Führt der Klimawandel zu vermehrten Überschwemmungen? Globale Prognosen hierzu sind nicht robust genug

Immer wieder lesen und hören wir die pauschale Behauptung, der Klimawandel würde in Zukunft zu vermehrten Überschwemmungen führen. Das akzeptieren dann alle wie das Amen in der Kirche. Wir wollen heute der Frage nachgehen, inwieweit dieses Modell wissenschaftlich eigentlich abgesichert ist. Bei der Diskussion spielen zwei Elemente eine Rolle: Die Analyse der Überflutungstrends aus der Vergangenheit sowie theoretische Modellzukunftsprognosen. In vielen Fällen werden die beiden Elemente separat behandelt. Dabei wäre eine Kalibrierung der Modelle an der Historie der letzten Jahrtausende dringend notwendig, um die Prognosen zu validieren.

Im Jahr 2013 erschien in Nature Climate Change eine Modellierungsarbeit eines Teams um Yukiko Hirabayashi. Darin erkannten die Forscher, dass die Modelle ganz unterschiedliche Entwicklungen ausspuckten. Zudem gab es starke regionale Unterschiede. In einigen Gebieten der Erde sollte das Überflutungsrisiko zunehmen, in anderen abnehmen. Einen einheitlichen Trend konnten die Wissenschaftler nicht feststellen. Abstract:

Global flood risk under climate change
A warmer climate would increase the risk of floods1. So far, only a few studies2, 3 have projected changes in floods on a global scale. None of these studies relied on multiple climate models. A few global studies4, 5 have started to estimate the exposure to flooding (population in potential inundation areas) as a proxy of risk, but none of them has estimated it in a warmer future climate. Here we present global flood risk for the end of this century based on the outputs of 11 climate models. A state-of-the-art global river routing model with an inundation scheme6 was employed to compute river discharge and inundation area. An ensemble of projections under a new high-concentration scenario7 demonstrates a large increase in flood frequency in Southeast Asia, Peninsular India, eastern Africa and the northern half of the Andes, with small uncertainty in the direction of change. In certain areas of the world, however, flood frequency is projected to decrease. Another larger ensemble of projections under four new concentration scenarios7 reveals that the global exposure to floods would increase depending on the degree of warming, but interannual variability of the exposure may imply the necessity of adaptation before significant warming.

Selbst der IPCC räumte in seinem SREX-Bericht ein, dass verlässliche Überschwemmungsprognosen derzeit nicht möglich sind und betrachtet die Vorhersagen als “wenig vertrauenswürdig” (low confidence).  Ein Wissenschaftlerteam um Zbigniew Kundzewicz hat die großen Probleme in einem Artikel im Hydrological Sciences Journal noch einmal bestätigt. Der Anstieg der Überschwemmungs-Versicheurngsschäden geht auf die Steigerung der versicherten Werte zurück.  Abstract:

Flood risk and climate change: global and regional perspectives
A holistic perspective on changing rainfall-driven flood risk is provided for the late 20th and early 21st centuries. Economic losses from floods have greatly increased, principally driven by the expanding exposure of assets at risk. It has not been possible to attribute rain-generated peak streamflow trends to anthropogenic climate change over the past several decades. Projected increases in the frequency and intensity of heavy rainfall, based on climate models, should contribute to increases in precipitation-generated local flooding (e.g. flash flooding and urban flooding). This article assesses the literature included in the IPCC SREX report and new literature published since, and includes an assessment of changes in flood risk in seven of the regions considered in the recent IPCC SREX report—Africa, Asia, Central and South America, Europe, North America, Oceania and Polar regions. Also considering newer publications, this article is consistent with the recent IPCC SREX assessment finding that the impacts of climate change on flood characteristics are highly sensitive to the detailed nature of those changes and that presently we have only low confidence1 in numerical projections of changes in flood magnitude or frequency resulting from climate change.

Eine Kernaussage des Papers lautet:

“The scientific community needs to emphasize that the problem of flood losses is mostly about what we do on or to the landscape and that will be the case for decades to come.”

Wie sieht es mit den globalen Trends bei den Niederschlägen aus? Hier gibt es drei globale Datensätze, die alle etwas anderes anzeigen: Steigend, fallend, ohne Trend. Wenig hilfreich. Das Paper dazu stammt von Li et al 2014:

Decadal trends of global precipitation in the recent 30 years
Decadal trends of global precipitation are examined using the Climate Prediction Center (CPC) Merged Analysis of Precipitation (CMAP), Global Precipitation Climatology Project (GPCP), and National Centers for Environmental Prediction (NCEP)/National Center for Atmospheric Research (NCAR) reanalysis data. The decadal trends of global precipitation average diverge a decreasing trend for the CMAP data, a flat trend for the GPCP data, and an increasing trend for the reanalysis data. The decreasing trend for the CMAP data is associated with the reduction in high precipitation. The flat trend for the GPCP data is related to the offset between the increase in high precipitation and the decrease in low precipitation. The increasing trend for the reanalysis data corresponds to the increase in high precipitation.

Interessante Arbeit von Serena Ceoloa und Kollegen aus dem Jahr 2014 in den Geophysical Research Letters: Die Autoren fanden erhöhte Überschwemmungsschäden in Gebieten, in denen sich die Menschen in den letzten Jahren augebreitet haben, basierend auf der Auswertung von nächtlicher Beleuchtung (nighttime lights). Das bedeutet, dass die zunehmende Ansiedelung in den Flutgefahrengebieten unabhängig vom Klimawandel eine wichtige Ursache der Flutschäden ist.  Abstract:

Satellite nighttime lights reveal increasing human exposure to floods worldwide
River floods claim thousands of lives every year, but effective and high-resolution methods to map human exposure to floods at the global scale are still lacking. We use satellite nightlight data to prove that nocturnal lights close to rivers are consistently related to flood damages. We correlate global data of economic losses caused by flooding events with nighttime lights and find that increasing nightlights are associated to flood damage intensification. Then, we analyze the temporal evolution of nightlights along the river network all over the world from 1992 to 2012 and obtain a global map of nightlight trends, which we associate with increasing human exposure to floods, at 1 km2 resolution. An enhancement of exposure to floods worldwide, particularly in Africa and Asia, is revealed, which may exacerbate the projected effects of climate change on flood-related losses and therefore argues for the development of valuable flood preparedness and mitigation strategies.