Anywhere from 4.3 to 13.1 million people in the coastal United States will be at risk of inundation due to sea level rise by 2100, according to a new study that combines population growth projections with sea level rise forecasts.

The study, published in Nature Climate Change, uses population trends and sea level rise projections to conduct a county-by-county risk assessment across the U.S. The results show that anywhere from 4.3 to 13.1 million people are at risk of inundation by 2100, depending on how much sea level rise there is by then.

The study shows that previous research — using current population estimates plus future sea level rise projections — likely underestimates the risk exposure of coastal states.

The southeastern U.S. is a hotspot for sea level rise-related inundation risks

The new study found that the southeastern U.S. is a hotspot for sea level rise-related inundation risks, in part because population growth is high in coastal areas there.

States such as Georgia, South Carolina and Louisiana have more than 10 percent of their future coastal populations at risk under a scenario in which global sea level increases by 1.8 meters, or 5.9 feet, by 2100.

The study found that the southeastern U.S. contains 70 percent of the at risk population through 2100.

The coast line of Miami Beach is seen June 3, 2014 in Miami, Florida. Image: Joe Raedle/Getty Images

Florida faces the most risk under a 1.8-meter, or 5.9-foot, sea level rise scenario, with up to 6 million residents potentially affected. One million people each in California and Louisiana also could be impacted in such a scenario.

"There are 31 counties where more than 100,000 residents could be affected by 6 feet of sea level rise."

“The impact projections are up to three times larger than current estimates, which significantly underestimate the effect of sea level rise in the United States,” said study co-author Mathew E. Hauer, of the University of Georgia.

“In fact, there are 31 counties where more than 100,000 residents could be affected by 6 feet of sea level rise," he said in a press release.

Recent sea level rise projections from a variety of scientific groups show that sea level rise is likely to be around 1 meter, or 3.6 feet, by 2100, but there is considerable uncertainty concerning the stability of the Greenland and West Antarctic ice sheets as air and ocean temperatures go up. Faster melting of those ice sheets would boost sea level rise significantly.

The study found three counties that have "extreme exposure" to sea level rise-related inundation: Tyrrell and Hyde Counties in North Carolina, and Monroe County in Florida. These counties in particular could see "catastrophic impacts" from sea level rise by the year 2100.

Tyrell and Hyde Counties are home to abundant nature preserves on North Carolina's Outer Banks, while Monroe County is located at the southwestern tip of Florida, encompassing a swath of Everglades National Park as well as the Florida Keys.

More populated counties including Broward and Miami-Dade Counties in Florida, San Mateo in California, and Jefferson and Orleans in Louisiana are each expected to see more than 100,000 residents "potentially impacted" with a sea level rise of 0.9 meters, or about 3 feet.

The impact projections are up to three times larger than current estimates

The cities that would be most jeopardized by a higher-level of sea level rise, namely 1.8 meters, include Tampa-St. Petersburg, Florida and Charleston, South Carolina, the study found.

The new study follows research published on Feb. 22 that showed human-caused climate change has prompted the fastest rate of sea level rise than any in at least the past 2,800 years. Even greater water rises are slated for the future as mountain glaciers and polar ice caps melt and ocean waters continue to heat up and expand.

An important implication of Monday's study is the finding that estimated costs of adapting to sea level rise may be too low, since they don't take population growth and the associated installation of more long-lasting, vulnerable infrastructure into account.

"Coastal areas are growing faster than the country as a whole, and so we realized that there was a need to take account of this growth in a rigorous way," said study co-author Jason Evans of Stetson University in an email to Mashable.

Image: National Climate assessment.

"My message to everyone is that sea-level rise is real and that many people living on the coast are already grappling with increased tidal flooding and other issues related to sea-level rise that we’ve already experienced," Evans said.

The study warns that a lack of protections for coastal residents, from sea walls to managed retreat from rising tides, could lead to a population migration on par with the "Great Migration" of southern African Americans after the first World War.

Such a migration would have staggering costs.

If all the Americans exposed to sea level rise-related inundation by 2100 were to be relocated to safer areas, the cost of doing so would exceed $14 trillion dollars.

The study, by researchers at the University of Georgia and Stetson University in Florida, has some limitations. It does not take into account regional variations in the rate of sea level rise. It also does not incorporate changes in land elevation, which can increase local rates of sea level rise in areas like coastal Louisiana and the Chesapeake Bay region.

Ben Strauss, who directs the climate impacts program at the nonprofit research and journalism group Climate Central, told Mashable that the study's main contribution is in using population projections to study future sea level rise risk.

However, Strauss said, the lack of regional variations in sea level rise would affect the results out to the year 2100, and the study also "assumes that people will be moving to the shore essentially just as briskly" in the latter half of the century as in 2020, despite the evident effects of sea level rise expected by 2070.