Ice sheet change is expected to be a "slow" climate feedback. How rapidly ice sheets can disintegrate is one of the most uncertain and imporant climate issues. The dominant physical process causing ice sheet disintegration may be absorption of heat by the ocean (due to an increasing greenhouse effect), resulting melting of ice shelves, and thus an increased rate of discharge of ice from the ice sheet to the ocean. Once this process gets well underway, it may be difficult to prevent accelerating ice sheet disintegration under its own impetus ( http://pubs.iss.nasa.gov/docs/2005/2005_Hansen.pdf).
The figures above show the rate of mass loss from the Greenland and Antarctic ice sheets, based on a recent publication by Shepherd et al. (2012). The rate of mass loss from Greenland has increased during the past several years, as it has from Antarctica. This is one of the most important geophysical measurements being made, so it is important to get a follow-on gravity satellite into space. A planned European gravity satellite is not sufficiently capable to yield accurate ice sheet mass change, and a planned NASA follow-on gravity mission is low on NASA's priority list.
We thank Dr. Eric Rignot for providing us digital data of an earlier version of "input output method" data, and Drs. Andrew Shepherd and Erik Ivins for providing the digital Antarctica mass data of their paper. All other data were read off from Fig. 4 of their paper.
A. Shepherd et al. (2012) "A reconciled estimate of ice-sheet mass balance", Science, 338, 1183-1189.
E. Rignot et al. (2011) "Acceleration of the contribution of the Greenland and Antarctic ice sheets to sea level rise", GRL, 38, L05503 (5 pp.)