Chronic flooding drives cumulative exposure inequalities across global cities

Rapid urbanisation and climate change are increasing flood exposure in cities, but global assessments commonly analyse infrequent, high-magnitude events such as the 1 in 100-year flood. This focus can underestimate the cumulative impacts of chronic flood hazards. In this study, we provide the first global analysis of cumulative chronic flood exposure for the world’s cities. By incorporating high-resolution flood hazard data, with population and gross domestic product (GDP) per capita estimates, we compare cumulative chronic flood exposure across spatial scales and against more extreme flood events. Our definition of ‘chronic flooding’ combines the 1 in 5-year river, coastal, and rainfall flood events. We quantify ‘cumulative flooding’ by scaling chronic-event exposures to align with the probability of occurrence associated with rarer, high-magnitude flooding. We find that globally, 170 million people, representing a combined GDP of US$1.69 trillion, are exposed to cumulative chronic flooding events. Within this total, low and lower-middle-income regions experience disproportionate exposure density. This is particularly apparent in Sub-Saharan Africa, where exposure totals from frequent, low-magnitude flooding exceed those of higher-magnitude events. We identify that patterns of inequality also extend downward to city size. Cumulative exposure to chronic flooding is disproportionately concentrated in cities with fewer than 1 million inhabitants. These smaller cities account for 121 million exposed people, constituting a combined GDP of US$1.1 trillion at risk from cumulative chronic flooding. Collectively, smaller cities represent over 80% of the exposure estimates for more extreme flood events. We discover that wider regional trends of inequality also manifest and intensify across city sizes. Exposure in smaller cities is weighted towards low- and lower-middle-income countries in Sub-Saharan Africa and Eastern and South-Eastern Asia, regions where cities are among the world’s fastest growing but often have limited resources and infrastructure. Our analysis shows that, when aggregated to comparable occurrence likelihoods, exposure to cumulative chronic flooding can approach and exceed exposure estimates to more extreme events. In highlighting unequal exposure burdens across scales and magnitudes, these findings can complement prevailing approaches to flood risk.