Carlos Barria/Reuters Rising sea levels could threaten the homes of millions of people in coastal states around the nation by 2100. Remember how neighborhoods were still flooded two weeks after Hurricane Katrina went though New Orleans?

In low-lying island nations like Tuvalu and the Maldives, the term “climate migrant” is all too familiar. Rising sea levels have already forced some Pacific Ocean communities to flee from their homes and there are fears that several whole islands will be underwater in just a few decades.

But it’s not just island dwellers who need to worry about climate-related migration. As coastal areas are deluged over this century, millions of mainland Americans could be forced to flee inland, where they may overwhelm already crowded cities, according to new research from the University of Georgia.

“We typically think about sea-level rise as a coastal issue, but if people are forced to move because their houses become inundated, the migration could affect many landlocked communities as well,” said Mathew Hauer, the University of Georgia demographer who wrote the paper.

Using migration data from the Internal Revenue Service and climate migration models, Hauer concluded that a 6-foot increase in sea levels would cause every U.S. state to experience climate-related migration by 2100. Scientists are predicting a 6-foot global sea-level rise by the end of the century if greenhouse gas emissions are left unchecked.

In a study last year, Hauer had estimated that a 6-foot rise in regional sea levels would put 13 million people in more than 300 U.S. coastal counties at risk.

The new report, published in the journal Nature, finds that nine states could see population declines as rising waters force people to flee. Florida would be worst off with millions of people leaving the state.

Other states would be taking in climate migrants. Texas could absorb as many as 2.5 million internal migrants.

Several metropolitan centers, in particular, could see significant population boosts. Houston; Austin, Texas; Orlando, Florida; and Atlanta could receive more than 250,000 net migrants each, the report said. Other popular cities for climate migrants would likely include Phoenix and Las Vegas.

The incoming human flood might spell trouble for already packed cities, said Hauer. “Some of the anticipated landlocked destinations, such as Las Vegas, Atlanta and Riverside, California, already struggle with water management or growth management challenges,” he said.

Alissa Scheller/HuffPost

Threatened coastal communities and areas that could face an influx of new residents should start preparing immediately for these future risks, Hauer told HuffPost.

“For many areas, incorporating climate change scenarios into their long-range strategic plans is a really good start. This could include transportation infrastructure, affordable housing options, etc.,” he said.

Commenting on Hauer’s report in Nature, Jeroen Aerts, director of the Institute for Environmental Studies in the Netherlands, noted some potential weaknesses in the predictions: People may not, for instance, follow past immigration patterns when they are forced to migrate for climate reasons. Nonetheless, he said Hauer’s study is part of an important conversation about global warming’s impact on coastal communities.

“There is much we still do not know about how climate change will influence migration,” Aerts said.

Laurent Van der Stockt/Gamma-Rapho via Getty Images Survivors of Hurricane Katrina evacuate their homes on Sept. 3, 2005.

Hauer’s scenario assumes a 6-foot rise in sea levels by 2100. But new research suggests that the outlook might be even grimmer for America’s coastal residents.

Earlier this year, the National Oceanic and Atmospheric Administration upped its worst-case scenario for global sea-level rise to 8.2 feet by the year 2100 — an increase of about 1.5 feet from its last worst-case estimate issued in 2012. In a technical report published in January, the NOAA warned that the U.S. would be especially hard-hit in this extreme projection. Sea-level rise could actually reach up to 10 to 12 feet for all coastal U.S. states except Alaska.

Many major cities, including Boston, Miami, New York and Seattle, would be almost completely submerged in this scenario. Cape Canaveral and the Jefferson Memorial would be underwater, as would the San Francisco International Airport and President Donald Trump’s home away from home, Mar-a-Lago.

News site Climate Central has created images showing what could happen to the Palm Beach, Florida, club and other U.S. cities and landmarks in this extreme scenario. Scroll down for more.

NOAA’s extreme sea-level projection is unlikely but “it is possible,” William Sweet, lead author of the January report, told AFP. Several other recent studies have supported the notion that global sea-level rise by century’s end may be much worse than is currently expected.

There’s evidence that the massive ice sheets of Antarctica and Greenland may not be as stable as once thought and could start melting at a rapid rate if greenhouse gas emissions are not curbed. This ice melt could trigger a “’tectonic shift’ in expectations for the speed and severity of the sea level problem,” Ben Strauss, director of the program on sea level rise at Climate Central, told The Washington Post last year.

A study published in April by the U.S. Geological Survey also found that sea floor erosion, following the degradation of coral reefs, may further boost sea-level increases around the United States and elsewhere. Coral reefs, USGS researcher Kimberly Yates explained, play a critical role in protecting coastal areas from waves, storms and resulting seafloor erosion. “Coral reefs and shallow seafloor create a natural barrier against those coastal hazards,” Yates told HuffPost.

But reefs worldwide are collapsing from the damage of human activity and climate change. “As those reefs and the seafloor erode away and water depth gets deeper, large waves can move closer to ― or even hit ― the shore before they break up, where they can cause more erosion and damage along the coast,” Yates said. “This creates a cascading effect ... loss of coral reefs and seafloor increases water depth, which allows bigger waves to reach coastal areas, which causes more erosion both of the seafloor and along the coastline.”

In their research, Yates and her team found significant seafloor erosion in five large coral reef tracts near Florida and Hawaii and in the Caribbean. The erosion has caused water depths in some of these areas to increase to levels that were not predicted to occur until the end of the century, said Yates.

“At current rates, by 2100, the combination of sea-level rise and seafloor erosion could increase water depths by two to eight times more than what is predicted from sea-level rise alone,” she said.

Climate Central

Climate Central

Experts say that if the world doesn’t act to significantly reduce greenhouse gas emissions, the probability of these extreme scenarios goes up and up.

If we want even a chance to save our coastal cities, Strauss of Climate Central told The Washington Post, we need to be “doing even better” than the landmark Paris climate agreement.

In the U.S., however, President Trump, who has repeatedly called global warming a “hoax,” has been busy trying to roll back rules limiting emissions. His commitment to the Paris agreement remains up in the air.