Most sea ice occurs as pack ice, which is very mobile, drifting across the ocean surface under the influence of the wind and ocean currents and moving vertically under the influence of tides, waves, and swells. There is also landfast ice, or fast ice, which is immobile, since it is either attached directly to the coast or seafloor or locked in place between grounded icebergs. Fast ice grows in place by freezing of seawater or by pack ice becoming attached to the shore, seafloor, or icebergs. Fast ice moves up and down in response to tides, waves, and swells, and pieces may break off and become part of the pack ice. A third type of sea ice, known as marine ice, forms far below the ocean surface at the bottom of ice shelves in Antarctica. Occasionally seen in icebergs that calve from the ice shelves, marine ice can appear green due to organic matter in the ice.

Sea ice undergoes large seasonal changes in extent as the ocean freezes and the ice cover expands in the autumn and winter, followed by a period of melting and retreat in the spring and summer. Northern Hemisphere sea ice extent typically ranges from approximately 8 million square km in September to approximately 15 million square km in March. (One square km equals approximately 0.4 square mile.) Southern Hemisphere sea ice extent ranges from approximately 4 million square km in February to approximately 20 million square km in September. In

September 2007 the sea ice extent in the Northern Hemisphere declined to roughly 4.1 million square km, a figure some 50 percent below mean sea ice coverage for that time of year. Globally, the minimum and maximum sea ice extents are about 20 million square km and 30 million square km, respectively. Measured routinely using data obtained from orbiting satellite instruments, the minimum and maximum sea ice extent figures vary annually and by decade. These figures are important factors for understanding polar and global climatic variation and change.

ICE SaliNity, TEMpERatuRE, AND Ecological iNtEractioNs

As seawater freezes and ice forms, liquid brine and air are trapped within a matrix of pure ice crystals. Solid salt crystals subsequently precipitate in pockets of brine within the ice. The brine volume and chemical composition of the solid salts are temperature-dependent.

Liquid ocean water has an average salinity of 35 parts per thousand. New ice such as nilas has the highest average salinity (12-15 parts per thousand); as ice grows thicker during the course of the winter, the average salinity of the entire ice thickness decreases as brine is lost from the ice. Brine loss occurs by temperature-dependent brine pocket migration, brine expulsion, and, most importantly, by gravity drainage via a network of cells and channels. At the end of winter, Arctic first-year ice has an average salinity of 4-6 parts per thousand. Antarctic first-year ice is more saline, perhaps because ice growth rates are more rapid than in the Arctic, and granular ice traps more brine.

In summer, gravity drainage of brine increases as the ice temperature and permeability increase. In the Arctic, summer gravity drainage is enhanced by flushing, as snow and ice meltwater percolate into the ice. Consequently, after a few summers the ice at the surface is completely desalinated and the average salinity of Arctic multiyear ice drops to 3-4 parts per thousand. Antarctic multiyear ice is more saline because the snow rarely melts completely at the ice surface, and brine flushing is uncommon. Instead of percolating into the ice, snow meltwater refreezes onto the ice surface, forming a layer of hard, glassy ice. In contrast, even though it forms from platelets in seawater, marine ice contains little or no salt. The reasons for this remain unclear, but possible explanations include the den-sification of the ice crystals or their desalination by convection within the "mushy" crystal layer.

Because sea ice is porous and permeable and the brine held within it contains nutrients, sea ice often harbours rich and complex ecosystems. Viruses, bacteria, algae, fungi, and protozoans inhabit sea ice, taking advantage of the differences in salinity, temperature, and light levels. Algae are perhaps the most obvious manifestation of the sea ice ecosystem because they are pigmented and darken the ice. Algae are found at the top, bottom, and interior of Antarctic sea ice; however, they are found primarily at the bottom of Arctic sea ice, where they can occur as strands many metres in length. Sea ice algae are important as a concentrated food source for krill and other zooplankton. Melting sea ice rich in algae may also be important for seeding phytoplankton blooms in the previously ice-covered ocean.

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