Oliver Wetter

There are concerns that recent climate change is altering the frequency and magnitude of river floods in an unprecedented way1. Historical studies have identified flood-rich periods in the past half millennium in various regions of Europe2. However, because of the low temporal resolution of existing datasets and the relatively low number of series, it has remained unclear whether Europe is currently in a flood-rich period from a long-term perspective. Here we analyse how recent decades compare with the flood history of Europe, using a new database composed of more than 100 high-resolution (sub-annual) historical flood series based on documentary evidence covering all major regions of Europe. We show that the past three decades were among the most flood-rich periods in Europe in the past 500 years, and that this period differs from other flood-rich periods in terms of its extent, air temperatures and flood seasonality. We identified nine flood-rich periods and associated regions. Among the periods richest in floods are 1560–1580 (western and central Europe), 1760–1800 (most of Europe), 1840–1870 (western and southern Europe) and 1990– 2016 (western and central Europe). In most parts of Europe, previous flood-rich periods occurred during cooler-than-usual phases, but the current flood-rich period has been much warmer. Flood seasonality is also more pronounced in the recent period. For example, during previous flood and interflood periods, 41 per cent and 42 per cent of central European floods occurred in summer, respectively, compared with 55 per cent of floods in the recent period. The exceptional nature of the present-day flood-rich period calls for process-based tools for flood-risk assessment that capture the physical mechanisms involved, and management strategies that can incorporate the recent changes in risk.

The heat waves of 2003 in Western Europe and 2010 in Russia, commonly labelled as rare climatic anomalies outside of previous experience, are often taken as harbingers of more frequent extremes in the global warming-influenced future. However, a recent reconstruction of spring–summer temperatures for WE resulted in the likelihood of significantly higher temperatures in 1540. In order to check the plausibility of this result we investigated the severity of the 1540 drought by putting forward the argument of the known soil desiccation-temperature feedback. Based on more than 300 first-hand documentary weather report sources originating from an area of 2 to 3 million km2, we show that Europe was affected by an unprecedented 11-month-long Megadrought. The estimated number of precipitation days and precipitation amount for Central and Western Europe in 1540 is significantly lower than the 100-year minima of the instrumental measurement period for spring, summer and autumn. This result is supported by independent documentary evidence about extremely low river flows and Europe-wide wild-, forest- and settlement fires. We found that an event of this severity cannot be simulated by state-of-the-art climate models.

The heat waves of 2003 in Western Europe and 2010 in Russia, commonly labelled as rare climatic anomalies outside of previous experience, are often taken as harbingers of more frequent extremes in the global warming-influenced future. However, a recent reconstruction of spring–summer temperatures for WE resulted in the likelihood of significantly higher temperatures in 1540. In order to check the plausibility of this result we investigated the severity of the 1540 drought by putting forward the argument of the known soil desiccation-temperature feedback. Based on more than 300 first-hand documentary weather report sources originating from an area of 2 to 3 million km2, we show that Europe was affected by an unprecedented 11-month-long Megadrought. The estimated number of precipitation days and precipitation amount for Central and Western Europe in 1540 is significantly lower than the 100-year minima of the instrumental measurement period for spring, summer and autumn. This result is supported by independent
documentary evidence about extremely low river flows and Europe-wide wild-, forestand settlement fires. We found that an event of this severity cannot be simulated by state-of-the-art climate models.

The magnitudes of the largest known floods of the River Rhine in Basel since 1268 were assessed using a hydraulic model drawing on a set of pre-instrumental evidence and daily hydrological measurements from 1808. The pre-instrumental evidence, consisting of flood marks and documentary data describing extreme events with the customary reference to specific landmarks, was “calibrated” by comparing it with the instrumental series for the overlapping period between the two categories of evidence (1808–1900). Summer (JJA) floods were particularly frequent in the century between 1651–1750, when precipitation was also high. Severe winter (DJF) floods have not occurred since the late 19th century despite a significant increase in winter precipitation. Six catastrophic events involving a runoff greater than 6000 m 3 s‐1 are documented prior to 1700. They were initiated by spells of torrential rainfall of up to 72 h (1480 event) and preceded by long periods of substantial precipitation that saturated the soils, and/or by abundant snowmelt. All except two (1999 and 2007) of the 43 identified severe events (SEs: defined as having runoff > 5000 and < 6000 m 3 s ‐1) occurred prior to 1877. Not a single SE is documented from 1877 to 1998. The intermediate 121-year-long “flood disaster gap” is unique over the period since 1268. The effect of river regulations (1714 for the River Kander; 1877 for the River Aare) and the building of reservoirs in the 20th century upon peak runoff were investigated using a one-dimensional hydraulic flood-routing model. Results show that anthropogenic effects only partially account for the “flood disaster gap” suggesting that variations in climate should also be taken into account in explaining these features. Citation Wetter, O., Pfister, C., Weingartner, R., Luterbacher, J., Reist, T., & Trösch, J. (2011) The largest floods in the High Rhine basin since 1268 assessed from documentary and instrumental evidence. Hydrol. Sci. J. 56(5), 733–758.Les débits des plus grandes crues du Rhin à Bâle ont été évalués à partir d'un modèle hydraulique basé sur un ensemble de données proxy à partir de 1286 et de mesures hydrologiques depuis 1808. Les données proxy se composent de marques d'inondation et de preuves narratives indiquant le plus haut niveau de chaque crue par rapport à un système de points de repère situés dans le voisinage du vieux pont. Ce système fut maintenu jusqu'au 19ème siècle tardif, ce qui a permis de “calibrer” les données avec la série des mesures instrumentales pendant la période de chevauchement des deux catégories de données. Des crues étaient fréquentes dans la période estivale de 1651 à 1750 marquée par des précipitations accrues. Il n'y a pas eu de grande crue hivernale depuis la fin du 19ème siècle, bien que les précipitations dans cette saison aient bien augmenté depuis le 19ème siècle tardif. Six événements “catastrophiques” impliquant un débit de plus de 6000 m3 s-1 sont documentés avant 1700. Ils ont étéconditionnés par de longues périodes de précipitation saturant les sols et/ou par une fonte de neige abondante, et déclenchés par des pluies intensives d'une durée allant jusquà 72 heures (1480). Les 43 évènements “sévères” ayant un écoulement estimé compris entre 5000 et 6000 m3 s-1 ont eu lieu avant 1877, à l'exception de ceux de 1999 et 2007. Aucun évènement “sévère” n'a cependant été mesuré entre 1877 et 1998. Une telle “lacune de calamités” (disaster gap) n'est pas documentée depuis 1268. Les effets de deux corrections de rivières (la Kander en 1714 et l'Aar en 1877) et de la construction de réservoirs au 20ème siècle sur les débits extrêmes ont été analysés avec un modèle unidimensionnel numérique de propagation. Les résultats montrent que les effets anthropogéniques expliquent seulement un part de la “lacune de calamités”, ce qui suggère que des variations climatiques ont aussi joué un rôle.