New climate research shows 300 million people living along the world's coasts could be hit by devastating flooding by 2050, about three times more than previously estimated.
The figure could double to 630 million people affected by 2100 if little is done to rein in greenhouse gas emissions that continue to rise around the planet.
The study by the US-based Climate Central organisation, published on Tuesday by the journal Nature Communications, said major coastal cities such as India's Mumbai, China's Shanghai, and Thailand's Bangkok could be submerged over the next 30 years.
An estimated 237 million people threatened by rising sea waters live in Asia alone, the research said.
"These assessments show the potential of climate change to reshape cities, economies, coastlines, and entire global regions within our lifetimes," said Scott Kulp, lead author of the study.
Mumbai, one of the largest cities in the world, risks becoming completely inundated. Shanghai, with a population of 24 million, faces a similar fate three decades from now.
The scale of human movement will be unrivaled in history, Dina Ionesco of the International Organization for Migration told The New York Times.
"We've been trying to ring the alarm bells, we know that it's coming," she said.
Egypt's Alexandria and Basra, the second-largest city in Iraq, will be mostly underwater by 2050, the study said.
Southern Vietnam - with more than 20 million people - may become entirely flooded.
The study suggested nations must begin preparing now for millions of citizens to relocate internally.
"As shocking as these findings might be, there is a silver lining," said Climate Central's director of communications Peter Girard.
"They give us the knowledge we need to take action in time to protect millions, and to avoid the economic and political upheaval that a climate disaster on this scale could bring."