Global scale assessment of coastal flood risk reduction by coral reefs and mangroves

Rising future extreme sea levels (ESL) increase coastal flooding, putting people and assets at risk in low-elevation coastal zones. Coastal ecosystems such as mangroves and coral reefs can attenuate waves and storm surge, thereby reducing flood impacts along tropical and subtropical coastlines. Despite earlier studies of global flood risk, the benefits provided by the simultaneous presence of mangroves and coral reefs, under climate change, have not been quantified. By using process-based inundation modelling with and without these coastal ecosystems, and accounting for population and asset changes under future socio-economic conditions, this study estimates how the spatial distribution of these ecosystems reduces coastal flooding for a baseline period (1980–2014), for 2030 and for 2050 under the SSP5-8.5 scenario. When mangroves and reefs are included, our results show a reduction in global expected annual flood damage by USD 1.7 billion (12.9\%) in the baseline period and by USD 2.7 billion (13.9\%) by 2050. The presence of ecosystems reduces the mid-century annual expected flooded area with 7811 \unit{\square\kilo\meter} and reduces the mid-century annual affected assets and population by USD 45.8 billion and 1.6 million people per year, respectively. In relative terms, we find that benefits resulting from the presence of ecosystems are concentrated in countries with high climate vulnerability and low adaptation readiness, implying that ecosystem degradation would raise future flood risk most where adaptive capacity is lowest. Therefore, our results suggest that mangrove preservation/rehabilitation and reef conservation directly support adaptation to climate change.