Population in floodplains or close to sea level increased in US but declined in some counties—especially among Black residents

Abstract

Abstract Previous studies estimating US population vulnerable to climate change have assumed that population is uniformly distributed within the geographical unit of analysis, typically a census block. That assumption overstates vulnerable population in places where people have avoided the most hazardous areas. After using independent samples of housing location and building footprints to validate a revised approach, this letter estimates the US population close to sea level and in floodplains based on the assumption that population in a hazard zone is proportional to the fraction of buildings in the hazard zone, for the period 1990–2020. The building-based assumption reduces population estimates about 30%. Over the 30-year period, the US population below 1 m increased 14%–18% from 1.85 (±0.19) million in 1990 to 2.2 (±0.25) million; population below 3 m increased 31%. Sea level rise accounts for 75% of the increase in population below 1 m, but only 14% of the increase below 3 m. Nevertheless, in 21 counties, net emigration from land below 1 m was greater than 1% of county population. Although this emigration was particularly great in the 2000s after Hurricane Katrina, it totaled 170 000 for the entire 30-year period. Similarly, the US population of inland 100-year floodplains increased 7%, but several hundred counties saw a steady emigration throughout the 30-year period which totaled about 240 000. Black residents accounted for 19% of the population below 1 m but only 12% of the nation’s population in 2020, meaning that Black residents were 63% more likely to live below 1 m than the general population nationwide. This disproportionality is more attributable to high Black populations in the most vulnerable localities (especially New Orleans) than to historic segregation. Black people also are almost five times as likely to have emigrated from land below 1 m than the general population since 1990.