Ice Sheets

Land ice contained in the world's glaciers and ice sheets contributes directly to sea level rise through melt or the flow of ice into the sea (Figure 7.4). In contrast, when sea ice, which is already floating on the ocean surface, melts, it contributes only a negligible amount to sea level rise (Jenkins and Holland, 2007; Noerdlinger and Brower,

FIGURE 7.4 Outlet glaciers in Northwest Greenland. SOURCE: Photo by K. Steffen.

2007). If all of the water currently stored as ice on land surfaces around the world were to melt, sea levels would rise up to 230 feet (70 meters; Bamber et al., 2001; Lythe and Vaughan, 2001). It is important to note, however, that the estimated time scale for complete melting of the major ice sheets is on the order of hundreds to thousands of years (Gregory et al., 2004; Lambeck and Chappell, 2001; Meehl et al., 2007a).

The major ice sheets of Greenland and Antarctica contain the equivalent of 23 and 197 feet (7 and 60 meters) of sea level rise, respectively. Recent observations of these ice sheets have revealed that not only are they shrinking (e.g., Lemke et al., 2007), but their rate of loss may have increased over the last decade (Lemke, et al., 2007; Rignot et al., 2004, 2008; Thomas et al., 2006; Velicogna, 2009). In Greenland, ice sheet melt has increased 30 percent over the past 30 years (Mote, 2007). In both Greenland and Antarctica, many outlet glaciers are accelerating their seaward flow, hastening the delivery of ice to the surrounding seas (Howat et al., 2007; Rignot and Kanagaratnam, 2006; Rignot et al., 2008). In many cases, when an outlet glacier reaches the sea, a large floating portion extends into the surrounding water, forming long, thin ice tongues or larger, thicker ice shelves that buttress the outlet glacier and restrain some of its discharge. Many of these ice shelves and ice tongues have retreated, thinned, and weakened—and in some cases, collapsed suddenly as seen in Figure 7.5—which has allowed the glaciers that discharge into the surrounding bodies of water to flow much more rapidly (Rignot and Kanagaratnam, 2006; Rignot et al., 2004, 2008; Scambos et al., 2004).

The implications of the loss of floating ice are particularly significant in West Antarctica, where ice shelves are enormous and much of the ice rests on a soft, deformable bed of rock that lies below sea level. The disappearance of Antarctic ice shelves and the retreat of the ice sheet at the continent's margins would allow the surrounding sea water to flow into the ice-bedrock interface, eroding the ice further from underneath and enhancing its discharge. The time scales of these processes are not well known, but, with the equivalent of 11 feet (3.3 meters) of sea level stored in the West Antarctic Ice Sheet (Bamber et al., 2009), this potential instability is of great importance to future sea level rise.

Two mechanisms contribute to the accelerating ice sheet loss to the ocean: (1) increased surface melt (in Greenland) and the associated lubrication of the ice-bedrock interface by surface meltwater during summer, and (2) increased calving processes and thinning at the glacial termini induced by a warming ocean, which in turn leads to faster ice flow and thinning upstream. For the Greenland ice sheet, increased surface melting is associated with earlier onset and longer length of the melt season (Mote, 2007). In addition to the increase in melt runoff, meltwater from the ice sheet surface

FIGURE 7.5 Larsen-B Ice Shelf (left) January 31,2002, and (right) March 17, 2002. The 2,018-mile (3,250km) section of ice shelf, estimated to be over 10,000 years old and 650 feet (200 meters) thick, disintegrated in 6 weeks. White areas correspond to the ice shelf and glaciers on the Antarctic Peninsula, and dark blue/black indicates ocean. The light blue streaks (left panel) correspond to melt ponds on the ice; the larger areas of light blue (right panel) indicate where the ice shelf has collapsed and formed icebergs. Some of the glaciers that fed the ice shelf accelerated by eightfold within months of the collapse. SOURCE: MODIS imagery courtesy of NASA and the National Snow and Ice Data Center.

FIGURE 7.5 Larsen-B Ice Shelf (left) January 31,2002, and (right) March 17, 2002. The 2,018-mile (3,250km) section of ice shelf, estimated to be over 10,000 years old and 650 feet (200 meters) thick, disintegrated in 6 weeks. White areas correspond to the ice shelf and glaciers on the Antarctic Peninsula, and dark blue/black indicates ocean. The light blue streaks (left panel) correspond to melt ponds on the ice; the larger areas of light blue (right panel) indicate where the ice shelf has collapsed and formed icebergs. Some of the glaciers that fed the ice shelf accelerated by eightfold within months of the collapse. SOURCE: MODIS imagery courtesy of NASA and the National Snow and Ice Data Center.

can penetrate through crevasses or tunnels in the ice (moulins) to the bed, where it can lubricate the ice-bedrock interface, causing a summertime acceleration of glacier flow (Joughin et al., 2008; Zwally et al., 2002). This summer acceleration hastens the flow of ice toward the edges of the ice sheet, where it can melt or calve more rapidly. Recent paleoclimate reconstructions and modeling studies indicate that human GHG emissions have elevated Arctic air temperatures in recent decades by 2.5°F (1.4°C) above those expected from natural climate cycles (Kaufman et al., 2009), meaning that continued surface melting and melting of outlet glacier floating ice tongues can be expected.

Recent analysis of ICESat altimetry data (Pritchard et al., 2009) reveal that ice sheet thinning is mainly confined to the margins for both the Greenland and Antarctic ice sheets. This observation can be ascribed to ocean-driven melting, a mechanism supported by the recent discovery of a warming ocean around Greenland that appears to be contributing to year-round calving into the ocean (Hanna et al., 2009; Holland et al., 2008; Rignot et al., 2010; Straneo et al., 2010). An analysis of time-dependent changes in ice flow rates (Joughin et al., 2008) also suggests that ice-ocean interactions tend to dominate coastal ice losses. Numerical modeling (Nick et al., 2009) further supports this conclusion and suggests that tidewater outlet glaciers adjust rapidly to changing boundary conditions at the calving terminus. Expanded monitoring of both air and sea temperatures at high latitudes and an improved understanding of ice sheet dynamics will be needed to improve scientific knowledge of these processes.

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