More than 400 U.S. cities and towns with a combined population in the millions could eventually be overtaken by sea level rise, according to a study published last month in the Proceedings of the National Academy of Sciences.

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The study concludes that “there is no hope for Miami and New Orleans, even if we halt greenhouse gas emissions today,” the Hamilton Spectator reports.

“You don’t have to be afraid, you have to be worried,” said study co-author Anders Levermann. “It isn’t killing people. It’s slow, it’s eating up land, it’s eating up our cultural heritage. It’s a thing to worry about, to tackle. It will cost a lot of money.”

Levermann stressed that the communities we build today are tomorrow’s cultural heritage. “People bring life to cities. Cities like New York, New Orleans, Shanghai—it is the cultural heritage of the future,” he told Aulakh.

“But at the same time, we are also committing them to the sea by more and more emissions. For every degree of warming, eventually we get a sea level rise of two metres or more….So, basically, we could lose the cultural heritage we are building right now.”

Levermann expressed concern about how communities will react to abrupt climate change and extreme weather. “Societies aren’t necessarily as stable as we like to believe,” he said. “Abrupt climate change and weather extremities put tension on a society and change its well-being, and that may lead to a tipping for societies, even in the industrialized world.”