences in temperature between sub-periods and Ref. = Reference (see Extended Data Table 4)
Extended Data Table 6 Sign and level of significance of relative changes in flood occurrence. Sign and level of significance of relative changes in flood occurrence obtained using the test of equal proportions as well as sign and level of significance of flood trends obtained respectively using a Poisson regression model with years as covariate and a Chi-square test. Si = sign of the change/trend; - = negative change/trend; + = positive change/trend; S=Level of significance; *** = p < 0.001; ** = p < 0.01; ** = p < 0.1; NA = no result due to the shortness of the record in relation to the studied period (Extended Data Table 2); NA* = no result due to a too limited number of recorded events in relation to the studied period
Extended Data Table 7 Modified Mann-Kendall test of significance of warming/cooling trend. Modified Mann-Kendall test of significance of warming/cooling trend during the Holocene (9000–1000 BP), the Last Millennium (950–1850 CE), the Industrial Era (1800–2000 CE) and subperiods of the Industrial Era. Level of significance: *** = p < 0.001; ** = p < 0.01; ** = p < 0.1. Ref. = Reference (see Extended Data Table 4)

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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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