CASE STUDY 2 Habitat vulnerability from subsidence and sea rise, US Atlantic Coast Satellite data from 2007 and 2020 along 3,500km of the US Atlantic Coast reveal that most areas are sinking by more than 3mm per year, impacting farmlands, wetlands, forests and developed regions. Projections indicate that 58–100% of coastal marshes are losing elevation relative to sea level when accounting for subsidence.33 This study shows that land sinking plays a major role, it can cause marshes to sinkfaster than they can keep up, leading to submersion. Any assessment of marsh risk that ignores land sinking will likely underestimate how vulnerable these areas really are. Source: Ohenhen, L.O., Shirzaei, M., Ojha, C. et al. (2023). Hidden vulnerability of US Atlantic coast to sea-level rise due to vertical land motion. Nature Communications. vol. 14, no. 2038. https://doi.org/10.1038/s41467-023-37853-7. A Low-elevation wetlands (lower bound) Total area 725km2 Submerging 57.6% Maintaining 14.4% Aggrading 28.0%Total area 551km2 Submerging 57.6% Maintaining 12.4% Aggrading 30.0%Total area 725km2 Submerging 100% Maintaining 0% Aggrading 0%Total area 551km2 Submerging 100% Maintaining 0% Aggrading 0%B C D Low-elevation wetlands (upper bound)High-elevation wetlands (lower bound)High-elevation wetlands (upper bound)Latitude Longitude40ºN 35ºN 30ºN 80ºW 75ºW 70ºW Longitude40ºN 35ºN 30ºN 80ºW 75ºW 70ºW Longitude40ºN 35ºN 30ºN 80ºW 75ºW 70ºW Longitude40ºN 35ºN 30ºN 80ºW 75ºW 70ºWEnvironmental implications Environmentally, subsidence in coastal areas threatens vital ecosystems such as wetlands, mangroves and estuaries. These habitats are crucial for biodiversity and serve as natural buffers against storm surges and erosion.32 Despite mounting evidence, a comprehensive understanding of the full economic, social, health and environmental costs of land subsidence remain fragmented. The risks and costs associated with the interplay between subsidence, sea-level rise and extreme weather are still insufficiently quantified. Cities that fail to proactively manage these interconnected risks face escalating costs for drainage systems, infrastructure maintenance, urban planning and disaster response. Areas built on soft soils, reclaimed land or with excessive groundwater use, especially those also facing sea- level rise and extreme weather, are most at risk. While the specifics of subsidence vary by location, land and water use, and governance practices, the overarching trend is clear: without sustainable management, cities, businesses and communities will confront compounding challenges from sinking land and associated threats. The impacts of a sinking city accelerate, like an avalanche, if ignored. It pays to be proactive. Antonio Gómez-Palacio, Chair, DIALOG Resilient Economies: Strategies for Sinking Cities and Flood Risks 9

Resilient Economies Strategies for Sinking Cities and Flood Risks 2025