Sea Ice

Sea ice is frozen ocean water. It forms primarily in and near the polar regions, although it can grow closer to the equator, as far as 40 degrees north and 55 degrees south latitude. Sea ice has a strong seasonal variability. In the Northern Hemisphere, the annual maximum extent occurs in late winter (March), covering about 5 million sq. mi. (15 million sq. km) on average. It then melts during spring and summer to an annual minimum extent of about 2 million sq. mi. (7 million sq. km) in September. In the Antarctic, the annual maximum is about 7 million sq. mi. (19 million sq. km) during September, and the annual minimum is about 1 million 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.

Sea ice typically grows to an average level thickness of 3 to 6.5 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 30 mi. (50 km) or more in a day, although 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 meters below the surface).

Role of Sea Ice in Climate

Sea ice plays an important role in climate. It has a much higher albedo than the unfrozen ocean, meaning that as much as 80 to 90 percent of the sun’s energy is reflected by a snow-covered 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.

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 30 percent of the average summer ice extent (areal coverage) since the late 1970s, and recent years have seen record or near-record low summer extents. September 2007 had the lowest extent on record, at 1 million sq. mi. (4.3 million sq. km), 23 percent below any previous September in the satellite record (since 1979) and likely the lowest in several hundred years and perhaps longer. Reductions of sea ice extent during winter are less, but are still significant. Septembers since 2007 have also been very low. In addition to declines in extent, the ice has thinned dramatically and has lost much of the older, thicker perennial ice cover than once used to dominate the Arctic Ocean. Now, most of the Arctic Ocean is covered by seasonal ice, or ice that forms each winter and melts during the subsequent summer. On the basis of current trends and projects by climate models, the Arctic is likely to be largely sea ice–free during at least part of the 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, trends in Southern Hemisphere ice are showing a small increase, likely because of its remoteness relative to other continental land areas, the seasonal nature of the ice, changes in atmospheric circulation, 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.