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