4.4.3 Sea Ice Thickness

4.4.3.1 Sea Ice Thickness Data Sources and Time Periods Covered

Until recently there have been no satellite remote sensing techniques capable of mapping sea ice thickness, and this parameter has primarily been determined by drilling or by under-ice sonar measurement of draft (the submerged portion of sea ice).

Subsea sonar from submarines or moored instruments can be used to measure ice draft over a footprint of 1 to 10 m diameter. Draft is converted to thickness, assuming an average density for the measured floe, including its snow cover. The principal challenges to accurate observation with sonar are uncertainties in sound speed and atmospheric pressure, and the identification of spurious targets. Upward-looking sonar has been on submarines operating beneath arctic pack ice since 1958. US and UK naval data are now being released for science, and some dedicated arctic submarine missions were made for science during 1993 to 1999. Ice draft measurement by moored ice-profiling sonar, best suited to studies of ice transport or change at fixed sites, began in the Arctic in the late 1980s. Instruments have operated since 1990 in the Beaufort and Greenland Seas and for shorter intervals in other areas, but few records span more than 10 years. In the SH there are no data from submarines and only short time series from moored sonar.

Other techniques, such as electromagnetic induction sounders deployed on the ice surface, ships or aircraft, or airborne laser altimetry to measure freeboard (the portion of sea ice above the waterline), have limited applicability to wide-scale climate analysis of sea ice thickness. Indirect estimates, based on measurement of surface gravity waves, are available in some regions for the 1970s and 1980s (Nagurnyi et al., 1999 as reported in Johannessen et al., 2004), but the accuracy of these estimates is difficult to quantify.

Quantitative data on the thickness of antarctic pack ice only started to become available in the 1980s from sparsely scattered drilling programs covering only small areas and primarily for use in validating other techniques. Visual observations of ice characteristics from ships (Worby and Ackley, 2000) are not adequate for climate monitoring, but are providing one of the first broad pictures of antarctic sea ice thickness.