Figure 27.1 Basal melt rates, B, seaward of Antarctic (hollow circle) and Greenland (solid circle) grounding lines, versus thermal forcing, AT, from the ocean, which is the difference between the nearest in situ ocean temperature data and the seawater freezing point at a depth of 0.88 times the maximum grounding line ice thickness. The regression indicates that a 1°C increase in effective ocean temperature increases melt rate by 10myr-1. PIG, Pine Island; THW, Thwaites; SMI, Smith; KOH, Kohler; DVQ, DeVicq; LAN, Land; BYR, Byrd; DAV, David; NIN, Ninnis; MER, Mertz; TOT, Totten; DEN, Denman; SCO, Scott; LAM, Lambert; SHI, Shirase; JUT, Jutulstraumen; STA, Stancomb-Wills; SLE, Slessor; REC, Recovery; INS, Institute; RUT, Rutford; CAR, Carlson; EVA, Evans; OST, Ostenfeld; PET, Petermann; RYD, Ryder; ZAC, Zachariae Isstrom; 79N, 79 north glaciers.
increases in accumulation, surface melt and bottom melting, the large increase in bottom melting is the dominant factor in the long-term response of the Antarctic Ice Sheet (Warner & Budd, 1998; Huybrechts & De Wolde, 1999).
Unlike melting under a grounded ice sheet, processes beneath floating glaciers are governed by the transport of ocean heat and by the seawater freezing temperature dependence on pressure (Robin, 1979). This allows sensible heat to be obtained from the cold dense shelf waters resulting from sea-ice formation, as well as warm deep water that intrudes onto the continental shelf and flows into ice-shelf cavities. Bottom melting freshens and cools the seawater, adding buoyancy that drives upwelling as the ice shoals seaward. In some regions, the rising seawater-meltwater mixture drops below the in situ freezing point to form marine ice that can comprise a substantial part of ice-shelf volume (Oerter et al., 1992). Area-average melt rates for the large ice shelves are about 40cmyr-1 (Jacobs et al., 1992). Much of the actual melting occurs in the deepest parts of the subice-shelf cavities (Plate 27.1), however, where direct measurements are difficult to obtain.
Basal melt rates near grounding zones, calculated using remote sensing observations of Antarctic and Greenland glacier velocity and thickness distribution, and applying principles of mass conservation, range from less than 4myr-1 for several glaciers that flow into the Filchner-Ronne Ice Shelves to greater than 40myr-1 for Pine Island Glacier (Fig. 27.1). The wide range is consistent with earlier studies of several of the individual glaciers, using a variety of techniques (Potter & Paren, 1985; Smith, 1996; Rignot, 1998; Lambrecht et al., 1999; Rignot & Jacobs, 2002), and stems from differences in grounding line drafts, seawater temperatures, and ice topographies and velocities. Nevertheless, most of the melt rates calculated near grounding lines exceed the area-average rates for the largest ice shelves by one to two orders of magnitude.
The largest thermal forcing in the Antarctic, 4°C above the in situ melting point, is associated with Pine Island, Thwaites and Smith Glaciers that flow into the Amundsen Sea. This results from the nearly unaltered Circumpolar Deep Water that extends southward across the floor of the Antarctic continental shelf in Pine Island Bay (Jacobs et al., 1996). Modified Circumpolar Deep Water is known to upwell at other locations along the East Antarctic continental shelf, e.g. Shirase Glacier, and will increase grounding-line melt rates where its density allows it to intrude along the sea floor.
The potential impact of basal melting on short-term (<100yr) ice-sheet stability is greatest in regions where deep water has direct access to glacier grounding lines. Ocean temperatures seaward of Antarctica's continental shelf break have risen about 0.2°C over recent decades, sufficient to increase basal melting by 2myr-1 where that change has reached vulnerable grounding lines. This may account for the rapid thinning of ice shelves in the western Amundsen Sea (Rignot, 1998), which would explain the observed acceleration of their nourishing glaciers (Rignot et al., 2002) and the resulting negative mass balance of the entire basin (Rignot et al., in press).
Submarine melting is also significant near the terminus of tidewater glaciers, which control the mass budget of coastal Alaska, Patagonia and Greenland, with summer rates reaching 12 m day-1 at the terminus of LeConte Glacier, Alaska, or 60% of the estimated mass loss at the front (Motyka et al., 2003). High bottom melting could explain the high correlation between calving rate and water depth because the subglacial area in contact with the ocean increases in deeper waters. Even in the absence of large floating sections, a warmer ocean may therefore have a major influence on the evolution of glaciers and ice streams.
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