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Author: Kristin Meredith Schild
Publisher:
ISBN:
Category : Earthquakes
Languages : en
Pages : 0
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Author: Kristin Meredith Schild
Publisher:
ISBN:
Category : Earthquakes
Languages : en
Pages : 0
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Author: Mason Joseph Fried
Publisher:
ISBN:
Category :
Languages : en
Pages : 270
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The Greenland Ice Sheet rapidly lost mass over the last two decades, in part due to increases in ice loss from termini of large tidewater glaciers. Terminus melting and calving can drive glacier retreat and the pattern of ice sheet mass loss through reductions in resistive stresses near the glacier front and, in turn, increases in ice flow to the ocean. Despite their importance to ice sheet mass balance, factors controlling terminus positions are poorly constrained in ice sheet models, which fundamentally obscures sea level rise predictions. In this dissertation, I use a suite of novel observations and techniques to quantify controls on frontal ablation and terminus positions at tidewater glaciers in central west Greenland. Until recently, frontal ablation processes were obscured due to limited observations of submarine termini. Here, I use observations from multibeam echo sonar to show the morphological complexity of the submarine terminus face and identify previously unrecognized melting and calving processes. The terminus features numerous secondary subglacial plume outlets outside of the main subglacial channel system that drive and disperse large submarine melt rates across the glacier front. Submarine melting drives steep, localized terminus undercutting that can trigger calving by connecting to finely-spaced surface crevasses. In turn, large calving events cause the terminus face to become anomalously overcut. Incorporating observed outlet geometries in a numerical plume model, I estimate small subglacial discharge fluxes feeding secondary plume outlets that are reminiscent of a distributed subglacial network. Regional remote-sensing observations reveal that, for most glaciers in central west Greenland, seasonal terminus positions are more sensitive to glacial runoff than ice mélange or ocean thermal forcing. Shallow, serac-failing tidewater glaciers are most sensitive, where subglacial plumes melt the terminus and locally enhance retreat. Glaciers with large ice fluxes and deep termini retreat sporadically through full ice-thickness calving events less dependent on runoff. Together, these results provide process-oriented constraints on the shape of the submarine terminus face, the geometry of subglacial discharge and submarine melting, the influence of environmental forcing mechanisms and the impact that these variables have on terminus positions and dynamics in a warming climate.
Author: Sophie Ann Goliber
Publisher:
ISBN:
Category :
Languages : en
Pages : 0
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Since the 1980s, the Greenland ice sheet has been losing ice mass at an increased rate. Our current understanding of the complex physical processes that control dynamic mass loss is incomplete and, therefore, leads to a wide range of possible future contributions to sea level. Ice dynamics, or changes due to changes in ice flux, is dominated by the behavior of fast-moving outlet glaciers in Greenland. These glaciers are changing through melting of the terminus face and/or calving of icebergs; the combination of these processes and ice motion determines the position of a glacier terminus. In understanding how and why outlet glacier termini change over time compared to external forcing and internal glacier dynamics, we are able to move toward a better understanding of marine-terminating glaciers. In this dissertation, I use terminus traces to observe how and why marine-terminating glaciers change in order to better understand the mechanisms behind these complex heterogeneous changes in Greenland. I develop the largest database of manually-traced marine-terminating glacier terminus data for use in scientific and machine learning applications. These data have been collected, cleaned, assigned with appropriate metadata, including image scenes, and compiled so that they can be easily accessed by scientists. Then I use the location of the termini to identify features in the bed topography that inhibit the retreat of glaciers following the onset of ocean warming and widespread glacier retreat in the late 1990s. I find that the slope and lateral dimensions of bed features exhibit the strongest correlation to retreat and that the shape of the bed features allows different styles of terminus retreat, which may be indicative of how different ablation mechanisms are distributed across termini. Finally, I produce a time series of terminus morphological properties for four glaciers in western Greenland to identify the characteristics that are indicative of calving processes with the goal of categorizing glaciers by calving style. I find that a concave shape and low sinuosity are present at glaciers that calve via buoyant flexure, while the opposite is true at glaciers that are dominated by melt-induced calving via serac failure. I also find that glaciers do not persistently fit into single calving styles and may change over time. By studying how the terminus changes over time compared to external forcing and internal glacier dynamics, we are able to move toward a better understanding of marine-terminating glaciers
Author:
Publisher:
ISBN:
Category :
Languages : en
Pages : 205
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The Greenland Ice Sheet contains enough water to raise global sea levels by ~7 metres, but predictions of the actual potential future contribution in a warming climate vary widely. These can be improved through a better understanding of how the whole ice sheet and its outlet glaciers have responded to past and present climate fluctuations. Recent studies have observed that Greenland Ice Sheet outlet glaciers have been retreating and thinning at increasingly faster rates since the 1990s. However, few studies have investigated the behaviour of the numerous independent ice caps that surround the ice sheet, or the land-terminating outlet glaciers. In addition, recent retreat is rarely put into context with long-term twentieth century fluctuations. This study has mapped ice sheet outlet glaciers and margins, independent ice cap outlets and mountain/valley glaciers at 11 time steps between the Little Ice Age and 2009 in northwest and southwest Greenland. Length changes of different glacier classes and terminus environments are examined, and overall glacier fluctuations compared to regional air temperatures and precipitation. Glaciers in the northwest have retreated further than those in the southwest at most time periods, with the exception of 1943/53-1964 when southwest glaciers underwent their most rapid rate of retreat. Length changes in both regions are driven by air temperature and precipitation changes. Tidewater outlet glaciers have generally retreated shorter distances than land-terminating glaciers in both absolute and relative terms over long time periods. These results imply that recent rapid retreat of many tidewater outlet glaciers in Greenland is not unprecedented, and may represent natural cyclical fluctuations rather than a long-term shift in behaviour. Ice sheet outlet glaciers have retreated shorter relative distances than independent ice caps and mountain/valley glaciers. Ice sheet margins advanced in the southwest between 1964 and 2001, and a slight and a slight advance of independent glaciers was observed from ~1964-1987. It is unclear why this advance occurred. This study highlights the need for more research into the fluctuations of the independent ice caps and land-terminating glaciers in all regions of Greenland. In addition, more detailed research into the response of glaciers of all classes and terminus environments to climate change during the whole of the twentieth century is required to put recent changes into context.
Author: Taryn Elizabeth Black
Publisher:
ISBN:
Category :
Languages : en
Pages : 0
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Glacier retreat and mass loss are contributing to global sea-level rise and environmental change. One method to improve our understanding of how glaciers affect local and global environments is to measure historical and ongoing glacier retreat. In this dissertation, I use satellite images to measure glacier terminus change in Greenland and Alaska. In both regions, I quantify glacier retreat and advance on time scales ranging from seasons to decades.In northwest and central-west Greenland, I investigate multi-decadal outlet glacier retreat and its potential driving mechanisms from 1972 through 2021. Over this time period, glaciers in this region of Greenland represented nearly half of Greenland's total contribution to sea-level rise. I show that regional glacier retreat accelerated from 1996, and that this retreat is most sensitive to runoff and ocean temperatures. Because runoff and ocean temperatures can influence terminus positions through several mechanisms, it is likely that multiple mechanisms are contributing to the observed retreat in this region. I also examine multi-decadal glacier retreat, as well as seasonal terminus variations, for maritime glaciers in Kenai Fjords National Park, Alaska, from 1984 through 2021. I show that most glaciers retreated substantially, and those that did not were predominantly tidewater. Seasonally, tidewater, lake-terminating, and land-terminating glaciers all tended to retreat during the summer; in the winter, tidewater glaciers tended to advance, while lake-terminating glaciers continued to retreat, and land-terminating glaciers were more variable. Glacier change in Kenai Fjords National Park is driving changes in landcover and viewscapes, which affects local ecosystems and ecotourism. Finally, I characterize seasonal terminus variations around the full margin of Greenland at monthly and six-day temporal resolution. I show that nearly 75% of outlet glaciers exhibit terminus position seasonality, with seasonal retreat typically beginning in mid-May and continuing through early October. The onset and duration of the retreat period appears to be related to the timing of the onset of melt, while the magnitude of terminus position seasonality correlates with glacier velocity. Glacier dynamics are influenced by conditions at the terminus, and terminus position seasonality can affect projections of future ice-sheet mass balance.
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: James M. Lea
Publisher:
ISBN:
Category : Geological modeling
Languages : en
Pages : 0
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Significant uncertainty surrounds the influence of atmospheric and oceanic forcing on the fluctuations of tidewater glacier outlets of the Greenland ice sheet (GrIS), with the majority of studies focussing on dynamics over the last two decades. Although numerical model based projections exist anticipating the future dynamics of major GrIS outlets, these have been made using temporally limited model calibration periods (5 years) compared to the centennial timescales that they seek to predict over. The ability of these numerical models to simulate the centennial timescale dynamics of GrIS tidewater glaciers has therefore not been explicitly tested. This thesis seeks to calibrate a well-established one-dimensional tidewater glacier numerical model against post-Little Ice Age maximum (LIAmax) observations of a major tidewater glacier outlet of GrIS. The study site chosen is Kangiata Nunaata Sermia (KNS); the largest tidewater outlet in SW Greenland south of Jakobshavn Isbræ. This glacier is known to have undergone retreat of20 km since its LIAmax, though the timing of this retreat and response to climate forcing is currently poorly constrained. Utilising a range of source material, it is demonstrated that KNS is likely to have achieved its LIAmax by 1761, experiencing either one, or two multi-kilometre retreats by 1859, and retreats of a similar scale between 1921-1968, and 1997-2012. Terminus fluctuations of KNS were in phase with climate anomalies, where data were available for comparison (1871-2012). To allow accurate comparison to numerical model output, the accuracy of different methods of quantifying glacier terminus change was also evaluated. Two new methods were devised so observations could be matched with greater accuracy than existing methods allowed. Glacier sensitivity to climate forcing was evaluated using the numerical model.
Author: David Bryan Podrasky
Publisher:
ISBN:
Category : Glaciers
Languages : en
Pages : 234
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Outlet glaciers in Greenland, and elsewhere, have recently shown large variations in terminus position and ice flux. One example is the tidewater retreat of Jakobshavn Isbræ, which began in the late 1990s with high thinning rates, acceleration and collapse of the floating glacier tongue. The retreat has continued to the present, with glacier speeds more than doubling in two decades' time. A campaign of in-situ measurements was initiated in 2006 with the aim of determining the importance of short-term forcing as a control on the continuing evolution of the glacier. Three years of continuous GPS measurements along the centerline of Jakobshavn Isbræ reveal seasonal velocity variations due to seasonally varying terminus position. The relationship between glacier speed and surface melt is complex, with both speed-up and slowdown events in response to variations in the rate of surface melt. During a particularly long and intense melt season in 2007, a series of melt-driven slowdowns effectively reduced the mean ice flow over the whole year. On shorter timescales, the response to surface meltwater input is more predictable with diurnal velocity variations of 1-2 % that closely match changes in meltwater input. The influence of iceberg calving and tidal forcing is restricted to the lower 10 km of the glacier, imposing an upper limit on longitudinal stress coupling length of a few ice thicknesses. The response to these forcings does not exceed 5 % of mean flow. This is consistent with a glacier operating under high driving stresses. Ice sheet velocities as far as 120 km inland of the margin have responded to the continuing retreat with increases in speed. The flow has also rotated toward the centerline of the main channel. This speedup and channelization of flow are the result of evolving ice surface gradients as the glacier continues to respond to changes initiated at the periphery. This shows that ocean driven changes have led to increased ice flux far inland on the Greenland Ice Sheet, implying a continuing large-scale evolution of the Jakobshavn Isbræ drainage basin.
Author: Carlos R. Mechoso
Publisher: Cambridge University Press
ISBN: 1108492703
Category : Science
Languages : en
Pages : 359
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A comprehensive review of interactions between the climates of different ocean basins and their key contributions to global climate variability and change. Providing essential theory and discussing outstanding examples as well as impacts on monsoons, it a useful resource for graduate students and researchers in the atmospheric and ocean sciences.
Author: Cornelia Reinharter
Publisher:
ISBN:
Category :
Languages : en
Pages : 70
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