Environmental Engineering Reference
In-Depth Information
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 km
2
) in
September and March compared to when the sea-ice extent reached nearly
ice-free conditions (less than 1.0 million km
2
) near the end of the 21st cen-
tury (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 be-
tween 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 1°C, 2°C, 3°C, and 4°C
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
