Measurement of sea ice
Encyclopedia
Measurement of sea ice is important for safety of navigation
Navigation
Navigation is the process of monitoring and controlling the movement of a craft or vehicle from one place to another. It is also the term of art used for the specialized knowledge used by navigators to perform navigation tasks...

 and for monitoring the environment
Natural environment
The natural environment encompasses all living and non-living things occurring naturally on Earth or some region thereof. It is an environment that encompasses the interaction of all living species....

, particularly the climate
Climate
Climate encompasses the statistics of temperature, humidity, atmospheric pressure, wind, rainfall, atmospheric particle count and other meteorological elemental measurements in a given region over long periods...

. Record keeping of direct observations began over a thousand years ago, but was sparse until the 1950s. Truly comprehensive records began with the satellite
Satellite
In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....

 era in the late 1970s. Modern records include date about ice extent, ice area, concentration, thickness and the age of the ice. The trends in the records show a significant decline in Northern hemisphere
Northern Hemisphere
The Northern Hemisphere is the half of a planet that is north of its equator—the word hemisphere literally means “half sphere”. It is also that half of the celestial sphere north of the celestial equator...

 sea ice and a very small, statistically insignificant increase in the winter Southern hemisphere
Southern Hemisphere
The Southern Hemisphere is the part of Earth that lies south of the equator. The word hemisphere literally means 'half ball' or "half sphere"...

 sea ice.

Early observations

Records assembled by Vikings showing the number of weeks per year that ice occurred along the north coast of Iceland
Iceland
Iceland , described as the Republic of Iceland, is a Nordic and European island country in the North Atlantic Ocean, on the Mid-Atlantic Ridge. Iceland also refers to the main island of the country, which contains almost all the population and almost all the land area. The country has a population...

 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
Russia
Russia or , officially known as both Russia and the Russian Federation , is a country in northern Eurasia. It is a federal semi-presidential republic, comprising 83 federal subjects...

 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, 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 two types of proxies for sea ice extent. One is records kept by Antarctic whaler
Whaler
A whaler is a specialized ship, designed for whaling, the catching and/or processing of whales. The former included the whale catcher, a steam or diesel-driven vessel with a harpoon gun mounted at its bows. The latter included such vessels as the sail or steam-driven whaleship of the 16th to early...

s since the 1930s that document the location of all whales caught. Because whales tend to congregate near the sea ice edge to feed, their locations could be a proxy for the ice extent. A second possible proxy is the presence of a phytoplankton-derived
Phytoplankton
Phytoplankton are the autotrophic component of the plankton community. The name comes from the Greek words φυτόν , meaning "plant", and πλαγκτός , meaning "wanderer" or "drifter". Most phytoplankton are too small to be individually seen with the unaided eye...

 organic compound in Antarctic ice core
Ice core
An ice core is a core sample that is typically removed from an ice sheet, most commonly from the polar ice caps of Antarctica, Greenland or from high mountain glaciers elsewhere. As the ice forms from the incremental build up of annual layers of snow, lower layers are older than upper, and an ice...

s. Since phytoplankton grow most abundantly along the edges of the ice pack, the concentration of this sulfur-containing
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...

 organic compound has been proposed as an indicator of how far the ice edge extended from the continent. Currently, however, only the satellite record is considered sufficiently reliable for studying Antarctic sea ice trends.

Satellites

Useful satellite data concerning sea ice began in December 1972 with the Electrically Scanning Microwave Radiometer
Electrically Scanning Microwave Radiometer
The electrically scanning microwave radiometer was an instrument carried by the Nimbus-5 satellite, precursor to the scanning multichannel microwave radiometer and special sensor microwave/imager instruments....

 (ESMR) instrument. However, this was not directly comparable with the later SSMR/SSMI, and so the practical record begins in late 1978 with the launch of NASA’s Scanning Multichannel Microwave Radiometer
Scanning Multichannel Microwave Radiometer
The scanning multichannel microwave radiometer [pronounced simmer] was a five-frequency microwave radiometer flown on the Seasat and Nimbus 7 satellites. Both were launched in 1978, with the Seasat mission lasting less than six months until failure of the primary bus. The Nimbus 7 SMMR lasted from...

 (SMMR) satellite., and continues with the Special Sensor Microwave/Imager
Special Sensor Microwave/Imager
The special sensor microwave/imager is a seven-channel, four-frequency, linearly polarized passive microwave radiometer system. It is flown on board the United States Air Force Defense Meteorological Satellite Program Block 5D-2 satellites. The instrument measures surface/atmospheric microwave...

 (SSMI) and Advanced Microwave Scanning Radiometer (AMSR).

Since 1979, satellites have provided a consistent continuous record of sea ice.
Satellite images of sea ice are made from observations of microwave
Microwave
Microwaves, a subset of radio waves, have wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz and 300 GHz. This broad definition includes both UHF and EHF , and various sources use different boundaries...

 energy radiated from the Earth’s surface. Because ocean water emits microwaves differently than sea ice, ice “looks” different to the satellite sensor—see sea ice emissivity modelling
Sea ice emissivity modelling
With increased interest in sea ice and its effects on the global climate, efficient methods are required to monitor both its extent and exchange processes. Satellite-mounted, microwave radiometers, such SSMI, AMSR and AMSU, are an ideal tool for the task because they can see through cloud cover,...

. 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.

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.

The threshold–based approach may seem less accurate, but it has the advantage of being more consistent. When scientists are analyzing satellite data, it is easier to say whether there is or isn’t at least 15 percent ice cover in a pixel than it is to say, for example, whether the ice cover is 70 percent or 75 percent. By reducing the uncertainty in the amount of ice, scientists can be more certain that changes in sea ice cover over time are real.

A careful analysis of satellite radar altimetry
Radar altimeter
A radar altimeter, radio altimeter, low range radio altimeter or simply RA measures altitude above the terrain presently beneath an aircraft or spacecraft...

 echoes can distinguish between those backscattered from the open ocean, new ice or multi-year ice. The difference between the elevation of the echoes from snow/sea ice and open water gives the elevation of the ice above the ocean; the ice thickness can computed from this. The technique has a limited vertical resolution - perhaps 0.5m - and is easily confused by the presence of even small amounts of open water. Hence it has mostly been used in the Arctic, where the ice is thicker and more continuous.

Submarines

Starting in 1958 U. S. Navy submarines
Submarines in the United States Navy
There are two major types of submarines in the United States Navy: ballistic missile submarines and attack submarines. In the U.S. Navy, all combatant submarines are nuclear-powered. Ballistic subs have a single, strategic mission: carrying nuclear submarine-launched ballistic missiles...

 collected upward-looking sonar
Sonar
Sonar is a technique that uses sound propagation to navigate, communicate with or detect other vessels...

 profiles, for navigation and defense, and converted the information into estimates of ice thickness. Data from U. S. and Royal Navy submarines
Royal Navy Submarine Service
The Royal Navy Submarine Service is the submarine element of the Royal Navy. It is sometimes known as the "Silent Service", on account of a submarine being required to operate quietly in order to remain undetected by enemy sonar...

 available from the NSIDC includes maps showing submarine tracks. Data are provided as ice draft profiles and as statistics derived from the profile data. Statistics files include information concerning ice draft characteristics, keels, level ice, leads, undeformed and deformed ice.

Buoys

Buoys are placed on the ice to measure ice properties and weather conditions by the participants of the International Arctic Buoy Program
International Arctic Buoy Program
The International Arctic Buoy Program is headquartered at the Polar Science Center, Applied Physics Laboratory, University of Washington, in Seattle, Washington, United States...

 and its sister, the International Programme for Antarctic Buoys
International Programme for Antarctic Buoys
The International Programme for Antarctic Buoys is a component of the WCRP.-Programme:It aims to establish and maintain a network of drifting buoys in the Antarctic sea-ice zone which monitor ice motion, pressure and temperature...

. Buoys can have sensors to measure air temperature, atmospheric pressure
Atmospheric pressure
Atmospheric pressure is the force per unit area exerted into a surface by the weight of air above that surface in the atmosphere of Earth . In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point...

, buoy position ice growth/melt, ice temperature, ocean currents, sea ice motion, sea level pressure, sea surface temperature, skin temperature, surface air temperature, surface winds, and water temperature.

Upward looking Sonar

Upward looking sonar (ULS) devices can be deployed under polar ice over a period of months or even years, and can provide a complete profile of ice thickness for a single site.

Auxiliary observations

Auxiliary observations of sea ice are made from shore stations, ships, and from aircraft
Aircraft
An aircraft is a vehicle that is able to fly by gaining support from the air, or, in general, the atmosphere of a planet. An aircraft counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines.Although...

.

Although in recent years remotely sensed data has come to play a major role in sea ice analysis, it is not yet possible to compile a complete and accurate picture of sea ice conditions from this data source alone. Auxiliary sea ice observations play a major role in confirming remotely sensed ice information or providing important corrections to the overall picture of ice conditions.

The most important auxiliary sea ice observation is the location of the ice edge. Its value reflects both the importance of the ice edge location in general and the difficulty of accurately locating the ice edge with remotely sensed data. It is also useful to provide a description of the ice edge in terms of indications of freezing or thawing, wind-driven advance or retreat, and compactness or diffuseness. Other important auxiliary information includes the location of the icebergs, floebergs, ice islands, old ice, ridging and hummocking. These ice features are poorly monitored by remote sensing techniques but are very important aspects of the ice cover.

Sea ice extent

Sea ice extent is the area of sea with a specified amount of ice, usually 15%. To satellite microwave sensors, surface melt appears to be open water rather than water on top of sea ice. So, while reliable for measuring area most of the year, the microwave sensors are prone to underestimating the actual ice concentration and area when the surface is melting.

Sea ice area

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 National Snow and Ice Data Center, one of NASA’s Distributed Active Archive Centers, monitors sea ice extent using a threshold of 15 percent.

Sea ice concentration

Sea ice concentration is the percentage of an area that is covered with sea ice.

Sea ice thickness

Sea ice thickness increases over time, and increases when winds and currents push the ice together. The European Space Agency's Cryosat-2
CryoSat-2
CryoSat-2 is a European Space Agency environmental research satellite which was launched in April 2010. It provides scientists with data about the polar ice caps and tracks changes in the thickness of the ice with a resolution of about . This information is useful for monitoring climate change...

 satellite was launched in April 2010 on a quest to map the thickness and shape of the Earth's polar ice cover. It's single instrument - a SAR/Interferometric Radar Altimeter is able to measure the difference between the height of the surface of sea ice and the water in open leads, the "freeboard" of the ice. Since 7/8ths of the ice is below the waterline, the computation of the thickness is fairly simple.

Sea ice age

The age of the ice is another key descriptor of the state of the sea ice cover, since older ice tends to be thicker and more resilient than younger ice. Sea ice rejects salt over time and becomes less salty resulting in a higher melting point. A simple two-stage approach classifies sea ice into first year and multiyear ice. First-year is ice that has not yet survived a summer melt season, while multi-year ice has survived at least one summer and can be several years old.
See sea ice growth processes
Sea ice growth processes
Sea ice is a complex composite composed primarily of pure ice in various states of crystallization along with air bubbles and included pockets of brine...

.

Sea mass balance

Sea mass balance is the balance of how much the ice grows in the winter and melts in the summer. For Arctic sea ice virtually all of the growth occurs on the bottom of the ice. Melting occurs on both the top and the bottom of the ice. In the vast majority of cases all of the snow melts during the summer, typically in just a couple of weeks. The mass balance is a powerful concept since it is the great integrator of the heat budget. If there is a net increase of heat, then the ice will thin. A net cooling will result in thicker ice.

Making direct measurements of the mass balance is simple. An array of stakes and thickness gauges is used to measure ablation and accumulation of ice and snow at the top and bottom of the ice cover. In spite of the importance of mass balance measurements and the relatively simple equipment involved in making them, there are few observational results. This is due, in large part, to the expense involved in operating a long-term field camp to serve as the base for these studies.

Sea ice volume

There are no Arctic-wide or Antarctic-wide measurements of the volume of sea ice, but the volume of the Arctic sea ice is calculated using the Pan-Arctic Ice Ocean Modeling and Assimilation System (PIOMAS) developed at the University of Washington Applied Physics Laboratory/Polar Science Center. PIOMAS blends satellite-observed sea ice concentrations into model calculations to estimate sea ice thickness and volume. Comparison with submarine, mooring, and satellite observations help increase the confidence of the model results.

Cryosat-2
CryoSat-2
CryoSat-2 is a European Space Agency environmental research satellite which was launched in April 2010. It provides scientists with data about the polar ice caps and tracks changes in the thickness of the ice with a resolution of about . This information is useful for monitoring climate change...

, launched in April 2010, has the ability to measure thickness across the entire Arctic Ocean Basin. This allows relatively simple calculation of the volume of the sea ice.

Trends in the data

Reliable and consistent records for all seasons are only available during the satellite era, from 1979 onwards.

Use of 1979 to 2000 as a Baseline

Scientists use the 1979 to 2000 average because it provides a consistent baseline for year-to-year comparisons of sea ice extent.

Southern hemisphere

Wintertime Antarctic sea ice has a small positive trend that is statistically insignificant

Records before the satellite era are sparse. William K. de la Mare, 1997, in Abrupt mid-twentieth-century decline in Antarctic sea-ice extent from whaling records found a southwards shift in ice edge based on whaling records; these findings have been questioned, but later papers by de la Mare support the same conclusion.

Northern hemisphere

According to scientific measurements, both the thickness and extent of summer sea ice in the Arctic have shown a dramatic decline over the past thirty years.

See also

  • Sea ice emissivity modelling
    Sea ice emissivity modelling
    With increased interest in sea ice and its effects on the global climate, efficient methods are required to monitor both its extent and exchange processes. Satellite-mounted, microwave radiometers, such SSMI, AMSR and AMSU, are an ideal tool for the task because they can see through cloud cover,...

  • Sea ice growth processes
    Sea ice growth processes
    Sea ice is a complex composite composed primarily of pure ice in various states of crystallization along with air bubbles and included pockets of brine...

  • Sea ice thickness
    Sea ice thickness
    Sea ice thickness is an important climate-related variable whose determination from satellite measurements is still an unsolved problem. While ice concentration is often used as a marker for climate change, the more important variable is sea ice volume which can be determined by multiplying...

  • Sea ice concentration
    Sea ice concentration
    Sea ice concentration is a useful variable for climatescientists and nautical navigators. It is defined as the area ofsea ice relative to the total at a given point in the ocean....


External links

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