TU Delft: Überschwemmungen in Europa zeigen während der vergangenen 150 Jahre keinen Anstieg von finanziellen Schäden oder Opferzahlen

Die TU Delft gab am 31. Mai 2018 per Pressemitteilung eine gute Nachricht bekannt: Während der letzten 150 Jahre haben sich weder die Opferzahlen, noch die finanziellen Schäden im Zusammenhang mit Überschwemmungen in Europa erhöht, wenn man entsprechende Korrekturen für demographische und ökonomische Veränderungen berücksichtigt:

No increase in losses in Europe from floods in the past 150 years

Extreme hydrological events are generally predicted to become more frequent and damaging in Europe due to warming climate. Researchers from TU Delft and Rice University (Houston) have now shown that, correcting for economic and demographic changes, there has been no increase in financial losses and fatalities from floods in the last 150 years. They have reported on their findings in Nature Communications.

Correcting

An analysis of long-term trends in flood losses should account for changes in size and distribution of population and assets. Without correcting reported losses for changes in exposure, studies (logically) report a significant upward trend in losses. ‘Such ‘normalization’ processes have also proven to be important for explaining trends in other natural hazards’, says Dominik Paprotny, researcher at TU Delft and lead author of the paper in Nature Communications.

So adverse consequences of floods change are influenced by both natural and socio-economic trends and interactions. In Europe, previous studies of historical flood losses that were corrected for demographic and economic growth (‘normalized’) have been limited, leading to an incomplete representation of trends in losses over time. ‘After adjusting nominal losses for demographic and economic growth, no significant trends in flood losses, both on European scale and for individual countries were observed.’

Less fatalities

Paprotny and his colleagues utilized a gridded reconstruction of flood exposure in 37 European countries and a new database of damaging floods since 1870. ‘Our results indicate that, after correcting for changes in flood exposure, there has been an increase in annually inundated area and number of persons affected since 1870, but we have also found a substantial decrease in flood fatalities. For more recent decades we found a considerable decline in financial losses per year. We estimate, however, that there is large underreporting of smaller floods beyond most recent years, and show that underreporting has a substantial impact on observed trends.’

Changes

Extreme hydrological events are generally predicted to become more frequent and damaging in Europe due to warming climate and there seems to be large consensus regarding the trajectory of future climatic developments. ‘There is however less confidence in the changes in flood losses as a result of climate change so far’, says Paprotny. ‘Qualitative and quantitative hydrological studies for Europe have indicated no general continental-wide trend in river flood occurrences, extreme precipitation, or annual maxima of runoff. However, substantial variations between different catchments have been observed, ranging from an increase in north-western Europe to no trend or a decrease in other parts of the continent. Similar findings were reported for storminess along the European coasts.’

Bereits im September 2017 hatte eine Forschergruppe um Glenn Hodgkins in einer Studie Trends großer Fluten in Europa und Nordamerika berechnet. Nur wenige Flüsse zeigten einen signifikanten Langzeittrend, der jedoch aufgrund der hohen Anzahl an Datensätzen auch bei einem Zufallsexperiment aufgetreten wäre. Stattdessen scheint das Flutgeschehen in Europa und Nordamerika eng an den Ozeanzyklus der Atlantischen Multidekadenoszillation gebunden zu sein. Hier der Abstract:

 Climate-driven variability in the occurrence of major floods across North America and Europe
Concern over the potential impact of anthropogenic climate change on flooding has led to a proliferation of studies examining past flood trends. Many studies have analysed annual-maximum flow trends but few have quantified changes in major (25–100 year return period) floods, i.e. those that have the greatest societal impacts. Existing major-flood studies used a limited number of very large catchments affected to varying degrees by alterations such as reservoirs and urbanisation. In the current study, trends in major-flood occurrence from 1961 to 2010 and from 1931 to 2010 were assessed using a very large dataset (>1200 gauges) of diverse catchments from North America and Europe; only minimally altered catchments were used, to focus on climate-driven changes rather than changes due to catchment alterations. Trend testing of major floods was based on counting the number of exceedances of a given flood threshold within a group of gauges. Evidence for significant trends varied between groups of gauges that were defined by catchment size, location, climate, flood threshold and period of record, indicating that generalizations about flood trends across large domains or a diversity of catchment types are ungrounded. Overall, the number of significant trends in major-flood occurrence across North America and Europe was approximately the number expected due to chance alone. Changes over time in the occurrence of major floods were dominated by multidecadal variability rather than by long-term trends. There were more than three times as many significant relationships between major-flood occurrence and the Atlantic Multidecadal Oscillation than significant long-term trends.