FIGURE 7.5 Larsen-B Ice Shelf (left) January 31, 2002, and (right) March 17, 2002. The 2,018-mile (3,250-km) 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

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