SCIENCE
10/13/2015 07:28 pm ET Updated Oct 14, 2015

More Than 400 U.S. Cities May Be 'Past The Point Of No Return' With Sea Level Threats

But there are still cities that could be saved by reducing carbon emissions.
A woman walks atop an earthen levee on the Mississippi River in the Lower 9th Ward as the Steamboat Natchez passes in Au
Mario Tama/Getty Images
A woman walks atop an earthen levee on the Mississippi River in the Lower 9th Ward as the Steamboat Natchez passes in August 2015 in New Orleans. The city is ringed by hundred of miles of levees to protect against flooding.

Millions of Americans live in places where it's too late to slow the threat of rising sea levels, a new study warns, and researchers are hoping those findings will serve as a call to action for cities that can still be saved by cutting carbon emissions. 

The study, published Monday in the journal Proceedings of the National Academy of Sciences, examines how much rising sea levels will affect cities across the United States over time if carbon emissions stay the same or decrease. The most startling finding is that 414 towns and cities have already passed their lock-in date, or the point at which it's guaranteed that more than half the city's populated land will eventually be underwater no matter how much humans decrease carbon emissions; it's just a matter of when.

That's "the date where we let the genie out of the bottle, when it’s past the point of no return," lead study author Benjamin Strauss of Climate Central told The Huffington Post.

Of those 414 cities, New Orleans stands to be one of the most compromised.

"Even in a best-case carbon emissions scenario, 98 percent of populated land in New Orleans would be below the future sea level," Strauss said, because it's so flat and low-lying. "So it’s really just a question of building suitable defenses or eventually abandoning the city."

Those defenses could include higher levees around the city, but that's not an ideal solution, Strauss said.

"How deep a bowl do you want to live in?" he asked. "We already saw with [Hurricane] Katrina what can happen when a levee is breached, and the higher the water gets and the taller the levee gets, the more catastrophic a levee breach would become."

Conditions in New Orleans could be even worse than the study predicts, he noted, as it didn't take into account the fact that New Orleans is already sinking. 

Low, flat Miami, another city that's past its lock-in date, is also of great concern because of the type of land beneath it. 

"The extra problem that Miami has is that it’s sitting on porous limestone, or, in other words, the bedrock underneath Miami is a lot like Swiss cheese," he said. "Water can just go through it and so building levees is not going to be effective in South Florida."

In every scenario in the study except the two most extreme ones, Florida contains at least 40 percent of the people living on potentially affected land.

How quickly these situations would play out remains a mystery.

"Think of a pile of ice in a warm room," Strauss said. "It’s very easy to know that it’s going to melt, but it’s harder to say just how quickly, and we have a similar problem with sea level projections."

While the future looks bleak in some cities, Strauss emphasized that many cities can be saved if people take swift action against carbon emissions. 

"The most interesting thing to me is there are a great deal of cities where our carbon choices make a huge difference," he said. "For example, if you look at Philadelphia, under business as usual, land that accounts for more than 100,000 people could be submerged. But you divide that total by 10 with an extreme carbon cut. The very biggest difference of all is for New York City, where you can avoid submergence of land where one and a half million people live."

There's similar promise in Virginia Beach, Sacramento and Jacksonville, Florida, he said.

"To me this is really a question of our American legacy and American heritage," Strauss said. "Are we going to let the ocean take a state-sized bite out of America? If we make extreme efforts to cut carbon, we can avoid that."

You can find out how much rising sea levels will affect your city -- or any city -- under different carbon emissions scenarios using this map tool from Climate Central. 

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