Get ready: seven foot sea level rise this century

If there is one question about climate science that went unanswered in the 2007 IPCC report, it was sea level rise. As the EPA notes, the report estimates a two-foot rise this century…at the most. 

But that estimate did not include the possibility of degradation of the polar ice sheets, which in the last two years has been observed by NASA's astounding GRACE satellite:

We use monthly measurements of time-variable gravity from the GRACE
(Gravity Recovery and Climate Experiment) satellite gravity mission to
determine the ice mass-loss for the Greenland and Antarctic Ice Sheets
during the period between April 2002 and February 2009. We find that
during this time period the mass loss of the ice sheets is not a
constant, but accelerating with time, i.e., that the GRACE observations
are better represented by a quadratic trend than by a linear one,
implying that the ice sheets contribution to sea level becomes larger
with time.

The IPCC could not estimate sea level rise, it said, because the unknowns were too great, so it assumed that warming would result in the same sort of sea level rise we saw in the 20th century. As authors Rob Young and Orrin Pilkey write on the Yale Environment 360:

In the 20th century, sea level rise was primarily due to thermal
expansion of ocean water. Contributions of melting mountain glaciers
and the large ice sheets were minor components. But most climate
scientists now believe that the main drivers of sea level rise in the
21st century will be the melting of the West Antarctic Ice Sheet (a
potential of a 16-foot rise if the entire sheet melts) and the
Greenland Ice Sheet (a potential rise of 20 feet if the entire ice cap
melts).

[edit]

Seeking to correct the IPCC’s failure to come up with a comprehensive
forecast for sea level increase, a number of state panels and
government committees have produced sea level rise predictions that include an
examination of melting ice sheets. For example, sea level rise panels
in Rhode Island and Miami-Dade County have concluded that a minimum of
a three- to five-foot sea level rise should be anticipated by 2100. A
California report assumes a possible 4.6-foot rise by 2100, while the
Dutch assume a 2.5-foot rise by 2050 in the design of their tidal gates.

The authors, professors at Duke and Western Carolina University, think that for the purposes of urban planning, a seven foot rise is a prudent estimate. 

Miami tops the list of most endangered cities in the world, as measured
by the value of property that would be threatened by a three-foot rise.
This would flood all of Miami Beach and leave downtown Miami sitting as
an island of water, disconnected from the rest of Florida. Other
threatened U.S. cities include New York/Newark, New Orleans, Boston,
Washington, Philadelphia, Tampa-St Petersburg, and San Francisco.

They're just saying…that pictures like this one, of the sea hitting St. Ives in Cornwall last September, will become a lot more common in years to come.