Sea Ice
Sea ice is formed when ocean water is cooled below its freezing temperature of approximately -2°C or 29°F. Such ice extends on a seasonal basis over great areas of the ocean. Sea ice is important to the study of oceans because it impacts oceanic chemical and physical properties, density structure, oceanic dynamics, and exchanges between the ocean and the atmosphere. It covers over 20 million square kilometers of the ocean at any given time, greatly limiting the exchange of heat, moisture, and momentum between the atmosphere and ocean and reflecting most of the solar radiation incident upon it.
Left Image. Close-up photograph of sea ice floes in the Bering Sea, between Alaska and Siberia, in February 1981. Sea ice floes in Alaska that look like flat sheets of ice and snow floating in the sea. Note the small rims around the edges of many of the floes. These rims form as the ice floes jostle against each other, moved by winds, waves, and currents. The larger floes in the photo are 5-10 m across. Credit: Claire Parkinson/NASA GSFC.
Right Image. This aerial photo shows shows new sea ice production south of St. Lawrence Island in the Bering Sea. Sea ice often forms down-wind of coastlines as illustrated in this aerial photo. The wind maintains an open area of sea water, allowing new ice to form. The colder the air temperature and the stronger the wind the greater the heat loss from the exposed ocean surface and the greater the ice production. Credit: D.Cavalieri, NASA/GSFC
During the process of sea ice formation, salt is released to the underlying ocean. This salt flux makes the upper ocean more dense, which may result in the deepening of the mixed layer and, in some instances, overturning and even denser bottom water formation. During the process of sea ice melt, relatively fresh water is introduced into the sea making the upper layer of ocean more stable and less likely to overturn. This process is particularly important when North Atlantic sea ice is transported toward the equator, a region known for large-scale deep-ocean convection.
This illustration shows the many forms of ice in the natural environment. At high elevations and/or high latitudes, snow that falls to the ground can gradually build up to form thick consolidated ice masses called glaciers. Glaciers flow downhill under the force of gravity and can extend into areas that are too warm to support year-round snow cover. The snow line, called the equilibrium line on a glacier or ice sheet, separates the ice areas that melt on the surface and become snow free in summer (net ablation zone) from the ice areas that remain snow covered during the entire year (net accumulation zone). Snow near the surface of a glacier that is gradually being compressed into solid ice is called firn.
Ice exists in the natural environment in many forms. Ice sheets are the largest forms of glaciers in the world and have smaller outlet glaciers or ice streams near their margins. In some places where the ice sheets reach the ocean, floating glacier tongues are formed. Icebergs are floating ice masses that have broken away from ice shelves, glacier tongues, or directly from the grounded ice sheet in some locations. Sea ice, which is produced when saline ocean water is cooled below its freezing temperature of approximately -2°C or 29°F, extends on a seasonal basis over great areas of the ocean.
Sea ice and icebergs are both carried by winds and currents into warmer waters. Melt water from sea ice, ice shelves, glacier tongues, and icebergs does not contribute to sea level rise, because these ice masses already displace an equivalent amount of sea water. However, sea level rise is caused by the flow of grounded glacial ice into the ocean and by surface or subsurface melt water discharged from the glacier, if the sum of those amounts exceeds the amount of ice accumulated from snowfall on the glacier or ice sheet. Credit: NASA GSFC, Graphic courtesy of Christopher Shuman, Claire Parkinson, Dorothy Hall, Robert Bindschadler, and Deborah McLean.
Measuring Ice
Sea ice is measured from space using both active and passive sensors operating at a variety of wavelengths from visible to infrared to microwave. The passive sensors operating at visible wavelengths such as Landsat ETM+ and Terra and Aqua MODIS provide the highest spatial resolution, typically from 15 meters to 1 kilometer. Active sensors, like radars and lasers, send a signal out and receive it back, whereas passive sensors passively receive radiation coming to the instrument from elsewhere.
This image of Antarctica shows a giant iceberg next to the Drygalski Ice Tongue. The Ross Sea is packed with sea ice. The image was collected by the Moderate Resolution Imaging Spectroradiometer (MODIS) instruments on NASA's Aqua and Terra Satellites. The giant B-15A iceberg has menaced the Drygalski Glacier Tongue since December 2004. At 122 kilometers (76 miles) in length by 28 kilometers (17 miles) in width, the bullying iceberg charged with great momentum towards the ice tongue, threatening to shatter the floating extension of the Davis Glacier. A scant three miles from Drygalski, B-15A ground to a stop, most likely grounded in the shallower waters near the shore. In the weeks that followed, the iceberg rotated free, until finally it began to drift past the ice tongue into the Ross Sea. Just when it looked as if Drygalski might escape a collision, B-15A delivered a glancing blow, knocking the end of the ice tongue loose.
Microwave sensors have the advantages that they can "see" in darkness as well as light and that, at particular microwave wavelengths, they are also able to see through clouds. Passive microwave sensors have resolutions ranging from about 5 kilometers to 50 kilometers depending on the particular microwave wavelength used. Because of their ability to see through clouds and darkness, passive microwave sensors have been used to provide a long-term climate record. The particular sensors include the Nimbus 7 Scanning Multichannel Microwave Radiometer (1978-1987), the series of DMSP Special Sensor Microwave Imagers (1987-present), and the more recent Aqua Advanced Microwave Scanning Radiometer for EOS (2002-present). Active sensors operating at visible wavelengths include the ICESat Geoscience Laser Altimeter System that provides information on sea ice thickness, whereas RADARSAT, an active microwave sensor, provides sea ice information at higher spatial resolutions (~ 100 meters) than the passive microwave systems.
Pictured are two images of sea ice in the Beaufort Sea off the north coast of Alaska. The image on the left is a black and white RADARSAT satellite image showing 4 classifications of sea ice. There is one classification for multiyear ice and three classifications for first year and younger ice. Red arrows at the top of the image highlight 'frost flowers', small faint white areas (newer ice) on top of multiyear ice. The multi-colored image on the right is from the MISR (Multi-angle Imaging SpectroRadiometer) instrument. This instrument shows 6 classifications. One classification is for clouds, 2 for multiyear ice, 2 for first year ice and one last classification for mostly thin, younger ice. The areas that are faint white in the SAR image are indicated in red in the MISR image. Refer to caption for more information about these areas.
Determining the amount and type of sea ice in the polar oceans is crucial to improving our knowledge and understanding of polar weather and long term climate fluctuations. These views from two satellite remote sensing instruments; the synthetic aperture radar (SAR) on board the RADARSAT satellite and the Multi-angle Imaging SpectroRadiometer (MISR), illustrate different methods that may be used to assess sea ice type. Sea ice in the Beaufort Sea off the north coast of Alaska was classified and mapped in these concurrent images acquired March 19, 2001 and mapped to the same geographic area.
RADARSAT SAR classifies sea ice types primarily by how the surface and subsurface roughness influence radar backscatter. In the SAR image, on the left, white lines delineate different sea ice zones as identified by the National Ice Center. Regions of mostly multiyear ice (A) are separated from regions with large amounts of first year and younger ice (B-D), and the dashed white line at bottom marks the coastline. In general, sea ice types that exhibit increased radar backscatter appear bright in SAR and are identified as rougher, older ice types. Younger, smoother ice types appear dark to SAR. Near the top of the SAR image, however, red arrows point to bright areas in which large, crystalline "frost flowers" have formed on young, thin ice, causing this young ice type to exhibit an increased radar backscatter. Frost flowers are strongly backscattering at radar wavelengths (cm) due to both surface roughness and the high salinity of frost flowers, which causes them to be highly reflective to radar energy.
Surface roughness is also registered by MISR, although the roughness observed is at a different spatial scale. Five classes of sea ice were found based upon the classification of MISR angular data. Very smooth ice areas that are predominantly forward scattering are colored red. Frost flowers are largely smooth to the MISR visible band sensor and are mapped as forward scattering. Some areas that may be first year or younger ice between the multi year ice floes are not discernible to SAR, illustrating how MISR potentially can make a unique contribution to sea ice mapping.
Credit: Image courtesy NASA/GSFC/LaRC/JPL, MISR Team. RADARSAT image courtesy NOAA Satellite Active Archive. Figure reprinted courtesy of IEEE.
Satellite observations of sea ice are used in a variety of ways. They are used on for navigation by ships operating in polar seas and for scientific studies by researchers interested in Earth system science and global change. Typically, the high spatial resolution measurements are used to study sea ice processes including the interaction of sea ice with the underlying ocean and the overlying atmosphere. They are also used to study sea ice kinematics and dynamics by measuring the displacement of a particular sea ice feature such as an ice floe from day to day. The lower resolution measurements provide near global coverage, and these data are used to study long-term trends in both the Northern Hemisphere and Southern Hemisphere sea ice covers.
The polar sea ice cover is very dynamic and is forced by winds and ocean currents. This image of the Arctic sea ice cover on March 1, 2003, obtained from the Aqua Advanced Microwave Scanning Radiometer for EOS (AMSR-E), shows the combination of both temperature and the emissivity of sea ice at 89 GHz. Patterns of leads (linear openings in the sea ice) appear darker than the surrounding thick sea ice. Generally, these areas of thin ice have a higher temperature because of the warmer sea water below. The 89 GHz channel used in generating this image provides the highest spatial resolution of about 5 km. Even at this spatial resolution individual ice flows can be observed. The green, brown, and white areas over land indicate increasing elevation. The dark circle over the pole is an area that is beyond the field of view of the instrument. Credit: NASA GSFC, Alvaro Ivanoff.
Trends we Observe
Based on the global passive microwave sea ice data sets collected since late 1978, sea ice extent has decreased in the Northern Hemisphere at the rate of approximately 3.0+0.4% per decade, whereas sea ice extent in the Southern Hemisphere has actually been increasing, at a rate of approximately 1.0+0.5% per decade. Both trends are statistically significant. Upon examining data back to the early 1970s (some of lesser quality), it's found that both the Northern Hemisphere and Southern Hemisphere have reductions in ice extent since the early 1970s, the Northern Hemisphere more so than the Southern Hemisphere.