Klimaextreme können nur in ihrem langfristigen Kontext korrekt bewertet werden. Heute stellen wir Ihnen Arbeiten aus den Alpen zur Überschwemmungsgeschichte vor. Wir beginnen mit einer Studie von Wilhelm et al. 2016 in Climate of the Past aus den französischen Alpen. Die Forscher untersuchten die Flutgeschichte der letzten 1000 Jahre und fanden eine Häufung der Flutereignisse während der kalten Kleinen Eiszeit. Flutereignisse von hoher Intensität wurden sowohl in der Kleinen Eiszeit als auch in der Mittelalterlichen Wärmeperiode gefunden. Interessanterweise konnte für das 20. Jahrhundert trotz Erwärmung kein Trend in Häufigkeit und Intensität der Fluten ausgemacht werden. Hier der Abstract:
Frequency and intensity of palaeofloods at the interface of Atlantic and Mediterranean climate domains
Mediterranean climatic influences was explored by studying a lake sequence (Lake Foréant) of the Western European Alps. High-resolution sedimentological and geochemical analysis revealed 171 event layers, 168 of which result from past flood events over the last millennium. The layer thickness was used as a proxy of intensity of past floods. Because the Foréant palaeoflood record is in agreement with the documented variability of historical floods resulting from local and mesoscale, summer-to-autumn convective events, it is assumed to highlight changes in flood frequency and intensity related to such events typical of both Atlantic (local events) and Mediterranean (mesoscale events) climatic influences. Comparing the Foréant record with other Atlantic-influenced and Mediterranean-influenced regional flood records highlights a common feature in all flood patterns that is a higher flood frequency during the cold period of the Little Ice Age (LIA, AD 1300–1900). In contrast, high-intensity flood events are apparent during both the cold LIA and the warm Medieval Climate Anomaly (MCA, AD 950–1250). However, there is a tendency towards higher frequencies of high-intensity flood events during the warm MCA. The MCA extremes could mean that under the global warming scenario, we might see an increase in intensity (not in frequency). However, the flood frequency and intensity in the course of the 20th century warming trend did not change significantly. Uncertainties in future evolution of flood intensity lie in the interpretation of the lack of 20th century extremes (transition or stable?) and the different climate forcing factors between the two periods (greenhouse gases vs. solar and/or volcanic eruptions).
Weiter mit dem Mondsee in Österreich. Tina Swierczynski und Kollegen berichteten 2013 in Climate of the Past über eine Untersuchung der Flutfrequenz in der Jungsteinzeit. Sie fanden dabei starke Schwankungen. Besonder häufig waren die Überschwemmungen in einer Phase die vor knapp 6000 Jahren begann und um 4500 Jahren endete. Hier der Abstract:
Late Neolithic Mondsee Culture in Austria: living on lakes and living with flood risk?
Neolithic and Bronze Age lake dwellings in the European Alps became recently protected under the UNESCO World Heritage. However, only little is known about the cultural history of the related pre-historic communities, their adaptation strategies to environmental changes and particularly about the almost synchronous decline of many of these settlements around the transition from the Late Neolithic to the Early Bronze Age. For example, there is an ongoing debate whether the abandonment of Late Neolithic lake dwellings at Lake Mondsee (Upper Austria) was caused by unfavourable climate conditions or a single catastrophic event. Within the varved sediments of Lake Mondsee, we investigated the occurrence of intercalated detrital layers from major floods and debris flows to unravel extreme surface runoff recurrence during the Neolithic settlement period. A combination of detailed sediment microfacies analysis and μXRF element scanning allows distinguishing debris flow and flood deposits. A total of 60 flood and 12 debris flow event layers was detected between 7000 and 4000 varve years (vyr) BP. Compared to the centennial- to millennial-scale average, a period of increased runoff event frequency can be identified between 5900 and 4450 vyr BP. Enhanced flood frequency is accompanied by predominantly siliciclastic sediment supply between ca. 5500 and 5000 vyr BP and enhanced dolomitic sediment supply between 4900 and 4500 vyr BP. A change in the location and the construction technique of the Neolithic lake dwellings at Lake Mondsee can be observed during the period of higher flood frequency. While lake dwellings of the first settlement period (ca. 5800–5250 cal. yr BP) were constructed directly on the wetlands, later constructions (ca. 5400–4700 cal. yr BP) were built on piles upon the water, possibly indicating an adaptation to either increased flood risk or a general increase of the lake level. However, our results also indicate that other than climatic factors (e.g. socio-economic changes) must have influenced the decline of the Mondsee Culture because flood activity generally decreased since 4450 vyr BP, but no new lake dwellings have been established thereafter.
Die nächste Studie wurde am schweizerischen Oeschinensee von Benjamin Amann und Kollegen durchgeführt und reicht 1200 Jahre zurück. Erschienen ist die Arbeit im Mai 2015 in den Quaternary Science Reviews. Die Autoren fanden starek natürliche Schwankungen im Hochwasser. Einen stabilen Zusammenhang zwischen Temperatur und Niederschlägen konnten sie nicht finden. Das hat auch damit zu tun, weil sich die Zusammenhänge im Verlauf eines 60-jährigen Ozeanzyklus umkehrten. Allgemein ereigneten sich die stärksten Flutereignisse während Kältephasen. Aussagen über die zukünftige Überflutungsentwicklung lassen sich angesichts der Komplexität der Zusammenhänge nicht machen, erklären die Autoren. Hier die Kurzfassung:
A millennial-long record of warm season precipitation and flood frequency for the North-western Alps inferred from varved lake sediments: implications for the future
The recent warming of the global climate is well recognized. However, does a warmer climate also mean a moister climate? Does dry get drier and wet get wetter? There are important questions as they relate to changes in the water cycle and impacts the water resources as well as the frequency and intensity of storms and floods in the near future. In Europe, regional climate models do not show consistent and robust results for future hydroclimatic changes and how extreme events will evolve in response to future climate change.
Paleo-hydroclimatic data from natural archives are one of the few means to assess such changes in the longer context. Here, we present an annually-resolved record of warm season (MJJA) precipitation and summer flood frequency from the varved (annually laminated) sediments of proglacial Lake Oeschinen (46°30′N–7°44′E, 1580 m, NW Swiss Alps) back to AD 884. These data sets are inferred from the thickness of annual sediment deposits and the occurrence of flood event layers in the sediments. The chronology of the sediment record is based on multiple varve counts and validated with historical floods chronicled in written documents (back to the 14th century) and 14C AMS dates.
The precipitation record shows pronounced interannual to centennial variability with humid warm season phases between AD 920–950, AD 1100–1180, AD 1300–1400, AD 1590–1650, AD 1700–1790, AD 1820–1880, and AD 1960–2008. Driest conditions are reconstructed for AD 960–1080, AD 1250–1300 and for AD 1880–1900. Our precipitation record is consistent with the few multi-centennial warm-season precipitation records available for Europe.
We did not find a persistent relationship between warm-season precipitation and temperature. In contrast, results show that the relation between precipitation and temperature has oscillated between positive correlations (warmer gets wetter, cooler gets drier) and negative correlations (warmer gets drier, cooler gets wetter) with a highly significant (χ2 = 99%) multidecadal (60–70 yrs) periodicity over the last millennium. Possible explanations for this phenomenon are changes in the weather type statistics or the within-weather-type variability, which influence the combinations between precipitation and temperature over continental central Europe and operate at multidecadal scales. Such multidecadal effects might also be important for precipitation scenarios in the Alpine area under future warming.
Our record of flood frequency suggests more frequent floods under cool and humid climate during the warm seasons. This picture is consistent with other studies from small and medium size catchments at mid- and high elevations in the Alpine area. However, the 13th century reveals a period with high flood frequency during warm and moderately dry (average precipitation) conditions. This anomalous situation is currently not understood; nonetheless, this is also one out of several possible scenarios for the future. From the different combinations found in our record, we conclude that the relation between floods, precipitation and temperature and, in consequence, future projections remain poorly constrained.
–The precipitation reconstruction is representative for most of Western Europe.
–The relation PP–TT follows a multidecadal periodicity over the last millennium.
–Intense rainfall events were generally more frequent during wet and cool summers.
Eine ganze Reihe von Seen in den Zentralalpen haben Anselmetti et al. 2014 untersucht und dabei die Flutgeschichte der letzten 10.000 Jahre rekonstruiert. Die Studie erschien im Fachblatt Georeview. Dabei fanden die Autoren auch einen Zusammenhang mit den Aktivitätsschwankungen der Sonne, was sie dankenswerterweise gleich in den Totel schrieben:
Holocene flood frequency as reconstructed by lake sediments from multiple archives: A record influenced by solar forcing and atmospheric circulation patterns
The frequency of large-scale heavy precipitation events in mountain ranges is expected to undergo substantial changes with current climate change. Hence, knowledge about the past natural variability of floods caused by heavy precipitation constitutes important input for climate projections and natural hazard analyses. We present a comprehensive Holocene (10,000 years) reconstruction of the flood frequency in the Central European Alps combining 15 lacustrine sediment records. These records provide an extensive catalog of flood deposits, which were generated by flood-induced underflows delivering terrestrial material to the lake floors. The multi-archive approach allows suppressing local weather patterns, such as thunderstorms, from the obtained climate signal.
Auch Pena et al. 2015 fanden eine starke Sonnensignatur in der schweizerischen Flutgeschichte, die sie Hydrology and Earth System Sciences dokumentierten:
Influence of solar forcing, climate variability and modes of low-frequency atmospheric variability on summer floods in Switzerland
The higher frequency of severe flood events in Switzerland in recent decades has given fresh impetus to the study of flood patterns and their possible forcing mechanisms, particularly in mountain environments. This paper presents a new index of summer flood damage that considers severe and catastrophic summer floods in Switzerland between 1800 and 2009, and explores the influence of external forcings on flood frequencies. In addition, links between floods and low-frequency atmospheric variability patterns are examined. The flood damage index provides evidence that the 1817-1851, 1881-1927, 1977-1990 and 2005-present flood clusters occur mostly in phase with palaeoclimate proxies. The cross-spectral analysis documents that the periodicities detected in the coherency and phase spectra of 11 (Schwabe cycle) and 104 years (Gleissberg cycle) are related to a high frequency of flooding and solar activity minima, whereas the 22-year cyclicity detected (Hale cycle) is associated with solar activity maxima and a decrease in flood frequency. The analysis of low-frequency atmospheric variability modes shows that Switzerland lies close to the border of the principal summer mode. The Swiss river catchments situated on the centre and southern flank of the Alps are affected by atmospherically unstable areas defined by the positive phase of the pattern, while those basins located in the northern slope of the Alps are predominantly associated with the negative phase of the pattern. Furthermore, a change in the low-frequency atmospheric variability pattern related to the major floods occurred over the period from 1800 to 2009; the summer principal mode persists in the negative phase during the last cool pulses of the Little Ice Age (1817-1851 and 1881-927 flood clusters), whereas the positive phases of the mode prevail during the warmer climate of the last 4 decades (flood clusters from 1977 to present).