Früher war das Wetter immer klasse. Dann kam der Klimawandel, und jetzt spielt das Wetter komplett verrückt. In dieser Art argumentieren etliche dem Klimaalarm verfallene mediale Protagonisten. Oft wird dann noch auf theoretische Klimamodellierungen hingewiesen, die eine schwarze Zukunft skizzieren. Der Blick zurück in die Klimageschichte öffnet hier die Augen: Extremwetter hat es auch früher schon gegeben, zum Teil sogar intensiver als heute. Rekonstruktionen des Extremwetters zeigen eine starke natürliche Variabilität auf. Der reflexhafte Aufschrei beim Auftreten eines Sturmes, einer Überschwemmung, einer Dürre etc. mit dem Hinweis auf den Klimawandel ist daher kurzsichtig und irreführend.
Beispiel Überschwemmungen in Italien. Marco Marani und Stefano Zanetti beschrieben im Januar 2015 in Water Resources Research eine Datenreihe zu extremen Niederschlagsereignissen in Padua für die vergangenen 300 Jahre. Die Autoren fanden keinen Langzeittrend, dafür aber typische Zyklen, gemäß denen der Extremregen schwankte. Insbesondere scheint der Ozeanzyklus der NAO (Nordatlantische Oszillation) eine große Rolle zu spielen. Abstract:
Long-term oscillations in rainfall extremes in a 268 year daily time series
We analyze long-term fluctuations of rainfall extremes in 268 years of daily observations (Padova, Italy, 1725–2006), to our knowledge, the longest existing instrumental time series of its kind. We identify multidecadal oscillations in extremes estimated by fitting the GEV distribution, with approximate periodicities of about 17–21, 30–38, 49–68, 85–94, and 145–172 years. The amplitudes of these oscillations exceed the changes associated with the observed trend in intensity. This finding implies that even if climatic trends are absent or negligible, rainfall and its extremes exhibit an apparent nonstationarity if analyzed over time intervals shorter than the longest periodicity in the data (about 170 years for the case analyzed here). These results suggest that because long-term periodicities may likely be present elsewhere, in the absence of observational time series with length comparable to such periodicities (possibly exceeding one century), past observations cannot be considered to be representative of future extremes. We also find that observed fluctuations in extreme events in Padova are linked to the North Atlantic Oscillation: increases in the NAO Index are on average associated with an intensification of daily extreme rainfall events. This link with the NAO global pattern is highly suggestive of implications of general relevance: long-term fluctuations in rainfall extremes connected with large-scale oscillating atmospheric patterns are likely to be widely present and undermine the very basic idea of using a single stationary distribution to infer future extremes from past observations.
Das zweite Beispiel kommt aus den Italienischen Mittelmeeralpen von Wilhelm et al. 2017, das im Fachblatt Natural Hazards and Earth System Sciences erschien. Die Wissenschaftler identifizierten im Valle d’Aosta 76 Hochwasserereignisse in den letzten 270 Jahren, wovon 8 von Erdbeben und 68 von Extremregen ausgelöst wurden. Natürliche Klimavariabilität am Werk. Abstract:
A multi-centennial record of past floods and earthquakes in Valle d’Aosta, Mediterranean Italian Alps
Mediterranean Alpine populations are particularly exposed to natural hazards like floods and earthquakes because of both the close Mediterranean humidity source and the seismically active Alpine region. Knowledge of long-term variability in flood and earthquake occurrences is of high value since it can be useful to improve risk assessment and mitigation. In this context, we explore the potential of a lake-sediment sequence from Lago Inferiore de Laures in Valle d’Aosta (Northern Italy) as a long-term record of past floods and earthquakes. The high-resolution sedimentological study revealed 76 event layers over the last ca. 270 years; 8 are interpreted as most probably induced by earthquakes and 68 by flood events. Comparison to historical seismic data suggests that the recorded earthquakes are strong (epicentral Medvedev–Sponheuer–Kárník (MSK) intensity of VI–IX) and/or close to the lake (distance of 25–120 km). Compared to other lake-sediment sequences, Lago Inferiore de Laures sediments appear to be regionally the most sensitive to earthquake shaking, offering a great potential to reconstruct the past regional seismicity further back in time. Comparison to historical and palaeoflood records suggests that the flood signal reconstructed from Lago Inferiore de Laures sediments represents the regional and (multi-)decadal variability of summer–autumn floods well, in connection to Mediterranean mesoscale precipitation events. Overall, our results reveal the high potential of Lago Inferiore de Laures sediments to extend the regional earthquake and flood catalogues far back in time.