6.4.1.6 How Much Did the Earth Warm During the Previous Interglacial?

Globally, there was less glacial ice on Earth during the Last Interglacial, also referred to as “Last Interglaciation” (LIG, 130 ± 1 to 116 ± 1 ka; Stirling et al., 1998) than now. This suggests significant reduction in the size of the Greenland and possibly Antarctic Ice Sheets (see Section 6.4.3). The climate of the LIG has been inferred to be warmer than present (Kukla et al., 2002), although the evidence is regional and not necessarily synchronous globally, consistent with understanding of the primary forcing. For the first half of this interglacial (~130–123 ka), orbital forcing (Box 6.1) produced a large increase in NH summer insolation. Proxy data indicate warmer-than-present coastal waters in parts of the Pacific, Atlantic, and Indian Oceans as well as in the Mediterranean Sea, greatly reduced sea ice in the coastal waters around Alaska, extension of boreal forest into areas now occupied by tundra in interior Alaska and Siberia and a generally warmer Arctic (Brigham-Grette and Hopkins, 1995; Lozhkin and Anderson, 1995; Muhs et al., 2001, CAPE Last Interglacial Project Members, 2006). Ice core data indicate a large response over Greenland and Antarctica with early LIG temperatures 3°C to 5°C warmer than present (Watanabe et al., 2003; NGRIP, 2004; Landais et al., 2006). Palaeofauna evidence from New Zealand indicates LIG warmth during the late LIG consistent with the latitudinal dependence of orbital forcing (Marra, 2003).

There are AOGCM simulations available for the LIG, but no standardised intercomparison simulations have been performed. When forced with orbital forcing of 130 to 125 ka (Box 6.1), with over 10% more summer insolation in the NH than today, AOGCMs produce a summer arctic warming of up to 5°C, with greatest warming over Eurasia and in the Baffin Island/northern Greenland region (Figure 6.6) (Montoya et al., 2000; Kaspar et al., 2005; Otto-Bliesner et al., 2006a). Simulations generally match proxy reconstructions of the maximum arctic summer warmth (Kaspar and Cubasch, 2006; CAPE Last Interglacial Project Members, 2006) although may still underestimate warmth in Siberia because vegetation feedbacks are not included in current simulations. Simulated LIG annual average global temperature is not notably higher than present, consistent with the orbital forcing.

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).