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Mass loss from the world’s ice sheets is one of the largest sources of uncertainty in sea-level rise projections for the 21st century. One way to improve sea-level rise projections is to better understand the processes driving past ice-sheet mass loss. This dissertation investigates past changes in ice flow for two regions: (1) Helheim and Kangerlussuaq Glaciers, two fast-flowing tidewater glaciers in Southeast Greenland, and (2) the Allan Hills Blue Ice Area, a slow-flowing blue ice area in East Antarctica. For both regions, I constrain changes in ice-sheet dynamics using geophysical observations and interpret those changes using numerical ice-flow models. At Helheim and Kangerlussuaq, I examine seasonal and interannual variations in surface velocity, elevation, and terminus position from 2001 to 2016. I show that glacier dynamics depend on the extent of floating ice near the terminus. Helheim’s grounded terminus calved small, nontabular icebergs, while Kangerlussuaq’s floating ice tongue calved large, tabular icebergs. Furthermore, terminus-driven, seasonal speedups and dynamic thinning were generally larger at Helheim than at Kangerlussuaq, likely due to its grounded rather than floating ice tongue. To interpret the observed changes at Helheim and Kangerlussuaq, I use inverse methods to investigate changes in basal conditions under the two glaciers. The basal shear stress under Helheim and Kangerlussuaq decreased or remained relatively constant during terminus-driven speedup events, suggesting that changes in the stress balance were generally supported outside of the region of fast flow. Finally, I use the inferred basal shear stresses to help constrain the form of the basal sliding law. At the Allan Hills Blue Ice Area, I combine ice-penetrating radar data, an ice-flow model, and age constraints to determine a potential site to drill a million-year-old ice core. I also show that thickness anomalies in the englacial stratigraphy suggest that glacier velocity was 30% of present-day values during the last glaciation. While the dynamics of the Allan Hills Blue Ice Area are likely unimportant for sea-level rise projections, an ice core from the region could provide insight into the past stability of the Ross Sea Sector and West Antarctic Ice Sheet.
Ice streams are the primary pathway by which Antarctic ice is evacuated to the ocean. Because the Antarctic ice sheets lose mass primarily through oceanic melt and calving, ice-stream dynamics exert a primary control on the mass balance of the ice sheets. Thus, changes in melt rates at the ice-sheet margins, or in accumulation in the ice-sheet interiors, affect ice-sheet mass balance on timescales modulated by the response time of the ice streams. Even abrupt changes in melt at the margins can cause ice-stream speedup and resultant thinning lasting millennia, so understanding the upstream propagation of marginally forced changes across timescales is key for understanding the ice sheets’ ongoing contribution to sea-level rise. This dissertation is comprised of three studies that use observations and models to understand changes to Antarctic ice-stream dynamics on timescales from decades to millennia. The first chapter synthesizes remotely sensed observations of Smith, Pope, and Kohler glaciers in West Antarctica to investigate the causes and extent of their retreat. These glaciers have displayed some of the largest measured grounding-line retreat, most rapid thinning, and largest speedup amongst Antarctic ice streams. This retreat has drawn interest in their stability both in its own right and as a harbinger of future changes to larger neighboring ice streams. In this study, recent melt rates were determined using flux divergence estimates derived from observations of ice thickness and surface velocity. Out-of-balance melt at the beginning of the study period indicates that the imbalance of this system predates the beginning of satellite velocity observations in 1996. Throughout much of 1996-2010, there was both greater melt over the ice shelves than flux across the grounding line, implying loss of floating ice and elevated melt forcing, and greater grounding-line flux than accumulation, implying adjustment of the grounded ice in response to the ongoing imbalance. The grounding line position of Kohler glacier, and a large melt channel that is unlikely to be a steady-state feature, suggest that the perturbation to this system began on Kohler glacier sometime around the 1970s. Viscosity of the ice shelves, inferred using a numerical model, indicates that weakening of the Crosson ice shelf was necessary to allow the observed speedup, though it is unable to determine whether the weakening was a cause or effect of the ongoing retreat. The second chapter uses a suite of numerical model simulations to determine the dominant drivers of the recent retreat of Smith, Pope, and Kohler glaciers, and extends those simulations that best match observations to evaluate likely future retreat. Similar to the findings of previous studies, the distribution of sub-shelf melt is found to be the primary control on the rate of grounding-line retreat, while the shelf-averaged melt rate exerts a secondary control. The model simulations indicate that, despite ongoing imbalance, the grounding-line position in 1996 was not inherently unstable, but rather elevated melt at the grounding line was required to cause the observed retreat. A weakening of the ice-shelf margins was found to hasten the onset of grounding-line retreat and led to greater speedup. However, without increases in melt beyond 1996 levels, marginal weakening was insufficient to initiate grounding-line retreat. All simulations that capture the observed retreat continue to lose mass until at least 2100, suggesting that ice in this basin may contribute over 8 mm to global mean sea level by 2100. The magnitude of thinning deep in the catchment suggests that the retreat of Kohler and Smith glacier may hasten the destabilization of the neighboring Thwaites glacier catchment. The third chapter uses the timescale of the recently drilled South Pole Ice Core (SPICEcore) and nearby geophysical observations to infer the history of ice flow near the South Pole during the last 10,000 years. The South Pole is located 180 km from the nearest ice divide and drains from the East Antarctic plateau through Academy glacier/Foundation ice stream. As a result, ice flow near the South Pole is potentially affected by the dynamics of these ice streams, and so the history of ice flow in this region has the potential to inform understanding of how marginally forced changes affect the ice-sheet interior. Because the South Pole is far from an ice divide, the accumulation record in SPICEcore incorporates both spatial variations in accumulation upstream and temporal variations in regional accumulation. Comparison between the SPICEcore accumulation record, derived by correcting measured layer thicknesses for thinning, with an accumulation record derived from new GPS and radar measurements upstream, yields insight into past ice flow and accumulation. When ice speeds are modeled as increasing by 15% since 10 ka, the upstream accumulation explains 77% of the variance in the SPICEcore-derived accumulation (vs. 22% without speedup). This correlation is only expected if the ice-flow direction and spatial pattern of accumulation were stable throughout the Holocene. The 15% speedup in turn suggests a slight (3-4%) steepening or thickening of the ice-sheet interior and provides a new constraint on the evolution of the East Antarctic Ice Sheet following the glacial termination.
Our realisation of how profoundly glaciers and ice sheets respond to climate change and impact sea level and the environment has propelled their study to the forefront of Earth system science. Aspects of this multidisciplinary endeavour now constitute major areas of research. This book is named after the international summer school held annually in the beautiful alpine village of Karthaus, Northern Italy, and consists of twenty chapters based on lectures from the school. They cover theory, methods, and observations, and introduce readers to essential glaciological topics such as ice-flow dynamics, polar meteorology, mass balance, ice-core analysis, paleoclimatology, remote sensing and geophysical methods, glacial isostatic adjustment, modern and past glacial fluctuations, and ice sheet reconstruction. The chapters were written by thirty-four contributing authors who are leading international authorities in their fields. The book can be used as a graduate-level textbook for a university course, and as a valuable reference guide for practising glaciologists and climate scientists.
Measuring, monitoring, and modeling technologies and methods changed the field of glaciology significantly in the 14 years since the publication of the first edition of Fundamentals of Glacier Dynamics. Designed to help readers achieve the basic level of understanding required to describe and model the flow and dynamics of glaciers, this second edition provides a theoretical framework for quantitatively interpreting glacier changes and for developing models of glacier flow. See What’s New in the Second Edition: Streamlined organization focusing on theory, model development, and data interpretation Introductory chapter reviews the most important mathematical tools used throughout the remainder of the book New chapter on fracture mechanics and iceberg calving Consolidated chapter covers applications of the force-budget technique using measurements of surface velocity to locate mechanical controls on glacier flow The latest developments in theory and modeling, including the addition of a discussion of exact time-dependent similarity solutions that can be used for verification of numerical models The book emphasizes developing procedures and presents derivations leading to frequently used equations step by step to allow readers to grasp the mathematical details as well as physical approximations involved without having to consult the original works. As a result, readers will have gained the understanding needed to apply similar techniques to somewhat different applications. Extensively updated with new material and focusing more on presenting the theoretical foundations of glacier flow, the book provides the tools for model validation in the form of analytical steady-state and time-evolving solutions. It 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.
Dynamics of Ice Sheets and Glaciers presents an introduction to the dynamics and thermodynamics of flowing ice masses on Earth. Based on an outline of general continuum mechanics, the different initial-boundary-value problems for the flow of ice sheets, ice shelves, ice caps and glaciers are systematically derived. Special emphasis is put on developing hierarchies of approximations for the different systems, and suitable numerical solution techniques are discussed. A separate chapter is devoted to glacial isostasy. The book is appropriate for graduate courses in glaciology, cryospheric sciences, environmental sciences, geophysics and related fields. Standard undergraduate knowledge of mathematics (calculus, linear algebra) and physics (classical mechanics, thermodynamics) provide a sufficient background for successfully studying the text.
Fundamentals of Glacier Dynamics presents an introduction to modelling the flow and dynamics of glaciers. The emphasis is more on developing and outlining procedures than on providing a complete overview of all aspects of glacier dynamics. Derivations leading to frequently-used equations are presented step-by-step to allow the reader to grasp the mathematical details and approximations involved and gain the understanding needed to apply similar concepts to different applications. The first four chapters discuss the background and theory needed for glacier modelling. The central part of the book discusses simple analytical solutions and time-evolving numerical models that are used to study general aspects of glacier dynamics and important feedback mechanisms. The final three chapters discuss applications specific to smaller mountain glaciers, the Greenland Ice Sheet, and the Antarctic Ice Sheet, respectively. This book will be suitable for graduate courses in geophysics and will also serve as a reference volume for scientists active in all aspects of glaciology and related research. Standard undergraduate mathematics and physics are sufficient background for studying the text.
Dynamics of Snow and Ice Masses gives an outline of snow and ice studies with an emphasis on essential properties and processes. The monograph also treats the dynamical aspects of snow and ice masses. The text covers topics such as the flow and temperature of ice sheets and shelves, the numerical modeling of ice-sheet changes; the structure of glaciers, the experimental creep behavior of ice, flow law of glacier ice, and advance and retreat of glaciers. Also covered are topics such as sea ice - the physics of its growth, drift, and decay; iceberg deterioration, sources, drift, and drift patterns; and freshwater ice growth, motion, and decay. The book is recommended as a textbook for graduate-level students of snow and ice studies and as reference for climatologists.
This book presents the concepts and tools of ice mechanics, together with examples of their application in the fields of glaciology, climate research and civil engineering in cold regions. It starts with an account of the most important physical properties of sea and polar ice treated as an anisotropic polycrystalline material, and reviews relevant field observations and experimental measurements. The book focuses on theoretical descriptions of the material behaviour of ice in different stress, deformation and deformation-rate regimes on spatial scales ranging from single ice crystals, those typical in civil engineering applications, up to scales of thousands of kilometres, characteristic of large, grounded polar ice caps in Antarctica and Greenland. In addition, it offers a range of numerical formulations based on either discrete (finite-element, finite-difference and smoothed particle hydrodynamics) methods or asymptotic expansion methods, which have been used by geophysicists, theoretical glaciologists and civil engineers to simulate the behaviour of ice in a number of problems of importance to glaciology and civil engineering, and discusses the results of these simulations. The book is intended for scientists, engineers and graduate students interested in mathematical and numerical modelling of a wide variety of geophysical and civil engineering problems involving natural ice.
Our appreciation of glaciological processes in Antarctica suffers from a lack of observations in regions where numerical models indicate the ice sheet to be susceptible to ocean and/or atmospheric warming. The solution lies in the use and development of glacier geophysics. In this volume we present a series of papers that demonstrate how geophysics can be deployed in Antarctica to comprehend: (1) boundary conditions that influence ice flow such as subglacial topography, the distribution of basal water and ice-sheet rheology; (2) phenomena that might affect ice-flow processes, such as complex internal ice-sheet structures and the proposition of large stores of hitherto unappreciated groundwater; and (3) how glacigenic sediments and formerly glaciated terrain on, and surrounding, the continent can inform us about past ice-sheet dynamics. The volume also takes a historical view on developments leading to current knowledge, examines active ice-sheet processes, and points the way forward on how geophysics can advance quantitative understanding of Antarctic ice-sheet behaviour.
Knowledge of processes, dynamics, and the ongoing mass-balance of polar ice sheets is essential if we are to understand the response of the cryosphere to a changing climate. Here we present a series of hypotheses and associated observations and interpretations addressing the West Antarctic and Greenland ice sheets. Specific attention is paid to the grounding line, ice-shelf mass balance, and crystal orientation fabrics in streaming ice. Initially, GLAS ICESat laser altimetry data is used in an accurate and rapid method of grounding line location. The method exploits the high surface-slope at the grounding line relative to the flat ice-shelf and ice streams. Validation is performed using ground-based observations and comparisons between the modern grounding-line and past estimates indicate that the Siple Coast grounding line has been largely static for at least several decades. During this time the ice streams have been undergoing large changes in flow speed indicating that the grounding-line position is insensitive to such changes. In order to address the mass balance of the Ross Ice Shelf, a divergence method assuming steady-state is used to estimate the spatial distribution and magnitude of basal melting (Mb). An area average rate of -0.08 +/- 0.01 m/a is estimated indicating that accretion dominates the sub-ice-shelf environment with rates of Mb=-0.32 +/- 0.01 m/a estimated in the centre of the ice shelf. Our estimates of accretion are an order of magnitude higher than previous studies and we caution that this is likely due to the divergence method misinterpreting past non-steady-state behavior of the ice streams. High melt rates (1.3 +/- 0.1 m/a) are observed at the ice shelf front. The ice front is further investigated using spatial and temporal elevation changes from GLAS ICESat laser altimetry data. Melt rates are observed to increase exponentially as the front is approached, from zero at approximately 40 km from the front to an average of 2.7 +/- 0.9 m/a within the front kilometer. Melt estimates are best fit by the relationship Mb=2.1exp(x/11800) m/a. Melt at the front is modeled as a combination of tidally-induced mixing and the ascension of buoyant water from beneath the ice shelf, indicating a relationship between melt profile and calving history. In the final section of this study, active-source seismic data are reported from an upstream location on Greenland's fastest-flowing outlet glacier, Jakobshavn Isbrae. Englacial reflectivity in these data reveal the development of complex and alternating crystal orientation fabrics, which we associate with changes in impurity loading brought about by climactic changes. These fabrics likely result in strain localization and therefore have implications for predictive ice sheet modeling.