Measuring the compound flood risk from economic growth, sea-level rise and land subsidence on insurance affordability in China's coastal cities

Coastal flood insurance is increasingly challenged by the combined effects of climate change, socioeconomic development and land subsidence. Yet how these compound risks may affect future insurance affordability remains poorly understood at fine spatial scales. Here, we assess scenario-based coastal flood insurance premiums and affordability across China's coastal counties under SSP2-RCP4.5 and SSP5-RCP8.5 for 2030, 2050, and 2080. We combine probabilistic coastal flood hazard maps, sea-level rise projections, land subsidence, existing protection standards, gridded socioeconomic projections, and fixed-asset exposure estimates. Annual average losses are converted into loaded insurance premiums, and affordability is measured as the ratio of per-capita premium to estimated per-capita disposable income. Our results show that future losses increase more rapidly than inundated area, reflecting the combined effects of changing flood hazards and SSP-based socioeconomic development on exposed asset values. Under SSP5-RCP8.5, national annual average losses rise from USD 1.7 billion in 2030 to USD 7.5 billion in 2080 under the median sea-level-rise case without land subsidence. Land subsidence further amplifies losses and premiums, especially in economically developed coastal areas. Multiple counties experience persistent increases in premium-to-income burdens from 2030 to 2080, with the strongest affordability pressure under high-end sea-level rise and subsidence. Scenario analysis shows that premiums depend strongly on protection standards, premium loading and payout design. These findings highlight the need to coordinate insurance design with adaptive protection, subsidence management and coastal development policy.