13 Million in US Could Become Climate Refugees: Top Counties Affected
Credit: Jon Bilous | Shutterstock.com

More than 13 million Americans could become climate refugees by 2100 if the worst sea-level rise comes to pass, new research suggests.

Rising seas caused by climate change could permanently flood hundreds of U.S. counties, according to the study. The hardest-hit county will be Miami-Dade, Florida, where 2 million people could be forced to relocate. In fact, Florida is home to about half of these potential U.S. climate refugees.

"The Great Migration of southern African-Americans from the South into the North is pretty much the same kind of magnitude we're talking about it if we don't adequately address sea-level rise," said study co-author Mathew Hauer, a demography doctoral candidate at the University of Georgia in Athens. [See Which Counties Could House the U.S. Climate Refugees]

However, city, county, state and local governments can take steps to mitigate some of these effects, while global work to stem climate change could mean the worst-case scenario never comes to pass, the researchers wrote in the paper, which was published today (March 14) in the journal Nature Climate Change.

Growing coastal population

Past studies have provided estimates, based on current population levels, of how many people climate change would affect. However, some of the fastest-growing areas of the country are situated on the coasts, in low-lying areas vulnerable to climate change.

To get a better sense of how many people would be affected in the future if current growth rates are sustained, Hauer and his colleagues analyzed census data from counties around the continental United States. The team looked at historical growth rates in coastal regions, and then used those numbers to project future population levels. Based on the projections, the study found that population in coastal areas could be up to three times that of current levels.

The team then combined those estimates with estimates of sea-level rise from the National Oceanic and Atmospheric Administration (NOAA).

In the NOAA's worst-case scenario, the oceans will rise about 6 feet (1.8 meters), on average, by 2100. These NOAA estimates account for permanent coastal flooding but not other potential hazards of climate change, such as increased hurricane numbers and intensity or storm surges.

If there is no climate change mitigation, 13 million people would be forced to move because their homes become partially submerged under water, the researchers found. In a better scenario, with only 3 feet (0.9 m) of sea-level rise, only 4.2 million people would be forced to move, the study found.

Mitigation

Of course, the millions displaced are not a foregone conclusion. For one, historical growth rates may not be a good predictor for future growth rates; some low-lying coastal areas are already so densely populated that they are unlikely to grow in the future, while others may be actively discouraging growth in vulnerable areas or taking steps to mitigate climate impacts, Hauer said.

Right now, climate change policy is often driven from the bottom up, by county- and city-level zoning laws, Hauer said.

Still, counties, cities and states can take a variety of steps to avoid a forced mass-migration, he said.

"You have protection, you have accommodation and you also have retreat," Hauer told Live Science.

For instance, in some areas, sea walls or marsh restoration may be sufficient to prevent the worst effects of sea-level rise, while in other places, zoning laws that discourage future building altogether may be the smartest approach, Hauer said.

"New York City is going to be able to adapt differently than, say, New Orleans," Hauer said.

Follow Tia Ghose on Twitter and Google+. Follow Live Science @livescience, Facebook & Google+. Original article on Live Science.