The ability to detect monthly mass changes through space-geodetic missions provides the capability to assess changes in the ice sheets of Antarctica and Greenland. Decreases in mass imply that the cryosphere is melting and increasing the volume of the oceans whereas increases in mass imply increased precipitation and snow accumulation. The contribution of ongoing isostatic adjustment to GRACE estimates of mass balance changes is significant - over 100% of the mass loss in the case of some reported values (Velicogna and Wahr, 2006). Nonetheless, space-geodetic techniques provide the most accurate information on polar melting that is currently available.
How fast are Antarctica and Greenland melting and how is such melting contributing to rising sea level? The total mass variation sensed by the satellites is the combination of isostatic adjustment, present-day melting, snow accumulation and variations in atmospheric pressure, atmospheric water vapour and ocean mass. Many of these are "removed" using global and/or incomplete models with deficiencies either known, suspected or unquantified. Research is underway to understand the accuracy limitations of the techniques and how to improve them.