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

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160 The U.S. Climate Science Program Observations during the past several decades document substantial retreat and thinning of the Arctic sea ice cover: retreat is accelerating, and it is expected to continue. 6.1 INTRODUCTION The most defining feature of the surface of the Arctic oceAn* and adjacent seas is its sea ice cover, which waxes and wanes with the seasons, and which also changes in extent and thickness on interannual and longer time scales. These changes in ice cover are related to climate, notably temperature changes (e.g., Smith et al., 2003), and themselves affect atmospheric and hydrographic conditions in high latitudes (Kinnard et al., 2008; Steele et al., 2008). Observations during the past several decades document substantial retreat and thinning of the Arctic sea ice cover: retreat is accelerating, and it is expected to continue. The Arctic oceAn may become seasonally ice free as early as 2040 (Holland et al., 2006a; Comiso et al., 2008; Stroeve et al., 2008). A reduction in sea ice will promote Arctic warming through a feedback mechanism between ice and its reflectivity (the ice-albedo feedback mechanism), and this reduction will thus influence weather systems in the northern high and perhaps middle latitudes. Changes in ice cover and freshwater flux out of the Arctic oceAn will also affect oceanic circulation of the north AtlAntic, which has profound influence on climate in Europe and North America (Seager et al., 2002; Holland et al., 2006b). Furthermore, continued retreat of sea ice will accelerate coastal erosion owing to increased wave action. Ice loss will modify the Arctic oceAn food web and its large predators, such as polar bears and seals, that depend on the ice cover. These changes, in turn, will affect indigenous human populations that harvest such species. All of these possibilities make it important to know how fast Arctic ice will diminish and the consequences of that reduction, a task that requires thorough understanding of the natural variability of ice cover in the recent and longer term past. *For bold terms, refer to Glossary; for italic terms, refer to Plate 1; for geologic ages, refer to Plate 2. Chapter 6 6.2 BACKGROUND OF ARCTIC SEA-ICE COVER 6.2.1 Ice Extent, Thickness, Drift, and Duration Arctic sea ice cover grows to its maximum extent by the end of winter and shrinks to a minimum in September. For the period of reliable satellite observations (1979–2007), extremes in Northern Hemisphere ice extent are 16.44×106 square kilometers (km 2) for March 1979 and 4.28×106 km 2 for September 2007 (http://nsidc.org/data/seaice_index/; Stroeve et al., 2008). Ice extent is defined as the region of the ocean of which at least 15% is covered by ice. The ice cover can be broadly divided into a perennial ice zone where ice is present throughout the year and a seasonal ice zone where ice is present only seasonally. A considerable fraction of Arctic sea ice is perennial, which differs strongly from Antarctic sea ice which is nearly all seasonal. Ice concentrations in the perennial ice zone typically exceed 97% in winter but fall to 85–95% in summer. Sea ice concentrations in the seasonal ice zone are highly variable, and in general (but not always) they decrease toward the southern sea ice margin. The thickness of sea ice, which varies markedly in both space and time, can be described by a probability distribution. For the Arctic oceAn as a whole, the peak of this distribution (as thick as the ice ever gets) is typically cited at about 3 meters (m) (Serreze et al., 2007b), but growing evidence (discussed below) suggests that during recent decades not only is the area of sea ice shrinking, but that it is also thinning substantially. Although many different types of sea ice can be defined, the two basic categories are first-year ice, which represents a single year’s growth, and multiyear ice, which has survived one or more melt seasons. Undeformed first-year ice typically is as much as 2 m thick. Although in general multiyear ice is thicker (greater than 2 m), first-year ice that is locally pushed into ridges can be as thick as 20–30 m. Under the inf luence of winds and ocean currents, the Arctic sea ice cover is in nearly constant motion. The large-scale circulation principally consists of the Beaufort Gyre, a mean annual clockwise motion in the western Arctic oceAn with a drift speed of 1–3 centimeters per
second, and the Transpolar Drift, the movement of ice from the coast of SiberiA east across the pole and into the north AtlAntic by way of FrAM StrAit, which lies between northern GreenlAnd and SvAlbArd. Ice velocities in the Transpolar Drift increase toward FrAM StrAit, where the mean drift speed is 5–20 centimeters per second (Figure 6.1) (Thorndike, 1986; Gow and Tucker, 1987). About 20% of the total ice area of the Arctic oceAn is discharged each year through FrAM StrAit, the majority of which is multiyear ice. This ice subsequently melts in the northern north AtlAntic, and since the ice is relatively fresh compared with sea water, this melting adds freshwater to the ocean in those regions. PACIFIC OCEAN Alaska CANADA BS Past Climate Variability and Change in the Arctic and at High Latitudes SIBERIA Beaufort Gyre Transpolar Drift 6.2.2 Influences on the Climate System Seasonal changes in the amount of heat at the surface (net surface heat flux) associated with sea ice modulate the exchange and transport of energy in the atmosphere. Ice, as sheets or as sea ice, reflects a certain percentage of incoming solar radiation back into the atmosphere. The albedo (reflectivity) of ice cover ranges from 80% when it is freshly snow covered to around 50% during the summer melt season (but lower in areas of ponded ice). This high reflectivity contrasts with the dark ocean sur- FS GREENLAND ATLANTIC OCEAN ASIA EUROPE Figure 6.1. Overview map of the Arctic and adjacent regions showing the extent of sea ice: magenta and blue lines = September and March medians for 1979–2000, respectively; white field = September 2007 extent. Major circulation systems are schematically shown by green arrows. BS = Bering Strait, FS = Fram Strait. (Courtesy National Snow and Ice Data Center, Boulder, CO.) 161
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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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100 The U.S. Climate Science Progra
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The U.S. Climate Science Program Ch
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Page 169: CHAPTER 6 ABSTRACT Past Climate Var
Page 173 and 174: EXTENT, IN MILLION SQUARE KILOMETER
Page 175 and 176: Until recently, and for logistical
Page 177 and 178: 1990; Fyles et al., 1994), the nort
Page 179 and 180: FREQUENCY 50 45 40 35 30 25 20 15 1
Page 181 and 182: Seas around icelAnd provide a rare
Page 183 and 184: at least 14 Ma (Darby, 2008). Sever
Page 185 and 186: T. quinqueloba, IN SPECIMENS PER GR
Page 187 and 188: 7 6 5 4 3 2 1 Past Climate Variabil
Page 189 and 190: SEA-ICE COVERAGE, IN NUMBER OF MONT
Page 191 and 192: QUARTZ WEIGHT, IN PERCENT 6 5 4 3 2
Page 193: 6.5 SUMMARY Geological data indicat
Page 201 and 202: GLOSSARY GLOSSARY 8 ka cold event A
Page 203 and 204: CAPE Project Circum-Arctic PaleoEnv
Page 205 and 206: Eccentricity Out of roundness (elli
Page 207 and 208: Innuitian sector The ice sheet that
Page 209 and 210: Moraine Landforms (typically ridges
Page 211 and 212: Positive feedback In climate studie
Page 213: Trough-mouth fans Undersea deposits
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Past Climate Variability and Change in the Arctic and at High Latitudes

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