Sinking Cities: Latent Flood Risk in Africa's Coastal Megacities

Africa’s coastal cities face a heightened flood risk from relative sea level rise (RSLR), a combined effect of sea-level rise (SLR) and localized vertical land motion (VLM). However, observations of VLM remain critically scarce across Africa. Here, we present the first comprehensive, high-resolution estimate of VLM for 20 major African coastal cities (home to over 90 million people), derived from an analysis of Sentinel-1 radar datasets. We find widespread, spatially variable subsidence with median rates reaching 6.0 mm yr⁻¹ in Alexandria and 5.0 mm yr⁻¹ in Lagos, several times faster than natural background processes such as glacial isostatic adjustment, and well above the assumptions used in IPCC projections. Revising IPCC AR6 sea-level projections using up-to-date VLM increases RSLR and flood exposure substantially. For instance, in Alexandria, refined SSP2-4.5 projections raise RSL by 35.8% and the flood-exposed area by ~ 15% by 2050. Across the 20 cities, an extreme sea-level event could expose > 7 million people, > 1 million buildings and ~ USD 180 billion in assets by mid-century. Our results reveal a latent vulnerability masked by globally averaged models and demonstrate a scalable, transferable framework for risk assessment in data-scarce regions of the Global South, providing decision-ready evidence for adaptation and resilience planning.