cover is projected to decrease. Note that late 20th century and early 21st century observations suggest a slight but significant increase in Antarctic sea-ice extent, contrary to that simulated by the models. This increase is associated with stratospheric ozone depletion and is expected to reverse as ozone returns to normal levels. The predicted decrease will occur more slowly than in the Arctic, particularly in the Ross Sea where temperatures are expected to remain cooler. This is attributed to the fact that the Southern Ocean stores much of its heat increase at depths below 1 km, while in the Arctic Ocean and subpolar seas the heat remains in the upper 1 km (Gregory, 2000; Bitz et al., 2007). In addition, horizontal heat transport poleward of about 60ºN increases in many models (Holland and Bitz, 2003). These differences in the depth where heat is accumulating in the high-latitude oceans influence the relative rates of sea ice decay in the Arctic and Antarctic so that ice loss is faster in the Arctic (IPCC, 2007a).

A comparison of the last 20 years of the 21st century under the SRES A1B scenario with the last 20 years of the 20th century shows a decrease in the sea-ice concentration in both summer (JFM) and winter (JAS) by the end of the 21st century (Meehl et al., 2007; Sen Gupta et al., 2009). (See Figure 4.17.)

IPCC models predict a loss in sea-ice cover in summer and winter ranging from 10-50% in winter, and 33% to total loss in summer by the end of 2100. This is associated with a global warming ranging from 1.7-4.4ºC above late 20th century values. The volume of sea ice is also reduced, and

FIGURE 4.17 Multi-model mean sea-ice concentration (%) for January to March (JFM) and June to September (JAS), in the Antarctic for the periods (a) 1980 to 2000 and (b) 2080 to 2100 for the SRES A1B scenario. The dashed white line indicates the present-day 15% average sea-ice concentration limit. Source: Modified from Flato et al. (2004); IPCC (2007a, their Figure 10.14).

FIGURE 4.17 Multi-model mean sea-ice concentration (%) for January to March (JFM) and June to September (JAS), in the Antarctic for the periods (a) 1980 to 2000 and (b) 2080 to 2100 for the SRES A1B scenario. The dashed white line indicates the present-day 15% average sea-ice concentration limit. Source: Modified from Flato et al. (2004); IPCC (2007a, their Figure 10.14).



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