While there are many troubling consequences of the global warming we are imposing on ourselves, one of the most inexorable is the increase in ocean volume that leads to rising seas around the world. These changes are already bringing coastal flooding with increasing frequency, loss of coastal land, and related trouble such as salt-water incursion into groundwater, and much worse is in store. Stefan Rahmstorf at RealClimate recently reviewed several recent papers on sea level rise and among other concerns noted that even if we manage to limit warming to just 2 degrees Celsius, the expected eventual sea-level rise is 25 meters (82 feet) over thousands of years. Part of this increase in volume comes from the melting of land-bound ice: mountain and polar glaciers and the major icesheets of Greenland and Antarctica. The other part of the increase comes from the slow steady heating of the ocean as a whole - warmer water occupies more volume than colder water. As long as our planet is subject to climate forcings that bring it away from the pre-industrial energy balance, the ocean will continue warming even if the surface temperature starts to cool again once we have brought CO2 under control. Even while the surface of Earth has warmed by about 1 degree C since pre-industrial times so far, the temperature increase in the deep ocean has been measured at only one or two hundredths of a degree; eventually the deep ocean should catch up to the surface so we will see on the order of 100 times present sea level rise from that cause - unless we can return surface temperatures to pre-industrial levels first.
Sea level rise (SLR) has accelerated over the past few centuries - see this post by Tamino on analysis of satellite and tide-gauge measures. Presently sea level is rising at close to 3 mm/year - 1 foot per century. The IPCC has warned of up to 1 meter of sea level rise by 2100, implying a rate increasing to 10 mm/year or more. James Hansen in a recent paper argued for the possibility of several meters in 50 to 100 years due to a major acceleration in ice-sheet loss: this would imply a rate on the order of 30-50 mm/year of SLR. Even at 3 mm/year the inexorable rise is troubling; 10 times that amount would be devastating. Is there anything we can do about it? What does a look at some of the numbers in this problem tell us?