A recent Nature Geoscience study, "High rates of sea-level rise during the last interglacial period," (subs. req'd) finds that sea levels could rise twice what the IPCC had project for 2100,. This confirms what many scientists have recently warned (and here), and it matches the conclusion of a study earlier this year in Science.
For the record, five feet of sea level rise would submerge some 22,000 square miles of land just on the Atlantic and Gulf Coasts (farewell, southern Louisiana and Florida) - and displace more than 100 million people worldwide. And, of course, sea levels would just keep rising, most likely even more next century than this century.
The researchers base their finding on their analysis of the rate of sea level rise during the last warm or interglacial period (the Eemian, about 120,000 years ago), when seas rose 1.6 meters (5 feet) per century. Why look at the rate of Eemian sea level rise? Becaause that's the last time the planet was as warm as it soon will be again: "such rates of sea-level rise occurred when the global mean temperature was 2 °C higher than today, as expected again by AD 2100."
Indeed, if we don't reverse emissions' trends very soon (and stay below 450 ppm of carbon dioxide), the planet might well warm 3°C or more by 2100. The Eemian warming was driven by "changes in orbital parameters from today (greater obliquity and eccentricity, and perihelion), known as the Milankovitch cycle." Current warming is driven by human emissions of greenhouse gases.
Here is the entire abstract from the article - note that the Eemian is also called "Marine Isotope Stage 5":