This chapter follows the agreed TAR template for WGII regional assessments. Rather than assessing all possible impacts of climate change, emphasis is placed on eight key regional concerns. These concerns are chosen for the Arctic and the Antarctic on the basis of earlier findings by the IPCC (Everett and Fitzharris, 1998) and on discussions at various workshops of the TAR. They represent what are considered to be the most important impacts of future climate change in the Arctic and Antarctic in the wider global perspective. Although some other impacts (e.g., Arctic glaciers and terrestrial Antarctic biota) may be very important locally, space does not permit a comprehensive review.

Changes in the polar climate will have a direct impact on the great ice sheets, ice caps, and glaciers of the polar regions. How each responds will depend on several climatological parameters; some will grow, whereas others shrink. We have high confidence that their overall contribution to rising sea level will be positive, with glaciers and the Greenland ice sheet shrinking. The contribution from Antarctica, however, is uncertain at present. There is a high likelihood that increasing temperatures over the continent and changing storm tracks will cause increased precipitation and thickening of the ice sheet, but there still exists at low confidence the possibility that the West Antarctic ice sheet will retreat dramatically in coming centuries (Vaughan and Spouge, 2000). Such a change would not result from recent and future climate change (Bentley, 1998) but more probably from continuing readjustment to the end of the last glacial period (Bindschadler, 1998), as a result of internal dynamics of the ice sheet (MacAyeal, 1992), or as a result of ice shelf-ocean interaction. This subject and the general issue of sea-level rise are reviewed more comprehensively in TAR WGI Chapter 11; we include only a summary of the main points here.

The Greenland ice sheet suffers melting in summer at its margin. There is a trend toward an increase in the area and duration of this melt (Abdalati and Steffen, 1997). This trend is likely to continue. Airborne altimetric monitoring has shown that over the period 1993-1998, the Greenland ice sheet was slowly thickening at higher elevations; at lower elevations, thinning (about 1 m yr^-1) was underway (Krabill et al., 1999, 2000). If warming continues, the Greenland ice sheet will shrink considerably, as occurred in previous interglacial periods (Cuffey and Marshall, 2000), and if the warming is sustained, the ice sheet will melt completely (see TAR WGI Chapter 11).

Over the Antarctic ice sheet, where only a few limited areas show summer melting (Zwally and Fiegles, 1994), a slight thickening is likely as precipitation rates increase (e.g., Ohmura et al., 1996; Smith et al., 1998a; Vaughan et al., 1999). In the past decade there has been some change in the ice cover in local areas (e.g., on the Antarctic Peninsula; see Section 16.2.2), but the majority of the Antarctic ice sheet appears, from satellite altimetry, to be close to a state of balance (Wingham et al., 1998). Only the Thwaites and Pine Island glacier basins show any spatially coherent trend, but it is not yet known if their thinning is related to a decrease in precipitation or some dynamic change in the ice sheet. Chinn (1998) reports that recession is the dominant change trend of recent decades for glaciers of the Dry Valleys area of Antarctica. The future of glaciers in the Arctic will be primarily one of shrinkage, although it is possible that in a few cases they will grow as a result of increased precipitation.

This report is concerned primarily with the impacts of climate change on particular regions. An important question is: How will changes in glacial ice in the polar regions impact local human populations and ecological systems, and what will be the socioeconomic consequences? The short answer is that impacts on ice systems will be substantial, but because the populations of humans and other biota within polar region are low, impacts may be relatively minor. In Antarctica, the continental human population is only a few thousand. A few localities may undergo changes such as that at Stonington Island on the Antarctic Peninsula (Splettoesser, 1992), where retreat of the ice sheet has left the station stranded on an island. In general, however, changes to ice sheets will directly cause few significant life-threatening problems.

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