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 figure above shows the rate of mass loss from the Greenland and Antarctic ice sheets, based on updates of Rignot et al. (2011) and Velicogna et al. (2014). 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.
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.)
I. Velicogna et al. (2014) "Regional acceleration in ice mass loss from Greenland and Antarctica using GRACE time-variable gravity data, GRL, 41, 8130-8137.
Our Essay: "Update of GreenlnadIce Sheet Mass Loss: Exponential?" (2012/12/26)