The United States is rich enough, industrialized enough, and far enough from the tropics that the rising temperatures of our changing climate aren't going to make any place uninhabitable. But a side effect of those rising temperatures—rising oceans—most certainly will. Already, an ever-growing list of places is facing what's called "nuisance flooding," in which even a high tide can leave streets underwater. Major storms just make matters worse. And, by the end of this century, the expected rise of the oceans may be over five times what we saw last century.

As a result of this, many areas of the country will simply become uninhabitable, lost to the sea. Well over a third of the United States' population lives in counties that are currently on the coast, and over 10 million currently live on land that will be lost to a sea-level rise of 1.8 meters. They'll have to go somewhere—and people who might otherwise move to the coast will have to find some place else to relocate. All of which will change the dynamics of the typical relocation of people within the US.

A new study released in PLOS ONE tries to estimate what that will mean for the rest of the country. Their results suggest that coastal regions will be far from the only ones affected by sea-level rise. A huge number of counties far from the coast—some deep in the US interior—will see dramatic changes in the number of people relocating there.

People movers

The new work was done by Caleb Robinson, Bistra Dilkina, and Juan Moreno-Cruz, who are all from different institutions in the US and Canada. The researchers took advantage of a new way of analyzing population movements they had developed earlier. Models of human migration typically evaluate a number of features: population at the source of migration, population at potential destinations, proximity of a destination, and the appeal of potential destinations in between. There are ways of estimating how much effect each of these has in order to create an equation that estimates future migration. But the researchers here decided to use a neural network trained on actual data from US population movements.

Of course, climate change will dramatically alter the dynamics of things compared to typical migrations. Part of this can be handled simply; the researchers could simply prevent their model from moving people into coastal counties that would end up underwater. But it's a bit more challenging to figure out how populations already living at the coast will respond to having the area where they're living end up underwater. To get an estimate of this, the researchers trained their neural network on the population changes that occurred following some major hurricanes that struck the US.

To an extent, that'll be accurate, as at least some of the people moving away from the coast will have been driven off by storm surges or damaged infrastructure. But others will almost certainly be leaving because the local government simply won't be able to maintain infrastructure in the face of ever-rising tides. So, that should be viewed as a limitation of this work.

The authors ran this model using data from NOAA that tracks the likely impacts of sea-level rise on coastal counties. They did evaluations using projected sea-level rise of 0.9 and 1.8 meters. The lower is about the same as the (conservative) IPCC estimate of one meter by century's end, and the higher one is closer to a more recent NOAA estimate of up to 2.5 meters if emissions continue to grow unchecked.

Where’d they go?

At 1.9 meters, the effects are dramatic but regional. Because the West Coast generally slopes upwards at a steeper angle than the East, there was less migration inland to the Mountain West—everywhere between Colorado and California was largely unaffected. But that's about the only region that was.

Nearly every county from Denver east saw an increase in the population migrating into it. There were some general rules. Rising seas should dramatically accelerate the urbanization of the US, for example. Another trend is that places that are currently unpopular for migration—off the coast but not necessarily urban—suddenly see their population rise simply because they're not far from the counties people are forced to leave.

Beyond those general trends, the effects varied by region. Right now, for example, Austin is the fastest-growing city in Texas, with people largely drawn by economic opportunities. But, as the present Gulf Coast makes way for the future one, people with no choice but to leave their homes are more likely to wind up in the Houston or Dallas areas. Further east along the Gulf, many of the migrants will stay near the retreating coast, leading to rapid growth in areas that might ultimately be at risk as sea-level rise continues into the next century. This is most dramatic in Florida, where 1.8 meters of sea-level rise will lead to a rapidly shrinking core centered on Orlando. Yet the model also predicts that both Orlando's population and population density will expand dramatically, rather than seeing people give up on Florida.

By contrast, heading up the Eastern Seaboard sees more people moving further inland, often to much smaller cities than the ones they'll be vacating. And the entire Midwest sees a significant increase in population thanks to the rising oceans.

While the details are likely to be rough estimates rather than solid predictions, the overall picture puts the lie to the idea that someone can live far enough inland that they'll be unaffected by climate change. While there are unaffected regions, they're generally sparsely populated already, so the vast majority of Americans will end up needing to get used to having new neighbors. And, while the changes may increase the economic activity, as businesses will need to relocate to where workers are, there are attendant issues of the need for new infrastructure and schools, as well as the risk of rising housing prices.

Our estimates of the areas affected and the magnitude of the effect will undoubtedly improve with time and as the first instances of climate-driven relocations are studied. But planning for housing and infrastructure may need decades of lead time, and the changes predicted by this model start becoming dramatic shortly after mid-century.

PLOS ONE, 2019. DOI: 10.1371/journal.pone.0227436 (About DOIs).