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Improving projections of sea level contribution from the Greenland ice sheet by modeling calving dynamics
Full text linkThe contribution of the Greenland ice sheet to global sea-level rise has increased rapidly during the last two decades and is currently ∼ 0.8 mm/year. As observations show a clear, accelerating increasing trend in both global temperature and ice mass loss from the Greenland ice sheet, how much mass the Greenland ice sheet is going to lose over the next century and beyond is one of the most urgent questions in understanding the implication of climate change. Estimating future ice sheet contributions to sea-level rise is currently an active area of research and numerical ice sheet modeling is our best tool to address this question.This thesis provides an estimate of sea-level contribution from Greenland with a new generation ice sheet model that fully accounts for changes of 200+ Greenland glaciers. First, we introduce modeling of calving dynamics which is one of the most important processes contributing to mass loss from outlet glaciers around the coast of Greenland. We test and compare calving laws in an ice sheet model and assess which calving law has better predictive abilities for each glacier. We then apply the best calving law to Nioghalvfjerdsfjorden and Zachariae Isstrøm glaciers in northeast Greenland to investigate the response of these fast-changing glaciers to future climate forcing. We extend our model to the entire Greenland ice sheet to estimate the future sea-level contribution from Greenland. Compared to previous studies, we calibrate our model at the individual glacier scale with a moving boundary capability to better constrain the retreat of marine-terminating glaciers. We find that the Greenland ice sheet will contribute 79.2 to 167 mm to sea-level between 2007 and 2100 under the most extreme warming scenarios. Our simulations show that discharge from ice dynamics will contribute to the total mass loss from Greenland more than previously estimated, implying that future scientific focus should remain on not only atmospheric processes but also the ice front of marine-terminating glaciers
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Projecting the evolution of Totten Glacier, East Antarctica, over the 21st century using ice-ocean coupled models
Full text linkTotten Glacier, the primary ice discharger of the East Antarctic Ice Sheet (EAIS), contains 3.85 m sea level rise equivalent ice mass (SLRe) and has displayed dynamic change driven by interaction of its ice shelf with the Southern Ocean. To project Totten Glacier's evolution, it is critical that sub-shelf ocean processes are properly resolved in dynamic ice sheet models. First, we combine an ocean box model with a buoyant plume parameterization to create PICOP, a novel melt parameterization that resolves sub-shelf vertical overturning and produces melt rates that are in excellent agreement with observations. We then use this parameterization to make century-scale mass balance projections of the EAIS, forced by surface mass balance and ocean thermal anomalies from ten global climate models. Although increased snowfall offsets ice discharge in high emission scenarios and results in ~10 mm SLRe gain by 2100, significant grounded ice thinning (1.15 m/yr) and mass loss (~6 mm SLRe) from Totten Glacier is projected. To investigate whether PICOP misses important processes, such as the advection of warm water into the ice shelf cavity, we develop a fully coupled ice-ocean model and find that warm water is able to access Totten Glacier's sub-shelf cavity through topographic depressions along the central and eastern calving front. By mid-century in high emission scenarios, warm water intrusions become strong enough to overcome topographic barriers and dislodge Totten Glacier's southern grounding line, triggering abrupt acceleration in ice discharge (+185%). Overall, the timing and extent of Totten Glacier's retreat is predominately controlled by the sub-shelf ocean circulation, highlighting the importance of studying dynamic glaciers in fully coupled ice-ocean models
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Modeling the evolution of coupled ice flow dynamics and subglacial hydrology for Petermann Glacier, Northern Greenland, on seasonal, inter-annual, and centennial time-scales
Full text linkPetermann Glacier is a major outlet glacier of northern Greenland that drains a marine-based basin vulnerable to destabilization from enhanced oceanic and atmospheric forcings. Satellite observations show significant grounding line retreat of ∼7 km in a central region of the glacier, with at least 1 km of retreat elsewhere along the grounding line. This representsa significant shift from the glacier’s previously stable grounding line position mapped in the 1990s. Satellite observations also show a seasonal ice acceleration for Petermann of 15% in the summer, from 1,250 to 1,500 m/yr measured close to the grounding line. We use a subglacial hydrology model (GlaDS) and an ice sheet model (ISSM) with asynchronous coupling to
evaluate the role of subglacial hydrology as a physical mechanism explaining the seasonal speedup of ice velocity. Results show an excellent agreement between the observed and modeled velocity in terms of timing and magnitude when an applied lower limit on effective pressure of 6% of ice overburden pressure is imposed in the ice flow model. We conclude that
seasonal changes in subglacial hydrology are sufficient to explain the observed seasonal speed up of Petermann Glacier. Current projections of glacier dynamics under 21st century climate forcings do not include seasonality or subglacial hydrology, so it is unknown if either will play any important role in evolving glacier dynamics under different climate change scenarios.
We use climate forcings through 2100 to investigate how the subglacial hydrologic system may evolve in a warmer climate, and to test if including hydrology changes the stability of Petermann under future climate scenarios using ISSM and the GlaDS model in both an asynchronous and synchronous coupled configuration. Results show that including subglacial
hydrology in projections of Petermann’s evolution yield larger predictions of future sea level rise by the end of the century. However, modeled results of both present day and future ice dynamics with and without subglacial hydrology included do not reproduce the observed grounding line retreat. To better understand grounding line migration of Petermann, we apply a newly published theory of seawater intrusion below grounded ice. By incorporating ocean driven basal melting in the grounding zone, we achieve a significantly improved match to the observed grounding line behavior that previous model setups failed to reproduce. This underscores the importance of considering ocean-driven melting to accurately capture grounding line behavior. These studies contribute to a deeper understanding of the observed behavior of Petermann Glacier, particularly its seasonal acceleration and grounding line migration. Subglacial hydrology and seawater intrusion both emerge as influential short time scale processes on ice dynamics, with potential long term implications on glacier stability and sea level rise
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
Détermination de propriétés des glaciers polaires par modélisation numérique et télédétection,
No full textIce sheets are amongst the main contributors to sea level rise. They are dynamic systems; they gain mass by snow accumulation, and lose it by melting at the ice-ocean interface, surface melting and iceberg calving at the margins. Observations over the last three decades have shown that the Greenland and Antarctic ice sheets have been losing more mass than they gain. How the ice sheets respond to this negative mass imbalance has become today one of the most urgent questions in understanding the implications of global climate change. The Intergovernmental Panel on Climate Change (IPCC) has indeed identified the contribution of the ice sheets as a key uncertainty in sea level rise projections. Numerical modeling is the only effective way of addressing this problem. Yet, modeling ice sheet flow at the scale of Greenland and Antarctica remains scientifically and technically very challenging. This thesis focuses on two major aspects of improving ice sheet numerical models. The first consists of determining non-observable ice properties using inverse methods. Some parameters, such as basal friction or ice shelf hardness, are difficult to measure and must be inferred from remote sensing observations. Inversions are developed here for three ice flow models of increasing complexity: MacAyeal/Morland’s shelfy-stream model, Blatter/Pattyn’s higher order model and the full-Stokes model. The inferred parameters are then used to initialize large-scale ice sheet models and to determine the minimum level of complexity required to capture ice dynamics correctly. The second aspect addressed in this work is the improvement of dataset consistency for ice sheet modeling. Available datasets are often collected at different epochs and at varying spatial resolutions, making them not readily usable for numerical simulations. We devise here an algorithm based on the conservation of mass principle and inverse methods to construct ice thicknesses that are consistent with velocity measurements. This approach therefore avoids the artificial mass redistributions that occur in existing algorithms for mapping ice thickness, hence considerably improving ice sheet model initialization. The advances made here are important steps towards the ultimate objective of accurate characterization of ice sheets and the realistic modeling of their evolution.Les calottes polaires, ou inlandsis, sont parmi les principaux contributeurs à la montée des océans. Ces systèmes dynamiques gagnent de la masse par accumulation de neige, et en perdent par fonte au contact de l’océan et à la surface, ainsi que par le vêlage d’icebergs. Depuis plus de trois décennies, les observations ont montré que les calottes polaires de l’Antarctique et du Groenland perdent plus de masse qu’ils n’en gagnent. L’évolution des glaciers suite à ce déséquilibre de masse est devenue aujourd’hui l’une des problématiques les plus importantes des implications du changement climatique. Le Groupe d’experts intergouvernemental sur l’évolution du climat (GIEC) a identifié la contribution des glaciers comme l’un des facteurs clés d’incertitude de prédiction de l’élévation du niveau des mers. La modélisation numérique est le seul outil efficace pour répondre à cette question. Cependant, modéliser l’écoulement de glace à l’échelle du Groenland ou de l’Antarctique représente un défi à la fois scientifique et technique. Deux aspects clés de l’amélioration de la modélisation des glaciers sont abordés dans cette thèse. Le premier consiste à déterminer certaines propriétés non mesurables de la glace par méthode inverse. La friction ou la rigidité des barrières de glace, sont des paramètres qui ne peuvent être mesurés directement et doivent donc être déduits à partir d’observations par télédétection. Nous appliquons ici ces inversions pour trois modèles d’écoulement de glace de complexité croissante: le modèle bidimensionnel de MacAyeal/Morland, le modèle dit d’ordre supérieur de Blatter/Pattyn et le modèle full-Stokes. Les propriétés ainsi calculées sont ensuite utilisées pour initialiser des modèles grande-échelle et pour déterminer le degré de complexité minimum nécessaire pour reproduire correctement la dynamique des glaciers. Le second aspect abordé dans ce travail est l’amélioration de la consistance des données pour la modélisation numérique. Les données disponibles sont souvent issues de campagnes de mesures s’étalant sur plusieurs années et dont résolutions spatiales varient, ce qui rend leur utilisation pour des simulations numériques difficiles. Nous présentons ici un algorithme basé sur la conservation de la masse et les méthodes inverses pour construire des épaisseurs de glace qui sont consistantes avec les mesures de vitesse. Cette approche empêche la redistribution artificielle de masse qu’engendrent généralement les autres méthodes de cartographie de l’épaisseur de glace, ce qui améliore considérablement l’initialisation des modèles d’écoulement de glace. Les avancées présentées ici sont des étapes importantes afin de mieux caractériser de manière précise les glaciers et de modéliser leur évolution de manière réaliste
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