Climate scientist James Hansen and his colleague Makiko Sato have released a new discussion paper with updated data on ice sheet mass loss from Greenland and Antarctica, with implications for possible multi-metre sea level rise this century. It makes for some interesting reading - there is a link to Hansen's website and the paper at the end.
The thesis that Hansen has put forward for several years is that Ice Sheet collapse is a non-linear process: that with the inclusion of amplifying climate feedbacks it is likely to follow an exponential rate of acceleration - a doubling rate. It might be a 10 year doubling time, or less. This will lead to extensive sea level rise, perhaps in the order of 5 metres this century.
But accurate data measurements of ice mass loss via laboriously estimating mass input and output has only been available since the early 1990s, and accurate satellite measurements (Gravimetry) via the GRACE satellites since 2000.
What these measurements show is that ice mass loss from both Greenland and Antarctica are accelerating, but the data for the time period is still too short to determine whether ice sheet mass loss will follow a somewhat linear path, or an exponential path doubling every 10 years or shorter time period.
Caption: Figure 2: Greenland (a) and Antarctic (b) mass change deduced from gravitational field measurements by Velicogna (2009) and best-fits with 5-year and 10-year mass loss doubling times. From NASA: Earth's Climate History: Implications for Tomorrow Hansen and Sato July 2011
Update 14 Jan 2013: Matt Owens from Fairfax Climate Watch had a similar idea in reviewing Hansen's discussion paper. Read his initial analysis: Sea level rise could crimp GDP; US direct losses could top 1/4 trillion per year during 2040-2050. On 11 January 2013 he updated his analysis with a new article: Surprising negative feedback could mean epic disaster. Both are worthwhile reading.