Are these ice sheets melting?
NASA: There is some evidence of melting in Greenland on the margins, but accumulation near the center. The net melting is still quite uncertain.
NSIDC: A few glaciers in Greenland and Antarctica are increasing in speed; this may be due to increased melt at the surface or the loss of floating ice at their ocean fronts. A smaller number of glaciers in Antarctica are slowing. This contrasts with the melting of smaller glaciers and ice caps, which we know are contributing to sea level rise. But to date, the combined effect of all these glacier changes on sea level is too small to detect. In fact, it is not known to what extent the large ice sheets contribute to present-day sea level rise. The ongoing ICESat mission, which measures ice sheet elevation changes, is intended to help resolve this issue.
The total potential rise represented by the mass of ice in the Greenland Ice Sheet is about 7 meters, and for the Western Hemisphere portion of Antarctica, about 5 meters. These two ice sheets are thought to be more likely to change their ice outflow due to global warming; however, no plausible scenario can be constructed that leads to a significant (several-foot) rise in sea level from these ice sheets in less than a few centuries.
An additional effect expected from global warming is increased snow accumulation at high elevations on these ice sheets. This may partly offset sea level rise due to increased melting and outflow. Please see NSIDC's State of the Cryosphere site for additional information.
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