(CNN) Hundreds of millions of people worldwide are at risk of losing their homes as entire cities sink under rising seas over the next three decades, according to researchers.

The findings, published Tuesday in the journal Nature Communications , put nearly three times as many people in coastal areas at risk from flooding than previously thought, and are the result of new advances in elevation modeling technology.

Global sea levels are expected to rise between two to seven feet (0.6 meters to 2.1 meters) -- and possibly more -- over the course of the 21st century.

And by 2050, land that is currently home to about 300 million people will fall below the elevation of the average annual coastal flood -- meaning they could face severe floods at least once a year. By 2100, land that is home to 200 million people could sit permanently below the high tide line, rendering those coastal areas all but unlivable.

"The results indicate that, yes, a great deal more people are on vulnerable land than we thought," said Benjamin Strauss, one of the study's co-authors and CEO of non-profit organization Climate Central. He added that these affected regions need to take immediate action to avoid the impending "economic and humanitarian catastrophe."

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