Abstract. Two ice-dynamic transitions of the Antarctic ice sheet -the boundary of grounded ice features and the freelyfloating boundary -are mapped at 15-m resolution by participants of the International Polar Year project ASAID using customized software combining Landsat-7 imagery and ICESat/GLAS laser altimetry. The grounded ice boundary is 53 610 km long; 74 % abuts to floating ice shelves or outlet glaciers, 19 % is adjacent to open or sea-ice covered ocean, and 7 % of the boundary ice terminates on land. The freelyfloating boundary, called here the hydrostatic line, is the most landward position on ice shelves that expresses the full amplitude of oscillating ocean tides. It extends 27 521 km and is discontinuous. Positional (one-sigma) accuracies of the grounded ice boundary vary an order of magnitude ranging from ±52 m for the land and open-ocean terminating segments to ±502 m for the outlet glaciers. The hydrostatic Correspondence to: R. Bindschadler (robert.a.bindschadler@nasa.gov) line is less well positioned with errors over 2 km. Elevations along each line are selected from 6 candidate digital elevation models based on their agreement with ICESat elevation values and surface shape inferred from the Landsat imagery. Elevations along the hydrostatic line are converted to ice thicknesses by applying a firn-correction factor and a flotation criterion. BEDMAP-compiled data and other airborne data are compared to the ASAID elevations and ice thicknesses to arrive at quantitative (one-sigma) uncertainties of surface elevations of ±3.6, ±9.6, ±11.4, ±30 and ±100 m for five ASAID-assigned confidence levels. Over one-half of the surface elevations along the grounded ice boundary and over one-third of the hydrostatic line elevations are ranked in the highest two confidence categories. A comparison between ASAID-calculated ice shelf thicknesses and BEDMAP-compiled data indicate a thin-ice bias of 41.2 ± 71.3 m for the ASAID ice thicknesses. The relationship between the seaward offset of the hydrostatic line from the grounded ice boundary only weakly matches a Published by Copernicus Publications on behalf of the European Geosciences Union.

Antarctic sea ice that has been affected by supercooled Ice Shelf Water (ISW) has a unique crystallographic structure and is called platelet ice. In this paper we synthesize platelet ice observations to construct a continent‐wide map of the winter presence of ISW at the ocean surface. The observations demonstrate that, in some regions of coastal Antarctica, supercooled ISW drives a negative oceanic heat flux of −30 Wm−2 that persists for several months during winter, significantly affecting sea ice thickness. In other regions, particularly where the thinning of ice shelves is believed to be greatest, platelet ice is not observed. Our new data set includes the longest ice‐ocean record for Antarctica, which dates back to 1902 near the McMurdo Ice Shelf. These historical data indicate that, over the past 100 years, any change in the volume of very cold surface outflow from this ice shelf is less than the uncertainties in the measurements.

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Changes of Larsen Ice Shelf, Antarctica, and the surrounding glaciers after its collapse in 1995 were investigated using satellite radar imagery, with emphasis on changes in the glaciers which previously nourished the ice shelf north of Seal Nunataks and now calve directly into the sea. The large glaciers retreated several kilometres inland of the previous grounding line. The velocity field of Drygalski Glacier, the largest glacier in this area, was mapped by means of interferograms derived from pairs of European Remote-sensing Satellite synthetic aperture radar images from 1995 and 1999. The main part of the glacier showed a significant acceleration of flow over these 4 years, with an increase of velocity up to three-fold at the terminus. Similar accelerations were observed by means of interferometry on several other grounded glaciers, suggesting that the removal of ice shelves could lead to an effect on eustatic sea level. For Larsen B, the northernmost surviving part of Larsen Ice Shelf, the retreat of the ice front to October 2000 is documented.

The boundary of grounded ice and the location of ice transitioning to a freely floating state are mapped at 15-m resolution around the entire continent of Antarctica. These data products are produced by participants of the International Polar Year project ASAID using customized software combining Landsat-7 imagery and ICESat laser altimetry. The grounded ice boundary is 53 610 km long; 74% of it abuts to floating ice shelves or outlet glaciers, 19% is adjacent to open or sea-ice covered ocean, and 7% of the boundary are land terminations with bare rock. Elevations along each line are selected from 6 candidate digital elevation models: two created from the input ICESat laser altimetry and Landsat data, two from stereo satellite imagery, and two from compilations of primarily radar altimetry. Elevation selection and an assignment of confidence in the elevation value are based on agreement with ICESat elevation values and shape of the surface inferred from the Landsat imagery. Elevations along the freely-floating boundary (called the hydrostatic line) are converted to ice thicknesses by applying a firn-correction factor and a flotation criterion. The relationship between the seaward offset of the hydrostatic line from the grounding line only weakly matches a prediction based on beam theory. Airborne data are used to validate the technique of grounding line mapping, elevation selection and ice thickness derivation. The mapped products along with the customized software to generate them and a variety of intermediate products are available from the National Snow and Ice Data Center

The areal changes of the northern Larsen Ite Shelf (LIS), Antarctic Peninsula, between March 1986 and March 1997 have been analyzed, based on synthetic aperture radar images of the European remote-sensing satellites ERS-1 and ERS-2 and on Landsat images. This analysis is complemented by data on ice motion and surface mass balance which have been obtained during several field campaigns since the early 1980s. After a period of retreat, coinciding with atmospheric warming and with decreasing net accumulation at the surface due to melt losses, the two northernmost sections of LIS disintegrated almost completely within a few days in January 1995. Recent observations of the ice-shelf section north of Jason Peninsula, which is presently the northernmost section of LIS, show increased summer melt and intensification of the rifting processes, probably causing accelerated retreat of this section in the near future. The retreat and the disintegration event of LIS indicate high sensitivity of ice shelves to prolonged perturbations of the mass balance.

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Abstract. This is an investigation to quantify the influence of the sub-ice platelet layer on satellite measurements of total freeboard and their conversion to thickness of Antarctic sea ice. The sub-ice platelet layer forms as a result of the seaward advection of supercooled ice shelf water from beneath ice shelves. This ice shelf water provides an oceanic heat sink promoting the formation of platelet crystals which accumulate at the sea ice-ocean interface. The build-up of this porous layer increases sea ice freeboard, and if not accounted for, leads to overestimates of sea ice thickness from surface elevation measurements. In order to quantify this buoyant effect, the solid fraction of the sub-ice platelet layer must be estimated. An extensive in situ data set measured in 2011 in McMurdo Sound in the southwestern Ross Sea is used to achieve this. We use drill-hole measurements and the hydrostatic equilibrium assumption to estimate a mean value for the solid fraction of this sub-ice platelet layer of 0.16. This is highly dependent upon the uncertainty in sea ice density. We test this value with independent Global Navigation Satellite System (GNSS) surface elevation data to estimate sea ice thickness. We find that sea ice thickness can be overestimated by up to 19 %, with a mean deviation of 12 % as a result of the influence of the sub-ice platelet layer. It is concluded that within 100 km of an ice shelf this influence might need to be considered when undertaking sea ice thickness investigations using remote sensing surface elevation measurements.