As many as 13.1 million people living along U.S. coastlines could face flooding by the end of the century because of rising sea levels, according to a new study that warns that large numbers of Americans could be forced to relocate to higher ground.

The estimated number of coastal dwellers affected by rising sea level is three times higher than previously projected, according to study published Monday in the science journal Nature Climate Change.

If protective measures are not implemented, the study says, coastal residents could be forced to move in numbers mirroring the scale of the Great Migration of African-Americans from Southern states during the 20th century. “We've been underestimating what those potential impacts could be,” said Mathew Hauer, a coauthor of the study. He is an applied demographer at the University of Georgia in Athens and a doctoral candidate in the school's geography department.

Rising sea levels, widely believed to be the result of climate change, are threatening to wipe out some of the world's island nations, such as the Maldives in South Asia, that scientists say could vanish under water this century.

The study's authors acknowledge that the degree to which sea levels will rise remains an uncertainty and that predicting flood patterns is challenging.

Some studies have used current population data to estimate the populations at risk of future rising sea levels, but the current study factored in population growth.

“Coastal communities are among some of the most rapidly growing in the United States, so we have to think about the anticipated expansion of those populations that is likely to occur in this century,” Hauer said.

He and his colleagues combined environmental data, such as elevations and flood risks for specific locations, with small-scale population projections for coastal states and projected sea level rise from the National Oceanic and Atmospheric Administration.

Their findings revealed that if the sea level rose about 3 feet by the year 2100, about 4.2 million people in U.S. coastal regions would be at risk of flooding. But if the sea level were to rise by about 6 feet, which lies at the higher end of projections by NOAA, then the number of those at risk of flooding would reach 13.1 million.

The study shows that the southeastern United States accounts for 70 percent of the potential populations that could be affected, with Florida accounting for almost half. States such as Georgia, South Carolina and Louisiana have more than 10 percent of future coastal populations at risk under the 6-foot scenario, according to the study.

Scientists say that implementing protective measures, such as raising homes and roadways, building wetlands as buffers against rising tides and constructing levees and sea walls, are key to preparing for sea level rise.

ann.simmons@tribpub.com