I did some back-of-an-envelope calcs last night with a friend and based on the area of the worlds oceans (omitting the fact that this surface area increases as it rises) and an average of 500m thickness of ice on the West Antarctic land sliding into the sea would roughly increase the global sea levels by 5m.
The ice on land is only going to slide into the sea at the rate of flow of a glacier, which of course can vary, lots have been speeding up.
One variable which would seem to delay sea level rise is in increase in snowfall in the Antarctic -- a meter of extra ice on the West Antarctic alone would reduce sea levels by roughly 2mm assuming all the water comes from evaporation from the sea. And over 1m per year has been measured according to this paper:
A doubling in snow accumulation in the western Antarctic Peninsula since 1850
Elizabeth R. Thomas British Antarctic Survey, Cambridge, UK
Gareth J. Marshall British Antarctic Survey, Cambridge, UK
Joseph R. McConnell Desert Research Institute, Reno, Nevada, USA
http://www.agu.org/pubs/crossref/2008/2007GL032529...
Which has been written about here:
Thomas et al. analyzed a medium depth ice core drilled at a high accumulation site (Gomez) on the south-western Antarctic Peninsula (73.59°S, 70.36°W, 1400 m) (see map , Figure 1). If you want the details, the core was drilled in January 2007 using an electromechanical, 104 mm diameter drill to a depth of 136 m. As seen in the figure below, the snow accumulation (measured in meters of water equivalent per year, mweq y-1), has as the title of the article suggests, been rising like a rocket. In their own words, the authors state “Annual accumulation has more than doubled in the last 150 years: the mean for 1855–1864 was 0.49 mweq y-1while for 1997–2006 it was 1.10 mweq y-1. At the beginning of the record annual accumulation is relatively stable until about 1930 when it begins to increase steadily. Following a slight reduction in accumulation in the late 1960s, the most rapid increase occurs in the latter part of the record with the mean accumulation rate from the mid-1970s onwards increasing to 0.95 mweq y-1. Note that for the post-1980 period even the lowest annual accumulation values are still greater than the highest accumulation values from the first half of the record (1855–1924).†This huge increase may be unique to the Gomez area, but other cores sites certainly show increases in accumulation as well.
http://www.agu.org/pubs/crossref/2008/2007GL032529...
So this increase in snowfall is potentially (probably?) offsetting the rise in sea levels by something in the order of mm's per year and currently the sea level is rising at the rate of about 3mm per year -- so a more rapid rise is being stored up for the future -- when the ice slides into the sea.
The issue with the predictions of between 0.5m or 1m of global sea level rise in the next hundred years is I'm not sure if these figures take into account the effect of an extra 27cm rise, which could happen in 40 years, unpinning the WAIS (as the first article in this thread proposed) and then this triggering an increased glacial flow which could lead to a more rapid rise in sea levels?
This article on Real Climate, about a recent paper which I think might try to take this into effect, includes:
In summary, they estimate that including dynamic ice sheet processes gives projected SLR at 2100 somewhere in the 80 cm to 2 meter range, and suggest that 80 cm should be the 'default' value. This is remarkable in a number of ways - first, these are the highest estimates of sea level rise by 2100 that has been published in the literature to date, and secondly, while they don't take into account the full uncertainty in other aspects of sea level rise considered by IPCC, their numbers are significantly higher in any case.
http://www.realclimate.org/index.php/archives/2008...
The 382 comments on that article are also interesting.