Environmental Engineering Reference
In-Depth Information
A1B emission scenario until 2100, and then holding the forcing fixed to
2200. In some of the models, the MOC was seen to maintain an equilibrium
strength that was similar to that projected for 2100; in other models the MOC
strengthened somewhat from their nadir early in the 22nd century. Too few
of the higher end AR4 climate models have been integrated far enough into
the future to assess whether persistently high greenhouse gas concentrations
will cause a permanent change to the strength of the MOC.
Sea Ice Beyond 2100
Climate model simulations suggest that in the decades following 2100
the Arctic may be perennially ice-free (Winton, 2006a,b; Eisenman and
Wettlaufer, 2009). However, on the millennium scale the system may oscil-
late between being totally ice-covered or having ice only along the land
margins (Ridley et al., 2008). Only two IPCC models predict a year-round
ice-free Arctic in the decades after 2100. However, these are the models
that have the most sophisticated sea ice components. The scenario within
which these models lose their Arctic ice is the 1% per year CO
2
increased
to quadrupling, a concentration of 1,120 ppm, after which although atmo-
spheric CO
2
is kept constant temperatures continue to rise. These models,
one initiated from pre-industrial conditions and the other from present-day
conditions are run for nearly 300 years; quadrupling occurs at 140 years.
Both models exhibit a gradual linear decline in September sea-ice loss be-
coming ice free when the average polar temperature is -9°C. In March, the
transition to an ice-free state is also linear until polar temperatures reach
-5°C, at which point one model experiences an abrupt transition, associ-
ated with that model's ice-albedo feedback mechanism, while in the other
it remains linear and the ocean heat flux plays a larger role. The tempera-
ture at which the Arctic becomes ice free in these models is 13°C above
present-day values (Winton, 2006a,b). The ice-albedo, convective cloud,
and ocean heat transport feedbacks all play necessary roles in the loss of
the winter sea ice (Abbot et al., 2009b). However, the ice-albedo feedback
plays a key role.
While an ice-free Arctic may present new economic opportunities it will
also likely have profound impacts on climatological and ecological systems
locally and globally. A few of these are mentioned here. From the physical
standpoint, the loss of Arctic sea ice means that the mediating influence of
sea ice on energy flux exchanges between the atmosphere and ocean will
no longer prevail and the Arctic atmosphere will warm. Model studies sug-
gest that these two impacts will affect the effectiveness of the overturning
