Regionales Flutrisiko wird von Ozeanzyklen mitgesteuert

Wenn Flüsse über die Ufer treten und ganze Landstriche unter Wasser setzen herrscht Katastrophenalarm. Tritt eine solche Flut auf, ist heutzutage reflexhaft der Klimawandel als Bösewicht identifiziert. Dabei wird vergessen, dass man das Geschehen im langfristigen Kontext der letzten Jahrhunderte und Jahrtausende sehen muss.

Bereits 2013 hat ein Team um Yukiko Hirabayashi das Überflutungsrisiko in verschiedenen Teilen der Erde simuliert. Während es in einigen Regionen wohl steigt, sinkt es in anderen ab:

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.

Najibi & Devineni 2018 zeigten, dass Trends in den Überflutungen stark an die Ozeanzyklen wie NAO, AMO und PDO gekoppelt sind:

Recent trends in the frequency and duration of global floods
Frequency and duration of floods are analyzed using the global flood database of the Dartmouth Flood Observatory (DFO) to explore evidence of trends during 1985–2015 at global and latitudinal scales. Three classes of flood duration (i.e., short: 1–7, moderate: 8–20, and long: 21 days and above) are also considered for this analysis. The nonparametric Mann–Kendall trend analysis is used to evaluate three hypotheses addressing potential monotonic trends in the frequency of flood, moments of duration, and frequency of specific flood duration types. We also evaluated if trends could be related to large-scale atmospheric teleconnections using a generalized linear model framework. Results show that flood frequency and the tails of the flood duration (long duration) have increased at both the global and the latitudinal scales. In the tropics, floods have increased 4-fold since the 2000s. This increase is 2.5-fold in the north midlatitudes. However, much of the trend in frequency and duration of the floods can be placed within the long-term climate variability context since the Atlantic Multidecadal Oscillation, North Atlantic Oscillation, and Pacific Decadal Oscillation were the main atmospheric teleconnections explaining this trend. There is no monotonic trend in the frequency of short-duration floods across all the global and latitudinal scales. There is a significant increasing trend in the annual median of flood durations globally and each latitudinal belt, and this trend is not related to these teleconnections. While the DFO data come with a certain level of epistemic uncertainty due to imprecision in the estimation of floods, overall, the analysis provides insights for understanding the frequency and persistence in hydrologic extremes and how they relate to changes in the climate, organization of global and local dynamical systems, and country-scale socioeconomic factors.

Trends in der Wasserführung von Flüssen wurden von Gudmundsson et al. 2019 analysiert:

Observed Trends in Global Indicators of Mean and Extreme Streamflow
This study investigates global changes in indicators of mean and extreme streamflow. The assessment is based on the Global Streamflow Indices and Metadata archive and focuses on time series of the annual minimum, the 10th, 50th, and 90th percentiles, the annual mean, and the annual maximum of daily streamflow. Trends are estimated using the Sen‐Theil slope, and the significance of mean regional trends is established through bootstrapping. Changes in the indices are often regionally consistent, showing that the entire flow distribution is moving either upward or downward. In addition, the analysis confirms the complex nature of hydrological change where drying in some regions (e.g., in the Mediterranean) is contrasted by wetting in other regions (e.g., North Asia). Observed changes are discussed in the context of previous results and with respect to model estimates of the impacts of anthropogenic climate change and human water management.

Trends variieren stark von Region zu Region. Aus den Conclusions:

A striking result is that in most cases the sign of regional trends is consistent across all indices. This implies that the entire flow distribution is changing upward or downward in the respective regions, indicating generally wetter or drier conditions. In other words, increasing low flows are in most cases associated with increasing high flows (and vice versa), contradicting the common notion that flood and drought risk may increase simultaneously. Another feature of the results is that for some regions (West North America, East North America, and North‐East Brazil) the sign of the trends has varied with respect to the considered period, suggesting low‐frequency variability in the baseline climate signal and that care is needed in the interpretation of the associated change patterns. Among all considered regions, South Europe/Mediterranean had the strongest signal with consistent negative trends in all indices throughout all considered time periods. Other regions with predominantly negative trends include Southern Africa, South Australia/New Zealand, and potentially South Asia. In addition, Northeastern Brazil experienced drying conditions for the last two time periods but had a consistent wetting trend for the first period. Consistent wetting trends were observed in Central North America, Southeastern South America, North Europe, and North Asia, although the trend weakens for the last period in Central North America and Southeast South America. Overall, these wetting trends are not equally visible in all regions and throughout all indices.