Urbanization under extreme climate events leads to synchronized decreases in flood protection and increases in vulnerability

Cities will be especially prone to flood hazards with increasing climate change. Artificial landscapes lose ecological functions that protect against floods. Simultaneously, urban densification leads to increased flood vulnerability. The aim of this research is to identify relationships between increasing flood hazard subject to climate change, and increasing flood vulnerability subject to growing urban populations. We combine a flood inundation model and a dynamic coupled human nature systems model to assess the supply and demand of flood protection for different flooding event intensities in 2018 and 2050. We compare two cities: Lisbon (Portugal) and Potsdam (Germany). Results show increased flood risk over time with event intensity. Flood hazard increases due to the artificialization of land, while flood vulnerability increases due to population densification. Flood protection supply and demand diverge which creates a reinforcing feedback loop, exacerbating flood risk in both cities. These results show the multiplicative effect that land cover artificialization and population growth have on flood risk under a changing climate. This highlights the importance of land use policy incorporating flood regulating ecological features especially in vulnerable locations as the climate changes. • Flood hazard and vulnerability increase, reinforcing a feedback on flood risk. • Flood hazard and vulnerability are used to represent loss of the flood protection ecosystem service. • Urban densification leads to artificialization and loss of surface roughness. • Diversity of green landcovers with greater roughness increases flood protection service.