4.6 Changes and Stability of Ice Sheets and Ice Shelves
New and improved observational techniques, and extended time series, reveal changes in many parts of the large ice sheets. Greenland has experienced mass loss recently in response to increases in near-coastal melting and in ice flow velocity more than offsetting increases in snowfall. Antarctica appears to be losing mass at least partly in response to recent ice flow acceleration in some near-coastal regions, although with greater uncertainty in overall balance than for Greenland. Shortcomings in forcing, physics and resolution in comprehensive ice flow models have prevented them from fully capturing the ice flow changes.
The ice sheets of Greenland and Antarctica hold enough ice to raise sea level about 64 m if fully melted (Bamber et al., 2001; Lythe et al., 2001). Even a modest change in ice sheet balance could strongly affect future sea level and freshwater flux to the oceans, with possible climatic implications. These ice sheets consist of vast central reservoirs of slow-moving ice drained by rapidly moving, ice-walled ice streams or rock-walled outlet glaciers typically flowing into floating ice shelves or narrower ice tongues, or directly into the ocean. Ice shelves often form in embayments, or run aground on local bedrock highs to produce ice rumples or ice rises, and friction with embayment sides or local grounding points helps restrain the motion of the ice shelves and their tributaries. About half of the ice lost from Greenland is by surface melting and runoff into the sea, but surface melting is much less important to the mass balance of Antarctica. Dynamics of the slow-moving ice and of ice shelves are reasonably well understood and can be modelled adequately, but this is not so for fast-moving ice streams and outlet glaciers. Until recently (including IPCC, 2001), it was assumed that velocities of these outlet glaciers and ice streams cannot change rapidly, and impacts of climate change were estimated primarily as changes in snowfall and surface melting. Recent observations show that outlet glacier and ice stream speeds can change rapidly, for reasons that are still under investigation. Consequently, this assessment will not adequately quantify such effects.