Melting of Northern Greenland during the last interglacial

The Greenland ice sheet (GrIS) is losing mass at an increasing rate, making it the primary contributor to global eustatic sea level rise. Large melting areas and rapid thinning at its margins has raised concerns about its stability. However, it is conceivable that these observations represent the transient adjustment of the fastest reacting parts of the ice sheet, masking slower processes that dominate the long term fate of the GrIS and its contribution to sea level rise.


Melting of Northern Greenland during the last interglacial
New approach to simulate past Greenland ice sheet IPCC 2007(Otto-Bliesner et al., Science, 2006 454 cores), the next glacial period would not be expected to start within the next 30 kyr (Loutre and Berger, 2000;Berger and Loutre, 2002;EPICA Community Members, 2004). Sustained high atmospheric greenhouse gas concentrations, comparable to a mid-range CO 2 stabilisation scenario, may lead to a complete melting of the Greenland Ice Sheet (Church et al., 2001) and further delay the onset of the next glacial period (Loutre and Berger, 2000;Archer and Ganopolski, 2005).
Abrupt climate changes have been variously de ned either simply as large changes within less than 30 years (Clark et al., 2002), or in a physical sense, as a threshold transition or a response that is rapid compared to forcing (Rahmstorf, 2001;Alley et al., 2003). Overpeck and Trenberth (2004) noted that not all abrupt changes need to be externally forced. Numerous terrestrial, ice and oceanic climatic records show that large, widespread, abrupt climate changes have occurred repeatedly throughout the past glacial interval (see review by Rahmstorf, Figure 6.6. Summer surface air temperature change over the Arctic (left) and annual minimum ice thickness and extent for Greenland and western arctic glaciers (right) for the LIG from a multi-model and a multi-proxy synthesis. The multi-model summer warming simulated by the National Center for Atmospheric Research (NCAR) Community Climate System Model (CCSM), 130 ka minus present (Otto-Bliesner et al., 2006b), and the ECHAM4 HOPE-G (ECHO-G) model, 125 ka minus pre-industrial (Kaspar et al., 2005), is contoured in the left panel and is overlain by proxy estimates of maximum summer warming from terrestrial (circles) and marine (diamonds) sites as compiled in the syntheses published by the CAPE Project Members (2006) and Kaspar et al. (2005). Extents and thicknesses of the Greenland Ice Sheet and eastern Canadian and Iceland glaciers are shown at their minimum extent for the LIG as a multi-model average from three ice models (Tarasov and Peltier, 2003;Lhomme et al., 2005a;Otto-Bliesner et al., 2006a). Ice core observations (Koerner, 1989;NGRIP, 2004) indicate LIG ice (white dots) at Renland (R), North Greenland Ice Core Project (N), Summit (S, Greenland Ice Core Project and Greenland Ice Sheet Project 2) and possibly Camp Century (C), but no LIG ice (black dots) at Devon (De) and Agassiz (A) in the eastern Canadian Arctic. Evidence for LIG ice at Dye-3 (D) in southern Greenland is equivocal (grey dot; see text for detail).
Scaled modern precipitation and temperature as forcing   2007(Otto-Bliesner et al., Science, 2006 454 cores), the next glacial period would not be expected to start within the next 30 kyr (Loutre and Berger, 2000;Berger and Loutre, 2002;EPICA Community Members, 2004). Sustained high atmospheric greenhouse gas concentrations, comparable to a mid-range CO 2 stabilisation scenario, may lead to a complete melting of the Greenland Ice Sheet (Church et al., 2001) and further delay the onset of the next glacial period (Loutre and Berger, 2000;Archer and Ganopolski, 2005).
Abrupt climate changes have been variously de ned either simply as large changes within less than 30 years (Clark et al., 2002), or in a physical sense, as a threshold transition or a response that is rapid compared to forcing (Rahmstorf, 2001;Alley et al., 2003). Overpeck and Trenberth (2004) noted that not all abrupt changes need to be externally forced. Numerous terrestrial, ice and oceanic climatic records show that large, widespread, abrupt climate changes have occurred repeatedly throughout the past glacial interval (see review by Rahmstorf,  (2006) and Kaspar et al. (2005). Extents and thicknesses of the Greenland Ice Sheet and eastern Canadian and Iceland glaciers are shown at their minimum extent for the LIG as a multi-model average from three ice models (Tarasov and Peltier, 2003;Lhomme et al., 2005a;Otto-Bliesner et al., 2006a). Ice core observations (Koerner, 1989;NGRIP, 2004) indicate LIG ice (white dots) at Renland (R), North Greenland Ice Core Project (N), Summit (S, Greenland Ice Core Project and Greenland Ice Sheet Project 2) and possibly Camp Century (C), but no LIG ice (black dots) at Devon (De) and Agassiz (A) in the eastern Canadian Arctic. Evidence for LIG ice at Dye-3 (D) in southern Greenland is equivocal (grey dot; see text for detail).
Scaled modern precipitation and temperature as forcing ( Fig. 4a). In this model, Eemian summer climate is approximately 4.1 • C warmer than preindustrial. However, the temparture anomaly pattern is similar to that of the IPSL CM4 (not shown). The simulated precipitation of CCSM3 does not match observations very well (Fig.  4c). Southern and eastern Greenland receive too little precipitation while the north is too wet. The precipitation distribution changes considerably with 130 ka boundary conditions (Fig. 4d).

Simulated Greenland ice sheet cover
Running the ice sheet model for 10,000 years with preindustrial climate forcing (0 ka) from IPSL CM4, initialized with cold ice, yields a fully glaciated Greenland (Fig. 5a). Compared to 8 Fig. 2. Seasonal averages of forcing fields from IPSL CM4, interpolated onto the ice sheet model grid. Temperature data has been corrected with a fixed lapse rate of 6.5 C km 1 to adjust for height differences between ice sheet and climate model. the only long ice core to become ice-free is NEEM. This occurs after 8300 model years, and defines an upper limit for sea-level rise of 5.9 m. Ice at Dye-3 becomes thinner, but does not disappear until the end of the 10 000 yr simulation.
Melting of the ice margins results in reduced ice thickness in the remaining ice sheet (Fig. 5, right). After 6000 yr, ice at the location of today's Greenland summit is about 400 m thinner in the Eemian than in the modern simulation, which compares well to estimates based on total gas content at GRIP (Raynaud et al., 1997;Cuffey and Marshall, 2000) and 18 O (Masson-Delmotte et al., 2011). Thinning at NGRIP is simulated to be less than 200 m, in agreement with a negligible reduction in elevation found for the late Eemian at this site (Svensson et al., 2011). NEEM experiences a thinning of approximately 300 m, Dye-3 of 100 m and a slight thickening is simulated at Camp Century.