Sea ice, which both originates in the ocean and melts into it, consists largely of frozen ocean water, unlike glaciers, icebergs, and ice shelves that may float in the ocean but originate on land and consist of frozen freshwater.
Every year, Arctic sea ice expands during the long, dark winter, coming to cover nearly the entire Arctic Ocean, which spans an area of approximately 14,060,000 square kilometres (5,430,000 square miles), by March. In winter, ice can reach a thickness of an average of 2.5 metres in central parts of the Arctic Ocean.
The brightness and whiteness of sea ice ensures that 80 percent of the sunlight that hits it reflects right back into space, making this feature of the Arctic environment critical for both keeping the region cool and moderating worldwide temperatures. From March to September, melting sea ice exposes the ocean, which, by contrast, absorbs 90 percent of sunlight and causes Arctic temperatures to rise.
Sea ice plays a part in the worldwide “conveyor belt” circulation system in the ocean, because the water below it has a higher concentration of salt and is denser than the surrounding water, which makes it sink. A current is created when surface water replaces the sinking salt water, which then moves towards the equator along the ocean floor, carrying its colder temperature along with it. At the same time, warm water from the equator travels above it towards the poles. This is called thermohaline circulation.
Sea ice floats on the ocean’s surface, where it is pushed around by wind, waves, and ocean currents. Commonly defined as an area where sea ice covers more than 15 percent of the ocean, the sea ice “edge” is not a single line, as would be expected, but a transition area between the open ocean and sea ice.