The U.S. Climate Change Science Program has released its report on Abrupt Climate Change, and it includes a chapter on “Rapid Changes in Glaciers and Ice Sheets and their Effects on Sea Level” (PDF), written by a team of leading scientists, and whose lead author is director of CIRES (where I work at CU) Koni Steffen.

What does the report say about our ability to predict future sea level rise (pp. 92-93)?

Considerable effort is now underway to improve the models, but it is far from complete, leaving us unable to make reliable predictions of ice-sheet responses to a warming climate if such glacier accelerations were to increase in size and frequency. It should be noted that there is also a large uncertainty in current model predictions of the atmosphere and ocean temperature changes which drive the ice-sheet changes, and this uncertainty could be as large as that on the marginal flow response.

So when asked . . . how high will sea levels rise in the 21st century?

. . . the scientifically correct answer, according to this report, is “we don’t know.” It could be large, but it also could be similar to that of the 20th century (and I am implying nothing of probability here). Of course, such a situation lends itself to cherrypicking and political Rorschaching. So the proper response might be “Well, what do you want it to be?”