There's a new paper out in Science this week, which compares the current state of the world with that of the last interglacial period (LIG), when sea levels were much higher than now. The paper is by Jeremy S. Hoffman, Peter U. Clark, Andrew C. Parnell, and Feng He. Their findings suggest that we could already be committed to around a six to nine metre rise in sea level. (That's about 20 to 30 feet higher for the metric deficient.) From the introduction to the paper:
The last interglaciation [LIG, 129 to 116 thousand years ago (ka)] was one of the warmest periods during the last 800,000 years (1), with an associated sea-level rise of 6 to 9 m above present levels (2). As such, the LIG provides an important target for validating global climate models used for climate-change projections (3, 4), as well as for understanding the sea-level response to a warm climate.