21 Climate Action created the Digital Coast Mapper to aggregate a range of data—on coastal flooding, storm surge, and long-range inundation impacts—using a method of visualizing climate risk that was initially developed for New York in the wake of Hurricane Sandy. Anywhere along the New York waterfront ranks high for flood risk, and nearly 1.5 million New Yorkers live in the federally designated 100-year floodplain, threatened by once-in-ahundred-year extreme storms. Maintained by FEMA as part of the National Flood Insurance Program, or NFIP, floodplain maps are meant to define mortgage risks, set insurance rates, and establish building and land-use regulations. But in New York and elsewhere, the actual risks of floods and storms reach farther than most people think, and farther than the prevailing federal flood maps have described. In a study published in February 2023 in the journal Nature Climate Change, researchers found that on federal maps, the underestimations of flood risks missed about eight million homes and overvalued the housing market by about $200 billion. The analysis, by researchers from the Environmental Defense Fund and the nonprofit organizations Resources for the Future and the First Street Foundation, found that many overvalued properties are concentrated in coastal counties with no flood-risk disclosure laws and where there is less concern about climate change. The unpriced flood risks endanger the stability of the housing market, local governments, and homeowners, particularly in low-income areas. The old floodplain maps don’t account for sea level rise, rainfall, or riverine flooding, and they can be hampered by outdated data and models. They can also create a false sense of security, by focusing on whether a property is inside or outside of a floodplain, without addressing risk to individual properties. And FEMA hasn’t significantly updated many of its maps for decades. In cities such as New York, billions of dollars in property values and government revenue are at stake, to say nothing of lives and livelihoods. By the year 2050, according to a report published by the city’s comptroller in October 2022, rising tides and more frequent storms will put significant essential infrastructure and properties amounting to upward of $242 billion in current market value at risk of coastal flooding. Sobering Statistics The consequences of geospatial data gaps can be devastating. When Hurricane Harvey dumped up to 47 inches of rain in Harris County, Texas, in 2017, more than two-thirds of the homes that flooded were outside FEMA’s Special Flood Hazard Area, which designates areas where flood insurance coverage is mandatory. This means that most of those homeowners were uninsured or heavily underinsured. New maps, delayed again in September 2023, will likely show Harris County’s 100-year floodplain growing from around 150,000 acres to 200,000 acres. The maps, built in consultation with the county, will be FEMA’s first to depict urban flooding and will incorporate new rainfall estimates from NOAA, data that had not been updated since the 1960s. NOAA created the Coastal Flood Exposure Mapper to help communities kick-start conversations about coastal risks. The mapper enables users to explore maps that show people, places, and natural resources exposed to coastal flood hazards.
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