Calving from tidewater glaciers and ice shelves accounts for around half the mass loss from both polar ice sheets, yet the process is not well represented in prognostic models of ice dynamics. Benn and others proposed a calving criterion appropriate for both grounded and floating glacier tongues or ice shelves, based on the penetration depth of transverse crevasses near the calving front, computed using Nye's formula. The criterion is readily incorporated into glacier and ice-sheet models, but has not been fully validated with observations. We apply a three-dimensional extension of Benn and others' criterion, incorporated into a full-Stokes model of glacier dynamics, to estimate the current position of the calving front of Johnsons Glacier, Antarctica. We find that two improvements to the original model are necessary to accurately reproduce the observed calving front: (1) computation of the tensile deviatoric stress opening the crevasse using the full-stress solution and (2) consideration of such a tensile stress as a function of depth. Our modelling results also suggest that Johnsons Glacier has a polythermal structure, rather than the temperate structure suggested by earlier studies.
A new three-dimensional finite-element model of the steady-state dynamics of temperate glaciers has been developed and applied to Johnsons Glacier, Livingston Island, Antarctica, with the aim of determining the velocity and stress fields for the present glacier configuration. It solves the full Stokes system of differential equations without recourse to simplifications such as those involved in the shallow-ice approximation. Rather high values of the stiffness parameter B (∼0.19–0.23MPaa1/3) are needed to match the observed ice surface velocities, although these results do not differ much from those found by other authors for temperate glaciers. Best-fit values of the coefficient k in the sliding law (*2.2–2.7 x 103m a–1MPa–2) are also of the same order of magnitude as those found by other authors. The results for velocities are satisfactory, though locally there exist significant discrepancies between computed and observed ice surface velocities, particularly for the vertical ones. This could be due to failures in the sliding law (in particular, the lack of information on water pressure), the use of an artificial down-edge boundary condition and the fact that bed deformation is not considered. For the whole glacier system, the driving stress is largely balanced by the basal drag (80% of the driving stress). Longitudinal stress gradients are only important in the divide areas and near the glacier terminus, while lateral drag is only important at both sides of the terminal zone.
Aldegondabreen is a small valley glacier, ending on land, located in the Grønfjorden area of Spitsbergen, Svalbard. Airborne radio-echo sounding in 1974/75, using a 440 MHz radar, revealed a polythermal two-layered structure, which has been confirmed by detailed ground-based radio-echo sounding done in 1999 using a 15 MHz monopulse radar. The 1999 radar data reveal an upper cold layer extending down to 90m depth in the southern part of the glacier, where the thickest ice (216 m) was also found. A repeated pattern of diffractions from the southern part of the glacier, at depths of 50–80 m and dipping down-glacier, has been interpreted as an englacial channel which originates in the temperate ice. From joint analysis of the 1936 topographic map, a digital elevation model constructed from 1990 aerial photographs and the subglacial topography determined from radar data, a severe loss of mass during the period 1936–90 has been estimated: a glacier tongue retreat of 930 m, a decrease in area from 8.9 to 7.6 km2, in average ice thickness from 101 to 73 m and in ice volume from 0.950 to 0.558 km3, which are equivalent to an average annual balance of –0.7 mw.e. This is comparable with the only available data of net mass balance for Aldegondabreen (–1.1 and –1.35m w.e. for the balance years 1976/77 and 2002/03) and consistent with the 0.27˚C increase in mean summer air temperature in this zone during 1936–90, as well as the warming in Spitsbergen following the end of the Little Ice Age (LIA), and the general glacier recession trend observed in this region.
Nowadays, there is an uprising social pressure on big companies to incorporate into their decisionmaking process elements of the so-called social responsibility. Among the many implications of this fact, one relevant one is the need to include this new element in classic portfolio selection models. This paper meets this challenge by formulating a model that combines goal programming with "goal games" against nature in a scenario where the social responsibility is defined through the introduction of a battery of sustainability indicators amalgamated into a synthetic index. In this way, we have obtained an efficient model that only implies solving a small number of linear programming problems. The proposed approach has been tested and illustrated by using a case study related to the selection of securities in international markets.
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