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Author: Ellyn Mary Enderlin
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Languages : en
Pages :
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Taken together, the results of our studies indicate that although the observed dynamic changes are likely triggered by changes in external forcing, the dynamic response of each glacier is largely determined by internal controls (i.e., shape, rheology, basal sliding, etc.). Thus, uncertainty in the parameterizations of these internal controlling factors will strongly limit our ability to confidently predict future dynamic change in the absence of improved observational constraints.
Author: Ellyn Mary Enderlin
Publisher:
ISBN:
Category :
Languages : en
Pages :
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Taken together, the results of our studies indicate that although the observed dynamic changes are likely triggered by changes in external forcing, the dynamic response of each glacier is largely determined by internal controls (i.e., shape, rheology, basal sliding, etc.). Thus, uncertainty in the parameterizations of these internal controlling factors will strongly limit our ability to confidently predict future dynamic change in the absence of improved observational constraints.
Author: Daniella N. Della-Giustina
Publisher:
ISBN:
Category : Glaciers
Languages : en
Pages : 78
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The most recent report from the Intergovernmental Panel on Climate Change cites ice sheet dynamics as the greatest source of uncertainty for predicting current and future rates of sea level rise. This has prompted the development and use of ice sheet models that are capable of simulating the flow and evolution of ice sheets and their corresponding sea level contribution. In the Arctic, the Greenland ice sheet appears to be responding to a warming climate more quickly than expected. In order to determine sea level contribution from Greenland, it is necessary to capture the regional dynamics of the fast flowing outlet glaciers that drain the ice sheet. This work has developed a novel regional model capable of simulating an outlet glacier, and its associated drainage basin, as a mode of using the Parallel Ice Sheet Model. Specifically, it focuses on modeling the Jakobshavn Isbrae as a demonstration. The Jakobshavn Isbrae is one of the world's fastest flowing outlet glaciers, and accounts for nearly 5% of ice loss from the Greenland Ice Sheet. Additionally, the Jakobshavn Isbrae has been widely studied for several decades, and a wealth of remotely sensed and in situ data is available in this region. These data are used as model input and for model validation. We have completed a parameter study in this work to examine the behavior of the regional model. The purpose of this study was not to tune the model to match observations, but rather to look at the influence of parameter choices on the ice dynamics. Model results indicate that we have identified the subset of the model parameter space that is appropriate for modeling this outlet glacier. Additionally, we are able to produce some of this more interesting features that have been observed at Jakobshavn, such as the development and disintegration of a floating ice tongue and the distribution of observed surface velocities. We validate these model results by comparison with recent spatially rich measurements of ice surface speeds, as well as ice geometry.
Author: Twila Moon
Publisher:
ISBN:
Category : Ice sheets
Languages : en
Pages : 116
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Outlet glacier ice dynamics, including ice-flow speed, play a key role in determining Greenland Ice Sheet mass loss, which is a significant contributor to global sea-level rise. Mass loss from the Greenland Ice Sheet increased significantly over the last several decades and current mass losses of 260-380 Gt ice/yr contribute 0.7-1.1 mm/yr to global sea-level rise (~10%). Understanding the potentially complex interactions among glacier, ocean, and climate, however, remains a challenge and limits certainty in modeling and predicting future ice sheet behavior and associated risks to society. This thesis focuses on understanding the seasonal to interannual scale changes in outlet glacier velocity across the Greenland Ice Sheet and how velocity fluctuations are connected to other elements of the ice sheet-ocean-atmosphere system. 1) Interannual velocity patterns Earlier observations on several of Greenland's outlet glaciers, starting near the turn of the 21st century, indicated rapid (annual-scale) and large (>100%) increases in glacier velocity. Combining data from several satellites, we produce a decade-long (2000 to 2010) record documenting the ongoing velocity evolution of nearly all (200+) of Greenland's major outlet glaciers, revealing complex spatial and temporal patterns. Changes on fast-flow marine-terminating glaciers contrast with steady velocities on ice-shelf-terminating glaciers and slow speeds on land-terminating glaciers. Regionally, glaciers in the northwest accelerated steadily, with more variability in the southeast and relatively steady flow elsewhere. Intraregional variability shows a complex response to regional and local forcing. Observed acceleration indicates that sea level rise from Greenland may fall well below earlier proposed upper bounds. 2) Seasonal velocity patterns. Greenland mass loss includes runoff of surface melt and ice discharge via marine-terminating outlet glaciers, the latter now making up a third to a half of total ice loss. The magnitude of ice discharge depends in part on ice-flow speed, which has broadly increased since 2000 but varies locally, regionally, and from year-to-year. Research on a few Greenland glaciers also shows that speed varies seasonally. However, for many regions of the ice sheet, including wide swaths of the west, northwest, and southeast coasts where ice loss is increasing most rapidly, there are few or no records of seasonal velocity variation. We present 5-year records of seasonal velocity measurements for 55 glaciers distributed around the ice sheet margin. We find 3 distinct seasonal velocity patterns. The different patterns indicate varying glacier sensitivity to ice-front (terminus) position and likely regional differences in basal hydrology in which some subglacial systems do transition seasonally from inefficient, distributed hydrologic networks to efficient, channelized drainage, while others do not. Our findings highlight the need for modeling and observation of diverse glacier systems in order to understand the full spectrum of ice-sheet dynamics. 3) Seasonal to interannual glacier and sea ice behavior and interaction Focusing on 16 northwestern Greenland glaciers during 2009-2012, we examine terminus position, sea ice and ice m??lange conditions, seasonal velocity changes, topography, and climate, with extended 1999-2012 records for 4 glaciers. There is a strong correlation between near-terminus sea ice/mélange conditions and terminus position. In several cases, late-forming and inconsistent sea ice/mélange may induce sustained retreat. For all of the 13-year records and most of the 4-year records, sustained, multi-year retreat is accompanied by velocity increase. Seasonal speedup, which is observed across the region, may, however, be more heavily influenced by melt interacting with the subglacial hydrologic system than seasonal terminus variation. Projections of continued warming and longer ice-free periods around Greenland suggest that notable retreat over wide areas may continue. Sustained retreat is likely to be associated with multi-year speedup, though both processes are modulated by local topography. The timing of seasonal ice dynamics patterns may also shift.
Author: Tristan O. Amaral
Publisher:
ISBN: 9781085569576
Category : Glaciers
Languages : en
Pages : 104
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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.
Author: Denis Felikson
Publisher:
ISBN:
Category :
Languages : en
Pages : 292
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The Greenland Ice Sheet has been losing mass at an accelerating rate since 2003, in part due to changes in ice sheet dynamics. As ocean-terminating outlet glaciers retreat, they initiate thinning that diffuses inland, causing dynamic mass loss from the ice sheet interior. Although outlet glaciers have undergone widespread retreat during the last two decades, the inland extent of thinning and, thus, the mass loss is heterogeneous between glacier catchments. There remains a lack of a unifying explanation of the cause of this heterogeneity and accurately projecting the sea-level rise contribution from the ice sheet requires improvement in our understanding of what controls the upstream diffusion of thinning, initiated by terminus retreat. In this dissertation, I use observations and modeling to identify limits to the upstream diffusion of dynamic thinning for ocean-terminating glaciers draining the Greenland Ice Sheet. I start by using diffusive-kinematic wave theory to describe the evolution of thinning and I calibrate a metric that identifies how far upstream a thinning perturbation can diffuse from glacier termini. This metric is calculable from the observed glacier bed and surface topography and I use it to predict inland thinning limits for the majority of Greenland's outlet glaciers. I find that inland thinning limits often coincide with subglacial knickpoints in bed topography. These are steep reaches of the bed that are located at the transition between the portion of the bed that is below sea level and the upstream portion that is above sea level. I use the predicted thinning limits to help identify individual glaciers that have the largest potential to contribute to sea-level rise in the coming century. Finally, I use higher-order numerical modeling to validate the predicted thinning limits from the first-order kinematic wave model, and to investigate the timing and magnitude of glacier mass loss over the coming century. I find that glaciers that have small ice fluxes but are susceptible to thin far into the interior of the ice sheet have the potential to contribute as much to sea-level rise as their higher-flux counterparts. These lower-flux glaciers are often not discussed in literature but will be significant contributors to sea-level rise by 2100.
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Languages : en
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The causes of recent dynamic thinning of Greenland's outlet glaciers have been debated. Realistic simulations suggest that changes at the marine fronts of these glaciers are to blame, implying that dynamic thinning will cease once the glaciers retreat to higher ground. For the last decade, many outlet glaciers in Greenland that terminate in the ocean have accelerated, thinned, and retreated. To explain these dynamic changes, two hypotheses have been discussed. Atmospheric warming has increased surface melting and may also have increased the amount of meltwater reaching the glacier bed, increasing lubrication at the base and hence the rate of glacier sliding. Alternatively, a change in the delicate balance of forces where the glacier fronts meet the ocean could trigger the changes. Faezeh Nick and colleagues5 present ice-sheet modeling experiments that mimic the observations on Helheim glacier, East Greenland, and suggest that the dynamic behaviour of outlet glaciers follows from perturbations at their marine fronts. Greenland's ice sheet loses mass partly through surface melting and partly through fast flowing outlet glaciers that connect the vast plateau of inland ice with the ocean. Earlier ice sheet models have failed to reproduce the dynamic variability exhibited by ice sheets over time. It has therefore not been possible to distinguish with confidence between basal lubrication from surface meltwater and changes at the glaciers' marine fronts as causes for the observed changes on Greenland's outlet glaciers. But this distinction bears directly on future sea-level rise, the raison d'etre of much of modern-day glaciology: If the recent dynamic mass loss Greenland's outlet glaciers is linked to changing atmospheric temperatures, it may continue for as long as temperatures continue to increase. On the other hand, if the source of the dynamic mass loss is a perturbation at the ice-ocean boundary, these glaciers will lose contact with that perturbation after a finite amount of thinning and retreat. Therefore, the first hypothesis implies continued retreat of outlet glaciers into the foreseeable future, while the second does not -- provided the bedrock topography prohibits a connection between the retreating glacier and the ocean. Nick and coauthors test the physical mechanisms implied in each hypotbesis in an innovative ice-flow model, and use that model to try to match a time series of observations from Helheim glacier, one of Greenland's three largest outlet glaciers. Along with many observations, the simulations strongly support the contention that the recent retreat of Greenland's outlet glaciers is the result of changes at their marine fronts. Further, the simulations confirm the earlier hypotheses that bedrock topography largely controlled Helheim glacier's rapid acceleration and retreat in 2004 and 2005, and its deceleration and stabilization in 2006. Finally, the current work implies that if requirements of observational data (high-resolution bed topography) and computational resources (fine computational grid resolution) can be met, improved predictive capability for ice-sheet models is attainable. With respect to the concerns raised by the IPCC, this study signals progress.
Author: C.J. van der Veen
Publisher: CRC Press
ISBN: 1439835675
Category : Science
Languages : en
Pages : 399
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The Intergovernmental Panel on Climate Change Fourth Assessment Report (2007) acknowledged that current ice sheet models do not adequately treat the dynamic response of ice sheets to climate change. This second edition addresses these issues through the addition of new chapters covering glacier instabilities, the interpretation of observations, and ice sheets and sea level. Another new chapter covers glacier mass balance. The text also provides the necessary background and theoretical foundation for developing more realistic ice sheet models, which is essential for better integration of data and observations as well as for better model development.
Author: Nicole-Jeanne Schlegel
Publisher:
ISBN:
Category : Ice sheets
Languages : en
Pages : 0
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The Greenland Ice Sheet, which extends south of the Arctic Circle, is vulnerable to melt in a warming climate. Complete melt of the ice sheet would raise global sea level by about 7 meters. Prediction of how the ice sheet will react to climate change requires inputs with a high degree of spatial resolution and improved simulation of the ice-dynamical responses to evolving surface mass balance. No Greenland Ice Sheet model has yet met these requirements. A three-dimensional thermo-mechanical ice sheet model of Greenland was enhanced to address these challenges. First, it was modified to accept high-resolution surface mass balance forcings. Second, a parameterization for basal drainage (of the sort responsible for sustaining the Northeast Greenland Ice Stream) was incorporated into the model. The enhanced model was used to investigate the century to millennial-scale evolution of the Greenland Ice Sheet in response to persistent climate trends. During initial experiments, the mechanism of flow in the outlet glaciers was assumed to be independent of climate change, and the outlet glaciers' dominant behavior was to counteract changes in surface mass balance. Around much of the ice sheet, warming resulted in calving front retreat and reduction of total ice sheet discharge. Observations show, however, that the character of outlet glacier flow changes with the climate. The ice sheet model was further developed to simulate observed dynamical responses of Greenland's outlet glaciers. A phenomenological description of the relation between outlet glacier discharge and surface mass balance was calibrated against recent observations. This model was used to investigate the ice sheet's response to a hypothesized 21st century warming trend. Enhanced discharge accounted for a 60% increase in Greenland mass loss, resulting in a net sea level increment of 7.3 cm by year 2100. By this time, the average surface mass balance had become negative, and widespread marginal thinning had caused 30% of historically active calving fronts to retreat. Mass losses persisted throughout the century due to flow of dynamically responsive outlets capable of sustaining high calving rates. Thinning in these areas propagated upstream into higher elevation catchments. Large drainage basins with low-lying outlets, especially those along Greenland's west coast and those fed by the Northeast Greenland Ice Stream, were most susceptible to dynamic mass loss in the 21st century.
Author: Lei Yang
Publisher:
ISBN:
Category :
Languages : en
Pages : 160
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The co-variability of glacier ice discharges and climate variability is also examined by using Polar MM5 V1 modeled summer temperature and April-September Positive Degree Day (PDD) anomalies. Ice discharges from south Greenland glaciers are found to be sensitive to temperature change. Based on sensitivities of ice discharge to melt index anomalies, time series of total ice discharge from 28 major glaciers since 1958 are modeled. The global sea level rise contribution from Greenland ice sheet during past 50 years is estimated be ∼0.6 mm yr-1 in average.
Author: Anker Weidick
Publisher:
ISBN:
Category : Glaciers
Languages : en
Pages : 88
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