FIGURE 4.13 Arctic September sea-ice extent (× 106 km2) from observations (thick red line) and 13 IPCC AR4 climate models, together with the multi-model ensemble mean (solid black line) and standard deviation (dotted black line). Models with more than one ensemble member are indicated with an asterisk. Inset shows 9-year running means. Source: Stroeve et al. (2007).

FIGURE 4.13 Arctic September sea-ice extent (× 106 km2) from observations (thick red line) and 13 IPCC AR4 climate models, together with the multi-model ensemble mean (solid black line) and standard deviation (dotted black line). Models with more than one ensemble member are indicated with an asterisk. Inset shows 9-year running means. Source: Stroeve et al. (2007).

on the emission scenario used by the models but may also depend on the natural variability simulated by the individual models. Figure 4.15 taken from Wang and Overland (2009) illustrates this using six IPCC models that simulate the observed mean minimum and seasonality of sea ice very well. Two SRES scenarios are represented: the A1B, which reaches CO2 concentration of 720 ppm by the end of the 21st century, and A2, which reaches 850 ppm at the same time.

The predicted reduction in sea-ice extent is expected to be accompanied by reduction in sea-ice thickness in summer and winter as more areas are replaced by first year ice. Ice in the Northern Hemisphere is expected to thin dramatically as the projected reduction in sea-ice volume is about twice that of the sea-ice extent reduction. Using the same six models in Figure 4.15, Wang and Overland (2009) compare the ice thickness at the time when the



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