J. A. Bohlander

The Link Between Climate Warming and Break-Up of Ice Shelves in the Antarctic Peninsula

A review of in situ and remote-sensing data covering the ice shelves of the Antarctic Peninsula provides a series of characteristics closely associated with rapid shelf retreat: deeply embayed ice fronts; calving of myriad small elongate bergs in punctuated events; increasing flow speed; and the presence of melt ponds on the ice-shelf surface in the vicinity of the break-ups. As climate has warmed in the Antarctic Peninsula region, melt-season duration and the extent of ponding have increased. Most break-up events have occurred during longer melt seasons, suggesting that meltwater itself, not just warming, is responsible. Regions that show melting without pond formation are relatively unchanged. Melt ponds thus appear to be a robust harbinger of ice-shelf retreat. We use these observations to guide a model of ice-shelf flow and the effects of meltwater. Crevasses present in a region of surface ponding will likely fill to the brim with water. We hypothesize (building on Weertman (1973), Hughes (1983) and Van der Veen (1998)) that crevasse propagation by meltwater is the main mechanism by which ice shelves weaken and retreat. A thermodynamic finite-element model is used to evaluate ice flow and the strain field, and simple extensions of this model are used to investigate crack propagation by meltwater. The model results support the hypothesis.

Mapping the grounding zone of the Amery Ice Shelf, East Antarctica using InSAR, MODIS and ICESat

Abstract We use a combination of satellite techniques (interferometric synthetic aperture radar (InSAR), visible-band imagery, and repeat-track laser altimetry) to develop a benchmark map for the Amery Ice Shelf (AIS) grounding zone (GZ), including its islands and ice rises. The break-in-slope, as an indirect estimate of grounding line location, was mapped for the entire AIS. We have also mapped ∼55% of the landward edge and ∼30% of the seaward edge of the ice shelf flexure boundary for the AIS perimeter. Vertical ice motion from Global Positioning System receivers confirms the location of the satellite-derived GZ in two regions. Our map redefines the extent of floating ice in the south-western AIS and identifies several previously unmapped grounded regions, improving our understanding of the stresses supporting the current dynamical state of the ice shelf. Finally, we identify three along-flow channels in the ice shelf basal topography, approximately 10 km apart, 1.5 km wide and 300–500 m deep, near the southern GZ. These channels, which form at the suture zones between ice streams, may represent zones of potential weakness in the ice shelf and may influence sub-ice-shelf ocean circulation.

From ice-shelf tributary to tidewater glacier: continued rapid recession, acceleration and thinning of Rohss Glacier following the 1995 collapse of the Prince Gustav Ice Shelf, Antarctic Peninsula

Glasser, N. F., Scambos, T. A., Bohlander, J., Truffer, M., Pettit, E., Davies, B. J. (2011). Journal of Glaciology, 57 (203), 397-406.

Glaciological characteristics of Institute Ice Stream using remote sensing

We assess the ice flow of Institute Ice Stream (IIS; 81.5°S, 75°W) and the adjacent Ronne Ice Shelf using satellite images and geophysical parameters from recent continent-wide compilations. Landsat image pairs from the 1980s and 1990s are used to determine ice velocity. Peak speed is 398 ± 10 m a−1. Several mappings using images spanning an eleven-year period indicate this speed and the pattern of ice flow throughout the mapped portion of the stream is constant to within ± 20 m a−1. Combining catchment extent (141 700 km2) with surface accumulation, mass input to IIS is 25.1 ± 2 Gt a−1. Mean ice thickness across the grounding line is 1177 m. Mass flux to the Ronne Ice Shelf, determined from these values and our velocity profile, is 22.7 ± 2 Gt a−1. Topographic mapping using photoclinometry, coupled with ice thickness and ice velocity, permits an assessment of driving force versus flow speed. This indicates wide variations in basal resistance. Despite evidence of present-day near-balance and constant speed in the ice stream trunk, a recent change in outflow is implied by folding of shelf streaklines near Korff Ice Rise. This may be a result of changing shelf thickness or erosion of Doake Ice Rumples.

Ice Flow and Morphology of Prestrud Inlet and Western Sulzberger Ice Shelf, West Antarctica

Ice velocity measurements for the period 1986-1989 are derived from three pairs of Landsat images covering the Prestrud Inlet and western Sulzberger Ice Shelf, West Antarctica. An image cross-correlation technique is used to track surface features and determine ice displacement. Maximum speed for the Prestrud Inlet during this period is 170 ± 25 ma-1. Similar speeds are found for Kiel Glacier, the primary source of ice flowing into the Prestrud. Streakline relationships between Ross Ice Shelf flow and flow from Prestrud Inlet suggest little change in relative flux over the past few centuries. The western Sulzberger Ice Shelf divides into two outlet regions. Maximum ice speed in the westernmost outlet is 307 ± 25 ma-1. The eastern outlet has speeds near 80-100 ma-1. Comparisons with vectors from previous studies indicate little change in velocity since 1972 for the Sulzberger. This study found no evidence of melt ponds in the Landsat image record, although surface melting does occur.