Growing compound-flood risk, driven by both climate change and land subsidence, challenges flood risk reduction in major delta cities

Low-lying deltas host some of the world’s fastest-growing cities yet are exposed to floods driven by the compound actions of tide, storm surge, rain, and river flows. Most previous studies of compound floods are partial, while here, we estimate future compound floods in Shanghai for all relevant driving factors. We use a dynamically linked atmosphere, ocean, and coast model (AOCM) that incorporates all flood drivers, including sea-level rise (SLR), sea-surface temperature rise, and land subsidence. Simulations forced by baseline conditions and IPCC RCP2.6, -4.5, and -8.5 scenarios show that by 2100, the inundation extent of the 200-year event could increase by up to 80%, reflecting subsidence (34% [28%–41%]) and climate change (29% [20%–37%] due to SLR and 37% [26%–44%] due to more intense tropical storms), respectively. Land subsidence and SLR create a dangerous “polder effect” if defenses fail, which must be considered in adaptation in Shanghai and other deltaic cities.