SEA ICE IS frozen ocean water. It forms primarily in and near the polar regions, though it can grow closer to the equator as far as 40 degrees N latitude and 55 degrees S latitude. Sea ice has a strong seasonal variability. In the Northern Hemisphere, the annual maximum extent occurs in late winter (March), covering about 5,791,532 sq. mi. (15 million sq. km.) on average. It then melts during spring and summer to an annual minimum extent of about 2,702,715 sq. mi. (7 million sq. km.) in September. In the Antarctic, the annual maximum is about 7,335,941 sq. mi. (19 million sq. km.) during September, and the annual minimum is about 1,158,306 sq. mi. (3 million sq. km.) in February or March. Overall, roughly 10 percent of the world's ocean area is covered with sea ice at some point during the year.

The point at which the ocean begins to freeze is a function of the salt content of the water, which for typical ocean salinities is around 29 degrees F (minus 1.6 degrees C). The saline nature of the ocean makes the formation of sea ice distinct from freshwater ice growth in lakes and rivers. Freshwater becomes less dense as it approaches the freezing point, keeping the coldest water at the surface and allowing ice to form as soon as the surface cools to the freezing point. However, the presence of salt changes the charac ter of near-freezing water such that as saline water nears the freezing point, it continues to increase in density. Thus, cooling surface waters will become denser and sink. This means that there is overturning, and subsurface waters must also cool before ice can begin to form.

Sea ice typically grows to an average level thickness of 3 to 6 ft. (1 to 2 m.) in the Antarctic and 10 to 13 ft. (3 to 4 m.) in the Arctic. The ice is thinner in the Antarctic because most ice melts during the austral summer, whereas in the Arctic a significant fraction (~40 percent) remains through the summer and can grow over several years. A larger ocean heat flux at the bottom of the ice in the Antarctic also keeps the ice thinner.

However, thicker ice is not uncommon because of the effect of ice motion. Most sea ice is almost constantly in motion mainly because of the force of winds and ocean currents (other factors include the Coriolis effect, the slope of the ocean surface, and the internal structure of the ice). The speed of sea ice motion varies considerably; it can move 31 mi. (50 km.) or more in a day, though 1.2 mi. (2 km.) per day is typical. The motion of the ice can result in convergence between different parts of the ice cover, causing the ice to pile up into features called ridges. Ridges may easily rise 16 to 33 ft. (5 to 10 m.) above the surrounding level ice (and many tens of m. below the surface).

Sea ice plays an important role in climate. It has a much higher albedo than the unfrozen ocean, meaning that 60 to 70 percent of the sun's energy is reflected by the sea ice surface, whereas the unfrozen ocean reflects less than 10 percent of the sun's energy, resulting in much less energy absorption where ice is present. Sea ice is also a physical barrier between the ocean and atmosphere. This prevents the transfer of heat and moisture between the two and during winter. Thus, sea ice keeps the polar regions cooler and drier than they would be without ice. Sea ice also reduces fetch and dampens waves, limiting coastal erosion.

Sea ice has important effects on wildlife and human activities. Polar bears, seals, and other creatures rely on the ice to traverse and hunt, and during summer, the sea ice edge is a fertile area for phytoplankton and other microorganisms. Native communities in the Arctic are intimately tied into the presence of sea ice, and it plays an important role in their traditional culture and ways of life, such as hunting and transporta tion. Navigation of surface ships is severely limited or curtailed altogether, and for ships that do sail in or near ice-infested waters, it represents a significant hazard. Finally, sea ice plays a role in military operations, providing a useful cover for submarine activities.

Because of its location near the poles, the thin nature of the ice cover, and its interaction with the ocean and the atmosphere, sea ice is a sensitive indicator of the climate state. Sea ice in the Arctic has been decreasing dramatically over the past several decades. Overall, the Arctic has lost approximately 20 percent of the average summer ice extent since the late 1970s. Reductions during winter are less but are still significant. On the basis of current trends and projects by climate models, the Arctic may be ice free during summer by 2050 or earlier. This reduction in sea ice has been linked to warming temperatures resulting from the anthropogenic emission of greenhouse gases, though other factors also play a role. Unlike in the Arctic, there has been no significant trend seen in Southern Hemisphere ice, likely because of its remoteness relative to other continental land areas, the seasonal nature of the ice, and a greater ocean influence.

Changes in Arctic sea ice cover will have profound effects on climate, human activities, and wildlife, some of which are already being felt. Polar bears and other animals may be endangered, as well as the traditions of native communities. Less ice may also have benefits by opening up shipping routes and facilitating extraction of natural resources. Nonetheless, most effects are expected to be negative, and their implications for future climate will extend to regions far beyond the Arctic.

SEE ALSO: Animals; Climate Change, Effects.

BIBLIOGRAPHY. Dan Lubin and Robert Massom, Polar Remote Sensing, Volume I: Atmosphere and Oceans (Springer Praxis Books, 2006); O.M. Johannessen, R.D. Muench, and J.E. Overland, eds., The Polar Oceans and Their Role in Shaping the Global Environment (American Geophysical Monograph 85, 1994); Norbert Untersteiner, ed., The Geophysics of Sea Ice (Plenum Press, 1986); Peter Wadhams, Ice in the Ocean (Gordon and Breach Science Publishers, 2000).

Walt Meier Julienne Stroeve University of Colorado

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