| 英文摘要: | Recent major flood disasters have shown that single extreme events can affect multiple countries simultaneously1, 2, 3, which puts high pressure on trans-national risk reduction and risk transfer mechanisms4, 5, 6. So far, little is known about such flood hazard interdependencies across regions7, 8 and the corresponding joint risks at regional to continental scales1, 9. Reliable information on correlated loss probabilities is crucial for developing robust insurance schemes5 and public adaptation funds10, and for enhancing our understanding of climate change impacts9, 11, 12. Here we show that extreme discharges are strongly correlated across European river basins. We present probabilistic trends in continental flood risk, and demonstrate that observed extreme flood losses could more than double in frequency by 2050 under future climate change and socio-economic development. We suggest that risk management for these increasing losses is largely feasible, and we demonstrate that risk can be shared by expanding risk transfer financing, reduced by investing in flood protection, or absorbed by enhanced solidarity between countries. We conclude that these measures have vastly different efficiency, equity and acceptability implications, which need to be taken into account in broader consultation, for which our analysis provides a basis. Major river floods are typically driven by large-scale atmospheric circulations8, 13, 14. As a result, single flood episodes can affect vast areas in a short period of time, irrespective of economic and political boundaries1, 3. This was demonstrated in June 2013 by the blocking of the planetary waves of the atmospheric flow regime in the Northern Hemisphere2, which led to extensive flooding and €12 billion losses15 in nine different countries across central and eastern Europe. Understanding the risk posed by large-scale floods is of growing importance, as their impacts are rising owing to socioeconomic development6, 16, and their frequency and intensity may increase under a changing climate1, 9, 12, 17. Well-devised risk management of climate-related extremes, including floods, is therefore considered to be an important pillar of climate adaptation18. Rising flood losses already force insurance companies to increase their capital base and may lead to more years of below-zero profitability5. Uninsured risks are a growing concern, as a lack of financial means for relief, recovery and reconstruction negatively affects the wellbeing of people, the economy and a country’s budget6, 19. Accurate information on the joint probability of flood losses that takes into account spatial correlations between river basins across different countries is essential for developing insurance mechanisms5 and public compensation schemes10 robust to present and future extreme losses. This information is especially required and informative in the European Union (EU), where international disaster financing is increasingly connected through insurance regulations21, climate change adaptation strategies20, and a joint compensation mechanism between member states10. So far, methods for producing large-scale flood risk estimates have either been based on specific hazard event scenarios, or are upscaled from lower to higher spatial levels by summation of basin-level risk16, 22, 23, 24. In both cases, natural correlation between events is neglected (that is, full spatial independence across river basins is assumed) and reliable estimates of extreme losses cannot be made. Hence, flood risk projections available to the disaster risk reduction community do not accurately represent geographical risk patterns and are not probabilistic in nature. We demonstrate here that natural dependencies among risks in different regions can be accounted for (Methods), and we present probabilistic projections of flood risk in the EU. We find monthly peak river discharges in the 1,007 sub-basins to be a good proxy for the occurrence of reported damaging flood events17 on a European scale, as shown in Supplementary Fig. 1. The results show high positive cross-correlations in observed peak discharges between the river sub-basins in Europe, indicating a large degree of spatial interdependence in extreme river flows. Spearman’s correlations are significant (α=0.05) in 63% of all sub-basins, and in 98% of the sub-basins showing strong correlations (that is, r >0.7; Supplementary Table 1). Strong positive cross-correlations in peak discharge occur between basins in central and eastern Europe, following the patterns exhibited during the 2002 and 2013 floods across multiple countries in this region (Fig. 1a). Peak discharges in this area are often linked to the atmospheric circulation pattern Vb, or Genoa Low; that is, a low-pressure system travelling from the Atlantic southeast across the Mediterranean towards central Europe1. High-to-strong cross-correlations amongst southern European basins (Fig. 1b) are known to be caused by the occurrence of regular Mediterranean depressions25, whereas regional negative cross-correlations are also observed under the influence of Atlantic depressions26. We also find high-to-strong correlations in peak discharges amongst basins in western European countries, which have been linked to the occurrence of atmospheric rivers and extra-tropical cyclones13 (Fig. 1c). On the basis of the peak discharge correlations, we assigned countries to 5 main regions, which are used for computing country-specific losses and the required compensation payments (Fig. 1d; Methods). In this study, the correlations were computed over the entire time series for which discharge data were available (1990–2011; Methods). The results may vary depending on the selected time periods, because some of the atmospheric circulation patterns and resulting peak discharges show seasonal variation (Supplementary Fig. 2); and the circulation patterns, and hence rainfall distributions and intensities, may be influenced by climate change1, 9, 13, 17. Uncertainty in these changes, however, remains high12.
| http://www.nature.com/nclimate/journal/v4/n4/full/nclimate2124.html
|
