FIGURE 4.14 Time series of August, September and October (ASO) season average Arctic sea ice extent. Dark solid lines are the model ensemble means and the shaded areas show the range of the ensemble members. Observed sea ice is in black. SOURCE: Washington et al., 2009, their Figure 3.

FIGURE 4.14 Time series of August, September and October (ASO) season average Arctic sea ice extent. Dark solid lines are the model ensemble means and the shaded areas show the range of the ensemble members. Observed sea ice is in black. SOURCE: Washington et al., 2009, their Figure 3.

simulated sea-ice extent reaches present-day values (4.6 million km2) in September and March compared to when the sea-ice extent reached nearly ice-free conditions (less than 1.0 million km2) near the end of the 21st century (see Figure 4.16). When sea-ice extent reaches present-day observed values in March much of the central Arctic is covered by sea ice less than 2.5 m thick. By the end of the century this sea ice is less than 2.0 m thick. In September over the same period sea ice moves from being less than 1.2 m thick to nearly ice-free conditions.

The mechanisms involved in reducing sea-ice cover are all positively correlated with temperature increase, giving rise to a linear relationship between annual Arctic sea-ice area reduction and global-averaged surface air temperature. According to one set of estimates, if GHG emissions continue to increase, corresponding temperature increases of 1ºC, 2ºC, 3ºC, and 4ºC are associated with Arctic sea-ice area reductions of 13%, 25%, 36% and 50% respectively (e.g., Gregory et al., 2002: Figure 4). Greater reductions are expected for summer compared to winter. For summer these values are on the order of 24% per degree warming resulting in an ice-free summer



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