ARTICLE IN PRESS Quaternary Science Reviews 25 (2006) 1383–1400 Last Interglacial Arctic warmth confirms polar amplification of climate change CAPE-Last Interglacial Project MembersÃ,1 Received 15 June 2005; accepted 23 January 2006 Abstract The warmest millennia of at least the past 250,000 years occurred during the Last Interglaciation, when global ice volumes were similar to or smaller than today and systematic variations in Earth’s orbital parameters aligned to produce a strong positive summer insolation anomaly throughout the Northern Hemisphere. The average insolation during the key summer months (M, J, J) was ca 11% above present across the Northern Hemisphere between 130,000 and 127,000 years ago, with a slightly greater anomaly, 13%, over the Arctic. Greater summer insolation, early penultimate deglaciation, and intensification of the North Atlantic Drift, combined to reduce Arctic Ocean sea ice, allow expansion of boreal forest to the Arctic Ocean shore across vast regions, reduce permafrost, and melt almost all glaciers in the Northern Hemisphere. Insolation, amplified by key boundary condition feedbacks, collectively produced Last Interglacial summer temperature anomalies 4–5 1C above present over most Arctic lands, significantly above the average Northern Hemisphere anomaly. The Last Interglaciation demonstrates the strength of positive feedbacks on Arctic warming and provides a potentially conservative analogue for anticipated future greenhouse warming. r 2006 Elsevier Ltd. All rights reserved. 1. Introduction Here we present quantitative estimates of circum-Arctic LIG summer air and sea-surface temperatures recon- Planetary warm times in the recent geological past structed from proxy records preserved in terrestrial and inform the debate over Earth’s response to the continuing marine archives. These reconstructions demonstrate that build up of radiatively active atmospheric trace gases. Arctic summer air temperatures averaged ca 4–5 1C above Strong positive feedbacks in the Arctic are expected to present for most of the Arctic, well above the planetary amplify future greenhouse warming (Holland and Bitz, LIG average, although with coherent spatial patterns in the 2003), but the cumulative effect of these feedbacks remains magnitude of warmth. Arctic summers were warm enough debated (Serreze and Francis, 2006). Past planetary warm to melt all glaciers below 5 km elevation except the times provide a testing ground for the debate over polar Greenland Ice Sheet, which was reduced by ca 20–50% amplification of climate change. During the Last Inter- (Cuffey and Marshall, 2000; Otto-Bliesner et al., 2006). In glaciation (LIG) Earth was warmer than at any time in at addition, the margins of permanent Arctic Ocean sea ice least the past 250,000 years, with global temperatures retracted well into the Arctic Ocean basin and boreal 0–2 1C above present (e.g., CLIMAP Project Members, forests advanced to the Arctic Ocean coast across vast 1984; Bauch and Erlenkeuser, 2003). Circum-Arctic regions of the Arctic currently occupied by tundra, warming is recognized to have been substantial during although the central Arctic Ocean basin remained covered the LIG (Lauritzen and Anderson, 1995), but quantitative by permanent sea ice (Spielhagen et al., 2004). These reconstructions for this region are lacking. boundary condition changes reduced Earth’s albedo and altered the exchange of heat, moisture and trace gases ÃCorresponding author. G.H. Miller. Tel.: +1 303 492 6962. between the land, ocean and atmosphere, collectively E-mail address: [email protected]. amplifying insolation-driven (Fig. 1) circum-Arctic inter- 1A full list of authors appears in Appendix A. glacial warmth. 0277-3791/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.quascirev.2006.01.033 ARTICLE IN PRESS 1384 CAPE-Last Interglacial Project Members / Quaternary Science Reviews 25 (2006) 1383–1400 sea level reached present, and certainly was over long before sea level fell below present ca 116 ka (Zagwijn, 1996; Grøsfjeld et al., 1996). Despite these modest temporal differences, sea level provides a powerful tool for correlat- ing the LIG across much of the globe, and associated coral reefs offer the most secure dating of the LIG. The LIG has received less attention in the Arctic than at mid-latitudes, largely because of difficult access and paucity of sites. A decade ago, a circum-Arctic summary confirmed that the LIG was warmer than present (Lauritzen and Anderson, 1995), although quantitative estimates of warmth were not then possible. In the intervening decade, many new sites have been described from both marine and terrestrial archives, systematic recovery of sea-floor sediment from the Arctic Ocean has provided new evidence for the state of the polar ocean during the LIG, and new training sets, coupled to improved statistical tools, allow more precise quantifica- tion of many climate proxies (ter Braak and Juggins, 1993; Birks, 1998). For many Arctic regions, reliable transfer functions for summer temperatures are now available for Fig. 1. Monthly insolation anomalies (deviations from the present) for the pollen, chironomids, diatoms, dinoflagellates, planktonic time periods from 25 ka to present (upper panel) and from 140 to 115 ka and benthonic foraminifera, alkenones, and several ice- (lower panel) at 651N, showing the larger insolation anomaly for the Last core parameters. Consequently, it is now possible to Interglaciation relative to the present interglaciation. Anomalies are expressed as W mÀ2; time is in thousands of years before present. describe quantitatively at least some aspects of the LIG climate in the Arctic. 2. The Last Interglaciation 3. Methods The LIG was recognized more than a century ago in 3.1. Dating the Last Interglaciation northern Europe (Harting, 1875). Subsequently, it was defined from pollen records as the Eemian, a time of We capitalize on the uniqueness of the Late Quaternary deciduous forests bounded by periods of tundra (Jessen climate record and recent advances in geochronology to and Milthers, 1928). A half-century later, the Eemian was define LIG sites with reasonable certainty. Arctic sites with correlated with marine isotope stage (MIS) 5 (Emiliani, continuous accumulation since the LIG demonstrate that 1955), and eventually with the much shorter MIS 5e only the LIG shows evidence of summer temperatures (Shackleton, 1969; Mangerud et al., 1979; Shackleton et al., comparable to, or higher than the Holocene. Materials 2002). The LIG was securely placed in an absolute time suitable for high-precision U/Th analyses are rare in the frame when correlated to coral terraces on stable platforms Arctic, but advances in trapped-charge dating (optically that were amenable to high-precision uranium-series dating stimulated luminescence, infra-red-stimulated lumines- (Broecker and Van Donk, 1970; Gallup et al., 2002). We cence, thermoluminescence) provide independent verifica- define the LIG temporally as the penultimate interval of tion of LIG ages for many sites, although the dates often minimum global ice volume, when sea level was at or above lack sufficient precision to subdivide the interglaciation present. It is now widely accepted that sea level reached (Berger and Anderson, 2000; Murray and Funder, 2003). modern levels 13072 ka (thousands of years ago), and Diagnostic tephra, especially in Alaska and the Nordic remained at or above this level until the inception of the Seas, provide additional temporal constraint (Haflidason last glaciation, as ice grew on the continents ca 116 ka et al., 2000; Bege´ t and Keskinen, 2003; Rasmussen et al., (Stirling et al., 1998; Henderson and Slowey, 2000; 2003a, b). LIG sites (Tables 1 and 2) are identified with Mcculloch and Esat, 2000; Gallup et al., 2002; Muhs moderate to high certainty by a combination of strati- et al., 2002). Although we use the term LIG as broadly graphic position, climate proxies, absolute dating, and correlative with both the Eemian and MIS 5e, we recognize relationship to stratigraphic marker horizons (e.g. diag- that these intervals are not precisely correlative. The first nostic tephra). half of the LIG coincides with an unusually strong Northern Hemisphere positive summer insolation anom- 3.2. Quantifying Last Interglacial summer temperatures aly, whereas summer insolation receipts are relatively low through the later half of the marine highstand (Fig. 1). At high northern latitudes, summer temperatures exert Peak Eemian warmth may have occurred slightly before the dominant control on glacier mass balance, unless ARTICLE IN PRESS CAPE-Last Interglacial Project Members / Quaternary Science Reviews 25 (2006) 1383–1400 1385 Table 1 Published sites with quantitative summer temperature estimates for peak LIG warmth from terrestrial sites around the circum-Arctic. Details of site location and reconstruction techniques are provided in the references cited. Some key cites with important qualitative LIG summer or winter temperature or precipitation are also included Proxy DT ( 1C) DT (1C) Ppt Reference Comments Summer Winter Scandinavia Finland Treeline position Warmer (Saarnisto et al., 1999) Boreal forest north of reconstructed from present limit; birch at pollen Arctic Ocean shore. Russia European Russia Pollen, plant 4(Devyatova, 1982) Boreal forest north of macrofossils (Grichuk, 1984) present limit. West central Siberia Pollen, plant 6 to 8 5 to 7 Wetter (Gudina et al., 1983) Boreal forest 800 km macrofossils (Grichuk, 1984) north of present