| Summary: | Using data from the first Earth-orbiting laser altimeter, the Geoscience Laser Altimeter System (GLAS), onboard the Ice, Cloud and land Elevation Satellite (ICESat), we analyze sea ice freeboard in the Arctic Ocean. We describe a new method for sea surface height retrieval, which relies on an algorithm that discriminates laser pulse reflections originating over leads or thin ice. The lead detection algorithm is based on surface reflectivity and analysis of parameters associated with the shape of reflected waveforms. Using knowledge of the local sea surface height and sea ice elevation, we estimate sea ice freeboard and present, for the first time, a time series of Arctic freeboard spanning 5 years between March 2003 and 2008. While the autumn (October-November) and winter (February-March) data illustrate the seasonal and interannual variations in freeboard, following the September 2007 record minimum sea ice extent, the autumn 2007 and winter 2008 spatially averaged freeboards are below the seasonal means at -4.5 cm and -6.8 cm, respectively. Over the observation period, mean freeboard has declined at a rate of similar to-1.8 cm/a during the autumn period and similar to-1.6 cm/a during the winter period, in the region bounded by the northern limit of ICESat coverage at 86 degrees N. Because of the short 5-year observation period, it is unclear whether these results represent a long-term, downward trend in Arctic freeboard or are part of a natural variability. Furthermore, since the variability of snow thickness is included in the ICESat freeboard signal, a decrease in the freeboard cannot wholly be attributed to a decrease in sea ice thickness.
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