Development of Satellite Sensing of Sea-Ice Thickness: A Major Contribution to Understanding Climate Change
Zwally, H. Jay
NASA Goddard Space Flight Center, UNITED STATES

Satellite observation of polar sea ice began with the advent of polar-orbiting meteorological satellites in the 1960s. Comprehensive information on the extent of sea ice was first shown in NOAA 30-day minimum-brightness composites that averaged-out cloud motion and showed the contrasting darker oceans and brighter sea-ice pack. Variations in grey-scale within the sea ice pack showed some areas of open water and thin ice. Similar information from high-resolution classified imagery was used by the US Navy for sea ice mapping, but was not available to the scientific community until US declassification of the Corona imagery in 1997, through the efforts of the Environmental Task Force established in 1992 by then Senator Al Gore. All-weather and all-season mapping of sea-ice extent and concentration began in December 1972 with the passive microwave imager (ESMR) on Nimbus-5. However, remote sensing of sea ice thickness was a major unfulfilled challenge, especially for the Arctic Ice Dynamics Joint Experiment (AIDJEX) in the 1970s. AIDJEX numerical models of sea-ice dynamics carried the ice-thickness-distribution (ITD) as a fundamental parameter. At that time, very limited information on thickness of the thinner ice types was obtained from grey-scale analysis of LandSat and aerial photography. During the cold war, extensive data on sea-ice draft (for estimation of thickness), was collected by submarine sonars in the Arctic Ocean, but that data remained mostly classified until about 1997 when data in the ''Gore-box'' was declassified.

Meanwhile in the late 1980s, Seymour Laxon was doing his PhD thesis on sea-ice measurements with satellite radar altimetry, interpreting the complex radar-return signals from the sea-ice pack. Before launch of the ERS-1 radar altimeter in 1991, ocean radar-altimeter measurements had been made to 72 degrees of latitude by SeaSat in 1978 and Geosat and Geosat-FO beginning in 1985. Because of the complexity of the radar signal from mixtures of sea-ice and open water in leads and polynyas (Fetterer, Laxon, Johnson, 1991), the use of altimetry in sea ice regions was initially limited to detection of the ice edge (Laxon, 1990). Potential measurement of the surface elevations of the sea ice and open water (or smooth-thin ice in leads) was very complicated by the mixed signals coming from the broad radar footprint. In addition, the altimeter tracker locked onto the strong signals from the leads, causing what appeared to be a sharply decreasing elevation below the ocean level called ''snagging''. Nevertheless somewhere along his pathway, Seymour Laxon got the seemingly-crazy idea that he could overcome those problems and figure out how to measure the sea-ice freeboard (height above the ocean surface). Somehow, Seymour and his colleagues developed a way to pick out the ocean- reference level from the initial part of the strong signals from the leads, and thereby measure sea-ice freeboard and estimate thickness. Working with ERS data, they produced the first satellite-derived maps of sea ice thickness (Laxon, Peacock, Smith, 2003). That paper in Nature, ''High interannual variability of sea ice thickness in the Arctic region'', was truly a scientific and technical landmark, achieving the long-sought goal of mapping sea-ice thickness from space and providing the primary scientific basis for ESA's CryoSat mission. One of the paper's conclusions was: ''Our results suggest that a continued increase in melt season length would lead to further thinning of Arctic sea ice''. In hindsight, that statement appears as an ominous forecast of the accelerated decline in the Arctic sea ice that was just beginning at that time.

Additional measurements of sea-ice freeboard and thickness that were made by the pioneering laser altimeter on ICESat (Zwally et al., 2008; Farrell et al., 2009; Kwok et al., 2009) provided the times series of Arctic ice thickness and volume (2003 to 2008) that enabled linking to thicknesses (1958 to 2000) derived over part of the Arctic Ocean from the de-classsified submarine-sonar data (Kwok and Rothrock, 2009).

While the area of Arctic sea ice remaining at the end of the summer melt season has declined to about 45% from the 1980s, the average thickness has declined to approximately 50% or less. Therefore, the volume of perennial ice is now down to nearly 20% of the volume in the 1980's, with thinner ice in both summer and winter. As the thickness of ice remaining from year to year continues to decrease, the ice pack may be passing a tipping point from which it will not recover, even without additional climate warming. The latest scientific contribution documenting these changes is ''CryoSat-2 estimates of Arctic sea ice thickness and volume'' (Laxon et al., accepted in mid-January 2013 for publication in GRL).

References:

Laxon, S. (1990). Seasonal and interannual variations in Antarctic sea ice extent as mapped by radar altimetry. Geophys. Res. Lett., 17 (10), 1553-1556.

N.R. Peacock, D.M. Smith (2003). High interannual variability of sea ice thickness in the Arctic region. Nature, 425, 947-950.

Laxon, S.W., K. A. Giles, A. L. Ridout, D. J. Wingham, R. Willatt, R. Cullen, R. Kwok, A. Schweiger, J. Zhang, C. Haas, S. Hendrick, R. Krishfield, N. Kurtz, S. Farrell, M. Davidson (2013). CryoSat-2 estimates of 1 Arctic sea ice thickness and volume. Geophys. Res. Lett.

Farrell, S.L., S. W. Laxon, D.C. McAdoo, D.Yi, H. J. Zwally (2009). Five years of Arctic sea ice freeboard measurements from the Ice, Cloud and land Elevation Satellite. J. Geophys. Res.-Oceans, 114, C04008, doi:10.1029/2008JC005074.

Fetterer, F.M., S. W Laxon, D.R. Johnson (1991). A comparison of Geosat altimeter and Synthetic Aperture Radar measurements over East Greenland pack ice. Int. J. Remote Sens., 12 (3), 569-583.

Kwok, R., G. F. Cunningham, M. Wensnahan, I. Rigor, H. J. Zwally, D. Yi (2009). Thinning and Volume Loss of the Arctic Ocean Sea Ice Cover: 2003 2008, J. Geophys. Res., 114, Co7oo5, doi:10.1029/2009JC005312.

Kwok, R., D. A. Rothrock (2009). Decline in Arctic sea ice thickness from submarine and ICESat records: 19582008.Geophys. Res. Lett 36, L15501, doi:10.1029/2009GL039035.

Zwally H. J., D. Yi, R. Kwok, Y. Zhao (2008), ICESat Measurements of Sea Ice Freeboard and Estimates of Sea Ice Thickness in the Weddell Sea, J. Geophys. Res., 113, C02S15, doi:10.1029/2007JC004284.