Measurement of sea ice

Measurement of sea ice is important for safety of navigation and for monitoring the environment, particularly the climate. Sea ice extent interacts with large climate patterns such as the North Atlantic oscillation and Atlantic Multidecadal Oscillation, to name just two, and influences climate in the rest of the globe. The amount of sea ice coverage in the arctic has been of interest for centuries, as the Northwest Passage was of high interest for trade and seafaring. There is a longstanding history of records and measurements of some effects of the sea ice extent, but comprehensive measurements were sparse till the 1950s and started with the satellite era in the late 1970s. Modern direct records include data about ice extent, ice area, concentration, thickness, and the age of the ice. The current trends in the records show a significant decline in Northern Hemisphere sea ice and a small but statistically significant increase in the winter Southern Hemisphere sea ice. Furthermore, current research comprises and establishes extensive sets of multi-century historical records of arctic and subarctic sea ice and uses, among others high-resolution paleo-proxy sea-ice records. The arctic sea ice is a dynamic climate-system component and is linked to the Atlantic multidecadal variability and the historical climate over various decades. There are circular changes of sea ice patterns but so far no clear patterns based on modeling predictions.

Methods of measuring sea ice

Early observations Records assembled by Vikings showing the number of weeks per year that ice occurred along the north coast of Iceland date back to A.D. 870, but a more complete record exists since 1600. More extensive written records of Arctic sea ice date back to the mid-18th century. The earliest of those records relate to Northern Hemisphere shipping lanes, but records from that period are sparse. Air temperature records dating back to the 1880s can serve as a stand-in (proxy) for Arctic sea ice, but such temperature records were initially collected at only 11 locations. Russia's Arctic and Antarctic Research Institute has compiled ice charts dating back to 1933. Today, scientists studying Arctic sea ice trends can rely on a fairly comprehensive record dating back to 1953, using a combination of satellite records, shipping records, and ice charts from several countries. In the Antarctic, direct data prior to the satellite record are even more sparse. To try to extend the historical record of Southern Hemisphere sea ice extent further back in time, scientists have been investigating various proxies for sea ice extent. One is records kept by Antarctic whalers that document the location of all whales caught and relate to sea ice observations directly. There seems to be an abrupt mid-twentieth-century decline in Antarctic sea-ice extent from whaling records, the direct global estimates of the Antarctic sea-ice cover from satellite observations, since the 1970 provide no clear trends. Because whales tend to congregate near the sea ice edge to feed, their locations could be a proxy for the ice extent. Other proxies use the presence of phytoplankton-derived organic compounds and other extremophiles traces in Antarctic ice cores and sediments. Since phytoplankton grow most abundantly along the edges of the ice pack, the concentration of this sulfur-containing organic compounds and their geochemistry provide indicators of how far the ice edge extended from the continent. There are further extensive sets of multicentury historical records of arctic and subarctic sea ice and uses, among others high-resolution paleo proxy sea-ice records.

Satellites

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