Identifying Ecosystem Surface Areas Available for Nature-Based Flood Risk Mitigation in Coastal Cities Around the World

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.