Nature-based risk mitigation is increasingly proposed as a strategy to cope with global changes that increase flood risks in coastal areas. However, loss of coastal ecosystems reduces their mitigating effect on coastal flood risks in many places around the world. Here, we identify global urban hot spots exposed to storm surge flood risks, where conservation of existing coastal ecosystems can contribute to nature-based risk mitigation. We present a global procedure identifying the most likely pathways followed by storm surges from the open sea toward 136 cities around the world and quantifying the extent of mangrove forests, salt marshes, seagrass meadows, and coral reefs along these storm surge pathways. Instead of being locally precise, our procedure aims to be globally applicable, in order to identify cities around the world where ecosystems can have a positive effect on storm surge risk mitigation and where further local-scale, more precise assessments are advised. Our results show that cities that combine large flood-exposed populations (> 400,000 people exposed to 1-in-100-year storm events) and large potential for nature-based risk mitigation (> 200 km2 of coastal ecosystems) are located in large river deltas and estuaries, such as Khulna (Ganges-Brahmaputra delta, Bangladesh), Guayaquil (Guayas delta, Ecuador), Ho Chi Minh City (Mekong delta, Vietnam), and New Orleans (Mississippi Delta, USA). Here, conservation of mangroves and salt marshes plays a key role. Cities with large flood-exposed populations and/or assets, but few ecosystems, are either located directly adjacent to coastlines or where former wetlands have been reclaimed, especially in European and Asian cities. Overall, 75% of the studied cities benefit from flood risk mitigation by present ecosystems and from additional ecosystem services. Hence, our study identifies global urban hot spots where more detailed local studies are advised on the effectiveness of conservation and (re-)creation of coastal ecosystems as a sustainable strategy for nature-based mitigation of increasing coastal flood risks.

中文翻译：

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识别可用于减轻世界各地沿海城市基于自然的洪水风险的生态系统表面积

越来越多地提出了基于自然的风险缓解措施，以应对全球变化，这种变化增加了沿海地区的洪水风险。但是，沿海生态系统的丧失降低了其对世界许多地方的沿海洪灾风险的缓解作用。在这里，我们确定了遭受风暴潮洪灾风险的全球城市热点，保护现有的沿海生态系统可有助于缓解基于自然的风险。我们提出了一个全球程序，确定了从公海向全球136个城市进行风暴潮之后最可能的路径，并量化了沿着这些风暴潮路径的红树林，盐沼，海草草甸和珊瑚礁的范围。我们的程序不仅要在本地保持精确，而且要在全球范围内适用，为了确定世界范围内的城市，其中生态系统可以对缓解风暴潮风险产生积极影响，并建议在当地进行更大规模的评估。我们的结果表明，大型河流位于洪水泛滥的人群（> 100,000人遭受100年暴风雨袭击的人口超过40万人）和基于自然的风险缓解潜力较大（> 200 km2的沿海生态系统）的城市三角洲和河口，例如Khulna（孟加拉国的Ganges-Brahmaputra三角洲），Guayaquil（厄瓜多尔的Guayas三角洲），胡志明市（越南的湄公河三角洲）和New Orleans（美国的密西西比三角洲）。在这里，保护红树林和盐沼起着关键作用。人口和/或资产遭受洪水泛滥但生态系统很少的城市，要么直接位于海岸线附近，要么被开垦为以前的湿地，尤其是在欧洲和亚洲城市。总体而言，目前研究的城市中有75％的城市受益于减轻洪水风险和其他生态系统服务。因此，我们的研究确定了全球城市热点地区，在这些热点地区，我们建议对海岸生态系统的保护和（再）创造的有效性进行更详细的本地研究，以此作为基于自然的减缓沿海洪灾风险的可持续策略。