The challenge of unprecedented floods and droughts in risk management.

Risk management has reduced vulnerability to floods and droughts globally, yet their impacts are still increasing. An improved understanding of the causes of changing impacts is therefore needed, but has been hampered by a lack of empirical data. On the basis of a global dataset of 45 pairs of events that occurred within the same area, we show that risk management generally reduces the impacts of floods and droughts but faces difficulties in reducing the impacts of unprecedented events of a magnitude not previously experienced. If the second event was much more hazardous than the first, its impact was almost always higher. This is because management was not designed to deal with such extreme events: for example, they exceeded the design levels of levees and reservoirs. In two success stories, the impact of the second, more hazardous, event was lower, as a result of improved risk management governance and high investment in integrated management. The observed difficulty of managing unprecedented events is alarming, given that more extreme hydrological events are projected owing to climate change. Unprecedented floods and droughts bring new challenges for risk reduction, as is clear from this analysis of the drivers of changing impacts in many cases worldwide, with implications for efficient governance and investment in integrated management.

风险管理已降低了全球范围内对洪水和干旱的脆弱性，然而它们的影响仍在增加。因此，需要更好地理解影响变化的原因，但这受到了缺乏实证数据的阻碍。基于在同一地区发生的45对事件的全球数据集，我们表明风险管理通常会降低洪水和干旱的影响，但在降低前所未有的、规模超出以往经验的事件的影响方面面临困难。如果第二次事件比第一次危险得多，其影响几乎总是更高。这是因为管理措施并非是为应对此类极端事件而设计的：例如，它们超出了堤坝和水库的设计水平。在两个成功的案例中，由于风险管理治理的改善以及对综合管理的大量投入，第二次更危险的事件的影响更低。鉴于气候变化预计会导致更极端的水文事件，观察到的管理前所未有的事件的困难令人担忧。 前所未有的洪水和干旱给降低风险带来了新的挑战，从对全球许多案例中影响变化的驱动因素的分析中可以清楚地看到这一点，这对综合管理中的高效治理和投资具有启示意义。