The Heinrich layers in North Atlantic sediment indicate that rapid changes in flow of one of the ice-age ice sheets have delivered large quantities of icebergs to the ocean, perhaps over centuries and with substantial effect on sea level. Among the modern ice sheets, attention is focused, although not exclusively, on the West Antarctic ice sheet because its bed is in many places below sea level, well lubricated, and deeper toward the center of the ice sheet. These characteristics allow the possibility of West Antarctic flow instabilities (Weertman, 1974; Oppenheimer, 1998; Alley and Bindschadler, 2000). A sea-level change of about 5 m (from the West Antarctic ice sheet alone) (Plate 8) to perhaps more than 10 m (including response of the East Antarctic ice sheet to loss of the West Antarctic ice) in a few centuries may be possible, even if unlikely (Intergovernmental Panel on Climate Change, 2001b).

No prediction of change is yet available, and large uncertainties are attached to possible rates and magnitudes of change. Sedimentary evidence from beneath the West Antarctic ice sheet indicates that the ice sheet shrank substantially or disappeared at least once after it formed, although at an unknown rate (Scherer et al., 1998). The current, locally rapid changes in the West Antarctic ice sheet are difficult to explain, but the average change across the ice sheet is small (Alley and Bindschadler, 2000). Modeled behavior includes the possibility of large and rapid ice-sheet changes (MacAyeal, 1992), but behavior in other models is more stable (Hulbe and Payne, 2001). The major southern sites of formation of oceanic deep-waters are close to the West Antarctic ice sheet, and deep-water formation involves interaction with floating extensions of the ice sheet called ice shelves (e.g., Schlosser et al., 1994). The freshwater delivery of Heinrich events to the North Atlantic was associated with greatly reduced formation of deep-water in the vicinity and very large climate anomalies well beyond the region of freshwater supply (Broecker, 1994). Thus, any large changes in the ice sheets could affect many aspects of climate in addition to sea level (Plate 8).

OUTLOOK

If the increase in atmospheric greenhouse gas concentration leads to a collapse of the Atlantic THC, the result will not be global cooling. However, there might be regional cooling over and around the North Atlantic, relative to a hypothetical global-warming scenario with unchanged THC. By itself, this reduced warming might not be detrimental. However, we



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