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

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112 The U.S. Climate Science Program Chapter 5 The Greenland Ice Sheet contains by far the largest volume of any present-day Northern Hemisphere ice mass. More-recent trends are toward warming temperatures, increasing snowfall, and more rapidly increasing meltwater runoff. 5.1 THE GREENLAND ICE SHEET 5.1.1 Overview The GreenlAnd ice Sheet* (Figure 5.1) contains by far the largest volume of any present-day Northern Hemisphere ice mass. The ice sheet is approximately 1.7 million square kilometers (km 2) in area, extending as much as 2,200 km north to south. The maximum ice thickness is 3,367 m, its average thickness is 1,600 m (Thomas et al., 2001), and its volume is 2.9 million km 3 (Bamber et al., 2001). Some of the bedrock beneath this ice has been depressed below sea level by the weight of the ice, and a little of this bedrock would remain below sea level following removal of the ice and rebound of the bedrock (Bamber et al., 2001). However, most of the ice that rests on bedrock is above sea level and so would contribute to sea-level rise if it were melted: if the entire ice sheet melted, it is estimated that sea-level would rise about 7.3 m (Lemke et al., 2007). The ice sheet consists primarily of old snow that has been squeezed to ice under the weight of new snow that accumulates every year. Snow accumulation on the upper surface tends to increase ice-sheet size. Ice sheets lose mass primarily by melting in low-elevation regions, and by forming icebergs that break off the ice margins (calving) and drift away to melt elsewhere. Sublimation, snowdrift (Box et al., 2006), and melting or freezing at the bed beneath the ice are minor terms in the budget, although melting beneath floating extensions called ice shelves before icebergs break off may be important (see section 5.1.2, below). Estimates of net snow accumulation on the GreenlAnd ice Sheet have been presented by Hanna et al. (2005) and Box et al. (2006), among others. Hanna et al. (2005) found for 1961–1990 (an interval of moderately stable conditions before more-recent warming) that surface snow accumulation (precipitation minus *For bold terms, refer to Glossary; for italic terms, refer to Plate 1; for geologic ages, refer to Plate 2. evaporation) was about 573 gigatons per year (Gt/yr) and that 280 Gt/yr of meltwater left the ice sheet. The difference of 293 Gt/yr is similar to the estimated iceberg calving flux within broad uncertainties (Reeh, 1985; Bigg, 1999; Reeh et al., 1999). (For reference, return of 360 Gt of ice to the ocean would raise global sea level by 1 millimeter (mm); Lemke et al., 2007.) More-recent trends are toward warming temperatures, increasing snowfall, and more rapidly increasing meltwater runoff (Hanna et al., 2005; Box et al., 2006). Large interannual variability causes the statistical significance of many of these trends to be relatively low, but the independent trends exhibit internal consistency (e.g., warming is expected to increase both melting and snowfall, on the basis of modeling experiments and simple physical arguments, and both trends are observed in independent studies (Hanna et al., 2005; Box et al., 2006)). Increased iceberg calving has also been observed in response to faster flow of many outlet glaciers and shrinkage or loss of ice shelves (see section 5.1.2, below, for discussion of the parts of an ice sheet) (e.g., Rignot and Kanagaratnam, 2006; Alley et al., 2005). The Intergovernmental Panel on Climate Change (IPCC; Lemke et al., 2007) found that “Assessment of the data and techniques suggests a mass balance of the GreenlAnd ice Sheet of between +25 and –60 Gt (–0.07 to 0.17 mm) SLE (sea level equivalent) per year from 1961–2003 and –50 to –100 Gt (0.14 to 0.28 mm SLE) per year from 1993–2003, with even larger losses in 2005.” Updates are provided by Alley et al. (2007b) (Figure 5.2) and by Cazenave (2006). Rapid changes have been occurring in the ice sheet, and in the ability to observe the ice sheet, so additional updates are virtually certain to be produced. The long-term importance of these trends is uncertain—short-lived oscillation or harbinger of further shrinkage? This uncertainty motivates some of the interest in the history of the ice sheet.
160°E 170°E 180° 170°W 160°W 150°W 140°W 130°W 120°W 110°W 80°N 100°W 70°N 150°E 140°E 80°W 130°E 120°E 110°E 100°E 90°E 80°E 70°W 70°E Past Climate Variability and Change in the Arctic and at High Latitudes 60°E 50°E Figure 5.1. Satellite image (SeaWiFS) of the Greenland Ice Sheet and surroundings, from July 15, 2000 (http://www.gsfc.nasa.gov/gsfc/earth/pictures/earthpic.htm). 60°W 80°N 40°E 60°N 60°W 70°N 40°W 20°E 10°E 0° 10°W 20°W 80°N 30°W 113
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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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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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186 The U.S. Climate Science Progra
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188 The U.S. Climate Science Progra
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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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Past Climate Variability and Change in the Arctic and at High Latitudes

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