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

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80 The U.S. Climate Science Program Chapter 3 The most consistent records of an Arctic Medieval Climate Anomaly come from the North Atlantic sector of the Arctic. Warmth during the Medieval interval is generally ascribed to lack of explosive volcanoes that produce particles that block the Sun and perhaps to greater brightness of the Sun. Additional evidence of Neoglacial seasonal cooling comes from several localities: a reduction in melt layers in the AGASSiz ice cap (Koerner and Fisher, 1990) and in GreenlAnd (Alley and Anandakrishnan, 1995); the decrease in δ 18O values in ice cores such as those from the devon iSlAnd (Fisher, 1979) and GreenlAnd (Johnsen et al., 1992) and indications of cooling from borehole thermometry (Cuffey et al., 1995); the retreat of large marine mammals and warm-water-dependent mollusks from the cAnAdiAn Arctic (Dyke and Savelle, 2001); the southward migration of the northern treeline across central cAnAdA (MacDonald et al., 1993), Eurasia (MacDonald et al., 2000b), and Scandinavia (Barnekow and Sandgren, 2001); the expansion of sea-ice cover along the shores of the Arctic oceAn on elleSMere iSlAnd (Bradley, 1990), in bAFFin bAY (Levac et al., 2001), and in the berinG SeA (Cockford and Frederick, 2007); and the shift in vegetation communities inferred from plant macrofossils and pollen around the Arctic (Bigelow et al., 2003). The assemblage of microfossils and the stable isotope ratios of foraminifers indicate a shift toward colder, lower salinity conditions about 5 ka along the eASt GreenlAnd ShelF (Jennings et al., 2002) and the western nordic SeAS (Koç and Jansen, 1994), suggesting increased influx of sea ice from the Arctic. Where quantitative estimates of temperature change are available, they generally indicate that summer temperature decreased by 1°–2°C during this initial phase of cooling. The general pattern of an early- to middle- Holocene Thermal Maximum followed by Neoglacial cooling forms a multi-millennial trend that, in most places, culminated in the 19th century. Superposed on the long-term cooling trend were many centennial-scale warmer and colder summer intervals, which are expressed to a varying extent and are interpreted with various levels of confidence in different proxy records. In northern Scandinavia, evidence for notable late Holocene cold intervals before the 16th century includes narrow tree rings (Grudd et al., 2002), lowered treeline (Eronen et al., 2002), and major glacier advances (Karlén, 1988) between 2.6 and 2.0 ka. An extended analysis of these many centennial-scale warmer and colder intervals in ruSSiA was published by Velichko and Nechaev (2005). 3.4.9c thE mEdiEVal climatE anomaly (mca) Probably the most oft-cited warm interval of the late Holocene is the Medieval Climate Anomaly (MCA), earlier referred to as the Medieval Warm Period (MWP). The anomaly was recognized on the basis of several lines of evidence in Western Europe, but the term is commonly applied to other regions to refer to any of the relatively warm intervals of various magnitudes and at various times between about 950 and 1200 AD (Lamb, 1977) (Figure 3.35). In the Arctic, evidence for climate variability, such as relative warmth, during this interval is based on glacier extents, marine sediments, speleothems, ice cores, borehole temperatures, tree rings, and archaeology. The most consistent records of an Arctic Medieval Climate Anomaly come from the north AtlAntic Sector of the Arctic. The summit of GreenlAnd (Dahl-Jensen et al., 1998), western GreenlAnd (Crowley and Lowery, 2000), Swedish Lapland (Grudd et al., 2002), northern SiberiA (Naurzbaev et al., 2002), and Arctic cAnAdA (Anderson et al., 2008) were all relatively warm around 1000 AD. During Medieval time, Inuit populations moved out of AlASkA into the eastern cAnAdiAn Arctic and hunted whale from skin boats in regions perennially ice-covered in the 20th century (McGhee, 2004). The evidence for Medieval warmth throughout the rest of the Arctic is less clear. However, some indications of Medieval warmth include the general retreat of glaciers in southeastern AlASkA (Reyes et al., 2006; Wiles et al., 2008) and the wider tree rings in some high-latitude tree-ring records from Asia and North America (D’Arrigo et al., 2006). However D’Arrigo et al. (2006) emphasized the uncertainties involved in estimating Medieval Climate Anomaly warmth relative to that of the 20th century, owing in part to the sparse geographic distribution of proxy data as well as to the less coherent variability of tree growth temperature estimates for this anomaly. Hughes and Diaz (1994) argued that the Arctic as a whole was not anomalously warm throughout Medieval time (also see Bradley et al., 2003b, and National Research Council, 2006). Warmth during the Medieval interval is generally ascribed to lack of explosive volcanoes that produce particles that block the Sun and perhaps to greater brightness of the Sun (Crowley, 2000; Goosse et al., 2005; also see Jansen et al., 2007). Warming around the north AtlAntic and adjacent regions may have been linked to changes in oceanic circulation as well (Broecker, 2001).
TEMPERATURE ANOMALY (°C) TEMPERATURE ANOMALY (°C) 0.8 0.6 0.4 0.2 0 –0.2 –0.4 –0.6 –0.8 –1.0 –1.2 200 400 600 800 1000 0.8 0.6 0.4 0.2 0 –0.2 –0.4 –0.6 –0.8 -1.0 CPS land with uncertainties EIV land with uncertainties EIV land + ocn with uncertainties Mann and Jones (2003) Esper et al. (2002) Moberg er al. (2005) HAD Instrumental Record CRU Instrumental Record Past Climate Variability and Change in the Arctic and at High Latitudes NORTHERN HEMISPHERE YEAR AD CPS land with uncertainties EIV land with uncertainties EIV land + ocn with uncertainties Mann and Jones (2003) Esper et al. (2002) Moberg er al. (2005) HAD Instrumental Record CRU Instrumental Record 1200 1400 1600 1800 2000 –1.2 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000 MCA YEAR AD CPS land + ocn with uncertainties Briffa et al. (2001) Crowley and Lowery (2000) Mann et al. (1999) Jones et al. (1998) Oerlemans (2005) Mann et al. (2003) Optimal Borehole Huang et al. (2000) Borehole Figure 3.35. Updated composite proxy-data reconstruction of Northern Hemisphere temperatures for most of the last 2,000 years, compared with other published reconstructions. Estimated confidence limits, 95%. All series have been smoothed with a 40-year lowpass filter. MCA = Medieval Climate Anomaly, about 950–1200 AD. The array of reconstructions demonstrates that the warming documented by instrumental data during the past few decades exceeds that of any warm interval of the past 2,000 years, including that estimated for the MCA. (Figure from Mann et al., 2008.) CPS = composite plus scale methodology; CRU = East Anglia Climate Research unit, a source of instrumental data; EIV = error-in-variables; HAD = Hadley Climate Center. [Reprinted by permission of National Academy of Sciences.] 81
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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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130 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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