Search

Two uncontrolled mosaics were assembled at an ~1 : 1000 000 scale, covering for the first time almost the entire Larsen Ice Shelf area, using Landsat TM images of 1986-89 and the Kosmos KATE-200 photographic products of late 1975. By comparing them, it was possible to estimate the change along the 600 km north-south ice-shelfs seaward margin where substantial calving has occurred in the recent past. In overall extent, the ice shelf has decreased by ~9300km2 since 1975. The interpretation of the available satellite data also allowed estimates of the ice-shelf’s inland boundary and detection of new surface ice features, especially in the very disturbed region south of Jason Peninsula, which may help to explain the significant calving events occurring in this region. Estimates of surface velocities were derived by photographically co-registering sequential imagery in the chaotic rifted-crevassed area east of Kenyon Peninsula, where conspicuous ice features could be clearly detected even after a 10.4 year time interval. The Landsat TM mosaic, though uncontrolled, provides a basic data set for future studies of global change of the major ice shelf on the Antarctic Peninsula which is probably very sensitive to any climatic trend.

The rapid retreat and disintegration of the Larsen Ice Shelf sector extending north of Seal Nunataks (65° S), documented from the mid 1970s onwards by remote sensing, is presented and related to the Antarctic Peninsula climatic warming recorded over several past decades. A 1975 KOSMOS satellite photograph and a series of LANDSAT MSS and TM images taken in 1978, 1979, 1986, 1988 and 1989 were used to monitor the retreat of the ice shelf between Seal Nunataks and Prince Gustav Channel. The ice shelf has decreased by more than 30% during the period 1975–89 within the Christensen Island to Cape Longing region. Measurements of the ice front position carried out in the field during late 1991 indicate that the recession between Lindenberg Island and Sobral Peninsula is still continuing, in some places at a rate of up to 2.5 km a−1.

[1] It is argued that Larsen Ice Shelf contains marine ice formed by oceanic freezing and other mechanisms. Missing basal returns in airborne radar soundings and observations of a smooth and healed surface coincide downstream of regions where an ocean model predicts freezing. Visible imagery suggests that marine ice currently stabilizes Larsen C Ice Shelf and implicates failure of marine flow bands in the 2002 Larsen B Ice Shelf collapse. Ocean modeling indicates that any regime change towards the incursion of warmer Modified Weddell Deep Water into the Larsen C cavity could curtail basal freezing and its stabilizing influence.

The breakup of ice shelves has been widely regarded as an indicator of climate change1, with observations around the Antarctic Peninsula having shown a pattern of gradual retreat, associated with regional atmospheric warming and increased summer melt and fracturing processes2, 3, 4, 5, 6, 7, 8, 9. The rapid collapse of the northernmost section of the Larsen Ice Shelf (Larsen A), over a few days in January 1995, indicated that, after retreat beyond a critical limit, ice shelves may disintegrate rapidly. Here we use a finite-element numerical model that treats ice as a continuum without fracture10 to examine the breakup history2 between 1986 and 1997 of the two northern sections of Larsen Ice Shelf (Larsen A and Larsen B), from which we establish stability criteria for ice shelves. Analysis of various ice-shelf configurations reveals characteristic patterns in the strain rates near the ice front which we use to describe the stability of the ice shelf. On Larsen A, only the initial and final ice-front configurations show a stable pattern. Larsen B at present exhibits a stable pattern, but if the ice front were to retreat by a further few kilometres, it too is likely to enter an irreversible retreat phase.

This paper presents the results of the glacier Inventory of James Ross and Vega islands based on topographic map sheets at 1:250 000 scale, Landsat multi-spectral scanner imagery in bands 4 and 7 at the same scale, and oblique air photographs obtained in 1979–80. Work was completed following the methodology proposed by the Temporary Technical Secretariat of the World Glacier Inventory. This is the first such inventory carried out in Antarctica.

A variety of data are used to investigate Larsen B, which is at present the northernmost section of the Larsen Ice Shelf, Antarctic Peninsula. Recently declassified U.S. Argon satellite photographs from 1963 Kosmos photographs from 1975, Landsat images from 1986,1988 and 1990, ERS-1/2 synthetic aperture radar images from 1992-97, Radarsat from 1998 and field surveys are used to analyze the areal extent, surface characteristics and dynamic behaviour of this ice-shelf section over more than three decades. Visible and radar imagery together with field observations are used synergistically to describe the ice-shelf morphology, including meltwater features and rifts. In contrast to the retreat of the ice-shelf sections in the north, Larsen B advanced steadily from 1963 to early 1995, when the area decreased significantly due to a major calving event. Analysis of different satellite images indicates that melting is proceeding further south in accordance with the regional warming trend. In addition? fracturing processes and rapid development of new rifts are observed, associated with recent acceleration of ice motion close to the front. All observations predict major calving events for this ice-shelf section in the near future.

The motion field of the northern Larsen Ice Shelf, Antarctic Peninsula, was analyzed, based on Landsat data from 1986 to 1989, Earth Resources Satellite (ERS) synthetic aperture radar (SAR) data from 1992 to 1997, and comparative field measurements along three transects. During this period the northern sections of the ice shelf showed steady retreat, which culminated in the disintegration of the two ice-shelf sections north of Seal Nunataks in January 1995. Velocities of these two sections were derived by cross-correlation, using SAR images of 1 year time intervals and Landsat images of 1-3 year intervals. A slight increase of velocity was observed as crevasses and rifts opened before the final disintegration. In addition, an interferometric motion analysis was carried out for the ice shelf around and south of Seal Nunataks, based on an image pair from the ERS-1/2 Tandem Mission in 1995. This analysis reveals a complex pattern of tidal flexure in the grounding zones, as well as rifting and shear zones on the ice shelf. In addition, the motion of the main input glaciers was derived.

Using a combination of satellite sensors, field measurements and satellite-uplinked in situ observing stations, we examine the evolution of several large icebergs drifting east of the Antarctic Peninsula towards South Georgia Island. Three styles of calving are observed during drift: ‘rift calvings’, ‘edge wasting’ and ‘rapid disintegration’. Rift calvings exploit large pre-existing fractures generated in the shelf environment and can occur at any stage of drift. Edge wasting is calving of the iceberg perimeter by numerous small edge-parallel, sliver-shaped icebergs, preserving the general shape of the main iceberg as it shrinks. This process is observed only in areas north of the sea-ice edge. Rapid disintegration, where numerous small calvings occur in rapid succession, is consistently associated with indications of surface melt saturation (surface lakes, firn-pit ponding). Freeboard measurements by ICESat indicate substantial increases in ice-thinning rates north of the sea-ice edge (from −1 to >30 m a−1), but surface densification is shown to be an important correction (>2 m freeboard loss before the firn saturates). Edge wasting of icebergs in ‘warm’ surface water (sea-ice-free, >−1.8°C) implies a mechanism based on waterline erosion. Rapid disintegration (‘Larsen B-style’ break-up) is likely due to the effects of surface or saturated-firn water acting on pre-existing crevasses, or on wave- or tidally induced fractures. Changes in microwave backscatter of iceberg firn as icebergs drift into warmer climate and experience increased surface melt suggest a means of predicting when floating ice plates are evolving towards disintegration.

The motion field of the northern Larsen Ice Shelf, Antarctic Peninsula, was analyzed using radar interferometry in combination with field measurements and finite-element model calculations. The ice shelf between Jason Peninsula and Seal Nunataks has been in steady retreat since January 1995. Model calculations suggest that the ice shelf is in a stage of irreversible retreat since the last calving events in summer 1998/99. The interferometric analysis is based on synthetic-aperture radar data of the tandem mission of European remote-sensing satellites ERS-1 and –2 in austral spring 1995. The phase contributions due to tidal motion were estimated from the vertical displacement at those parts of the grounding zone where the horizontal motion is close to zero" in order to separate the vertical and horizontal motion components over the ice shelf. Satellite-derived velocities compare well with the long-term field measurements along a transverse and a longitudinal profile. The real interferograms and synthetic interferograms, calculated from model velocities, show reasonable agreement over the main parts of the ice shelf, but differ in the boundary zones where the details are not resolved by the model.