Rising seas due to climate change could put millions more residents of U.S. coastal areas at risk than once thought, a new study suggests. Most previous studies that modeled the number of coastal residents under threat from sea-level rise didn’t factor in population growth, even though the coasts are among the fastest-growing areas in the country. In the new study, researchers designed a model that sought to address this problem. As the model boosted the sea level, it projected which coastal land areas would get submerged based on their geographical traits, such as elevation and historical flood risk. Then, with U.S. census population data for the affected coastal areas, the researchers could project how many people would live in each submerged area after a given time. A 1.8-meter sea-level rise—at the upper end of common sea-level rise projections—would submerge enough areas on the Pacific, Gulf of Mexico, and Atlantic coasts by 2100 to put 13.1 million people at risk, the team found. A 0.9-m rise—within the ranges projected by the United Nations—would affect 4.2 million people, the researchers report today in Nature Climate Change. Both figures are a factor of three higher than what researchers found for those scenarios using current population data. In Florida alone (Miami skyline, shown), 1.2 million people are at risk in the 0.9-m scenario, and 6.1 million in the 1.8-m scenario. The researchers warn that without proper precautions—from building storm-surge barriers to boosting disaster-preparedness programs—the impacts of sea-level rise could spur mass migration not seen in the United States since the early to mid-20th century, when millions of black Americans left the South for other regions—an event known as the Great Migration.