Calving processes and the dynamics of calving glaciers |
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| Institution: | 1. Earth and Atmospheric Sciences Department, Cornell University, Ithaca, NY 14850, USA;2. Geological Sciences, University of North Carolina, Chapel Hill, NC 27599, USA;1. Norwegian Polar Institute, Framsenteret, Tromsø 9296, Norway;2. British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, UK;3. Department of Geography, Durham University, Lower Mountjoy, South Road, Durham DH1 3LE, UK;4. Department of Earth and Space Sciences, University of Washington, Seattle, WA 98195-1310, USA;5. Laboratoire de Glaciologie, Université Libre de Bruxelles, Avenue F.D. Roosevelt 50, B-1050 Brussels, Belgium;6. CNRS, LGGE, F-38041 Grenoble, France;7. University Grenoble Alps, LGGE, F-38041 Grenoble, France;8. School of Geoscience, University of Edinburgh, Drummond Street, Edinburgh EH8 9XP, UK;9. Akvaplan-niva AS, Framsenteret, Tromsø 9296, Norway;10. Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research, Bremerhaven, Germany;11. Institute for Marine and Atmospheric Research, Utrecht University, P.O. Box 80005, 3508 TA Utrecht, The Netherlands;12. School of Geography, Politics, and Sociology, Newcastle University, Newcastle Upon Tyne NE1 7RU, UK;13. National Snow and Ice Data Center, Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO 80309-0449, USA |
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| Abstract: | Calving of icebergs is an important component of mass loss from the polar ice sheets and glaciers in many parts of the world. Calving rates can increase dramatically in response to increases in velocity and/or retreat of the glacier margin, with important implications for sea level change. Despite their importance, calving and related dynamic processes are poorly represented in the current generation of ice sheet models. This is largely because understanding the ‘calving problem’ involves several other long-standing problems in glaciology, combined with the difficulties and dangers of field data collection. In this paper, we systematically review different aspects of the calving problem, and outline a new framework for representing calving processes in ice sheet models. We define a hierarchy of calving processes, to distinguish those that exert a fundamental control on the position of the ice margin from more localised processes responsible for individual calving events. The first-order control on calving is the strain rate arising from spatial variations in velocity (particularly sliding speed), which determines the location and depth of surface crevasses. Superimposed on this first-order process are second-order processes that can further erode the ice margin. These include: fracture propagation in response to local stress imbalances in the immediate vicinity of the glacier front; undercutting of the glacier terminus by melting at or below the waterline; and bending at the junction between grounded and buoyant parts of an ice tongue. Calving of projecting, submerged ‘ice feet’ can be regarded as a third-order process, because it is paced by first- or second-order calving above the waterline.First-order calving can be represented in glacier models using a calving criterion based on crevasse depth, which is a function of longitudinal strain rate. Modelling changes in terminus position and calving rates thus reduces to the problem of determining the ice geometry and velocity distribution. Realistic solutions to the problem of modelling ice flow therefore depend critically on an appropriate choice of sliding law. Models that assume that basal velocities are controlled by basal drag can replicate much of the observed behaviour of calving glaciers with grounded termini, but an important limitation is that they cannot be used to model floating glacier termini or ice shelves. Alternative sliding laws that parameterise drag from the glacier margins provide more flexible and robust ways of representing calving in ice sheet models. Such models can explain a remarkable range of observed phenomena within a simple, unifying framework, including: downglacier increases in velocity and strain rates where basal and/or lateral drag diminishes; flow acceleration in response to thinning through time; the tendency for glaciers to stabilise at ‘pinning points’ in relatively shallow water or fjord narrowings; the constraints on ice shelf stability; and the contrasts in calving rates between tidewater and freshwater calving glaciers. Many unresolved issues remain, however, including the role played by the removal of backstress in the acceleration of retreating calving glaciers, and the controls on melting at and below the waterline. |
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