Past Climate Variability and Change in the Arctic and at High Latitudes

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72 The U.S. Climate Science Program Chapter 3 Landforms and fossils from the western Arctic and Bering Strait indicate vastly reduced sea ice during MIS 5. 3.4.6b marinE rEcords of marinE isotoPE stagE 5E Low sedimentation rates in the central Arctic oceAn and the rare preservation of carbonate fossils limit the number of sites at which MIS 5e can be reliably identified in sediment cores. MIS 5e sediments from the central Arctic oceAn usually contain high concentrations of planktonic (surface-dwelling) foraminifers and coccoliths, which indicate a reduction in summer sea-ice coverage that permitted increased biological productivity (Gard, 1993; Spielhagen et al., 1997, 2004; Jakobsson et al., 2000; Backman et al., 2004; Polyak et al., 2004; Nørgaard- Pedersen et al., 2007a,b). However, occasional dissolution of carbonate fossils complicates the interpretation of microfossil concentrations. Also, marine sediments from MIS 5a, slightly younger and cooler than MIS 5e, sometimes have higher microfossil concentrations than do MIS 5e sediments (Gard, 1986, 1987). Arctic oceAn sediment cores recently recovered from the loMonoSov ridGe, north of GreenlAnd, have revived the discussion of MIS 5e conditions in the Arctic oceAn. Unusually high concentrations of a subpolar foraminifer species, one which usually dwells in waters with temperatures well above freezing, were found in MIS 5e zones and interpreted to indicate warm interglacial conditions and much reduced sea-ice cover in the interior Arctic oceAn (Nørgaard-Pedersen et al., 2007a,b). Interpretation of these and other microfossils is complicated by the strong vertical stratification in the Arctic oceAn; today, warm Atlantic water (temperatures greater than 1°C) is in most areas isolated from the atmosphere by a relatively thin layer of cold (less than 1°C) fresher water; this cold water limits the transfer of heat to the atmosphere. It is not always possible to determine whether warm-water foraminifers found in marine sediment from the Arctic oceAn lived in warm waters that remained isolated from the atmosphere below the cold surface layer, or whether the warm Atlantic water had displaced the cold surface layer and was interacting with the atmosphere and affecting its energy balance. Landforms and fossils from the western Arctic and berinG StrAit indicate vastly reduced sea ice during MIS 5e (Figure 3.30). The winter sea-ice limit is estimated to have been as much as 800 km farther north than its average 20th- century position, and summer sea ice was likely to have been much reduced relative to present (Brigham-Grette and Hopkins, 1995). These reconstructions are consistent with the northward migration of treeline by hundreds of kilometers throughout much of AlASkA and nearby chukotkA and with the elimination of tundra from chukotkA to the Arctic oceAn coast (Lozhkin and Anderson, 1995). Sufficient data are not yet available to allow unambiguous reconstruction of MIS 5e conditions in the central Arctic oceAn. Key uncertainties are related to the extent and duration of Arctic oceAn sea ice. The vertical structure of the upper 500 m of the water column is also climatically important but poorly known, in particular whether the strong vertical stratification characteristic of the modern regime persisted throughout MIS 5e, or whether reduced sea ice and changes in the hydrologic cycle and winds destabilized this stratification and allowed Atlantic water to reside at the surface in larger areas of the Arctic oceAn. 3.4.7 Marine Isotope Stage 3 Warm Intervals The temperature and precipitation history of MIS 3 (about 70–30 ka) is difficult to reconstruct because of the paucity of continuous records and the difficulty in providing a secure time frame. The δ 18O record of temperature change over the GreenlAnd ice Sheet and other icecore data show that the north AtlAntic region experienced repeated episodes of rapid, highmagnitude climate change, that temperatures rapidly increased by as much as 15°C (reviewed by Alley, 2007 and references therein), and that each warm period lasted several hundred to a few thousand years. These brief climate excursions are found not only in the GreenlAnd ice Sheet but are also recorded in cave sediments in China (Wang et al., 2001; Dykoski, et al., 2005) and in high-resolution marine records off California (Behl and Kennett, 1996), and in the Caribbean Sea’s Cariaco Basin (Hughen et al., 1996.), the Arabian Sea (Schulz et al., 1998) and the Sea of Okhotsk (Nürnberg and Tiedmann, 2004), among many other sites. The ice-core records from GreenlAnd contain indications of climate change in many regions on the same time scale (for example, the methane trapped in ice-core bubbles was in part produced in tropi-
During 5e, Treeline migration of ~600 km northward; in many areas tundra eliminated from the arctic coast in NE Siberia Lozhkin and Anderson, 1995 QR During 5e, winter sea ice limit ~800 km north of present, Bering Sea ice free year around; Arctic Ocean nearly ice free some summers Brigham-Grette and Hopkins, 1995 QR cal wetlands and was essentially all produced beyond the GreenlAnd ice Sheet; Severinghaus et al., 1998). These ice-core records demonstrate clearly that the climate-change events were synchronous throughout widespread areas, and that the ages of events from many regions agree within the stated uncertainties. These events were thus hemispheric to global in nature (see review by Alley, 2007) and are considered a sign of large-scale coupling between the ocean and the atmosphere (Bard, 2002). The cause of these events is still debated. However, Broecker and Hemming (2001) and Bard (2002) among others suggested that they were likely the result of major and abrupt reorganizations of the ocean’s thermohaline circulation, probably related to ice sheet instabilities that introduced large quantities of fresh water into the north AtlAntic (Alley, 2007). Such large and abrupt oscillations, which were linked to changes in north AtlAntic surface conditions and probably to the large-scale oceanic circulation, persisted into the Holocene (MIS 1); the youngest was Past Climate Variability and Change in the Arctic and at High Latitudes Winter sea ice today Winter sea ice max 5e 0 250 500 KILOMETERS Beringia today Maps created by Bill Manley Figure 3.30. Winter sea-ice limit during MIS 5e and at present. Fossiliferous paleoshorelines and marine sediments were used by Brigham-Grette and Hopkins (1995) to evaluate the seasonality of coastal sea ice on both sides of the Bering Strait during the last interglaciation. Winter sea limit is estimated to have been north of the narrowest section of the strait, 800 km north of modern limits. Pollen data derived from Last Interglacial lake sediments suggest that tundra was nearly eliminated from the Russian coast at this time (Lozhkin and Anderson, 1995). In Chukokta during the warm interglaciation, additional open water favored some taxa tolerant of deeper winter snows. (Base map by William Manley, http://instaar.colorado.edu/QGISL/.) only about 8.2 ka (Alley and Ágústdóttir, 2005). However, it appears that the abrupt 8.2 ka cooling was linked to an ice-age cause, a catastrophic flood from a very large lake that had been dammed by the melting lAurentide ice Sheet. Within MIS 3, land ice was somewhat reduced compared with the colder times of MIS 2 and MIS 4, but Arctic temperatures generally were much lower and ice more extensive than in MIS 1 (with certain exceptions). Sea level was lower at that time, the coastline was well offshore in many places, and the increased continentality very likely contributed to warmer summer temperatures that presumably were offset by colder winter temperatures. For example, on the new SiberiAn iSlAndS in the eASt SiberiAn SeA, Andreev et al. (2001) documented the existence of graminoid-rich tundra thought to have covered wide areas of the emergent shelf while summer temperatures were perhaps as much as 2°C warmer than during the These ice-core records demonstrate clearly that the climate-change events were synchronous throughout widespread areas, and that the ages of events from many regions agree within the stated uncertainties. 73
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Past Climate Variability and Change
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Past Climate Variability and Change
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AUTHOR TEAMS FOR THIS REPORT Execut
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ACKNOWLEDGMENTS Many individuals ma
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TABLE OF CONTENTS Executive Summary
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2 The U.S. Climate Change Science P
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4 The U.S. Climate Change Science P
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6 The U.S. Climate Science Program
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122 The U.S. Climate Science Progra
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CHAPTER 6 ABSTRACT Past Climate Var
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second, and the Transpolar Drift, t
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EXTENT, IN MILLION SQUARE KILOMETER
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Until recently, and for logistical
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1990; Fyles et al., 1994), the nort
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FREQUENCY 50 45 40 35 30 25 20 15 1
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Seas around icelAnd provide a rare
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at least 14 Ma (Darby, 2008). Sever
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T. quinqueloba, IN SPECIMENS PER GR
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7 6 5 4 3 2 1 Past Climate Variabil
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SEA-ICE COVERAGE, IN NUMBER OF MONT
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QUARTZ WEIGHT, IN PERCENT 6 5 4 3 2
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6.5 SUMMARY Geological data indicat
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GLOSSARY GLOSSARY 8 ka cold event A
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CAPE Project Circum-Arctic PaleoEnv
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Eccentricity Out of roundness (elli
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Innuitian sector The ice sheet that
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Moraine Landforms (typically ridges
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Positive feedback In climate studie
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Trough-mouth fans Undersea deposits
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The U.S. Climate Change Science Pro
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