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"Jakobshavn Isbræ, a fast-flowing outlet glacier in West Greenland, began a rapid retreat in the late 1990's. The glacier has since retreated over 15 km, thinned by tens of meters, and doubled its discharge into the ocean. The glacier's retreat and associated dynamic adjustment are driven by poorly-understood processes occurring at the glacier-ocean interface. These processes were investigated by synthesizing a suite of field data collected in 2007-2008, including timelapse imagery, seismic and audio recordings, iceberg and glacier motion surveys, and ocean wave measurements, with simple theoretical considerations. Observations indicate that the glacier's mass loss from calving occurs primarily in summer and is dominated by the semi-weekly calving of full-glacier-thickness icebergs, which can only occur when the terminus is at or near flotation. The calving icebergs produce long-lasting and far-reaching ocean waves and seismic signals, including 'glacial earthquakes'. Due to changes in the glacier stress field associated with calving, the lower glacier instantaneously accelerates by ~3% but does not episodically slip, thus contradicting the originally proposed glacial earthquake mechanism. We furthermore showed that the pre-dominance of calving during summer can be attributed to variations in the strength of the proglacial ice mélange (dense pack of sea ice and icebergs). Sea ice growth in winter stiffens the mélange and prevents calving; each summer the mélange weakens and calving resumes. Previously proposed calving models are unable to explain the terminus dynamics of Jakobshavn Isbræ (and many other calving glaciers). Using our field observations as a basis, we developed a general framework for iceberg calving models that can be applied to any calving margin. The framework is based on mass continuity, the assumption that calving rate and terminus velocity are not independent, and the simple idea that terminus thickness following a calving event is larger than terminus thickness at the event onset. Although the calving framework does not constitute a complete calving model, it provides a guide for future attempts to define a universal calving law"--Leaf iii.
Snow and Ice-Related Hazards, Risks, and Disasters provides you with the latest scientific developments in glacier surges and melting, ice shelf collapses, paleo-climate reconstruction, sea level rise, climate change implications, causality, impacts, preparedness, and mitigation. It takes a geo-scientific approach to the topic while also covering current thinking about directly related social scientific issues that can adversely affect ecosystems and global economies. Puts the contributions from expert oceanographers, geologists, geophysicists, environmental scientists, and climatologists selected by a world-renowned editorial board in your hands Presents the latest research on causality, glacial surges, ice-shelf collapses, sea level rise, climate change implications, and more Numerous tables, maps, diagrams, illustrations and photographs of hazardous processes will be included Features new insights into the implications of climate change on increased melting, collapsing, flooding, methane emissions, and sea level rise
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.
The retreat and advance of marine-terminating outlet glaciers in Greenland plays a critical role in modulating ice sheet mass balance. However, the frontal ablation processes that regulate glacier terminus position are challenging to observe and thus difficult to represent in numerical ice flow models. Current models of the Greenland Ice Sheet rely upon simple iceberg calving and submarine melt parameterization to prescribe either a stable terminus position or iceberg calving rate, yet the relative accuracies and uncertainties of these criteria remain largely unknown at the ice sheet scale. Here, we evaluate six iceberg calving models from the literature against spatially and temporally diverse observations and model output from 50 marine-terminating outlet glaciers in Greenland. Five of six calving models successfully reproduce observed May/June terminus conditions with zero median model bias and low ice-sheet-wide uncertainty using fixed, spatially-optimized parameter values. However, when evaluated against time series observations from select glaciers, we find that calving models that predict a calving rate struggle to reproduce variations in observed terminus dynamics over seasonal and inter-annual time scales with single, optimized model parameters. Comparatively, calving models that prescribe a terminus position, rather than a calving rate, more accurately account for observed changes in terminus dynamics through time and are therefore less likely to generate glacier length and/or ice flux errors when employed in predictive ice flow models. Overall, our results indicate that the crevasse depth calving model reproduces observed terminus dynamics with high fidelity and should be considered a leading candidate for use in models of the Greenland Ice Sheet.
Ice composition has until now been mostly used for reconstructing the environment of the past. A great research effort is made today to model the climate system in which the ice cover at the earth sur face plays a prominent role. To obtain a correct model of the ice sheets, due attention must be paid to the physical processes operating at the interfaces, i. e. the boundary conditions. The idea behind the title of this book is that the study of ice composition can shed some light on the various processes operating at the ice bedrock and ice-ocean interfaces and more generally on glacier dy namics. The book is not intended as a treatise on some specialized topic of glaciology. It is mainly the product of the experience of the two authors gained over several years research on the subject. The two authors are both members of the same university department and personal friends. The book was prepared in the following way. After a first draft of the complete book had been written by the first author, it was put in the hands of the second. The final version sent to the publishers is therefore the result of ex tended discussion, while at the same time preserving the unity of style that would have been lost had the two authors written selected chapters of the book individually. The book is organized into two distinct parts.
A greater knowledge of the interactions between the Greenland Ice Sheet and climate is critical to understanding the possible impacts of future global warming, including ice sheet contribution to global sea-level rise and perturbations to ocean circulation. Recent acceleration, thinning and retreat of major tidewater glaciers in Greenland and Antarctica during the past two decades demonstrate the potential for ice sheets to respond to climate change much faster than previously assumed. One approach to understanding the role of atmospheric and oceanic warming to ice sheet dynamics is to investigate how ice sheets responded to past periods of climate change. This thesis uses benthic foraminifera as a proxy to reconstruct past changes in the temperature of the relatively warm West Greenland Current, to investigate the possible influence of ocean warming on ice sheet dynamics during the initial marine-based deglaciation phase, and throughout the Holocene, when the ice was positioned close to the present margin. This thesis finds that the marine-based ice sheet in central West Greenland collapsed rapidly due to a combination of high relative sea-level and ice sheet thinning due to climatic warming. Foraminiferal evidence does not support a major influence of ocean forcing on initial deglaciation. However, Holocene changes in the relative temperature of the West Greenland Current may have had a more significant influence on ice stream dynamics following the marine-based ice retreat, when outlet glaciers were positioned within coastal fjords. Changes in the relative temperature of the West Greenland Current are determined?upstream? by wider scale changes in the North Atlantic region.