With glaciers in Greenland and Antarctica melting into the ocean at up to double the speed of just a few years ago, all the water has to go somewhere -- and that could mean an end to America's coastlines as we know them.
The water wouldn't come crashing down city streets the way it did in the movie, "The Day After Tomorrow." Instead, according to the new issues of the journal Science, the oceans fed by rapidly melting glaciers would rise steadily over years -- a total of 13 to 20 feet by the end of this century, threatening huge chunks of American cities like New Orleans, Miami, Charleston, S.C., and New York.
In the future, storms could regularly swamp New York's subways and tunnels if barriers aren't built to hold back the rising tides, according to Vivien Gornitz, a sea level specialist for NASA. Even pricey riverfront properties in the borough of Manhattan may be under water periodically.
This is the sort of thing that happens in Venice, Italy. Each winter they have floods or high tides and have to put down temporary boardwalks as walkways to keep people out of the water.
In Miami, a team is trying to figure out how to keep the sea from spilling into the city's drinking water reserves.
"We know we've got to take some action," said Harvey Ruvin, who leads the team confronting the issue. "Certainly building a dike 500 miles long wouldn't work. It's too expensive. But maybe building a desalinization plant or moving our oil well fields further to the west [would work]."
Any of the solutions could run into the millions or billions of dollars, but scientists say the cost of inaction is much higher.
"There is no point waiting until we've reached that tipping point," said Tim Flannery, the author of "The Weather Makers." "It's then too late. It really is."
In Holland, where 50 percent of the land is below sea level, they are way ahead of the game. The Dutch spent $8 billion over 30 years to fortify the coastline with state-of-the-art dikes, dams, levees and a 6-mile-long hydraulic sea wall. That system is 50 times stronger than the defenses that failed New Orleans during Hurricane Katrina.
"To the Dutch standards, New Orleans was not very well-protected," said Huib de Vriend of Delta Hydraulics.
But the ocean can't be contained forever, and experts agree the only true way to protect the precious shoreline is to make serious cuts in the greenhouse emissions that are warming the Earth and melting the polar ice caps.