The history of past ice sheets
Understanding present-day melting patterns of polar ice caps and the contribution to present-day sea level variations requires first unravelling the puzzle of how the continents are still adjusting after the melting that has occurred since the Last Glacial Maximum around 10,000 years ago. This is a complicated field that involves field sampling to identify indicators of past sea levels, then generating numerical models that represent the amount and spatial locations of ice caps that can reproduce the observed sea level histories.
The mass of ice sheets deform the surface of the Earth - depressing the continents beneath the ice and causing slight uplift just outside the ice sheet perimeter - and reduce the volume of the oceans. Consequently, the melting of ice sheets has the reverse effect, causing uplift of continents and increases in ocean volumes. The processes can take thousands of years to reach equilibrium and the regions of the major ice sheets (Fennoscandia, Greenland, Laurentia, Antarctica) are still deforming as a result of melting that occurred thousands of years ago.
Dating of biological and geomorphologic samples yield information about the retreat of the ice caps and the rebound rates of regions previously covered with ice. Such data are inverted to generate models for the amount and timing of melting that has occurred.
The present-day glacial isostatic adjustment of Antarctica generates uplift of the continent that is detectable by space-geodetic observing techniques. The Gravity Recovery and Climate Experiment (GRACE) is sensitive to the induced gravity changes, while satellite altimetry and ground-based GPS can measure directly the vertical movement of the surface. These effects must be removed from GRACE and altimetry estimates of temporal changes in ice in order to estimate present-day mass balance changes that might be occurring as a result of global warming.