by Oliver Rordorf ’15
Sea ice is frozen sea water, hence the name. Sea ice forms in the oceans of the polar regions and occupies about 7% of the world ocean. This ice floats because the structure of ice is an open hexagonal structure with each oxygen atom at the center of a tetrahedron with oxygen bonded at each vertex. Hydrogen lies along these bonds and the length of the bonded hydrogen creates an open structure which is less dense than liquid water. When sea ice forms, the majority of salt is rejected, but fist year ice will still have a salinity of about ten parts per thousand (ppt) compared to sea water at 35ppt.
There are different classifications of sea ice according to whether or not it can drift and its age. Sea ice that is attached to land, and can therefore not drift, is called land-fast ice (or just fast ice). On the other hand, drift ice forms in large areas free of obstructions where it flows at the whim of wind and ocean currents. A particular ice mass receives a name according its age. “New ice” is defined as frozen sea water that has yet to form an ice pack. This ice is normally consists of frazil ice, loose needle-shaped ice crystals, or Nilas, very thin sea ice (>10cm thick). First-year sea ice is the next phase after new ice; it is thicker but is no older than one year. Lastly, “old sea ice” (also known as multi-year ice) is ice that has remained intact for at least one summer. Addtionally, within a sea ice pack there are areas of open water that form which are called leads (thin strips) and polynyas (larger and more uniform than leads an sometimes lasting several years).
Sea ice cycles through two phases throughout the year: the winter growing period and the summer melting period. During the winter ice forms and grows. The formation of sea ice occurs in different ways according to the roughness of the water. In calm water a thin layer of separate crystals in the shape of a disc form. Eventually, these crystals take on a hexagonal form with long spindly arms extending outwards. Finally, these crystals meet and grow together forming a thin layer of sea ice. In rough water, small ice crystals form but they remain on random collision tracks forming pancake ice, small coherent cakes of slush. Slowly these pancakes coalesce in times of calm ocean. Growth continues by these processes.
Sea ice that forms at the different poles is drastically different. In the Arctic the ice can last for multiple years because the Arctic Ocean is relatively sheltered by surrounding land and thus experiences lower wind speeds. On the other hand, in the Antarctic, ice can only last for one season because it has no protection from the high winds of the surrounding Southern Ocean. Because of the winds and currents, no sea ice that forms in the Antarctic drifts out of the Southern Ocean whereas in the Arctic the ice drifts into other oceans (North Atlantic). In the summer the sea ice melts. The degree and extent of melting is a positive feedback cycle; the more ice that melts the lower the area’s albedo (the reflectivity of a surface) and the more sunlight (and energy) is instead absorbed, warming the surface (of the ocean in this case) and leading to increased ice melting, which again decrease the surface area of high albedo ice.
As mentioned above, a major function of sea ice in the Earth system is its contribution to the Earth’s albedo. Normally the ocean absorbs much of the sun light, but when 7% of it is covered in white (the most reflective color) ice the albedo of the Earth is increased keeping the planet cooler. Due to the large role sea ice plays in the albedo of the earth, disturbances to this vital system could be one of the most significant changes in the Earth system. There have been current trends of sea ice thinning and less forming in the winter. (1) As the climate continues to warm, less ice will form and more energy will be absorbed than reflected by the polar oceans, causing the temperature to rise. (1) Once the sea ice melts and there is less relfection of light energy, all of the negatives of global warming will be exacerbated. (1) Furthermore, substantial deposits of oil are believed to exist in the Arctic and with less sea ice those deposits may be easier to access. (2) If we used this new fuel more radiatively active gases (greenhouse gases) would be released, warming the climate and melting more ice. From a shipping perspective the melting trend of sea ice is a positive because large vessels can now adopt an Artic path to make shipping routes shorter and therefore more efficient. (3)
(1) Winton, Michael. “Sea Ice–Albedo Feedback and Nonlinear Arctic Climate Change.” NOAA, n.d. Web. 13 Mar. 2014.
(2) “Locating Oil under Ice.” Scottish Marine Insitute. N.p., n.d. Web. 13 Mar. 2014.
(3) Oremus, Will. “Thanks to Global Warming, Arctic Shipping Has Quadrupled in the Past Year.” Slate Magazine. N.p., n.d. Web. 13 Mar. 2014.