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 the study published Monday in the science journal Nature Climate Change.

If protective measures are not implemented, the study says, large numbers of Americans could be forced to relocate in a migration 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, one of the co-authors of the study. He is an applied demographer at the University of Georgia, 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 traditional approach to assessing the effects of rising sea levels is to look at the current population and infrastructure, Hauer said. His study accounts for ongoing 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.

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

Their findings revealed that if the sea level rose 35.4 inches by the year 2100, some 4.2 million people in U.S. coastal regions would be at risk of flooding. But if the sea level were to rise by 70.9 inches, 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 Southeastern United States accounts for 70 percent of the potential populations that could be affected, according to the study. Florida accounts for almost half, Hauer said. States such as Georgia, South Carolina and Louisiana have more than 10 percent of coastal populations at risk under the 70.9-inch scenario, according to the study.

California does not fare too well either. Upward of 1 million people could be affected by sea level rise, Hauer said. Orange County, for example, is projected to see 225,720 residents potentially affected under the 70.9-inch sea level rise scenario, and ranks eighth on a list of 319 coastal counties whose populations are at risk. San Mateo County ranks seventh with a potential 249,020 residents at risk.

In September 2014, Democratic Gov. Jerry Brown signed a measure designed to help California prepare for rising sea levels after a report by the California State Assembly’s Select Committee on Sea Level Rise and the California Economy found that the state was “woefully unprepared” for potential changes caused by sea level rise.

The legislation created a statewide online database that allows California communities to have access to studies, modeling, inundation maps and other information about rising sea levels.

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, is key to preparing for sea level rise. Forecasting the potential number of people who might be forced to move because of sea level rise, as outlined in the new study, underscores the urgency to take action.

“These numbers could be useful for policymakers to make decisions about growth management strategies, or protective infrastructure,” Hauer said.