Severe floods triggered by intense precipitation are among the most destructive natural hazards in Alpine environments, frequently causing large financial and societal damage. Potential enhanced flood occurrence due to global climate change would thus increase threat to settlements, infrastructure, and human lives in the affected regions. Yet, projections of intense precipitation exhibit major uncertainties and robust reconstructions of Alpine floods are limited to the instrumental and

more »

... l period. Here we present a 2500-year long flood reconstruction for the European Alps, based on dated sedimentary flood deposits from ten lakes in Switzerland. We show that periods with high flood frequency coincide with cool summer temperatures. This wet-cold synchronism suggests enhanced flood occurrence to be triggered by latitudinal shifts of Atlantic and Mediterranean storm tracks. This paleoclimatic perspective reveals natural analogues for varying climate conditions, and thus can contribute to a better understanding and improved projections of weather extremes under climate change. M ean Central European summer temperatures are projected to increase under global climate change, while summer precipitation totals will likely decrease 1,2 . However, the frequency of climate extremes, such as intense precipitation events, is more difficult to project, as such events strongly depend upon season, location, and spatial extent 3-5 . Therefore, the analysis of long climate time series supports the identification of climatic processes naturally governing the occurrence of intense precipitation and thus improves the projection of these climate extremes. Instrumental measurements that cover the last 150 years reveal highly resolved precipitation records over space and time 6,7 , but are too short to detect the natural multi-decadal to centennial variability in the climate system. Longer time series can be reconstructed from historical documents 8,9 , as well as from geological archives such as riverine overwash deposits 10,11 . However, both approaches do not provide continuous records over the past millennia 11,12 , and thus only yield an incomplete palaeoclimatic picture. Lake sediments, in contrast, reflect past flood activity very accurately, as they record individual events as distinct sediment layers. These 'turbidites' or flood deposits provide a continuous flood archive over thousands of years 13 . The flood deposits are composed of terrigenous material, which is mobilized during intense precipitation in the catchment area and eventually deposited at the bottom of the next downstream lake 13-15 (Fig. 1) . Several studies have established flood records from single lakes 16-18 , however, these records may only reveal a local climate signal and/or may loose their pristine natural signal due to human activity in the corresponding catchment area 13,14 . Here we present a multi-archive Alpine flood reconstruction based on ten lacustrine sediment records, covering the past 2500 years. The 10 investigated lakes are situated north of the Central Alpine arc along a montane to Alpine transect, spanning an elevation gradient from 447 to 2068 m asl (Fig. 2) . This multi-lake compilation allows the extraction of a synoptic, rather than a merely local rainfall signal revealed by a single-lake study 14 . Results The complete flood reconstruction contains 842 dated flood layers (Fig. 3a) , deposited dominantly from midspring to late-fall, since the higher elevated lakes are ice-covered and/or receive precipitation in the form of snow OPEN SUBJECT AREAS: PALAEOCLIMATE LIMNOLOGY CLIMATE CHANGE ENVIRONMENTAL SCIENCES