4.4 Changes in Sea Ice
Sea ice is formed by freezing of seawater in the polar oceans. It is an important, interactive component of the global climate system because: a) it is central to the powerful ‘ice-albedo’ feedback mechanism that enhances climate response at high latitudes (see Chapter 2); b) it modifies the exchange of heat, gases and momentum between the atmosphere and polar oceans; and c) it redistributes freshwater via the transport and subsequent melt of relatively fresh sea ice, and hence alters ocean buoyancy forcing.
The thickness of sea ice is a consequence of past growth, melt and deformation, and so is an important indicator of climatic conditions. Ice thickness is also closely connected to ice strength, and so changes in thickness are important to navigability by ships, to the stability of the ice as a platform for use by humans and marine mammals, to light transmission through the ice cover, etc. Sea ice increases in thickness as bottom freezing balances heat conduction through the ice to the surface (heat conduction is strongly influenced by the insulating thickness of the ice itself and the snow on it). Most of the inhomogeneity in the pack results from deformation of the ice due to differential movement of individual pieces of ice (called ‘floes’). Open water areas created within the ice pack under divergence or shear (called ‘leads’) are a major contributor to ocean-atmosphere heat exchange (turbulent heat loss from the ocean in winter and shortwave heating in the summer). In some locations, due either to persistent ice divergence or to persistent upwelling of oceanic heat, open water areas within an otherwise ice-covered region can be sustained over much of the winter. These are called ‘polynyas’ and are important feeding areas for marine mammals and birds.
Under convergence, thin ice sheets may ‘raft’ on top of each other, doubling the ice thickness, and under strong convergence (for example, when wind drives sea ice against a coast), the ice buckles and crushes to form sinuous ‘ridges’ of thick ice. In the Arctic, ridges can be tens of metres thick, account for nearly half of the total ice volume and constitute a major impediment to transportation on, through, or under the ice. Although ridging is generally less severe in the Antarctic, ice deformation is still an important process in thickening the ice cover.
Near the shore, in bays and fjords, and among islands like those of the Canadian Arctic Archipelago, sea ice can be attached to land and therefore be immobile. This is termed ‘landfast’ ice. In the Arctic such ice (and in particular its freeze-up and breakup) is of special importance to local residents as it is used as a platform for hunting and fishing, and is an impediment to shipping.
Some climatically important characteristics of sea ice include: its concentration (that fraction of the ocean covered by ice); its extent (the area enclosed by the ice edge – operationally defined as the 15% concentration contour); the total area of ice within its extent (i.e., extent weighted by concentration); the area of multi-year ice within the total extent; its thickness (and the thickness of the snow cover on it); its velocity; and its growth and melt rates (and hence salt or freshwater flux into the ocean). Ice extent, or ice edge position, is the only sea ice variable for which observations are available for more than a few decades. Expansion or retreat of the ice edge may be amplified by the ice-albedo feedback.