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. 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. 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. Useful satellite data concerning sea ice began in December 1972 with the Electrically Scanning Microwave Radiometer (ESMR) instrument. However, this was not directly comparable with the later SMMR/SSMI, and so the practical record begins in late 1978 with the launch of NASA's Scanning Multichannel Microwave Radiometer (SMMR) satellite., and continues with the Special Sensor Microwave/Imager (SSMI). Advanced Microwave Scanning Radiometer (AMSR) and Cryosat-2 provide separate records. Since 1979, satellites have provided a consistent continuous record of sea ice.However, the record relies on stitching together measurements from a series of different satellite-borne instruments, which can lead to errors associated with intercalibration across the sensor changes.Satellite images of sea ice are made from observations of microwave energy radiated from the Earth's surface. Because ocean water emits microwaves differently from sea ice, ice 'looks' different from water to the satellite sensor—see sea ice emissivity modelling. The observations are processed into digital picture elements, or pixels. Each pixel represents a square surface area on Earth. The first instruments provided approximately 25 kilometers by 25 kilometers resolution; later instruments higher. Algorithms examine the microwave emissions, and their vertical and horizontal polarisations, and estimate the ice area. Sea ice may be considered in terms of total volume, or in terms of areal coverage. Volume is harder, because it requires a knowledge of the ice thickness, which is hard to measure directly; efforts such as PIOMAS use a combination of observations and modelling to estimate total volume. There are two ways to express the total polar ice cover: ice area and ice extent. To estimate ice area, scientists calculate the percentage of sea ice in each pixel, multiply by the pixel area, and total the amounts. To estimate ice extent, scientists set a threshold percentage, and count every pixel meeting or exceeding that threshold as 'ice-covered.' The common threshold is 15 percent.