6.4.3.3 What Is the Significance of Higher than Present Sea Levels During the Last Interglacial Period?

The record of eustatic sea level change can be extended into the time of the LIG. Direct sea level measurements based upon coastal sedimentary deposits and tropical coral sequences (e.g., in tectonically stable settings) have clearly established that eustatic sea level was higher than present during this last interglacial by approximately 4 to 6 m (e.g., Rostami et al., 2000; Muhs et al., 2002). The undisturbed ice core record of the North Greenland Ice Core Project (NGRIP) to 123 ka, and older but disturbed LIG ice in the Greenland Ice Core Project (GRIP) and Greenland Ice Sheet Project 2 (GISP2) cores, indicate that the Greenland Summit region remained ice-covered during the LIG (Raynaud et al., 1997; NGRIP, 2004). Similar isotopic value differences found in the Camp Century and Renland cores (Johnsen et al., 2001) suggest that relative elevation differences during the LIG in northern Greenland were not large (NGRIP, 2004). Interpretation of the Dye-3 ice core in southern Greenland is equivocal. The presence of isotopically enriched ice, possibly LIG ice, at the bottom of the Dye-3 core has been interpreted as substantial reduction in southern Greenland ice thickness during the LIG (NGRIP, 2004). Equally plausible interpretations suggest that the Greenland Ice Sheet’s southern dome did not survive the peak interglacial warmth and that Dye-3 is recording the growth of late-LIG ice when the ice sheet re-established itself in southern Greenland (Koerner and Fisher, 2002), or ice that flowed into the region from central Greenland or from a surviving but isolated southern dome (Lhomme et al., 2005a). The absence of pre-LIG ice in the larger ice caps in the eastern Canadian Arctic indicates that they melted completely during the LIG (Koerner, 1989).

Most of the global sea level rise during the LIG must have been the result of polar ice sheet melting. Greenland Ice Sheet models forced with Greenland temperature scenarios derived from data (Cuffey and Marshall, 2000; Tarasov and Peltier, 2003; Lhomme et al., 2005a), or temperatures and precipitation produced by an AOGCM (Otto-Bliesner et al., 2006a), simulate the minimal LIG Greenland Ice Sheet as a steep-sided ice sheet in central and northern Greenland (Figure 6.6). This inferred ice sheet, combined with the change in other arctic ice fields, likely generated no more than 2 to 4 m of early LIG sea level rise over several millennia. The simulated contribution of Greenland to this sea level rise was likely driven by orbitally forced summer warming in the Arctic (see Section 6.4.1). The evidence that sea level was 4 to 6 m above present implies there may also have been a contribution from Antarctica (Scherer et al., 1998; Overpeck et al., 2006). Overpeck et al. (2006) argued that since the circum-arctic LIG warming was very similar to that expected in a future doubled CO₂ climate, significant retreat of the Greenland Ice Sheet can be expected to occur under this future condition. Since not all of the LIG increment of sea level appears to be explained by the melt-back of the Greenland Ice Sheet, it is possible that parts of the Antarctic Ice Sheet might also retreat under this future condition (see also Scherer et al., 1998; Tarasov and Peltier, 2003; Domack et al., 2005 and Oppenheimer and Alley, 2005).