Frequent floods in the European Alps coincide with cooler periods of the past 2500 years

Abstract

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 historical 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.

Recent trends in the frequency and duration of global floods

“…Theoretical studies also discussed the fact that mean global precipitation intensity increased by 1-3 % (conditional on available energy budgets) in proportion to the 1 • C increasing rate of surface air temperature. Trenberth (1999), Trenberth et al (2003), Trenberth (2011), Schiermeier (2011), and Glur et al (2013 among others have also argued that an increase in air temperature will increase the atmospheric waterholding capacity (Clausius-Clapyron relationship), leading to more intense and frequent precipitation events. Hence, fluctuating precipitation regimes would interrupt the current balances of components within the hydrological cycle and human activities (Doherty et al, 2000;Dentener et al, 2006).…”

Tributaries affect the thermal response of lakes to climate change

“…4). High SSC events are usually associated with extreme floods (Fink et al, 2016), which are predicted to vary in alpine lake catchments with on-going climate change (Glur et al, 2013). The lack of constraints on extreme precipitation events introduces uncertainty into future flood frequency and magnitude predictions (CH2011, 2011).…”

“…Major (deep penetrating) river intrusion events typically occur due to flooding, which flush large sediment loads into the river (Fink et al, 2016). The frequency and volume of floods in the Alps are notoriously hard to predict, although a decrease in floods has occurred in association with recent warmer summers observed in the Alps (CH2011, 2011; Glur et al, 2013).…”

Cahokia’s emergence and decline coincided with shifts of flood frequency on the Mississippi River

“…During flood stages that hydrologically connect the main channel with floodplain lakes, floodplain lakes act as sediment traps that allow the suspended load of floodwaters to fall out of suspension (19,20). The composition of floodwater sediments usually differs from locally sourced sediment deposited during nonflood conditions, particularly in grain size distribution (19).…”

“…Across the wide floodplain of a low-gradient river like the Mississippi, overbank floods deposit well-sorted fine silt-and claysized sediments in distal floodplain lakes and depressions (21)(22)(23). Analysis of sediment records from multiple basins in similar geomorphic settings along the same river ensures that identified flood events are not due to localized erosion or flooding from tributaries (20).…”