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Sea Ice

pancake ice in the Arctic's Chukchi Seaicon-caption
The U.S. Coast Guard icebreaker Healy moves through pancake ice in the Arctic's Chukchi Sea.
(Photo by Carin Ashjian, Woods Hole Oceanographic Institution)

What is Sea Ice?

Sea ice is frozen seawater floating on the surface of the ocean. Unlike icebergs, which originate from land-based sources like glaciers and ice sheets, sea ice is formed entirely in the ocean.

Sea ice exists in the Arctic Ocean and the far northern and southern reaches of the Pacific, Atlantic, and Southern Oceans. It forms and melts each year with the seasons under the influence of several factors, including air and water temperature, the amount of sunlight reaching the ocean surface and the age of the ice.[ MORE ]

During the Northern Hemisphere winter, the extent of Arctic sea ice increases and generally reaches its maximum in March. As the days lengthen, sea ice extent gradually shrinks until reaching its minimum sometime in September. First-year ice—ice that has only existed for one winter season—generally melts more easily than older, multi-year ice because it is often thinner.

Warming in the Arctic is causing the annual extent of sea ice to shrink each winter. This is also shrinking the percentage of multi-year ice in the ice pack, which leaves it susceptible to further reductions each summer.[ LESS ]

Why is it important?

Sea ice plays an important role in Earth’s climate system. Because it is white, it reflects as much as 90 percent of the sunlight that lands on it. As the extent of sea ice in the Northern and Southern Hemispheres shrinks, it is exposing more of the ocean beneath. Because it is darker than sea ice, the ocean naturally absorbs more of the sunlight that lands on it, warming the water and causing additional melting to occur.

Sea ice is also an important part of the Arctic and Antarctic ecosystems. The bottom and interior of sea ice is riddled with brine-filled channels that serve as a home for bacteria, viruses, unicellular algae, diatoms, ice worms and small crustaceans. As the ice warms in spring and summer, these tiny organisms are released into the surface water, where they become food for a wide range of fish and shrimp, which in turn become food for larger animals.[ MORE ]

In the winter, these sympagic (ice-reliant) organisms continue to survive and to thrive, providing a possible model for scientists interested in understanding how life could exist inside or beneath the frozen surface on some moons of Jupiter and Saturn.

Terrestrial animals, such as penguins in the Antarctic and polar bears in the Arctic, also rely on sea ice to provide a platform on which to hunt and rest.[ LESS ]