State of the ice sheets

Unlike the Arctic, which is an ocean surrounded by land, Antarctica is land surrounded by ocean. While both the Greenland and West Antarctic Ice Sheets are changing in similar ways, the changes do not match expectations (see Chapter 5 for a discussion of the changes currently happening in Greenland).

Satellite monitoring of the West Antarctic Ice Sheet (WAIS) has revealed several changes in recent decades that seem to be responses to direct interaction with the surrounding ocean, facilitated by atmospheric changes that are ultimately the result of climate warming. These observations include ice sheet thinning near the coast (measured by satellite altimetry), ice stream acceleration (measured by InSAR1), grounding line retreat (measured using photographic records from the Landsat), and increased calving of large icebergs (measured by MODIS2), all of which could signal the beginnings of disintegration (see Figure 6.1, Plate 5).

Other more direct indications that climate change is already affecting Antarctica come from the collapse of ice shelves along the Antarctic Peninsula. Because this peninsula extends northward toward the southern tip of South America, scientists expected that the effects of global warming on Antarctica would be seen first. This prediction was confirmed by the 1995 collapse of the northernmost section of the Larsen A ice shelf, followed in 2002 by the collapse of Larsen B - an ice shelf that had been in place for at least the last 12,000 years (see Figure 6.2, Plate 6). As warming continues, we should expect similar dramatic changes further south

Figure 6.1 Map of Antarctica showing annual change in height of various locations of the ice sheet see Plate 5 for color version)

Note: The areas in blue show that extensive thinning is occurring in the coastal areas of the West Antarctic Ice Sheet.

Figure 6.2 Satellite image of the Antarctic Peninsula, showing the rapid break-up of the Larsen B ice shelf over about five weeks in 2002 (see Plate 6 for color version)

Note: The ice shelf had been in place for approximately 12,000 years. Following the disintegration, grounded ice streams that had been flowing into the ice shelf accelerated substantially.

(poleward) on the Antarctic Peninsula and even later on the main body of the Antarctic continent.

Feedback mechanisms that could accelerate melting

One of the reasons that the future behavior of the WAIS is so hard to predict is the existence of complex interactive mechanisms that link changes in the atmosphere and ocean to changes on the ice sheet, which can act to amplify the effects of warming. For example, as the circumpolar atmospheric circulation increases, the strength of the westerly Antarctic Circumpolar Current in the Southern Ocean increases. This response, along with the compensating easterly boundary current, combines to increase upwelling of warmer Circumpolar Deep Water onto the continental shelf. This additional heat is eventually delivered to the underside of the floating ice shelf and the ice sheet grounding line, driving faster ice flow and retreat of the grounding line.

Another important process results from warmer air temperatures, which produce meltwater ponds that act to speed up ice sheet disintegration in several ways. First, the pools ofmeltwater (such as those recently observed on the Greenland Ice Sheet) have a lower albedo than the ice itself and so absorb more radiation, further warming the surface. Second, when extensive meltwater occurs on an ice shelf, it can also act as a wedge, slicing the ice shelf into tall, thin icebergs that, like falling dominoes, push each other into the open ocean (a process thought to be responsible for the very rapid collapse of the Larsen B ice shelf).

Third, meltwater can travel through cracks and crevasses in the ice surface to the base of the ice sheet, a process that both warms deeper ice much faster than ordinary conductive warming and provides a lubricant that helps the ice flow faster over underlying rock. Recently, NASA's Ice Cloud and Land Elevation Satellite (ICEsat) has shown the existence of an extensive network of connected lakes under the WAIS ice streams (Fricker et al, 2007). These large volumes of water appear to move relatively rapidly under the ice, supporting the expectation that meltwater plays an important role in controlling ice sheet movement.

A final feedback mechanism involves the disintegration of the ice shelves, which are thought to 'buttress' on-land glaciers. The collapse of ice shelves removes this buttressing effect, accelerating the glaciers and leading to a net loss of grounded ice. This is happening now, for example, in the Amundsen Sea area.

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