Higher water

Real Climate talks about the recent finding that some Antarctic ice freezes from the bottom, but also notes the big picture with ice melt:

....there is a new assessment of the net mass balance of Antarctica and Greenland. Rignot et al have updated results, including those from the GRACE gravity measurement satellite, to the end of 2010 and show that the downward trend in ice mass is continuing (stronger in Greenland than in Antarctica). The net rise in sea level associated with this decline is about 1.3 mm/yr, which will likely accelerate with further warming. Complementary analyses of the surface mass balance of Greenland (Tedesco et al, 2011) also show that 2010 was a record year for melt area extent.

This rate of melting is more than was figured into the tabulated IPCC AR4 estimates of sea level rise, and any further acceleration will obviously make the discrepancy worse. Indeed, even in the highest forcing A1F1 scenario, the IPCC calculated only a 0.3 mm/year contribution from the ice sheets averaged over the whole 21st Century! This was clearly a gross underestimate.

Extrapolating these melt rates forward to 2050, “the cumulative loss could raise sea level by 15 cm by 2050″ for a total of 32 cm (adding in 8 cm from glacial ice caps and 9 cm from thermal expansion) – a number very close to the best estimate of Vermeer & Rahmstorf (2009), derived by linking the observed rate of sea level rise to the observed warming.