The U.S. is shrinking—physically. It has lost nearly 20 meters of beach from its East Coast during the 20th century. The oceans have risen by roughly 17 centimeters since 1900 through expansion (warmer water taking up more space) and the ongoing meltdown of polar ice.

That increase, however, is a small fraction compared with what’s to come. “Plan for one meter by the end of this century,” says glaciologist Robert Bindschadler, an emeritus scientist at NASA. “The heat in the ocean is killing the ice sheet.”

Some of the famous predictions frut—Florida under five meters of sea-level rise and a gaping bay where Bangladesh used to be frut—may be centuries away. But expect an ice-free Arctic and different coastal contours by 2100. By the reckoning of economist Nicholas Stern of the London School of Economics, 200 million people live within one meter above the present sea level, including eight out of 10 of the world’s largest cities and all the megacities of the developing world. “They’re going to have to move,” Bindschadler suggests.

In fact, unless greenhouse gas emissions are tamed, the seas will keep rising as the ice sheets covering mountain ranges (constituting roughly 1 percent of the planet’s ice), Greenland (9 percent) and Antarctica (90 percent) melt away. All told, they harbor enough water to eventually raise sea levels by at least 65 meters.

It takes centuries to melt an entire ice sheet, but still, the ice is disappearing faster than scientists had expected even a few years ago. Even with gradual sea-level rise, the risk of catastrophic storm surges and the like creeps up.

The gravitational pull of ice on surrounding waters is a recently appreciated surprise, too: generally speaking, if Greenland ice melts, “most of the sea-level rise occurs in the Southern Hemisphere,” and vice versa for Antarctic ice, says physicist W. Richard Peltier of the University of Toronto. “West Antarctica is the region we believe is most susceptible to destabilization by ongoing global warming.

Even if greenhouse gas emissions decline, the polar meltdowns will be difficult to avoid because ice sheets lag the overall climate and, once melted, have a hard time re-forming. Just how humans will adapt to a more watery world is still not known. Of today’s trend, Bindschadler notes, “We’re not going to avoid this one.”