Here’s another piece of evidence that climate change might be worse than scientists previously predicted.

The seas are rising, and will continue to rise, because hotter temperatures melt land-based ice and expand the volume existing ocean water takes up. But while much study has examined the shift in amount and warmth of seawater humans will face, there is another variable scientists must get right to assess the risk to humanity: just how many people live in low-lying areas. A new paper suggests previous estimates of land elevation – and, therefore, the number of at-risk people – were wrong.

The study, published Tuesday in the journal Nature Communications, corrects satellite elevation data, and it “reveals a developed global coastline three times more exposed to extreme coastal water levels than previously thought,” the authors warn. Even under an optimistic scenario in which heat-warming greenhouse emissions are restrained and Antarctic ice sheets remain stable, “the global impacts of sea-level rise and coastal flooding this century will likely be far greater than indicated by the most pessimistic past analyses.”

And that’s the rosy scenario. If Antarctica’s massive ice sheets became unstable – and there are ominous signs – land that is home to 300 million people would be at risk of yearly flooding by 2050, 50 million more than present projections. Some 230 million more people than current estimates would be vulnerable by 2100.

Eight Asian nations – China, Bangladesh, India, Vietnam, Indonesia, Thailand, the Philippines and Japan – account for some 70 percent of the people living on at-risk land. Under the newly calculated numbers, China’s population facing the extreme risk of falling below the high-tide line by 2100 roughly triples. Bangladesh, India, Indonesia and the Philippines see the number of people living below the projected high-tide line increase by five to 10 times. One-third of the populations of Bangladesh and Vietnam live in areas that – if emissions were not restrained and Antarctic ice proved unstable – would sink below the high-tide line. A massive slice of south Vietnam and large swaths of Ho Chi Minh City are in danger. Giant chunks of Bangkok could be washed away. Central Shanghai and Mumbai would see massive problems.

Millions already live behind protective dikes, levees and other flood-control systems; millions more will have to do so in the future – or surrender their cities. As the problem worsens, countries will probably be forced to do some of both. “Coastal communities worldwide must prepare themselves for much more difficult futures,” the study’s authors counsel. Even in the United States, “sea-level rise this century may induce large-scale migration away from unprotected coastlines.” In poorer countries, this retreat from the sea could lead to civil strife, instability and violence.

There is at least as much risk that scientists have been more optimistic than alarmist in their projections of what human-caused global warming will do. The choices the United States makes now will determine how safe the world is for generations to come. And, for the moment, the United States is failing these future generations.