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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.
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