1,721,276 research outputs found
A gradient-boosted tree framework to model the ice thickness of the world's glaciers (IceBoost v1.1)
Full text linkKnowledge of glacier ice volumes is crucial for constraining future sea level potential, evaluating freshwater resources, and assessing impacts on societies, from regional to global. Motivated by the disparity in existing ice volume estimates, we present IceBoost, a global machine learning framework trained to predict ice thickness at arbitrary coordinates, thereby enabling the generation of spatially distributed thickness maps for individual glaciers. IceBoost is an ensemble of two gradient-boosted trees trained with 3.7 million globally available ice thickness measurements and an array of 39 numerical features. The model error is similar to those of existing models outside polar regions and is up to 30 %-40 % lower at high latitudes. Providing supervision by exposing the model to available glacier thickness measurements reduces the error by a factor of up to 2 to 3. A feature-ranking analysis reveals that geodetic data are the most informative variables, while ice velocity can improve the model performance by 6 % at high latitudes. A major feature of IceBoost is a capability to generalize outside the training domain, i.e. producing meaningful ice thickness maps in all regions of the world, including on the ice sheet peripheries
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Ice grounding zone processes in Antarctica from satellite SAR interferometry and other data
No full textThis dissertation presents a comprehensive study on the dynamics of ice grounding zones in Antarctica’s ice shelves, with a particular focus on the Amery Ice Shelf and Berry Glacier. Using satellite radar interferometry and other data, this research examines the intricate processes of grounding line migration, ice dynamics, and basal melting. The findings highlight the significant impact of bed topography, subglacial hydrology, and seawater intrusions on ice grounding zone dynamics, challenging traditional models that assume static grounding line positions. In the Amery Ice Shelf region, the study investigates the Lambert, Mellor, and Fisher glaciers, where the analysis of Sentinel-1 data reveals extensive tide-induced grounding line migrations, with movements far exceeding previous estimates. Moreover, high speed of the seawater intrusions brings impressive basal melting in the ice grounding zone. These results underscore the importance of including detailed ice grounding zone processes in ice sheet models to better predict the glaciers’ responses to ocean warming. For Berry Glacier in West Antarctica, the study documents a rapid ice grounding zone retreat over a 25-year period, driven by increased basal melting from warm Circumpolar Deep Water incursions. This retreat has resulted in a marked increase in ice discharge and surface elevation changes, demonstrating the glacier’s accelerated response to oceanic forces. The research presented here contributes to a deeper understanding of the ice grounding zone processes in Antarctica and emphasizes the need for refined modeling approaches that account for the complex interactions between ice, ocean, and subglacial environments. These insights are crucial for improving sea level rise projections and assessing the broader impacts of climate change on polar ice sheets
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Bed topography of Greenland glaciers from high-resolution gravity data
Full text linkThe mass balance of glaciers is influenced by their bed elevation below sea level, surface melt, and ocean-induced ice melt at calving fronts. It is essential to know the glacier thickness, bed elevation and fjord bathymetry to interpret the glacier evolution in the ongoing warm climate. Traditional methods for mapping ice thickness from radar sounding fail in the terminal valleys occupied by glaciers and near glacier calving fronts because of challenging conditions: side returns, rough surface, warm ice, water inclusions. We had to explore new ways to infer ice thickness. The advent of modern airborne gravimeters capable of sub milligal precision made it possible to explore the usage of gravity. Such instrument had been used widely for oil and mineral surveys and we applied it to glacier ice.In this dissertation, we use airborne gravity data collected in August 2012 with a 0.5 mGal precision at a spatial resolution 750 m, combined with measurements of the fjord bathymetry and mass conservation reconstruction solution to obtain novel mapping of bed topography for several glaciers in Greenland. We use both 2D and 3D modeling to interpret the gravity data in these areas. The models are heavily constrained by geological information as much as we got, such as ocean bathymetry data, rock density information from Geological Survey of Denmark and Greenland with the selection of more optimized initial solutions. We use the gravity misfit to quantify the uncertainty of the inversion. The inversion significantly reduces the gravity misfit from the initial bed, as expected. The gravity misfit ranges from -3 to +3 mGal, the nominal precision of our bed mapping is about 60 m. Our study demonstrated the practical use of high-resolution airborne gravity to fill critical gaps in bed elevation in Greenland, especially in deep fjords that cannot be surveyed with deep radar sounders. The results provide more definite view of the bed topography of these major glaciers system than available previously, meanwhile, at a spatial resolution of 750 m along the trough and with an average precision of about 60 m. However, more precise rock information or supplementary data e.g., magnetic data is needed because of the difficulty in associating with space-varying geology/density.This study provides simple guidelines for utilizing gravity data to obtain glaciers bed topography and then to understand glaciers' evolution in ongoing warm climate for both ocean and atmosphere
A 75,000-y-old Scandinavian Arctic cave deposit reveals past faunal diversity and paleoenvironment
Full text linkDuring the last glacial period (~118 to 11.7 ka), the Arctic has been characterized by a major redistribution of flora and fauna as a consequence of extreme climatic fluctuations, with associated glacial advances and retreats, sea-level changes, and shifting sea ice extent. In the high-latitude regions of Northern Europe that are currently subject to rapid climate warming, we lack a comprehensive understanding of faunal biodiversity in the last glacial period due to the extreme rarity of preserved organic remains. Here, we present a stratified sediment deposit with a diverse faunal composition preserved in a bone-bearing layer in Arne Qvamgrotta, part of the Storsteinhola cave system (68.10° N 16.38° E) in Northern Norway. Chronological analyses of sediments and bones including radiocarbon, optically stimulated luminescence, uranium–thorium, and phylogenetic dating place the faunal assemblage in Marine Isotope Stage 5a (MIS 5a, Odderade interstadial, ~85 to 71 ka). Combining comparative osteology and bulk-bone metabarcoding, we identify 46 taxa, including mammals, birds, and fish, with several species not previously found in Fennoscandia. The fauna implies a nonanalogous cold-adapted coastal community, with close proximity to sea ice and nearby freshwater bodies. Mitogenome analyses of a subset of taxa identify extinct lineages which attest to a lack of habitat tracking and the absence of a local refugium during the subsequent fully glaciated periods. This faunal record demonstrates long-term faunal dynamics and coastal environmental conditions during MIS 5a in the European Arctic
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Short-Timescale Dynamics of Marine-terminating Glaciers in Western Greenland
Full text linkIceberg calving is a major component of glacier mass ablation that is not well understood due to a lack of detailed temporal and spatial observations. For better understanding, it is critical to examine processes occurring on the time scale of calving processes, sub-daily to sub-hourly. Current satellites are not able to observe the same location at time scales small enough to measure sub-daily phenomena. This research aims to increase the temporal resolution of ice speed and elevation measurements during the calving season to allow for analysis of short-term variations that are otherwise unobserved. We measure glacier speed and surface elevation at 3-minute intervals using a portable radar interferometer at three marine-terminating glaciers in West Greenland over two summer field campaigns. We detect diurnal variations in glacier speed caused by tidal height changes that propagate far inland, the effect of which varies by glacier but are consistent with simple models where basal stress is tidally modulated. We find no speed up from ice shedding off the calving face or the detachment of floating ice blocks, as expected. We detect a 30% speedup within a few hundred meters of the ice front that persists for days when calving removes full thickness grounded ice blocks. Within one ice thickness from the calving front, we detect strain rates 2 to 3 times larger than observable from satellite data, which has implications for studying iceberg calving as a fracturing process, in particular to select an appropriate value of the threshold tensile stress necessary for ice cliff failure
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Ocean-induced Melting of Greenland Ice Shelves
Full text linkThe Greenland glaciers have been experiencing ongoing acceleration and significant calving events during the last two decades. Ocean-induced melt is a potential trigger for destabilizing the glaciers and ice shelves, and consequently contributing to global sea level rise. However, its mechanism is still uncertain.In this dissertation, we employ observational and numerical methods to improve our under- standings of ocean-induced melt under major Greenland glaciers. Using improved remote sensing data, we calculate melt rates with an improved accuracy. We then employ the Mas- sachusetts Institute of Technology general circulation model (MITgcm) to study ice-ocean interactions beneath an ice shelf in a 2-D configuration at a high resolution. We include ther- mal forcing from the ocean, cavity shape, and for the first time subglacial water discharge at the grounding line. We optimize the heat and salt transfer coefficients to match observed results. The model replicates the general pattern of melting: high near the grounding zone, decreasing rapidly downstream. Melt increases below linear with subglacial discharge and above linear with thermal forcing from the ocean. Next, we investigate the role of the slope of the ice shelf draft in controlling ice shelf melt. The simulations indicate that the melt rate is sensitive to the slope, hence is larger for steeper ice shelves; and the location of the region of high melt migrates toward the grounding line as the slope becomes steeper. In the limit case of a vertical wall, no ice shelf, we know that the locus of ice melt undercuts the glacier.This study provides major new insights on the sensitivity of ice shelf melt to (1) subglacial water discharge: a direct product of ice sheet surface melt (2) thermal forcing from the ocean: a direct product of changes in ocean circulation as a result of wind forcing, and (3) a time-evolving cavity which affects the melt regimes: shallow, nearly flat cavities do not favor high melt; deep, steep cavities favor high melt. These results are important to interpret recent changes on the ice shelves and to inform ice sheet numerical models how to parameterize ice shelf melt in a changing climate
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Dynamics of Glaciers and Ice Sheets at the Ocean Margin from Airborne and Satellite Data
Full text linkThe modern contribution of glaciers and ice sheets to sea level rise increases with time and has largely been attributed to anthropogenic sources. The mass losses through the dynamic discharge of ice into the ocean has played a major role in the last two decades with a widespread acceleration of marine terminating glaciers. Recent studies have shown that these changes are closely linked to the ocean conditions, the bedrock and fjord topography. It is therefore crucial to document in details the evolution of glaciers and ice sheets, the bedrock topography and ocean properties to understand how and why the glaciers have been changing recently. Therefore, the aim of this thesis is to improve our understanding of ice dynamics and ice-ocean interaction by using a set of satellite and airborne remote sensing data over key regions. We describe the evolution of glacier dynamics and the detailed partitioning of the mass losses of the Queen Elizabeth Islands, Canada since the 1990s, which are major contributors to recent sea level rise. In Antarctica, we provide the first map of the sub-ice shelf bathymetry of the largest glaciers in the Amundsen Sea Embayment that reveals deep pathways for circumpolar warm water up to the grounding line of the glaciers. Finally, we assembled a comprehensive map of the bedrock and fjord topography of the southeastern coast of Greenland, and interpret the pattern of glacier retreat during the last 80 years, which was not possible before. The work proposed help to understand the recent deglaciation history of key regions in the Arctic, Greenland and Antarctica. The new mapping of sub-ice shelf, bedrock and fjord topography provides invaluable insights for mass balance calculation, ocean and ice-sheet modeling
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Modeling the dynamics of Thwaites Glacier, West Antarctica
Full text linkThwaites Glacier (TG), West Antarctica, has been experiencing rapid mass loss and ground- ing line retreat in the past few decades. The mass loss of TG is now responsible for 4% of global sea level rise. It is therefore crucial to simulate the future evolution of TG to make projections for future sea level rise. The cause of the dramatic changes is dynamic through the loss of buttressing from its ice shelf due to calving and ice shelf melting. In this thesis, we employ various numerical ice sheet models to study the calving dynamics of TG and the response of TG to enhanced ice shelf melting. We combine a two-dimensional ice flow model with the linear elastic fracture mechanics (LEFM) theory to model crevasse propagation and ice fracturing. We find that the combination of a full-Stokes (FS) model and LEFM produces surface and bottom crevasses that are consistent with the distribution of depth and width of surface and bottom crevasses observed, whereas the combinations of simplified models with LEFM do not. We find that calving is enhanced when pre-existing surface crevasses are present, when the ice shelf is shortened, or when the ice shelf front is undercut. We show that the FS/LEFM combination yields substantial improvements in capturing the stress field near the grounding line of a glacier for constraining crevasse formation and iceberg calving. We then simulate the evolution of TG under different ice shelf melt scenarios and different ice sheet model configurations. We find that the grounding line retreat and its sensitivity to ocean forcing is enhanced when a full-Stokes model is used, ice shelf melt is applied on partially floating elements, and a Budd friction is used. Initial conditions also impact the model results. Yet, all simulations suggest a rapid, sustained retreat along the same preferred pathway. The highest retreat rate occurs on the eastern side of the glacier and the lowest rate on a subglacial ridge on the western side. Combining the results, we find the differences among simulations are small in the first 30 years, with a cumulative contribution to sea level rise of 5 mm, similar to the current rate. After 30 years, the mass loss highly depends on the model configurations, with a 300% difference over the next 100 years, ranging from 14 to 42 mm
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The modulating effect of ocean thermal forcing on the retreat of Greenland's marine-terminating glaciers
Full text linkIn recent decades, tidewater glaciers in Greenland have exhibited a complex spatial pattern of retreat and contributed significantly to sea level rise. This development has been coincident with the warming of ocean waters around Greenland's continental shelf and within its fjords. Here, I use a combination of regional ocean state estimates, remotely-sensed data of glacier evolution, and novel observations of bathymetry and water temperature from NASA's Ocean Melting Greenland mission to quantify the role of warm, salty Atlantic Water in controlling the retreat of 226 marine-terminating glaciers from 1985 to present. Modeled ocean-induced undercutting of calving margins compared with ice advection and ice front change indicates that glacier perturbations are largely triggered by excess melt by the ocean. Subsequent ice front retreat is determined by the bed geometry underneath the ice and the progression of ice front undercutting after retreat: Shallow protrusions, submerged sills and colder, fresher water act to stabilize ice fronts, while deeper, warmer fjords tend to enhance retreat. Despite the role of the ocean in inducing the inland migration of glacier margins, calving processes still dominate the total ablation on the periphery of the ice sheet. This work highlights the role of ocean temperature variability in modulating the retreat of Greenland's glaciers
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