Geoscience Reference
In-Depth Information
observations in the ocean, in the atmosphere and at the surface is
clearly insufficient, and climatic models cannot satisfactorily
reproduce the processes by which the ice forms and disappears.
Figure 3.7.
Horizontal extension of the arctic ice field on average over the month of
September (annual minimum anomaly in percentage) from 1979 to 2012 (National
Snow and Ice Data Center)
However, the feedback mechanisms involving sea ice are well
known:
- the presence of sea ice evidently inhibits air-sea fluxes. After it
has formed during winter, the lower atmosphere cools greatly through
radiative loss. This cooling is reduced occasionally, by the arrival of
warm air within low pressure systems. The ice remains for a long
time, since its very high albedo prevents the solar energy from
penetrating into its depth and the water beneath. But, particle
deposition (in particular of black carbon transported from Asia) tends
to reduce this albedo. Indeed, the more the surface albedo diminishes,
the more the penetration of solar radiation increases, and warms the
upper layers of the ocean, inhibiting the formation of ice even more
(positive feedback). The ice formation at the beginning of the polar
winter, and its persistence late into spring are, therefore, necessary for
retaining an overall stable sea ice coverage from year to year. The
contrary has been observed over the last 10 years;
- in marginal ice regions, in free water areas formed within the ice
field under the effect of ice divergent movements, or also in channels
