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This thesis develops the technique of millimetre-wave radar at 94 GHz for close-range remote sensing of glaciers and terrestrial snow cover (the cryosphere). The capabilities of 94 GHz radar for cryosphere mapping are demonstrated using the 2ⁿᵈ generation All-weather Volcano Topography Imaging Sensor (AVTIS2), which maps 3D terrain from real-beam scanning. AVTIS2 acquires 3D point clouds of terrain and a comparison to co-located high density point clouds derived from Terrestrial Laser Scanner (TLS) data showed that AVTIS2 point cloud uncertainties were 1.5 m at 1.5 km and 3 m at 3 km. These values are smaller than other close-range radar systems used to map cryospheric terrain in 3D. Next, the distribution of Normalised Radar Cross Section (σ⁰) values over glacier ice at 94 GHz was found to be −17.0 < σ⁰ < −3.4; σ⁰ₘₑₐₙ = −9.9 dB and followed a log-normal distribution. These values are comparable to other terrain types at 94 GHz such as refrozen snow, wet snow and wet soil, hence glacier surfaces were found to be suitable targets for terrain mapping at 94 GHz. These fundamental results were used to apply the AVTIS2 94 GHz radar for monitoring snow cover and glacier calving. Field trials in Scotland showed that the transition of wet snow to a refrozen snowpack in response to reductions in air temperature could be detected from changes in 94 GHz σ⁰. Snow hazard features could also be identified, such as post-avalanche debris accumulations which manifest as localised increases in radar backscatter. Next, 3D AVTIS2 data was used to quantify glacier calving rates at the Hansbreen tidewater glacier in Svalbard. A time series of ice cliff morphology derived from the AVTIS2 3D data sets exposed the role of melt undercutting and demonstrated the critical role of the ocean on calving processes in Svalbard. The high accuracy 3D data acquired from AVTIS2 and the sensitivity of radar backscatter to surface changes has demonstrated the unique capabilities of 94 GHz radar for cryosphere mapping, paving the way forward for new applications and opportunities for close-range remote sensing of the cryosphere.

The courts have become an increasingly important venue for efforts to combat the impacts of climate change. Claims have brought on a range of legal grounds, but many climate lawsuits are based on the existence of a causal relationship between emissions of greenhouse gases and impacts that occur as a result. Such claims may be made to establish emitters' liability for climate change impacts, or to demonstrate that contributions to climate change cause harm, and therefore that greenhouse gas emissions should be curbed. Establishing causation in climate litigation requires synthesis of climate science and legal arguments. This thesis evaluates how legal arguments can align with developments in climate change attribution science, a subfield of climate science that can assess the contribution of anthropogenic influence to climate-related impacts. It then considers how attribution-science evidence has been used in past lawsuits, and responds to emerging scientific questions regarding the effects of climate change that have arisen in the courts. Legal scholars have identified attribution science as holding the potential to establish causation in the courts, but the analyses described in section 4 of this thesis found that the evidence presented in climate lawsuits often lags developments in climate science and that, in some cases, this may have hindered findings of specific causal relationships between greenhouse gases and their consequences. Responding to these findings, sections 5 and 6 report analyses that advance the state of knowledge in areas of attribution science that hold particular legal relevance. Section 5 studied anthropogenic influence on the glacial lake outburst flood hazard from Laguna Palcacocha in the Peruvian Andes, in the context of ongoing litigation (Lliuya v. RWE). It concluded that 85-105% (5-95% confidence interval) of the observed industrial-era warming in the region surrounding Laguna Palcacocha was attributable to anthropogenic climate change and that the overall retreat of the Palcaraju glacier since 1880 is entirely attributable to this warming. Moreover, it is virtually certain (>99% probability) that the observed glacier retreat cannot be explained by natural variability alone. Laguna Palcacocha has expanded in the wake of this glacier retreat, substantially increasing the outburst flood hazard to the downstream city of Huaraz. Finally, this thesis responds to the increased use of human rights-based arguments in climate litigation and develops methods that extend attribution science analyses from quantifying the effect of climate change on physical hazards to their impacts on health. These methods address limitations in existing approaches, including the representation of the climate-health relationship, choices in calculating counterfactual temperatures, differing approaches for assessing long-term trends and individual events, and estimating the effects of adaptation. The approach described in this section is then applied to quantify heat-related mortality attributable to climate change in the Canton of Zürich (Switzerland) over 1969-2018. We found a substantial burden of heat-related mortality attributable to climate change both within and outside of heatwaves. Overall, 1,7000 heat-related deaths were attributed to climate change, and adaptation was found to have avoided at least a further 700 deaths. The thesis spans climate science, glaciology, epidemiology, and legal analysis. Altogether, this research explains how science can strengthen legal causation arguments in climate litigation, and develops methods and analyses to this end. Scientific evidence has a central role to play in developing well-grounded climate lawsuits, and methodological developments can resolve questions raised in court.

The movement of water beneath ice sheets exerts an important, yet poorly understood, control on how ice masses respond to climatic warming. However, the subglacial realm of ice sheets is one of the most inaccessible environments on Earth. Consequently, little is known about the processes that operate beneath today's ice masses - and how these will evolve in the future. Subglacial landforms present in formerly-glaciated regions provide comparatively accessible records of glacial erosion, deposition, and sediment transport beneath ice sheets that have undergone deglaciation. This thesis investigates the potential of these landforms to reconstruct the flow of water beneath past ice sheets as analogues for how contemporary ice masses will evolve in a warming climate. A combination of geophysical approaches, including multibeam-bathymetric surveys, high-resolution 3D seismic-reflection data, conventional 3D seismic-reflection data, and geotechnical information from boreholes, is used to investigate the flow of water beneath ice sheets which covered western Europe and more expansive regions of the Antarctic continental shelf in the past. These data are first used to constrain the routing and fluxes of subglacial water beneath the retreating West Antarctic Ice Sheet. The impact of subglacial water flow on ice-sheet dynamics during deglaciation is then examined by imaging the internal structures of ancient channels incised by meltwater - tunnel valleys - in the North Sea. The unprecedented detail provided by the high-resolution 3D seismic-reflection data provides links between ice-sheet dynamics and subglacial meltwater flow during deglaciation. A numerical modelling approach constrains these linkages further by estimating the time that the meltwater channels take to form beneath deglaciating ice sheets. Finally, the sedimentation patterns resulting from subglacial water flow and other glacially-influenced processes during deglaciation are examined. Greater coverage of geophysical data on formerly glaciated continental margins, combined with chronological constraints from shallow drilling, will improve understanding of the hydrological systems and dynamics of former and contemporary ice sheets.

Predictions of ice-sheet mass loss, and therefore predictions of global sea level rise, depend sensitively upon how ice-sheet motion is incorporated into numerical models. Using field observations and numerical modelling, this thesis demonstrates that two frequently overlooked processes are central to describing borehole observations of fast ice-sheet motion --- intermediate-scale (< 25 m, ⪅2 km) interaction of ice motion with realistic or real bed topography, and modulation of these ice-motion patterns through a basal layer of temperate ice (much softer ice at the pressure-melting point). I first present a fibre-optic data set from a 1,043 m deep borehole drilled to the base of the fast-moving (>500 m a‾¹) marine-terminating Sermeq Kujalleq (Store Glacier) at the western margin of the Greenland Ice Sheet. This reveals hitherto unappreciated complexity in the processes behind fast ice-sheet motion. I observe substantial but isolated strain heating ~220 m beneath the surface within stiffer interglacial-phase ice where previously none was expected. Ice deformation within glacial-phase ice below 889 m is further observed to be strongly heterogeneous, with a possible high-strain interface demarcating the Last Glacial-Interglacial Transition. I also find a 73-m-thick temperate basal layer, notably thicker than the < 10-m-thick temperate layer just 8.9 km away, unexplained by existing theory, and interpreted to be important for the glacier's fast motion. To disentangle this observed complexity, I then model three isolated 3D domains from the Greenland Ice Sheet's western margin --- two from Sermeq Kujalleq and one from the land-terminating Isunnguata Sermia, all centred above a central borehole observation. By incorporating high-resolution realistic geostatistically simulated topography, I demonstrate that a layer of basal temperate ice with spatially highly variable thickness forms naturally in both marine- and land-terminating settings, alongside ice-motion patterns which are far more complex than previously considered. I show that temperate ice is expected to be vertically extensive in deep troughs, but to thin over bedrock highs. I further show that basal-slip rates are interconnected with this variability, reaching > 90% or < 5% of surface velocity dependent on setting. Last, I apply the assembled model to real high-resolution bed topography data produced by radio-echo sounding at Thwaites Glacier, Antarctica. This reveals a distinct pattern of ice motion controlled by rough topographic highs where basal slip rates are highly variable, and the landscape is predominantly erosive, with broader topographic basins where basal slip is high and uniform, and the landscape is predominantly depositional. This work further suggests the existence of basal temperate ice layer beneath Thwaites Glacier, at least at the rougher topographic highs. Overall, this thesis advances understanding of how ice sheets move, which may ultimately lead to improved parameterisations of ice-sheet motion for predictive models.

The Southern Hemisphere Westerly Winds play a critical role in the global climate system by modulating the upwelling and the transfer of heat and carbon between the atmosphere and the ocean. Since observations started, the core of the westerly wind belt has increased in strength and has contracted towards Antarctica. It has been proposed that these deviations are among the main drivers of the observed widespread warming in West Antarctica, threatening the stability of ice shelves, and ultimately contributing to global sea level rise. Over the last decades, it has been widely believed these atmospheric changes have occurred in response to recently increased greenhouse gas (GHG) concentrations and ozone depletion. However, the lack of long-term wind records in the Southern Hemisphere mid-latitudes hinders our ability to assess the wider context of the recently observed changes. This lack of a clear consistent timing limits our understanding of the causes of westerly wind changes and the roles they have played in driving recent environmental changes in Antarctica. Addressing these questions is crucial for future climate predictions. The answer to this conundrum could be found in diatoms preserved in Antarctic ice core layers. This project presents a thorough analysis of diatoms preserved in Antarctic ice to reconstruct past changes in the westerly wind belt and assess the role of winds in the changing Antarctic environment. A new method to extract and analyse these microscopic organisms preserved in ancient ice is designed, implemented, tested and validated. The spatial and temporal variability of the diatom record preserved in Antarctic ice is explored in 5 locations in the southern Antarctic Peninsula and Ellsworth Land. The regional study of these records reveal a direct link between changes in the intensity of westerly winds and the presence of diatoms in ice. This, therefore, presents a novel ice core wind proxy based on the abundance and diversity of diatoms preserved in ice cores. This novel proxy development enabled the reconstruction of wind variation over the last 140 years. This reconstruction provides first evidence to differentiate between past strengthening and/or migration of the westerly wind belt, a key component to understanding recent environmental changes in West Antarctica. Results from this project reveal the recent poleward intensification of the wind belt is unprecedented in the last 140 years and synchronous with the increase of greenhouse gases and ozone depletion in the atmosphere. In turn, these results imply that recent changes in winds have acted as the lynchpin linking variations in atmospheric GHG and stratospheric ozone concentrations to environmental changes in West Antarctica.

Recent changes at marine-terminating glaciers in Greenland have suggested a dynamic and sensitive response to climate via atmospheric and oceanic warming. Ice loss from marine-terminating glaciers has already contributed significantly to global sea level rise and this is likely to continue with future warming. However, understanding the processes connecting this warming to ice loss from marine-terminating glaciers remains confounded. This is in part due to the complex and dynamic interactions between various potential forcing mechanisms and also due to limited observations of marine-terminating glacier behaviour at sufficiently high spatial and temporal resolutions. This thesis combines remote sensing techniques and field-based observations to investigate ice loss at marine-terminating glaciers at a range of spatial and temporal scales. Here, the overarching aim is to better elucidate the dominant controls on ice loss at marine-terminating glaciers and thus the role they play in the sensitivity of the Greenland Ice Sheet to future environmental change. This thesis first aimed to address one of the primary gaps in observations regarding how key regions of the ice sheet have responded to environmental change, by deriving a detailed record of marine-terminating glacier terminus positions in southeast and northwest Greenland between 2000 and 2015. Through analysing these two key regions of ice loss alongside potential controls on retreat, the ambition was to better constrain the mechanisms driving region-wide change. The results revealed that irrespective of individual spatial or temporal variations in glacier terminus behaviour within or between these two regions, there was an overriding signal of marine-terminating glacier retreat (and thus ice loss) during the 21st Century. Overall, 97% of glaciers experienced terminus retreat, with mean annual retreat rates of -90 m a−1 and -70 m a−1 in the northwest and south-east respectively. This work also showed that changes in bed and fjord geometry were key controls on retreat, which prior to this work had only been indicated at a local scale. Whilst this first results chapter highlighted a clear and dominant signal of retreat and gave an indication to the important controls on this retreat, it was not possible to fully isolate key mechanisms of ice loss by analysing marine-terminating glacier behaviour on large spatial and temporal scales. To further improve understanding of the complex processes driving ice loss, this research next turned to an analysis of marine-terminating glacier behaviour at considerably smaller, and thus more detailed, temporal and spatial scales. Time-lapse imagery was used to generate a very high temporal resolution record of iceberg calving activity at Kangiata Nunaata Sermia (KNS), a large marine-terminating glacier in southwest Greenland. This was combined with meteorological data and modelled estimates of subglacial discharge to investigate the timing and magnitude of ice loss from the glacier terminus over the course of one melt season. The key finding from this work was that the seasonal evolution of subglacial hydrology exerted a critical control on the spatial pattern of iceberg calving across the KNS ice face and ultimately, critically impacted the rate and magnitude of ice loss. More specifically, the subglacial hydrological system was an important control on meltwater plume-focused submarine melting across the KNS ice face. At the start of the melt season, subglacial hydrology was in a relatively distributed state, whereby meltwater emerged through multiple, small portals at the grounding line. This promoted iceberg calving across the entire ice face, likely as a result of instabilities in the calving margin driven by submarine melt-enhanced terminus undercutting. Later in the melt season, the subglacial hydrological system evolved to a relatively more channelised configuration, with a focused discharge of meltwater through fewer portals at the grounding line. This led to a focusing of plume-enhanced submarine melting across the ice face, visible here by changes in terminus planform shape, with the formation of embayments and 'crenelated' terminus geometries. During this time there was also a substantial reduction in terminus-wide iceberg calving, coincident with a ∼50% decrease (0.014 km3 d −1 to 0.005 km3 d −1 ) in frontal ablation flux. Despite the importance of thorough analyses, it is important to recognise that the acquisition of detailed data required to conduct this level of investigation is often not possible. However, the time-lapse work at KNS revealed a potential solution to this problem. An important finding was the changes in the planform geometry of the KNS calving margin, which were strongly controlled by meltwater plume-focused submarine melting. Furthermore, emerging studies have shown morphology to be an important indicator of controls on ice loss, such as bed and fjord geometry. If these and further additional processes of ice loss can be attributed to characteristic morphometric changes observed across glacier calving margins, it may be possible to supplement existing regional scale observations with new types of data. In light of this, the final results chapter utilised a new method for analysing calving front morphology at marine-terminating glaciers. The aim of this work was to establish the potential of using calving front morphology to predict marine-terminating glacier retreat behaviour. However, despite a detailed analysis of the changes in morphology at the terminus of Narsap Sermia in southwest Greenland, during a period of sustained retreat, it was not possible to link specific morphometric change to patterns of ice loss. Whether this reflects the lack of direct process link between terminus planform morphology and calving, or the fact that morphometric changes were a result of interlinked processes and thus could not be isolated to individual controls, remains unclear. The findings of this thesis report on an exploration of marine-terminating glacier behaviour over different spatial and temporal scales. Each investigation has developed new insights advancing our understanding of the processes driving ice loss from marine-terminating glaciers. However, the work has also demonstrated that whilst both large-scale approaches and very detailed analyses are able to elucidate drivers of change, they cannot uniquely isolate key controls. This thesis therefore supports the emerging view that the individual processes of ice loss are too complex and interlinked to isolate effectively and thus, more generalised, large-scale parameterisations of the processes of ice loss may be the best way forward to advance future projections of Greenlandic marine-terminating glacier behaviour. Going forward, regional and ice-sheet wide observations of patterns of marine-terminating glacier behaviour will be essential to our knowledge of the key processes of ice loss, and an improved understanding of the future sensitivity and response of the Greenland Ice Sheet to environmental change.

The Greenland Ice Sheet is the largest ice mass in the Northern Hemisphere and has been losing mass at an increasing rate since the 1990s, with the greatest losses coming from marine-terminating sectors of the ice sheet where outlet glaciers discharge icebergs into the ocean. Jakobshavn Isbræ drains 7% of the ice sheet area and is one of the fastest flowing outlet glaciers. It has dramatically thinned, retreated and accelerated since the late 1990s. Since 2016 it has modestly slowed, readvanced and thickened, suggesting that it responds rapidly to changes in the water temperature in Ilulissat Icefjord. The complexity of its dynamical behaviour therefore makes it difficult to project its future evolution. This thesis takes a new approach to modelling Jakobshavn Isbræ, by running hindcast model experiments of the period 2009 to 2017 (inclusive) and assimilating all available data into the model. An iceberg calving rate was derived from satellite observations of velocity and terminus extent, and a method to drive a model of the glacier by the calving rate using the BISICLES ice sheet model was developed. A time-dependent inverse method was applied to assimilate observations into the model and different optimisation methods were considered. The calculated calving rate and optimised input fields were applied to drive hindcast model simulations. These experiments demonstrate that it is possible to model the evolution of Jakobshavn Isbræ by the regular assimilation of observations into the model. The model was dependent on regular assimilation, raising challenges for future projections of Jakobshavn Isbræ. The applicability of the model to future projections is discussed in this work. A linear viscous sliding law is poorly suited to modelling the future evolution of Jakobshavn Isbræ. ith additional parameter tuning, a regularised Coulomb sliding law may be more appropriate for future projections of the evolution of Jakobshavn Isbræ.

Ice penetrating radar (IPR) is a key tool in understanding the internal geometry and nature of glaciers and ice sheets, and has widely been used to derive bed topography, map internal layers and understand the thermal state of the cryosphere. Modern glacier and ice-sheet models facilitate increased assimilation of observations of englacial structure, including glacier thermal state and internal-layer geometry, yet the products available from radar surveys are often under-utilised. This thesis presents the development and assessment of radar processing strategies to improve quantitative retrievals from commonly acquired radar data. The first major focus of this thesis centres on deriving englacial velocities from zero-offset IPR data. Water held within micro- and macro-scale pores in ice has a direct influence on radar velocity, and significantly reduces ice viscosity and hence impacts the long-term evolution of polythermal glaciers. Knowledge of the radar velocity field is essential to retrieve correct bed topography from depth conversion processing, yet bed topography is often estimated assuming constant velocity, and potential errors from lateral variations in the velocity field are neglected. Here I calculate the englacial radar velocity field from common offset IPR data collected on Von Postbreen, a polythermal glacier in Svalbard. I first extract the diffracted wavefield using local coherent stacking, then use the focusing metric of negative entropy to deduce a local migration velocity field from constant-velocity migration panels and produce a glacier-wide model of local radar velocity. I show that this velocity field is successful in differentiating between areas of cold and temperate ice and can detect lateral variations in radar velocity close to the glacier bed. The effects of this velocity field in both migration and depth-conversion of the bed reflection are shown to result in consistently lower ice depths across the glacier, indicating that diffraction focusing and velocity estimation are crucial in retrieving correct bed topography in the presence of temperate ice. For the thesis' second major component I undertake an assessment of automated techniques for tracing and interpreting ice-sheet internal stratigraphy. Radar surveys across ice sheets typically measure numerous englacial layers that can be often be regarded as isochrones. Such layers are valuable for extrapolating age-depth relationships away from ice-core locations, reconstructing palaeoaccumulation variability, and investigating past ice-sheet dynamics. However, the use of englacial layers in Antarctica has been hampered by underdeveloped techniques for characterising layer continuity and geometry over large distances, with techniques developed independently and little opportunity for inter-comparison of results. In this paper, we present a methodology to assess the performance of automated layer-tracking and layer-dip-estimation algorithms through their ability to propagate a correct age-depth model. We use this to assess isochrone-tracking techniques applied to two test case datasets, selected from CreSIS MCoRDS data over Antarctica from a range of environments including low-dip, continuous layers and layers with terminations. We find that dip-estimation techniques are generally successful in tracking englacial dip but break down in the upper and lower regions of the ice sheet. The results of testing two previously published layer-tracking algorithms show that further development is required to attain a good constraint of age-depth relationship away from dated ice cores. I make the recommendation that auto-tracking techniques focus on improved linking of picked stratigraphy across signal disruptions to enable accurate determination of the Antarctic-wide age-depth structure. The final aspect of the thesis focuses on Finite-Difference Time-Domain (FDTD) modelling of IPR data. I present a sliced-3D approach to FDTD modelling, whereby a thin 3D domain is used to replicate modelling of full 3D polarisation while reducing computational cost. Sliced-3D modelling makes use of perfectly matched layer (PML) boundary conditions, and requires tuning of PML parameters to minimise non-physical reflections from the model-PML interface. I investigate the frequency dependence of PML parameters, and establish a relationship between complex frequency stretching parameters and effective wavelength. The resultant parameter choice is shown to minimise propagation errors in the context of a simple radioglaciological model, where 3D domains may be prohibitively large, and for a near-surface cross-borehole survey configuration, a case where full waveform inversion may typically be used.

Variation in the rate of meltwater input into the subglacial system of the Greenland Ice Sheet can force dynamic responses on a range of scales from hourly to interannual. Observations of the ice sheet dynamic response are commonly made either through ground-based Global Navigation Satellite System (GNSS) measurements, which can provide continuous and accurate point measurements, or through satellite remote sensing, which can provide regional-scale observations but at coarse temporal resolutions. This thesis investigates the potential of Uncrewed Aerial Vehicles (UAVs) to provide intermediate-level observations of the interactions between ice sheet hydrology and dynamics at a fast-flowing, marine terminating glacier in West Greenland. I first describe the development of a low- cost UAV suitable for deriving ice sheet velocity fields from Structure-from-Motion photogrammetry. In order to geolocate products without using ground control, image locations are determined directly using an on-board L1 GNSS receiver. I validate this method, showing that accuracies are sufficient for producing velocity fields in the ice sheet interior. Next, this method is used, alongside in-situ geophysical observations, to characterise the causes and dynamic influence of a rapid supraglacial lake drainage. I show that rapid drainage can induce a significant dynamic response up to 4 km away from the lake itself, and that fracture history can exert controls on interannual lake drainage behaviour. Finally, I upscale UAV ob- servations using satellite datasets over a ~3,000 km2 area, exploring dynamic controls on crevasse hydrology. I find that in compressive mean stress compressive regimes, crevasses are more likely to display ponding and rapid hydrofracture than in extensional regimes, where continuous slow drainage is typical. Continued high-resolution observations are necessary to further identify key controls on the hydrological influences of Greenland Ice Sheet dynamics.

Supraglacial debris thickness is a key control on the surface energy balance of debris-covered glaciers, which are common in temperate mountain ranges around the world. As such, it is an important input variable to the sorts of models that are used to understand and predict glacier change, which are essential for determining future water supply in glacierised regions and glacier contributions to sea-level rise. However, to quantify supraglacial debris thickness is difficult: making direct measurements is laborious and existing remote sensing approaches have not been thoroughly validated, so there is a general paucity of supraglacial debris thickness data. This thesis investigates methods of quantifying supraglacial debris thickness at local to regional scales. First, it makes in-situ field measurements of debris thickness at the local scale on glaciers in the Himalaya and the European Alps by manual excavation and by ground-penetrating radar (GPR). Second, it uses some of these field measurements to test and develop thermal remote sensing approaches to quantifying supraglacial debris thickness at the glacier scale. Third, it uses a dynamic energy-balance model in an inverse approach to quantify debris thickness on the glaciers of three watersheds in High Mountain Asia from thermal satellite imagery and high-resolution meteorological reanalysis data. At the local scale, GPR is found to be useful for measuring supraglacial debris thickness accurately and precisely, at least in the range 0.16-4.9 m. Debris thickness is highly variable over horizontal distances of < 10 m on individual glaciers due to gravitational reworking, which necessarily implies higher sub-debris ice melt rates than if debris thickness was spatially invariable. At the glacier scale, thermal remote sensing approaches can reproduce field measurements, and remote sensing estimates of supraglacial debris thickness can be used successfully to model sub-debris melting. If well-distributed field measurements are available, supraglacial debris thickness should be extrapolated using remote sensing-derived pseudo daily mean surface temperatures. Otherwise, it should be determined iteratively by minimising the mismatch between remotely sensed surface temperatures, preferably from night-time thermal images, and surface temperatures determined using a dynamic energy-balance model. At the regional scale, thermal satellite imagery and high-resolution meteorological reanalysis data can be used to provide reasonable estimates of supraglacial debris thickness. However, modelled uncertainties are not always able to explain ground-truth measurements, and there is a tendency towards underestimation due to problems associated with supraglacial ponds and ice cliffs and the spatial resolution of input data. The findings of this thesis will lead to improvements in the quantification of supraglacial debris thickness at a range of scales and, therefore, in the understanding and prediction of glacier change in temperate mountain ranges.

Volcanoes and glaciers can both pose a significant threat to life and property. Volcanoes can erupt suddenly, without warning, causing injury, death and damage to property. Glaciers generally present a hazard over longer timescales, melting or sliding into the oceans and contributing to sea-level rise. The movement of melt at volcanoes, and ice at and near the Earth's poles, can be investigated using microseismicity, emitted when these fluids and bodies release kinetic energy as they move. I use this microseismicity to study: melt moving from the deep crust and feeding Bardarbunga volcano, Iceland, before and after an eruption; icequakes at the bed of glaciers and ice sheets to study and constrain the physics of glacial sliding; and surface icequakes caused by crevassing, to see whether or not the crevasses observed are induced by hydrofracture. I use a combination of seismic observations and simple physical models to investigate these fundamental geophysical processes. Understanding the magmatic plumbing of a volcano is important for attempting to improve eruption forecasting. I analyse microseismicity before and after the Bardarbunga volcanic eruption, the largest eruption in Iceland in 230 years, to study possible pathways of melt from the deep crust to the shallow melt storage region. The seismicity and earthquake source mechanisms suggest that melt travels along a pathway that is approximately vertical, and laterally offset from the main shallow melt storage region by 12 km. However, it is also likely that an aseismic melt feed exists directly under Bardarbunga that we do not observe. These observations imply that volcanoes can be fed from depth, with lateral offsets of 10s kms, and that it is not adequate to monitor such volcanoes using seismicity alone. One critical process for constraining sea-level rise projections is glacial sliding. I describe how to detect and locate icequakes that originate at the bed of glaciers, which can be used to study glacial sliding. I analyse icequake source mechanisms for several glacial settings, comprising a range of spatial scales, in an attempt to unify the theory of stick-slip icequake failure, what it can tell us about glacial slip, and how such icequakes can be used to provide the first remotely measured values of bed shear modulus. The method used to remotely measure bed shear modulus will help constrain ice dynamics models and inform future passive cryoseismology studies of the Earth's ice sheets. Another poorly understood process is surface crevassing. Again, I analyse the source mechanisms of surface crevassing icequakes and show that they are tensile cracks, opening in the shallow subsurface. I present a novel method of obtaining depth estimates for these crevasse icequakes. Deriving crevasse depth is important since crevasse depth is usually limited by the stress distribution within the ice column, unless they are filled with water that can allow deeper propagation via hydrofracture. This mechanism is a possible pathway of meltwater propagating from surface to bed, lubricating the bed and exacerbating the movement of ice into the ocean. The same mechanism is also important for understanding how ice shelves, such as the Larson B ice shelf, break up, releasing onshore ice to flow unperturbed into the ocean. Such observations of crevasse fracture are therefore of great relevance for understanding key instabilities that could contribute to future sea-level changes. My results help inform our understanding of the aforementioned processes, which are critical for forecasting volcanic eruptions better, and informing and constraining ice dynamics models used for sea-level rise projections.

Accelerated mass loss from the Greenland Ice Sheet in recent decades has been concentrated around the coastal margins, where glacier ice fronts and the undersides of floating ice are in contact with warm ocean waters. The interaction of the ice sheet with a warming ocean leads to thinning of tidewater glaciers, which has been linked to increased glacier velocity, calving, retreat and subsequent mass loss. Our understanding of these ice-ocean interactions is limited, particularly regarding submarine melting of tidewater glacier ice fronts, which has been proposed as an initial trigger for glacier retreat and acceleration. Understanding the mechanisms promoting changes to tidewater glacier dynamics is critical, as they are currently absent from ice sheet models and present a large source of uncertainty in 21st century sea level rise predictions. This thesis develops two novel remote sensing techniques, to investigate both submarine melt rates under tidewater glacier floating ice tongues and iceberg freshwater fluxes from submarine melting, providing improved datasets and process understanding that can be used to constrain changes to the Greenland Ice Sheet in a warming world. The first result of this thesis is the development of a methodology to estimate submarine melting under floating ice tongues using satellite imagery. Submarine melt rates were derived by differencing along-flow ice tongue surface elevation, in combination with ice tongue velocity and changes in surface mass balance. Kangiata Nunaata Sermia (KNS), a large tidewater glacier in southwest Greenland, was used as a proof-of-concept study site. Mean submarine melt rates under the seasonal ice tongue at KNS in spring 2013 reach over 0.8 ± 0.3 m d−1, decreasing with distance down-fjord from the glacier grounding line and varying across-fjord. These variations in melt rate likely result from changes in ice tongue draft and fjord water temperature with depth, but may also reflect the strength of subglacial discharge plumes exiting beneath the glacier grounding line. Expanding upon these initial melt rate results, the thesis next explores the spatial and temporal variations in submarine melting at KNS from 2012 to 2014. Using the same methodology, derived melt rates vary significantly near the glacier grounding line, both spatially and temporally, with mean melt rates of 1.3 ± 0.6, 0.8 ± 0.3, and 1.0 ± 0.4 m d−1 across the ice tongue in 2012, 2013 and 2014, respectively. Areas of higher submarine melting correspond spatially with locations of subglacial discharge plumes and ice front calving activity observed during the melt season using time-lapse camera imagery, as well as to locations of modelled subglacial flow paths with large upstream catchment areas. These results suggest a dynamic subglacial hydrology system capable of rapidly re-routing subglacial discharge to different terminus outlets both within and between melt seasons. Furthermore, they provide an empirically-derived link between the presence of subglacial discharge plumes and areas of high spring submarine melting and calving along glacier termini. Moving to east Greenland, the final results chapter turns to Sermilik Fjord, where a new methodology for estimating freshwater fluxes from icebergs is developed. The amount, timing and location of meltwater produced via submarine melting of icebergs can significantly impact local fjord water circulation and heat budget, which has implications for glacier calving, retreat and acceleration in addition to nutrient fluxes and primary productivity. Previous methods for estimating iceberg meltwater fluxes have either been complex models, themselves reliant on limited field data and poorly constrained model parameters, or user-intensive, expensive remote sensing techniques. This thesis presents a simplified approach for deriving summer and autumn iceberg freshwater fluxes in 2017, using freely available Sentinel-2 satellite imagery to estimate iceberg velocity and seasonal changes in iceberg volume with distance down-fjord. Integrated 2-month full-fjord freshwater fluxes reach 1270 ± 650, 1200 ± 610, 3410 ± 1740, and 1150 ± 590 m3 s−1 for June-July, July-August, August-September and September-October, respectively. The estimated iceberg freshwater fluxes are highest across August and September, likely due to a combination of warming fjord water temperatures at depth as autumn approaches and increased calving into the fjord system in the months prior, and decrease with distance from the head of the fjord. The proportion of solid ice exiting the fjord averages 14% of the total ice volume calved into the fjord from Helheim Glacier, the largest tidewater glacier feeding into Sermilik Fjord, confirming that a significant volume of freshwater is released at depth along the length of the fjord. The volume of freshwater generated from iceberg melt is comparable to or greater than subglacial discharge volume throughout the year, and has important implications for fjord-scale circulation, submarine melt rates and primary productivity. In terms of future developments, this thesis presents two new methodologies focused on ice-ocean interactions: one to estimate submarine melt rates near tidewater glaciers and one to derive iceberg freshwater fluxes entering glacial fjords. These innovative methods have generated important new datasets in a challenging environment from which it has consistently proven difficult to derive estimates of submarine melt rates. The additional utility of these methods is their potential application to other fjord systems for constraining both fjord-scale and ice-sheet wide ice-ocean models, developments that are critical for understanding the sensitivity of the Greenland Ice Sheet and surrounding ocean basins to future climate change.

Trace gases play a variety of pivotal roles in atmospheric and earth surface processes despite being low in relative atmospheric abundance. Halogenated gases such as halocarbons contribute to the destruction of ozone; carbon dioxide (CO2), hydrogen (H2) and methane (CH4) can act as electron donors or acceptors to provide microbial energy; and CO2 links the chemical weathering of rocks to the global carbon cycle. However, there has been less research into the cycling of trace gases in glaciated environments. The subglacial environment can be isolated from the atmosphere and surface processes which would otherwise supply energy for microbial life and acidity for chemical weathering. It is unknown how such processes are driven when reactants become limited during long glaciations. The proglacial environment can be remote, particularly in regions such as the High Arctic, meaning fluxes of trace gases such as halocarbons can be particularly important due to distance from the major halocarbon source regions. Little research has been conducted on trace gas fluxes from the proglacial environment despite the ongoing expansion of these land surfaces as glaciers undergo widespread retreat in response to Arctic warming. This thesis explores novel sources, sinks and the roles of trace gases in both sub- and pro- glacial environments using a combination of laboratory experiments, field experiments and analysis of long-term atmospheric datasets. Laboratory rock grinding experiments were used as an analogue to glacial bedrock erosion and demonstrated abiotic production of several trace gases at rates significant to subglacial microbial metabolism and chemical weathering. Field flux chamber measurements on the proglacial forefield of a High Arctic glacier demonstrated that halocarbon fluxes were significant despite the immaturity of the recently exposed soils. Analysis of long-term, frequent halocarbon measurements in the ambient Arctic atmosphere indicated the influence of local halocarbon production, with evidence of marine processes in particular. In conclusion, trace gases are important for a variety of processes related to the marginal and rapidly changing glaciated environment. Understanding the role of trace gases, and their sources and sinks in these environments can help further our understanding of extreme life, the influence of chemical weathering on the carbon cycle and the influence of marginal soils on the halogen budget of the atmosphere.

Basal melting of ice shelves is causing accelerating mass loss from the Antarctic Ice Sheet, yet the oceanographic processes which drive this are rarely observed. This thesis uses new observations from phase sensitive radar and moored oceanographic instruments to describe the processes which drive rapid basal melting of the north-western Ross Ice Shelf. Oceanographic conditions at the mooring site are strongly influenced by the neighbouring Ross Sea Polynya. High Salinity Shelf Water fills the lower water column continuously, but during summer a southward flow ventilates the cavity bringing Antarctic Surface Water (AASW) to the site. Tides account for half of the flow speed variance, and low frequency variability is influenced by local winds, and eddies associated with sea ice production in the polynya. Four years of basal melt rate observations show a mean melt rate of 1.8 m y$^{-1}$ at the mooring site and a strong seasonal cycle driven principally by water temperature variations. Radar observations show that melt rates vary rapidly and continuously in response to flow speed variability, and rapid melting occurs only when flow speeds are high. Radar observations of melt rates from 78 sites on the Ross and McMurdo ice shelves show an area-averaged annual-mean basal melt rate of 1.35 m y$^{-1}$, implying a net basal mass loss of 9.6 Gt y$^{-1}$ from the region. Melt rates are highest near the ice front where annual-mean and short-term summer rates reached 7.7 m y$^{-1}$ and 53 m y$^{-1}$, respectively. The seasonal and spatial variations in melt rate are consistent with melting driven by the summer inflow of AASW. Observations of boundary layer water temperature, flow speed and melt rates indicate that melt rates scale linearly with current speed, but sub-linearly with temperature in the outer boundary layer, possibly due to the stabilising effects of melt water input. Existing melt rate parameterisations which account for flow speed can be tuned to match the observations when thermal driving is low, but overestimate melt rates at higher temperatures, implying the need for further refinements to the models.

The use of iceberg-keel ploughmarks as proxy indicators of past and present iceberg morphology, keel depth and drift direction has seldom been approached in the southern hemisphere. Using high-resolution multi-beam swath bathymetry of the mid-shelf Pine Island Trough and outermost Weddell Sea shelf regions of Antarctica, detailed analysis of >13,000 iceberg-keel ploughmarks was undertaken. By considering the draft of icebergs calved from Antarctica today, calculated from detailed satellite altimetric datasets by this work, almost all observed ploughmarks were interpreted to be relict features. In Pine Island Trough, ploughmark planform parameters and cross-sections imply calving of a large number of non-tabular icebergs with v-shaped keels from the palaeo-Pine Island-Thwaites ice stream. Geological evidence of ploughmark form and modern water depth distribution indicates calving-margin thicknesses (949 m) and subaerial ice cliff elevations (102 m) equivalent to the theoretical threshold predicted to trigger ice-cliff structural collapse and calving by marine ice-cliff instability (MICI) processes. Thus, ploughmarks provide the first observational evidence of rapid retreat of the palaeo-Pine Island-Thwaites ice stream, driven by MICI processes commencing ~12.3 cal ka BP. On the Weddell Sea shelf, ploughmark morphologies imply considerable variation in palaeo-iceberg shape and size, most likely reflecting calving from multiple source margins. In turn, an absence of grounded ice on the Weddell Sea shelf and a palaeo-oceanographic regime comparable to today are implied at the time of formation. Analysis of a 3D seismic cube of the Sea Lion Field area of the North Falkland Basin reveals iceberg-keel ploughmarks incised into the modern- and palaeo-seafloor, formed by icebergs of varying shape and size that most-likely calved from the Antarctic Ice Sheet during three past glacial periods (estimated ages ~18 - 26.5 ka BP, ~246 ka BP, ~9.8 Ma BP). Despite illustrating the possibility of iceberg drift into the North Falkland Basin today, the relict ploughmark age implies little risk to any seafloor structures in the area, which might be required for hydrocarbon production. By these analyses, the significance of iceberg-keel ploughmarks as indicators of palaeo-glaciology and palaeo-oceanography at the time of formation is emphasised.

Supraglacial lakes in the ablation zone of the Greenland Ice Sheet (GrIS) often drain rapidly (in hours to days) by hydraulically-driven fracture (“hydrofracture”) in the summer. Hydrofracture can deliver large meltwater volumes to the ice-bed interface and open-up surface-to-bed connections, thereby routing surface meltwater to the subglacial system, altering basal water pressures and, consequently, the velocity profile of the GrIS. The study of rapidly draining lakes is thus important for developing coupled hydrology and ice-dynamics models, which can help predict the GrIS’s future mass balance. Remote sensing is commonly used to identify the location, timing and magnitude of rapid lake-drainage events for different regions of the GrIS and, with the increased availability of high-quality satellite data, may be able to offer additional insights into the GrIS’s surface hydrology. This study uses new remote-sensing datasets and develops novel analytical techniques to produce improved knowledge of rapidly draining lake behaviour in west Greenland over recent years. While many studies use 250 m MODerate-resolution Imaging Spectroradiometer (MODIS) imagery to monitor intra- and inter-annual changes to lakes on the GrIS, no existing research with MODIS calculates changes to individual and total lake volume using a physically-based method. The first aim of this research is to overcome this shortfall by developing a fully-automated lake area and volume tracking method (“the FAST algorithm”). For this, various methods for automatically calculating lake areas and volumes with MODIS are tested, and the best techniques are incorporated into the FAST algorithm. The FAST algorithm is applied to the land-terminating Paakitsoq and marine-terminating Store Glacier regions of west Greenland to investigate the incidence of rapid lake drainage in summer 2014. The validation and application of the FAST algorithm show that lake areas and volumes (using a physically-based method) can be calculated accurately using MODIS, that the new algorithm can identify rapidly draining lakes reliably, and that it therefore has the potential to be used widely across the GrIS to generate novel insights into rapidly draining lakes. The controls on rapid lake drainage remain unclear, making it difficult to incorporate lake drainage into models of GrIS hydrology. The second aspect of this study therefore investigates whether various hydrological, morphological, glaciological and surface-mass-balance controls can explain the incidence of rapid lake drainage on the GrIS. These potential controlling factors are examined within an Exploratory Data Analysis statistical technique to elicit statistical similarities and differences between the rapidly and non-rapidly draining lake types. The results show that the lake types are statistically indistinguishable for almost all factors, except lake area. It is impossible, therefore, to elicit an empirically-supported, deterministic method for predicting hydrofracture in models of GrIS hydrology. A frequent problem in remote sensing is the need to trade-off high spatial resolution for low temporal resolution, or vice versa. The final element of this thesis overcomes this problem in the context of monitoring lakes on the GrIS by adapting the FAST algorithm (to become “the FASTER algorithm”) to use with a combined Landsat 8 and Sentinel-2 satellite dataset. The FASTER algorithm is applied to a large, predominantly land-terminating region of west Greenland in summers 2016 and 2017 to track changes to lakes, identify rapidly draining lakes, and ascertain the extra quantity of information that can be generated by using the two satellites simultaneously rather than individually. The FASTER algorithm can monitor changes to lakes at both high spatial (10 to 30 m) and temporal (~3 days) resolution, overcoming the limitation of low spatial or temporal resolution associated with previous remote sensing of lakes on the GrIS. The combined dataset identifies many additional rapid lake-drainage events than would be possible with Landsat 8 or Sentinel-2 alone, due to their low temporal resolutions, or with MODIS, due to its inferior spatial resolution.

Increasing surface runoff from the Greenland Ice Sheet due to a warming climate not only accelerates ice mass loss by altering surface mass balance, but may also lead to increased dynamic losses. This is because surface melt draining to the bed can reduce ice-bed coupling, leading to faster ice flow. Understanding the impact of surface melt on ice dynamics is important for constraining the contribution of the Greenland Ice Sheet to sea level rise. The aim of this thesis is to numerically model the influence of surface runoff on ice velocities. Three new models are presented: an updated supraglacial hydrology model incorporating moulin and crevasse drainage, along with lake drainage over the ice surface via channel incision; an ice sheet model implementing a numerically efficient formulation of ice flow; an adjoint code of the ice flow model based on automatic differentiation. Together with a subglacial hydrology model, these represent the key components of the ice sheet system. The supraglacial hydrology model is calibrated in the Paakitsoq region. Model output shows the partitioning of melt between different drainage pathways and the spatial distribution of surface drainage. Melt season intensity is found to be a relevant factor for both. A key challenge for simulations applying a coupled ice-flow/hydrology model is state and parameter initialization. This challenge is addressed by developing a new workflow for incorporating modelled subglacial water pressures into inversions of basal drag. A current subglacial hydrology model is run for a winter season, and the output is incorporated into the workflow to invert for basal drag at the start of summer in the Russell Glacier area. Comparison of the modelled subglacial system to observations suggests that model output is more in line with summer conditions than winter conditions. A multicomponent model integrating the main components of the ice sheet system is developed and applied to the Russell Glacier area. A coupled ice-flow/hydrology model is initialized using the proposed workflow, and driven using output from the supraglacial hydrology model. Three recent melt seasons are modelled. To a first order, predicted ice velocities match measured velocities at multiple GPS sites. This affirms the conceptual model that summer velocity patterns are driven by transitions between distributed and channelized subglacial hydrological systems.

The dynamic response of a faster-flowing Greenland Ice Sheet to climate change is modulated by feedbacks between ice flow and surface meltwater delivery to the basal environment. While supraglacial melt processes have been thoroughly examined and are well constrained, the response of the englacial and subglacial environment to these seasonal perturbations still represent the least-studied, understood, and parameterised processes of glacier dynamics due to a paucity of direct observation. To better understand these processes in the wake of a changing climate, novel in-situ geophysical experiments were undertaken on Store Glacier in west Greenland to quantify rates of englacial deformation and basal melting. The records produced from these experiments yield new insights into the thermodynamic setting of a major outlet glacier, and the physical mechanisms underlying and resulting from fast glacier motion. The deployment of autonomous phase-sensitive radio-echo sounders (ApRES) $\SI{30}{\kilo\metre}$ from the calving terminus of Store Glacier between 2014 and 2016 revealed high rates of both englacial deformation and basal melting, driven primarily by the dynamic response of the basal hydrological system to seasonal surface meltwater influxes. Thermodynamic modelling of this process revealed a convergence of large-scale basal hydrological pathways that aggregated large amounts of water towards the field site. The warm, turbulent water routed from surface melt contained and dissipated enough energy at the ice-bed interface to explain the observed high melt rates. Simultaneously, changes in the local strain field, involving seasonal variations in the morphology of internal layers, were found to be the result of far-field perturbations in downstream ice flow which propagated tens of kilometres upglacier through longitudinal stress coupling. When observed in multiple dimensions, the layer structure revealed complex internal reflection geometries, demonstrating ApRES as not just a monitor of depth changes in ice thickness, but also as an imaging instrument capable of characterising the subsurface environment within and beneath ice sheets. Altogether, the observations and analyses comprising this thesis provide new and significant insight and understanding into the structural, thermal, and mechanical processes tied to Store Glacier and its fast, complex, and dynamic ice flow.

Retreating glaciers and ice sheets provide a significant contribution to sea level rise, which will affect future populations and their activities. Accurate sea level projections are needed in order to best inform policy makers, but these projections are limited by our understanding of dynamical change at marine-terminating glaciers. Terrestrial time-lapse photography has proved to be a viable approach for obtaining high-detail observational records, and is used here to examine signals of dynamical change at two tidewater glaciers in Svalbard. Photogrammetric measurements were extracted using PyTrx (`Python Tracking'), a new photogrammetry toolbox that has been developed here for deriving velocities (e.g. glacier surface velocity), surface areas (e.g. supraglacial lake area, surfacing plume area), and line distances (e.g. terminus profiles). PyTrx has been created as a Python-alternative photogrammetry software, and offers additional functionality to the typical monoscopic feature-tracking toolboxes that are currently available. Subglacial hydrology and its relation to basal sliding were examined at Kronebreen, Svalbard. The results revealed a difference in flow efficiency between the north and south regions of the glacier tongue, which influences spatial patterns in surface velocities. Long-term changes in ice flow were concluded to be controlled by the location of effcient and inefficient drainage, and the position of regions where water is stored and released. Changes in terminus conditions and calving processes were examined at Tunabreen, a surge-type tidewater glacier. Observations suggested that atmospheric forcing plays a larger role in terminus stability than previously considered, and it is likely that terminus dynamics at Tunabreen are the product of a unique interplay between oceanic and atmospheric forcing which are shaped by the glacier's surge-type nature. Additionally, calving activity at Tunabreen can be characterised as high-frequency, low-magnitude events, and a high proportion of its long-term calving activity can be attributed to the rate of under-cutting at the terminus. In all, these studies demonstrate that long-term changes in glacier dynamics are dictated by the small changes in basal and terminus conditions, and how they vary from year-to-year. Future research now needs to be directed towards understanding how small-scale processes vary over multiple melt seasons, in order to establish how they operate at longer timescales. PyTrx provides an appropriate basis to continue this work and expand the capabilities of the toolbox.

Ice streams are fast flowing outlet glaciers through which over 90% of the ice stored within the Antarctic Ice Sheet drains. The dynamic behaviour of ice streams is therefore crucial in controlling the mass balance of the ice sheet. Over the past few decades, Antarctica has been losing mass. Much of this mass loss has been focussed around coastal regions of the Antarctic Ice Sheet. Some of the most dramatic changes such as grounding-line retreat, acceleration and surface elevation change have been observed in Pine Island Glacier (PIG) and its neighbouring ice streams. This is of particular concern because these ice streams account for 10% of the discharge from the west Antarctic Ice Sheet and therefore have the potential to contribute significantly to global sea-level rise. One of the key challenges in accurately forecasting this future sea-level rise is improving understanding of processes occurring at the beds of ice streams. This requires detailed knowledge of the properties and dynamics of the bed. This thesis aims to address this knowledge gap by investigating the spatial and temporal characteristics of the bed of PIG using high-resolution geophysical data acquired in its trunk and tributaries and beneath the ice shelf. The thesis begins by analysing radar-derived high-resolution maps of subglacial topography. These data show a contrasting topography across the ice-bed interface. These diverse subglacial landscapes have an impact on ice flow through form drag, controlled by the size and orientation of bedrock undulations and protuberances. The next chapter provides a quantitative analysis of these landscapes using Fast Fourier analysis of subglacial roughness. This analysis investigates the roughness signature of subglacial bedforms and the how the orientation and wavelength of roughness elements determine their correlation with ice dynamic parameters. The slow-flowing inter-tributary site is found to have a distinct signature comparable to 'ribbed' patterns of modelled basal shear stress and transverse 'mega rib' bedforms. Roughness oriented parallel to ice flow with wavelengths approaching mean ice thickness are found to have the highest correlation with ice dynamic parameters. The temporal stability of PIG is analysed using repeat radar measurements. No significant change is observed over a period of 3-6 years with no evidence of rapid erosion or the evolution of subglacial bedforms as observed in previous repeat measurements of ice-stream beds elsewhere in Antarctica. This suggests that the widespread deforming till layer detected in extensive seismic reflection surveys is in steady state. Lastly, the thesis explores geomorphological evidence of twentieth-century grounding-line retreat beneath PIG Ice Shelf using high-resolution geophysical data acquired from autonomous underwater vehicle surveys. Evidence of erosion, deposition, meltwater flow and post-glacial modification is observed in fine detail. The observed distribution of sediment supported previous surveys indicating a geological transition coinciding with the ridge that acted as a former stable grounding-line location. Metre-scale resolution images of recently deglaciated ice stream beds were found to reveal bedforms that are not detectable with traditional offshore bathymetric surveys. Together these findings reveal the role of short wavelength topography as both an influence on, and product of fast ice stream flow. It also highlights the spatial diversity of subglacial environments and the need to focus future research on tying detailed observations of ice-stream beds with knowledge of basal properties over time.

Ice shelves are important to the climate system as they control the release of fresh water from ice sheets into the ocean, with consequences for sea level rise and ocean dynamics. Channels modifying basal melt rates and structural integrity have been observed inscribed in the undersides of some ice shelves. Observations indicate that some channels run in a direction transverse to ice flow and it has been suggested that these form due to seasonal variations in ocean properties. This thesis analyses the effect of seasonal variability on ice shelves and meltwater plumes in the underlying ocean. A linear perturbation analysis on vertically integrated ice and plume models showed that seasonal forcing of subglacial discharge or ice flux can generate small ripples melted into the base of the ice shelf. These ripples did not develop into overdeepened channels, but the ripples caused by ice flux appear similar to basal terraces observed underneath some ice shelves. Code was developed to run 1-D nonlinear simulations with the vertically-integrated equations, producing similar results to the linear case. However, runs neglecting hydrostatic pressure gradients exhibited a feedback causing ice flux-generated ripples to grow into small proto-channels. A horizontally-integrated plume model was derived, incorporating the Coriolis force and transverse plume flow into a 1-D model which agreed well with a 3-D ocean simulation. Coupling this horizontally-integrated plume with a co-evolving ice shelf prevented proto-channels from forming. It appears unlikely that subglacial discharge or ice flux variations can give rise to observed transverse channels. A new approach was developed to predict the evolution of internal radar reflectors observed within ice shelves, using vertically-integrated models of ice flow. It is hoped this approach might have applications for inverse modelling and data assimilation.

In this thesis, I explore the flow dynamics associated with ice shelves confined within channels and the buttressing they provide to grounded ice. Ice shelves are the floating extensions of ice sheets and act as the interface between the ice sheet and the ocean. They form when ice flows out from the interior of the ice sheet towards the coast and begins to float as the ice thins. Ice shelves are often found within a channel or pinned in place by stationary bedrock outcrops. The interest in their dynamics is motivated by the buttressing effect they provide to the grounded ice, which strongly controls the rate of ice discharge and thereby the contribution to sea-level rise. I use a combination of mathematical modeling, fluid-mechanical laboratory experiments and geophysical data analysis to develop an improved understanding of ice-shelf flow dynamics. Initially, geophysical data in the form of Antarctic ice-surface velocity data is analysed, producing maps of strain rate, shear rate and strain orientation for Antarctic ice shelves. This allows the geophysical setting and flow processes to be explored, particularly by identifying areas where resistance to ice flow is generated and regions of the shelf that make no contribution to buttressing. Using the geophysical data, I find good agreement between a theoretical scaling relationship for ice flow at the ice-shelf calving front and data from Antarctic ice shelves. I proceed to develop an idealized mathematical model of an ice shelf confined to flow in a channel. By assuming shear-dominated dynamics within the shelf, analytical solutions are obtained for steady-state ice-shelf thickness profiles in parallel and diverging channels. This model is developed further to include both shear and extensional stresses, from which numerical solutions for steady-state shelves are calculated. The results from these two models are then compared. It is found that shear stresses dominate the dynamics throughout the majority of the shelf, with adjustment regions at the upstream and downstream boundaries where extensional dynamics become important. Output from these models is also compared with geophysical data and it is observed that there is good agreement between several features of the thickness profiles and velocity fields. In addition to the geophysical data, comparisons are made with fluid-mechanical laboratory experiments designed to simulate the flow of an ice shelf in a channel. The advantage of performing experiments of this kind is that parameters such as the fluid rheology can be varied, allowing for direct comparison with a range of parameters in the mathematical models. From these experiments, surface velocity fields and thickness profiles are collected, which are used to make comparisons with the models. Clear differences are observed in the velocity and strain-rate fields produced using fluids with different rheologies, for which there is qualitative agreement with the output from the mathematical models.

There is increasing recognition that the hydrology of the Greenland Ice Sheet plays an important role in the dynamics and therefore mass balance of the ice sheet. Understanding the hydrology of the ice sheet and being able to predict its future behaviour is therefore a key aspect of glaciological research. To date, the ice sheet’s hydrology has tended to be inferred from the analysis of surface velocity measurements, or modelled in a theoretical, idealised way. This study focuses on the development of a high spatial (100 m) and temporal (1 hour) resolution, physically based, time-dependent hydrological model which is applied to the ~2,300 km2 Paakitsoq region, West Greenland, and is driven, calibrated, and evaluated using measured data. The model consists of three components. First, net runoff is calculated across the ice sheet from a distributed, surface energy- balance melt model coupled to a subsurface model, which calculates changes in temperature, density and water content in the snow, firn and upper-ice layers, and hence refreezing. The model is calibrated by adjusting key parameter values to minimize the error between modelled output and surface height and albedo measurements from the three Greenland Climate Network (GC-Net) stations, JAR 1, JAR 2 and Swiss Camp. Model performance is evaluated in two ways by comparing: i) modelled snow and ice distribution with that derived from Landsat-7 ETM+ satellite imagery using Normalised Difference Snow Index (NDSI) classification and supervised image thresholding; and ii) modelled albedo with that retrieved from the Moderate- resolution Imaging Spectroradiometer (MODIS) sensor MOD10A1 product. Second, a surface routing / lake filling model takes the time-series of calculated net runoff over the ice sheet and calculates flow paths and water velocities over the snow / ice covered surface, routing the water into ‘open’ moulins or into topographic depressions which can fill to form supraglacial lakes. This model component is calibrated against field measurements of a filling lake in the study area made during June 2011. Supraglacial lakes are able to drain by a simulated hydrofracture mechanism if they reach a critical volume. Once water is at the ice / bed interface, discharge and hydraulic head within subglacial drainage pathways are modelled using the third model component. This consists of an adaptation of a component (EXTRAN) of the U.S. Environmental Protection Agency Storm Water Management Model (SWMM), modified to allow for enlargement and closure of ice-walled conduits. The model is used to identify how the subglacial hydrological system evolves in space and time in response to varying surface water inputs due to melt and lake drainage events, driven ultimately by climate data. A key output from the model is the spatially and temporally varying water pressures which are of interest in helping to explain patterns of surface velocity and uplift found by others, and will ultimately be of interest for driving ice dynamics models.

Hydrology is recognised as an important component of the glacial system in alpine environments. In particular, the subglacial drainage of surface meltwaters is known to exert a strong influence on the motion of glaciers and on their capacity to erode the underlying bedrock. This thesis examines the more poorly understood drainage system of the Greenland Ice Sheet, with specific focus on Leverett Glacier, a landterminating outlet glacier on the ice sheet’s western margin. Because of the vast size of the ice sheet, the influence of the drainage system could have wide ranging implications, most notably for sea level rise and continental scale landscape evolution. The thesis commences with an investigation into the morphology of the drainage system of the lower 14 km of Leverett Glacier. This is undertaken using a variety of field methods, including dye tracing and the monitoring of proglacial discharge, englacial water levels, surface melt rates and glacier motion. The data reveal that the drainage system of the glacier closely resembles that of alpine glaciers, undergoing an evolution from distributed to channelised drainage morphologies as the melt season progresses. Another aspect of the field data, the suspended sediment load evacuated from the subglacial system in the emerging proglacial river, is then examined to investigate the impact that this drainage system morphology has on the interaction between the glacier and the underlying bedrock or substrate. This demonstrates that the presence of large, efficient subglacial drainage channels allows for the removal of vast quantities of basal debris during much of the melt season, facilitating an erosion rate 1-2 orders of magnitude greater than previously proposed for ice sheet settings. The thesis then focuses on the relationship between discharge, water pressure and ice motion. Observations from Greenlandic and alpine glaciers demonstrate that glaciers generally decelerate through the melt season following a maximum velocity induced by the onset of melt in the spring. The data indicate that the evolution of the drainage system from a distributed to a channelised morphology occurs rapidly and so can only explain this trend in ice velocity during the early part of the melt season. Beyond this period, ice velocity patterns can instead be explained primarily by transient fluctuations in water pressure within the channelised drainage system. These transient pressure fluctuations result from the lag between changes to the rate of meltwater input to the glacier and the subsequent adjustment of channel cross section. This indicates that it is crucial to consider temporal variability in melt rate when seeking to link climate with the dynamics of ice sheets and glaciers. This process can be simulated, which is demonstrated by using the proglacial discharge record to model subglacial water pressure and ice velocity. In the following chapter, this model is built upon by considering how these variations in water pressure, originating in discrete subglacial channels, control sliding velocities across large areas of the glacier. Detailed examination of high-resolution ice velocity records from Leverett Glacier reveals that, in keeping with theory, horizontal ice velocity is dependent on both the volume of subglacial cavities and the rate-of-change of this volume. A simple model of subglacial water movement is then used to demonstrate how these changes in the cavity system could be driven by the pressure fluctuations predicted within the channelised drainage system. This enables a system scale model of glacier hydrology to be developed, which is presented in the final chapter, linking variations in surface melt rate to channel pressure, cavity volume and ultimately ice motion. In summary, this research has helped to illuminate the morphology and functioning of the drainage system of Leverett Glacier. This has improved our understanding of how hydrology influences both the motion of the Greenland Ice Sheet and its impact on the underlying topography, and enabled better prediction of how these processes are influenced by changes in climate.

The portfolio of compositions comprises six pieces: a chamber opera, an orchestral piece and four shorter chamber works. These pieces are diverse and distinct from one another but collectively explore aesthetic tensions relating to tonality, aura and ontology. The largest piece is a chamber opera setting Margaret Atwood's novel, The Handmaid's Tale, which has been flexibly scored as a series of fragments in order to reflect the quality of her text. The remaining pieces draw influence from poetry, landscape and the environment. They all encompass a series of material contrasts but attempt to simply contain these tensions in some way, leaving them partially unresolved. The thesis is a re-assessment of the music of the Ukrainian composer, Valentin Silvestrov, in particular, his 'metamusic' approach to composition that treats pre-existing styles as a form of musical metaphor. Through a series of comparisons with landscape and photography, I offer new vantage points for approaching the aesthetic issues present in his work, relating to aura, imitation and historical reference. The metaphors of landscape and photography might appear far removed from his work, but mediated by the work of the artist Gerhard Richter, offer a basis for critically analysing Silvestrov's approach. Furthermore, by drawing upon the theories of Walter Benjamin, Roland Barthes and the geographer, Stephan Harrison, I demonstrate how concepts from other disciplines can be recast in order to be effective for approaching both Silvestrov and Richter. As a form of conclusion, I consider the role of photography in the production of CD covers and how this relates to the reception of Silvestrov's metamusic in a commercial setting.

Ice streams play a fundamental role in the stability and dynamics of ice sheets. They are defined by their rapid flow and this is enabled by conditions and processes at the icebed interface. A significant limitation to our understanding of this environment is that most studies, of both contemporary and palaeo-ice streams, have focussed on only one or two, discrete spatial scales of analysis and so integration between scales is restricted. This thesis investigates palaeo-ice streams at multiple scales in order to examine their subglacial processes and characteristics, and to assess the links between and the application of different spatial scales of analysis. Seven palaeo-ice streams from the British and Laurentide ice sheets were investigated at the macroscale, which involved geomorphological mapping, spatial analysis of subglacial lineations and examination of bed characteristics. Two ice streams were also investigated at smaller scales, which included sedimentological analysis (mesoscale) and micromorphological analysis (microscale). Macroscale results showed that subglacial lineations display certain spatial characteristics, including: clustering according to elongation ratio; distribution of low elongation ratios throughout the ice streams; and a decrease in maximum elongation ratio towards the ice stream lateral margins. The latter of which is considered to reflect the transverse distribution of ice velocity. In some cases, a decline in subglacial lineation concentration and elongation ratio coincided with topographic obstacles at the ice stream bed. The most common bed characteristics identified were: widespread till, fine grained sedimentary bedrock with a moderate permeability, low relief and a flat topographic curvature. Key subglacial processes identified included deformation, which was observed at all three scales, and high pore water pressures, for which multiple lines of evidence were found at the meso and micro scales. Spatial variability in both strain and pore water pressure was also common. The multiscale approach allowed robust interpretations of fast flow mechanisms, which furthers knowledge of the sediment and landform characteristics that may result from these flow mechanisms. A summary of the processes that can be identified at each of the spatial scales is given.

This thesis analyses satellite-based radar data to improve our understanding of the interactions between the Antarctic Ice Sheet and the ocean in the Amundsen Sea Sector of West Antarctica. Over the last two decades, the European Remote Sensing (ERS) Satellites have provided extensive observations of the marine and cryospheric environments of this region. Here I use this data record to develop new datasets and methods for studying the nature and drivers of ongoing change in this sector. Firstly, I develop a new bathymetric map of the Amundsen Sea, which serves to provide improved boundary conditions for models of (1) ocean heat transfer to the ice sheet margin, and (2) past ice sheet behaviour and extent. This new map augments sparse ship-based depth soundings with dense gravity data acquired from ERS altimetry and achieves an RMS depth accuracy of 120 meters. An evaluation of this technique indicates that the inclusion of gravity data improves the depth accuracy by up to 17 % and reveals glaciologically-important features in regions devoid of ship surveys. Secondly, I use ERS synthetic aperture radar observations of the tidal motion of ice shelves to assess the accuracy of tide models in the Amundsen Sea. Tide models contribute to simulations of ocean circulation and are used to remove unwanted signals from estimates of ice shelf flow velocities. The quality of tide models directly affects the accuracy of such estimates yet, due to a lack of in situ records, tide model accuracy in this region is poorly constrained. Here I use two methods to determine that tide model accuracy in the Amundsen Sea is of the order of 10 cm. Finally, I develop a method to map 2-d ice shelf flow velocity from stacked conventional and multiple aperture radar interferograms. Estimates of ice shelf flow provide detail of catchment stability, and the processes driving glaciological change in the Amundsen Sea. However, velocity estimates can be contaminated by ocean tide and atmospheric pressure signals. I minimise these signals by stacking interferograms, a process which synthesises a longer observation period, and enhances long-period (flow) displacement signals, relative to rapidly-varying (tide and atmospheric pressure) ones. This avoids the reliance upon model predictions of tide and atmospheric pressure, which can be uncertain in remote regions. Ice loss from Amundsen Sea glaciers forms the largest component of Antarctica’s total contribution to sea level, yet because present models cannot adequately characterise the processes driving this system, future glacier evolution is uncertain. Observations and models implicate the ocean as the driver of glaciological change in this region and have focussed attention on improving our understanding of the nature of ice-ocean interactions in the Amundsen Sea. This thesis contributes datasets and methods that will aid historical reconstructions, current monitoring and future modelling of these processes.

The purpose of this thesis is to investigate the hydrology and dynamics of a land-terminating outlet glacier on the western margin of the Greenland Ice Sheet (GrIS). The investigations are motivated by uncertainty about the effect of meltwater on rates of ice flow in the GrIS and the possibility that hydrologically forced changes in ice velocity might increase mass loss from the ice sheet significantly in response to climate warming. The impact of meltwater on fluctuations in ice flow has been a research focus for glaciologists studying Alpine and Arctic glaciers for decades. In these settings, one of the main controls on the relationship between surface melting and ice velocity is the structure of the subglacial drainage system, which evolves spatially and temporally on a seasonal basis in response to inputs of meltwater from the glacier surface. In this thesis we present three years of field observations of glacier velocity, surface ablation and hydrology from a land-terminating glacier in west Greenland. These data are supplemented by satellite data and the use of simple models to constrain surface melting. We find that hydrologically forced ice acceleration occurs each year along a 115 km transect, first at sites nearest the ice sheet margin and at locations further inland following the onset of surface melting at higher elevations. At sites near the ice sheet margin, the relationship between surface melting and ice velocity is not consistent throughout the melt season, and ice velocity becomes less sensitive to inputs of meltwater later in the summer. This is explained by development in the efficiency of the subglacial drainage system, in a manner similar to Alpine glaciers. We perform a hydrological study which indicates that an efficient subglacial drainage system expands upglacier over the course of the melt season, in response to inputs of water from the ice sheet surface. At higher elevation sites, however, thicker ice and colder temperatures mean that it is harder to generate enough water to reach the ice-bed interface and this only occurs once enough water has accumulated to propagate fractures through thick ice to the bed. One mechanism which allows this is drainage of supraglacial lakes. Inter-annual comparison shows that increased rates of annual ablation lead to higher annual ice velocities. At high elevation sites (>1000 m), timing of drainage of meltwater to the ice-bed interface appears to be the main control on the the overall magnitude of summer acceleration. At lower elevations, although development in the structure of the subglacial drainage system limits the overall summer acceleration signal, short-term variability in meltwater input can sustain high ice velocities even once the subglacial drainage system has become channelised. Overall, the research presented in this thesis suggests that hydrologically-forced acceleration can increase mass loss from the GrIS in a warmer climate due to inland expansion of the area of the ice sheet bed which is subject to inputs of meltwater from the ice sheet surface. The relationship between surface melting and ice velocity is mediated, however, by the structure of the subglacial drainage system and variations in the rate of meltwater drainage to the ice bed interface. Insights from this work can help in the development of numerical ice sheet models which aim to predict the future contribution to sea-level rise from the Greenland Ice Sheet.

The influence of seasonal influx of supraglacial meltwater on basal water pressures and consequent changes in ice surface velocity has been a focus of research spanning over three decades. With a need to better include glacial hydrology within models of ice sheet evolution, the ability to predict where and when meltwater reaches the subglacial system is paramount for understanding the dynamics of large Arctic ice masses. The response of ice velocities to melt production suggests efficient transmission of meltwater from the supraglacial to subglacial hydrologic systems, and it has been shown that build-ups of stored meltwater in supraglacial lakes can force crevasse penetration through hundreds of metres of ice. This thesis presents a new modelling routine for prediction of moulin formation and delivery of meltwater to the ice-bed interface. Temporal and spatial patterns of moulin formation and drainage of supraglacial lakes are presented, and quantitative controls on crevasse propagation are investigated through a series of sensitivity tests. _J .' . The model is applied to two glacial catchments: the Croker Bay catchment of the Devon Ice Cap in High Arctic Canada; and the Leverett glacier catchment of the Greenland Ice Sheet. Through model application to these sites, sensitivities to crevasse surface dimensions, ice tensile strength, ice fracture toughness and air temperatures are investigated. Model predictions of moulin formation and melt transfer are compared with field observations and remotely sensed data, including ice surface velocities, proglacial discharge, dynamic flow regimes, and visible surface features. The inclusion of spatially distributed points of meltwater delivery to the 'subglacial system is imperative to fully understand the behaviour of the subglacial drainage system. Furthermore, dynamic response to future climatic change and melt scenarios, and the evolution of ice masses, cannot be fully understood without first understanding the glacial hydrologic processes driving many of these changes.

The complex record of glaciogenic landforms and sediments in Britain relating to the last British-Irish Ice Sheet provides the opportunity to reconstruct former ice extents, ice dynamics, retreat patterns and examine their links to climate change. Yet in Scotland, as in the rest of Britain, a previously fragmentary approach to palaeoglaciological research has limited our understanding of glacier dynamics and their relationship to climate, particularly during the Last Glacial-Interglacial Transition. The Monadhliath Mountains in the Central Scottish Highlands are dominated by an extensive plateau area that has received little research attention in the past. The few examples of research include work by British Geological Survey officers in the early 1900s and J.R. Young in the 1970s. These studies focussed primarily on the geomorphology and sedimentology of isolated valleys and therefore this PhD research provides the first systematic mapping of the region as a whole. Results of remote and field mapping demonstrate that two coalescent plateau icefields, together covering an area of c. 280 km2, occurred over the southwest and central sector of the Monadhliath Mountains during the Younger Dryas. Equilibrium line altitudes calculated for the icefield are of comparable magnitude to those reconstructed for nearby Younger Dryas ice masses, such as in Drumochter and Creag Meagaidh, but indicate slightly lower precipitation in the Monadhliath Mountains. ELAs of individual outlet glaciers rise steeply from west to east across the plateau, indicating a strong local precipitation gradient. Significant variations in the geomorphology on the plateau and within outlet valleys allowed an examination of former thermal regime and differences in ice dynamics during retreat. In-depth analysis of moraine retreat patterns enabled a detailed insight into palaeoglaciological controls on deglaciation for the first time, concluding that valley morphology and gradient were the most influential factors on the retreat dynamics of the plateau icefield.

Palaeoenvironmental investigations from the Lake Plav catchment of the Prokletije Mountains in Montenegro and Albania, allowed primarily climatic change and anthropogenic influences during the Late Holocene and particularly the Little Ice Age (LIA) to be identified. Three sediment cores were analysed, two from Lake Plav (904 m a.s.l., cores LPCA and LPCB) and one from the upper catchment site of Lake C in Buni i Jezerces (1754 m a.s.l., core BJC1). These sediments were analysed for a variety of proxies including pollen, ostracoda, organic content, magnetic susceptibility and particle size. Chronologies for each sediment core were constructed using AMS radiocarbon, 210Pb and 137Cs dating techniques. The lower sites provided a record of past flood events, anthropogenic influences, lake development and infilling that have occurred since c. AD 500. Core BJC1 provided longer-term data since c. 2720 BC, providing complementary records of Pediastrum and thermophilous arboreal types, identified following a catchment vegetation survey. Glacial geomorphological mapping of the Maja e Koljaet glacier in Buni i Jezerces, Albania, enabled a catchment specific palaeotemperature record to be constructed from AD 1859 to the present. Glacial features were dated using lichenometry before degree-day modelling enabled temperature reconstruction. The palaeotemperature reconstruction for the Albanian Little Ice Age glacial maximum (LIAGM) suggests that temperatures were 0.9°C below the 1980-2008 annual temperature mean. This work also provided the first record of glacial extent during the LIA in Albania, indicating that the Albania LIAGM occurred c. AD 1859, around a decade after the European LIAGM and two decades before that of Montenegro. Anthropogenic indicators were used to reconstruct human activity in the catchment, which suggested that arable farming was pursued throughout the Medieval Warm Period (MWP; c. AD 800-1090) and continued during a period of transition to the LIA, between c. AD 1090 and AD 1300. The LIA (c. AD 1300 - 1860) was characterised by an abrupt Alnus decline, thought to be the result of anthropogenic clearance of the floodplain and reduction of both arable and thermophilous types. During the LIA sedimentation rates were up to 1.41 + 0.17 cm yr-1 at Lake Plav causing lake infilling and shallowing allowing wetland expansion c. AD 1570. The result of lake infilling is highlighted during the early 20th century, when the lake extent fell by around 42% as a result of climatic amelioration post-LIA causing lake levels to fall and wetland indicators to decline. The inferred past climatic changes from the Lake Plav catchment are compared to data from around the Mediterranean and Southern Europe. This allows identification of the climatic influences affecting the site during the Late Holocene. Catchment records have provided evidence of cooler and wetter conditions coeval to the occurrence of solar minima such as the Wolf, Spörer and Maunder minima. Overall, the records suggest that continental atmospheric circulation patterns such as the North Sea-Caspian Pattern (NCP) and East Atlantic-West Russia pattern (EA-WR), dominated the site until the late 1800s, when records become more synchronous with the NAO index and Mediterranean/Southern European data.

This thesis considers the use of Interferometric Synthetic Aperture Radar (InSAR) for surveying temporal fluctuations in the velocity of glaciers in the Arctic region. The aim of this thesis is to gain a broader understanding of the manner in which the flow of both land- and marine-terminating glaciers varies over time, and to asses the ability of InSAR to resolve flow changes over timescales which provide useful information about the physical processes that control them. InSAR makes use of the electromagnetic phase difference between successive SAR images to produce interference patterns (interferograms) which contain information on the topography and motion of the Earth's surface in the direction of the radar line-of-sight. We apply established InSAR techniques (Goldstein et al., 1993) to (i) the 925 km2 LangjÖkull Ice Cap (LIC) in Iceland, which terminates on land (ii) the 8 500 km2 Flade Isblink Icecap (FIIC) in Northeast Greenland which has both land- and marine-terminating glaciers and (iii) to a 7 000 km2 land-terminating sector of the Western Greenland Ice Sheet (GrIS). It is found that these three regions exhibit velocity variations over contrasting timescales. At the LIC, we use an existing ice surface elevation model and dual-look SAR data acquired by the European Remote Sensing (ERS) satellite to estimate ice velocity (Joughin et al., 1998) during late-February in 1994. A comparison with direct velocity measurements determined by global positioning system (GPS) sensors during the summer of 2001 shows agreement (r2 = 0.86), suggesting that the LIC exhibits moderate seasonal and inter-annual variations in ice flow. At the FIIC, we difference pairs of interferograms (Kwok and Fahnestock, 1996) formed using ERS SAR data acquired between 15th August 1995 and 3rd February 1996 to estimate ice velocity on four separate days. We observe that the flow of 5 of the 8 outlet glaciers varies in latesummer compared with winter, although flow speeds vary by up to 20 % over a 10 day period in August 1995. At the GrIS, we use InSAR (Joughin et al., 1996) and ERS SAR data to reveal a detailed pattern of seasonal velocity variations, with ice speeds in latesummer up to three times greater than wintertime rates. We show that the degree of seasonal speedup is spatially variable and correlated with modeled runoff, suggesting that seasonal velocity changes are controlled by the routing of water melted at the ice sheet surface. The overall conclusion of this work is that the technique of InSAR can provide useful information on fluctuations in ice speed across a range of timescales. Although some ice masses exhibit little or no temporal flow variability, others show marked inter-annual, seasonal and even daily variations in speed. We observe variations in seasonality in ice flow over distances of ~ 10 km and over time periods of ~10 days during late-summer. With the aid of ancillary meteorological data, we are able to establish that rates of flow in western Greenland are strongly moderated by the degree of surface melting, which varies seasonally and secularly. Although the sampling of our data is insufficiently frequent and spans too brief a period for us to derive a general relationship between climate and seasonality of flow, we show that production of meltwater at the ice surface and its delivery to the ice bed play an important role in the modulation of horizontal flow speeds. We suggest that a similarly detailed investigation of other ice masses is required to reduce the uncertainty in predictions of the future Arctic land-ice contribution to sea level in a warming world.

In order to increase coverage, mass balance changes of the world’s ice sheets are increasingly derived from surface elevation changes measured via satellite. Across the percolation zone of the Greenland Ice Sheet, meltwater, percolation and refreezing cause a re-distribution of mass through densification which may result in elevation change with no associated mass loss. Therefore, densification processes need to be quantified, spatially and temporally, and accounted for in mass balance measurements. This thesis investigates the relationships between patterns of elevation change and temporally and spatially variable accumulation and densification processes. In doing so, it provides an important contribution to the validation of the European Space Agency’s CryoSat-2 mission by placing error bars on the accuracy to which changes in satellite-measured ice-mass surface elevation represent real changes in ice mass. Temporal variability in near-surface (<10 m) snowpack and firn density and structure was measured in snowpits, shallow cores and using a neutron probe in the spring and autumn of 2004 at ~1945 m elevation (T05, 69o 51N, 47o 15W) in the percolation zone of the Greenland Ice Sheet. Results show that average snowpack density increased by 26% from spring to autumn, with a 5% (7.6 cm) increase in elevation, and a corresponding 32% increase in mass. Spatial variability was investigated at 11 sites along two transects at spatial scales of 1 m – 10 km. Whilst there was little variability in small scale (1 - 100 m) density changes, ‘seasonal densification’ increased at lower elevations, rising to 47% 10 km closer to the ice sheet margin at 1860 m a.s.l. The spatial variability in seasonal densification was further investigated in spring 2006 at seven sites located at ~10 km intervals along a 57 km transect spanning a 350 m elevation range. Snowpits and shallow cores reveal no significant variation in spring (prior to melt) snowpack density but following summer melt and refreezing cycles, seasonal densification decreased with increasing elevation at 32 kg m-3 per 100 m. Measurements at three sites ranging in elevation from 1860 – 2015 m and spanning three melt-seasons show inter-annual variation in the seasonal densification gradient. In order to obtain a longer time series of mass balance, a 17 m core retrieved in spring 2004 was analysed for stratigraphy, density and ionic and isotopic concentrations to identify annual layers. Unfortunately, the seasonal melt cycle (whereby on average 10% of the snowpack undergoes melt), results in a complex stratigraphy and density and ionic concentrations that cannot be resolved into a seasonal signature. However, the δ18O and δ D isotopes show clear sinusoidal fluctuations, which have been used to derive annual mass balance from 1986 to 2003. These show a mean annual accumulation of 53.7 cm w.e. (s.d. 12.9 cm w.e.) although the accuracy of these measurements is compromised by the percolation of meltwater through more than more year’s snowpack. These findings confirm that estimates of mass balance cannot be calculated solely from observed changes in surface elevation. However, predicting spatial and temporal variations in densification is not straightforward because of the complex inter-annual variations in the processes of accumulation, melt, percolation and refreezing.

Petermann 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.

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.

The rapidly changing cryosphere motivates a better understanding of the physical processes governing ice-ocean boundaries. These processes are no more pronounced than in glacierized fjords where massive glaciers meet the ocean. This underwater acoustic environment is significantly louder than other ice-covered environments. The abundant acoustic information, coupled with the difficulty performing measurements on or near glacier termini, encourage the use of passive acoustic monitoring to observe the system. While progress has been made towards improved understanding of sound in glacierized fjords, there remain considerable gaps in the community's understanding of the temporal variability of the acoustic field and the influence of the acoustic propagation environment. To address these deficiencies, three field experiments were conducted in three glacierized fjords: LeConte Bay at the terminus of LeConte Glacier near Petersburg, Alaska; Hornsund Fjord at the terminus of Hansbreen Glacier in Svalbard, an archipelago in the Arctic Ocean; and Disenchantment Bay at the termini of Hubbard and Turner Glaciers near Yakutat, Alaska. Acoustic data were collected between October 2016 and May 2017 from an underwater hydrophone array moored 500 m from LeConte Glacier. Ambient noise levels (ANL) from recordings were clustered, revealing relationships between ambient noise and the speed of icebergs above the mooring and with calving events. In particular, the local acoustic field demonstrated a uniquely consistent period between late February and early April 2017 where a single cluster dominated observations. The beginning and end of this period coincided with the formation and breakup of a dense pack of icebergs in the fjord. Characterization of the underside of a brash ice surface was obtained using an inference procedure and time difference of arrival and transmission loss data from an acoustic propagation experiment performed in Hornsund Fjord in September 2017. The inferred surface was incorporated into a forward simulation of the environment using BELLHOP, a ray tracing code. The measured data and simulated results were compared, providing insight to the shape and reflection characteristics of brash ice. Nearly continuous acoustic data was collected over fifty-two hours in June 2021 on two hydrophone arrays moored in Disenchantment Bay. The effect of recording duty cycle on sampling the spectral and temporal variability of the acoustic field was analyzed by comparing clustered spectral shapes and observations from full and reduced duty cycle recordings of varying length from 1% to 99% of the full 59-minute recordings. A relationship between duty cycle and relative error in hourly cluster observations was determined, which may inform the sampling procedure used in future deployments.

Glaciers are among the most prominent indicators of climate change, since their behavior is directly linked to climatic variables such as temperature, precipitation, and solar radiation. Under the longterm trend of shrinkage due to recent and future climate warming, near-real-time glacier mass balance information and its prediction play a particular role: on scales of days to months, glaciers fulfill important functions concerning water supply, planning for hydroelectricity production, ecology, and mountain tourism. Due to this importance, near-real-time glacier mass balance information is also of interest to the broad media. However, supplying such near-real-time information, for example on glacier mass balance, is not simple. This is mainly for two reasons: first, acquiring up-to-date glacier observations comes with a high cost, because glaciers are often located in remote areas and a considerable amount of time and humanpower is required to access and observe them in situ. Second, there is the uncertainty that affects glacier mass balance models, which are often justified by physics, but still parametrized by statistical relations between mass change and meteorological variables. As a consequence of sparse and uncertain observations, model parameters are often not uniquely identifiable, or their spatial and temporal variability cannot be accounted for. A lack of observations thus associates the calculation of near-real-time glacier mass balance with high uncertainties. This thesis aims at treating the issue of uncertain observations and models by making use of available observations and their respective uncertainties. It does so by presenting Cryospheric Monitoring and Prediction Online (CRAMPON), a Bayesian framework that allows determining near-real-time glacier mass balances in an optimal fashion, i.e. by using all available direct and indirect mass balance information and by minimizing the uncertainties. Bayesian methods are widely used in fields like meteorology, hydrology, snow sciences, and oceanography, but applications in glaciology are sparse to date. CRAMPON builds upon a Sequential Importance Resampling (SIR) scheme, also known as Particle Filtering, which comprises three steps: first, a prior estimate of a glacier’s mass balance on a particular day is given through forward integration of a mass balance model ensemble. The forward integration is driven by gridded meteorological data, and accounts for the corresponding uncertainty. Second, this prior estimate is updated with observations. This is done by using various measurements, including daily point mass balance observations from cameras, as well as surface albedo and transient snow lines derived from optical satellites. The combination of observations ensures that both temporally frequent point observations and less frequent but spatially comprehensive observations complement each other. This second step results in a so-called posterior mass balance estimate. Third, a resampling technique is applied to ensure temporal stability of the particle filter. Here, CRAMPON focuses on (1) making the resampling technique compatible with an ensemble modeling approach, and (2) using the filter to estimate model parameter distributions. This statistical data assimilation approach ensures that, at any instance, the framework delivers an optimal estimate of the current mass balance of a glacier, given all observations and respecting all observation uncertainties. Special focus is put on handling variables in a probabilistic fashion. This allows calculating uncertainties for the near-real-time glacier mass balance estimates during model runtime. The daily estimates and their uncertainties are then used for predicting the glacier mass balances into the near future. This is achieved by using Consortium for Small-Scale Modelling (COSMO) numerical weather predictions with lead times of up to five days, and European Centre for Medium-Range Weather Forecasts (ECMWF) extended-range forecasts for lead times of up to one month. Analyses of the results show that (1) CRAMPON delivers up to 95% more accurate results than conventional approaches that model mass balance deterministically and with constant parameters, (2) the produced mass balances are in line with seasonal, glacier-wide mass balances obtained from interpolation of in situ observations, and (3) the combination of point mass balances and satellite information is helpful to reduce the uncertainty. The thesis also explores other options for improving near-real-time glacier mass balances. In particular, the possibility of (i) extrapolating glacier mass balance signals in space to otherwise unobserved glaciers, (ii) using short-term geodetic volume changes, (iii) predicting model parameters through machine learning, (iv) acquiring glacier melt observations with Unmanned Aerial Vehicles (UAVs), and (v) using Bayesian calibration for cases where model parameters cannot be uniquely identified, are tested and discussed. The daily, near-real-time estimates calculated by CRAMPON are provided at the resolution of individual glaciers, the results being summarized on an interactive web site.

In the ablation zone of land terminating sectors of the Greenland Ice Sheet (GrIS), water pressures at the bed control ice motion variability on diurnal and seasonal timescales. During the melt season, large volumes of surface meltwater access the ice-bed interface through moulins.Moulins are large vertical shafts that connect the supraglacial and subglacial drainage systems. Moulins form when a crevasse intersects a surface meltwater source that can drive hydrofracture to the bed of the ice sheet. Upon reaching the bed, meltwater can establish and sustain an efficient, channelized drainage system. Due to the technical impossibility of physically exploring underwater passages beneath the GrIS, the subglacial drainage system must be studied through geophysical methods. To date, measurements of water level variability within moulins and boreholes have proved to be critical for constraining models. However, direct hydrologic measurements from the GrIS are sparse, due to the remoteness and harsh conditions of the ice sheet. The work presented in this dissertation combines simple physically based mathematical models with direct measurements from the ablation portion of Sermeq Avannarleq, in west Greenland to advance our understanding of the influence of moulin geometry and life span on glacier dynamics. In Chapter 2, I investigate the moulin life cycle within several neighboring surface catchments within the GrIS ablation zone. A combination of remote sensing and ground observations of moulin locations over two to three years reveals an annual pattern of systematic formation and abandonment of moulins after they are advected down-glacier.In Chapter 3, I use a modified single conduit model to explore the role of moulin shape and size on hydraulic head variability within moulins. This model shows that only the englacial storage capacity within the range of water level fluctuations affects the oscillation range of moulin hydraulic head, which controls subglacial channel water pressure dynamics. Further, the model shows that depth-varying changes in englacial water storage control the temporal shape of the head oscillations. Finally, in Chapter 4, I simulate the moulin water level variability in a moulin we instrumented in 2017-2018 using the recently developed Moulin Shape (MouSh) model. The MouSh model requires additional subglacial baseflow to simulate an accurate diurnal range of head oscillation. We hypothesize that this additional baseflow is the result of strong network connectivity with other moulins through a channelized subglacial drainage system, potentially supplemented by basal or non-local, upstream inputs. Additional work is necessary to accurately characterize moulin positions and life cycles, and to determine whether the observed annual formation and abandonment is widespread. Such characterization would improve the simulation of moulin inputs in models. In addition, further knowledge of the shape of moulins around the equilibrium head elevation would improve englacial storage parameterization in subglacial hydrological models and aid predictions of coupling between meltwater and ice motion under future melt scenarios. Finally, this work suggests that the connectivity of the subglacial network needs further study, to improve our understanding on how local and non-local drivers influence subglacial water pressures and ice sliding.

The Greenland Ice Sheet (GrIS) has experienced increasing ice loss in the past decades and the trend is expected to continue due to climate change. However, large uncertainties about the magnitude of Greenland’s future ice loss remain. One process that contributes to this uncertainty are transient ice accelerations, so-called ice speed-up events, caused by a short-term increase in rain-/meltwater, which descends to the glacier bed and can enhance basal sliding. Here, we investigate this phenomenon focusing on two main objectives. First, we present an analysis of local conditions on the Russell glacier in Southwest Greenland, which confirms the importance of meltwater variability for ice speed-up events. We find the strongest influence in early summer with correlations between surface melt and ice velocities above 0.8 in May and identify a typical lag of zero to one day between meltwater peaks and their ice velocity responses. Contrastingly, we find rainfall to have a minor influence with only 4 out of 51 ice speed-up events, where daily increases in water input on the glacier are dominated by rainfall. Secondly, we study the atmospheric drivers of ice speed-up events using a Self-Organizing Maps (SOM) classification of synoptic weather situations over Greenland. Two main patterns that induce most ice speed-up events emerge: The first pattern is characterized by southerly advection of warm and moist air that increases local surface melting by enhanced downward longwave radiation and high sensible heat flux. The second one is a large-scale anticyclone that induces GrIS surface melting through strong downward shortwave radiation and descending air that warm adiabatically. Our study provides new insights into atmospheric drivers of short-term ice accelerations and contributes towards an effort to constrain the impact of ice speed-up events on GrIS mass loss under a changing climate, which can be achieved when more high-resolution ice velocity data is available.

Modeling of the climate system including, but not limited to, atmospheric dynamics, ocean dynamics, and ice dynamics, is one of the crucial scientific problems of the 21st century. This work focuses on modeling of iceberg calving, the process by which high stresses within ice cause fractures and eventual detachment of icebergs from glaciers. Iceberg calving causes approximately half of mass loss of the world's glaciers, but remains poorly understood and implemented in climate models. Existing model implementations of calving exist at a wide range of spatial and temporal scales, ranging from models that seek to resolve crevasse propagation on the scale of seconds or minutes to broad parameterizations of calving implemented in ice sheet scale climate models. In this work, we seek to develop an intermediate model that can run for year to decade timescales and determines calving based on the internal stresses within the ice. We first focus on crevasse advection, the memory of previously formed crevasses within the ice and their impact on calving behavior. Second, we implement the possibility for mixed mode calving, where ice can fail either via high tensile stresses, high shear stresses, or a combination of the two. Lastly, we include submarine melt to see the combined effect of melt and mixed mode calving on glacier stability. This model is developed using the highly flexible Python finite element library FEniCS and LEoPart, a particle tracking library developed for use with FEniCS. Our novel use of particles to track previously crevassed ice provides a computationally efficient method to track glacier parameters that does not diffuse over time. In initial model development, we identified a key numerical consideration related to the buoyant boundary condition on ice that can create unphysical results if not careful managed in a calving model. Once this issue was documented and addressed through a simple addition to the glacier momentum balance, including crevasse advection in the model, which should increase calving rates, reduces overall rate of glacier advance but is incapable of causing retreat in most cases. This indicates that other mass loss mechanisms, such as shear calving and submarine melt, are crucial to mass balance and should be included in future models whenever possible. When mixed-mode calving is included, we find that glacier behavior is highly unstable with regards to the shear strength of ice. Sharp transitions exists at shear strengths where the ice transitions from slow advance or retreat to rapid, catastrophic collapse. Including submarine melt causes further steady mass loss, but does not significantly alter the shear strength threshold necessary for rapid collapse. This shows that if models seek to model potential catastrophic glacier collapse, which is a point of current scientific contention, mixed mode failure including shear localization should be a key focus of modeling efforts.

Worldwide, glaciers are receding as a consequence of climate change. Due to the global glacier recession, glaciers are experiencing enhanced melting, which results in an increase in meltwater availability. This increased meltwater has the ability to alter the glacier's hydraulic conditions and ultimately affects the dynamics of glaciers. Glaciers move through a combination of basal motion and internal ice deformation. The motion at the ice-bed interface comprises of both ice sliding along the bed and the deformation of subglacial sediments. Sliding at the bed is partially controlled by the water pressure at the ice-bed interface and therefore, knowledge of the subglacial hydraulic system is paramount in order to predict glacier movement and fundamentally the future evolution of glaciers. To date, our understanding of the glacier’s hydrological system is limited due insufficient field observations. As part of my thesis, I used surface-based geophysical methods to detect and characterise an alpine glacier's hydrological system. The aim is to improve our understanding of the drainage network of temperate alpine glaciers by using active seismic and ground-penetrating radar (GPR) to determine the properties in space and time of the hydrological system. To detect and characterise englacial flow paths, I performed active seismic and GPR on Rhonegletscher (Switzerland). Both geophysical methods detected a hydrological flow path within the glacier. With the use of amplitude-versus-offset analysis from the 2017 active seismic data, I confirmed that the englacial conduit was water-filled, thereby indicating that this technique is able to characterise hydrological features of glaciers. From both geophysical datasets, I estimated the conduit thickness to be between 0.5 and 4 m. With the acquisition of a 2D GPR grid in 2018, the spatial extent of the englacial conduit was estimated to be approximately 14,000 m$^2$. The water within the englacial conduit was sourced by surrounding surface meltwater and morainal streams. These streams enter the glacier subglacially before flowing into an efficient conduit system indicating that surrounding melt water streams directly impact the glacier's hydrological system. Upon successfully detecting an englacial conduit in 2017 on Rhonegletscher, I performed repeated GPR surveys in 2018 and 2019 to detect seasonal and annual changes to the englacial conduit. I extracted the GPR reflectivity using an impedance inversion workflow. The workflow allowed an interpretation of the englacial conduit's infill material. During the summer melt season, a flowing, water-filled sediment-transporting englacial conduit was detected having a sinusoidal shape with shallow inclination and being 0.4 $\pm{}$ 0.35 m thick. The repeated GPR measurements revealed that the shape of the conduit did not dramatically alter throughout the melt season. During the winter periods, the englacial conduit transported neither water nor sediment. It was either physically closed, or it became very thin (< 0.1 m). Such detailed interpretations were only possible with the use of the impedance inversion workflow and synthetic GPR waveform modelling. Even though temperate glaciers have a dynamic hydrological system, I was able confirm that the englacial conduit reactivated in an identical position from 2018 to 2019. The observations into Rhonegletscher's englacial conduit were conducted using 2D geophysical data. Such 2D geophysical reflection data represents the current state-of-the-art in glaciological applications; however, 2D reflection data falsely assume that all reflections originate from the vertical plane of acquisition. Complex englacial structures or basal geometries cause this assumption to be violated. Therefore, for accurate subsurface imaging, 3D GPR acquisition is required. Consequently, I acquired a 3D GPR survey that provided unprecedented high-resolution and unaliased 3D images of the Rhonegletscher's drainage network situated in the lower ablation zone. With this dataset, I was able to confirm a long-standing theory regarding melt water flow pathways around overdeepenings. With the development of unmanned aerial vehicles, future 3D GPR surveys are looking to be carried out in a fast and efficient manner. The research carried out and presented in this thesis demonstrates that both active seismic and GPR can be used to successfully identify, characterise and monitor a temperate glacier's hydrological network. Such studies will have a substantial impact on future investigations of the glacier hydrological system, and I conclude that future 3D GPR surveys conducted by unmanned aerial vehicles have the potential to revolutionise the way GPR data are acquired and processed in cryosphere applications.

Abstract: The circum-Arctic is a major contributor to sea level rise. Between 1991 and 2010, 70 % of eustatic sea level rise was attributable to glacier mass loss, 62 % of which was from glaciers in the circum-Arctic (Alaska, Arctic Canada North, Iceland, Svalbard, Scandinavia, and the Russian Arctic). In addition, Arctic temperatures are expected to increase at 2.4 times the magnitude of projected global average warming over the next 100 years. An understanding of how circum-Arctic glaciers are responding to temperature increase, and how they will respond under future climate conditions, is crucial to helping island nations and low-lying coastal communities predict and mitigate the impacts of sea level rise. This thesis has two objectives. The first objective is to a) identify the most effective methodology to calculate regional mass balance trends in the circum-Arctic using spatially and temporally sparse datasets and b) use these data to determine past and present (1961-2016) circum-Arctic mass balance trends. To accomplish this, I explore spatially interpolated mass balance from prior studies and compare these results to specific mass balance calculated using only observational data. I then compare two different time periods from the specific mass balance dataset (1961-2016 and 2000-2016) to determine regional mass balance trends. I find that mass balance calculated through spatial interpolation and specific mass balance are statistically likely to derive from the same population in regions that contain observational mass balance data. However, qualitatively, the variability between the datasets appears to be different for regions in which ≥50 % of observational data are geodetic. In addition, the mean magnitude of mass loss appears different in glacier regions with only high-variability glaciological mass balance data. A comparison of 1961-2016 and 2000-2016 mean specific mass balance in each region determines that glacier mass balance in Arctic Canada North has decreased at the largest rate, followed by Alaska and Svalbard (-0.20, -0.14, and -0.12 m w.e. a-1, respectively). The second objective of this thesis is to: a) determine the circum-Arctic glacier mass balance sensitivity temperature and precipitation increase and then b) investigate the factors driving the sensitivity. To achieve this objective, I use a degree-day model (the Python Glacier Evolution Model, PyGEM) to simulate circum-Arctic mass balance sensitivity to 1-3 °C temperature and 4%°C-1precipitation increase between 2000 and 2100. The model simulations suggest that Iceland glaciers are the most sensitive to temperature and precipitation increase (-0.70 m w.e. a-1 °C -1) of all regions studied, and Arctic Canada North is the least sensitive (-0.39 m w.e. a-1 °C -1). These results suggest that the degree of continentality (how warm/wet a region is) and the proximity of accumulation season temperatures (the rain/snow threshold) is the primary driver of mass balance sensitivity; warm, wet, ‘maritime’ regions (Iceland, Scandinavia) are more sensitive to the same temperature increase than cold, dry, ‘continental’ regions (Arctic Canada North, the Russian Arctic). Secondary factors such as glacier size, altitude, slope, and surface albedo may also impact regional glacier mass balance sensitivity. Small glacier size, low glacier altitude, large surface albedo, and steep glacier slope may increase mass balance sensitivity, while large glacier size, high glacier altitude, small surface albedo, and slight glacier slope may decrease mass balance sensitivity. Overall, the results of this thesis provide incentive for future data collection in rapidly changing regions like Arctic Canada North, and provides a better understanding of how the circum-Arctic may change in response to future climate change.

The Greenland Ice Sheet (GrIS) is losing mass at accelerated rates in the 21st century, due in part to faster flow at large outlet glaciers. Chapter 2 presents work published in The Cryosphere (King et al., 2018). Here, we sample rapid changes in thickness and velocity at all large outlet glaciers to derive the first continuous, GrIS-wide record of total ice sheet discharge, or the volume of ice glaciers export, for the 2000-2016 period. We resolve a distinct pattern of seasonal variability with an amplitude of 6%, and analyze how seasonal to annual variability in the discharge time series relates to both meltwater runoff and glacier front position changes over the same period. We find that the annual magnitude of discharge is closely related to cumulative front position change (r2 = 0.79), averaging over 2 km of retreat since 2000. We find that larger seasonal quantities of runoff do not relate to increased annual discharge, although seasonal acceleration of ice discharge does closely coincide with the onset of the melt season. These results suggest that changes in glacier front position drive secular trends in discharge, whereas the impact of runoff is likely limited to the summer months when observed seasonal variations are substantially controlled by the timing of meltwater input.In Chapter 3, we extend our 2000-2016 discharge time series to the period 1985-2018, combining more than three decades of GrIS-wide observational products of outlet glacier velocity, elevation, and front position changes, and compare decadal variability in discharge with calving front position. We find that the close relationship between frontal change and ice discharge identified over the 2000-2016 record holds true for the 34-year record, and that increased glacier discharge can be attributed almost entirely to the retreat of glacier fronts, rather than inland ice sheet processes, such as changes in meltwater runoff. Discharge sensitivity to retreat is remarkably consistent across the ice sheet, with all main regions showing an increase in discharge of 4-5% per km of retreat. Most importantly, we show that widespread retreat between 2000 and 2005 resulted in a step-increase in discharge and a switch to a new dynamic state of sustained mass loss. The increase in discharge was large enough to alone ensure persistent negative ice sheet mass balance, even before including more recent negative trends in surface mass balance. This study is published in Nature Communications Earth & Environment (King et al., 2020). Lastly, we revisit seasonal impacts of meltwater on short temporal variability in glacier dynamics. Chapter 4 work describes how, in lieu of widespread direct observations of subglacial systems, we derive a novel runoff index that quantifies the potential of a given melt season to sustain fast outlet glacier flow. This index, "Basal Pumping Index'', is based on the distribution and relative magnitude of individual runoff events throughout the summer, and reflects the current understanding of how subglacial drainage systems evolve over time to modulate water pressures at the ice bed. We also categorize glaciers based on several unique patterns of variability, and discuss how particular glacier properties impact seasonality and sensitivity to our index. We compare net GrIS discharge between low and highly effective runoff years, and find that particular runoff distributions can increase total GrIS dynamic ice losses by 3% in the late summer/autumnal months. In total, our results provide new insights into the large scale, ice sheet-wide drivers of variability in Greenland's ice discharge and mass balance on seasonal to decadal timescales. These insights will guide projections of future glacier response to ongoing climate change, improving our estimates of Greenland's contribution to global sea level rise and impacts on ocean salinity and circulation.

Iceberg 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.

The objective of this study is to evaluate methods of hydrostratigraphic modeling using geophysics and well hydrographs at the eastern edge of the High Plains aquifer (HPA) in Platte and Colfax counties within Nebraska, USA. The HPA is very heterogeneous in the study area, being hosted by architecturally complex glacial sediments and having many irregular hydraulic boundaries. Further, the HPA exhibits local variations between unconfined and confined conditions. Pumping in such bounded aquifers can be unsustainable because of cost increases and lost agricultural productivity. Moreover, the large drawdowns typical of confined aquifers can contribute to well interference during heavy pumping. Mapping the HPA accurately at small (10’s of km2 ) to medium (100’s of km2 ) scales is vital to sustainable management. AEM modeling and well hydrograph interpretation methods were used to characterize the aquifer in the study area. A 2016 airborne electromagnetic (AEM) survey mapped the electrical resistivity of subsurface strata to depths of 300 m. This data was used in the present study to create 3D hydrostratigraphic models using cognitive-layer modeling and voxel-based geostatistical modeling approaches, both with their own advantages and disadvantages. Water-level hydrographs from piezometers near irrigated fields provide the basis for aquifer characterization at each site and for assessing the accuracy of the two AEM modeling approaches, which are applied commonly in Nebraska and elsewhere. The temporal pattern of water-level drawdown indicated possible boundaries and confinement. The existence of background displacement, size of displacement, and responses of nearby wells led to aquifer interpretations. Little correlation existed between the hydrograph interpretations and both of the modeling approaches, but the voxel model did show boundaries near many of the irrigation wells with bounded hydrograph signatures. Overall, the simple modeling approaches failed to adequately convert resistivity to accurate interpretations of subsurface stratigraphy, rendering both types of hydrostratigraphic models largely invalid here. Nevertheless, the results of this study lead to important future work recommendations: (1) modeling and quantifying uncertainty using more sophisticated methods, (2) applying different modeling approaches in different areas to fit hydrologic data, and (3) using hydrograph data and pumping tests to validate the results of hydrostratigraphic modeling. Advisor: Jesse Korus

Abstract: Airborne radio-echo sounding (RES) is a powerful tool to derive properties of glaciers and ice caps over spatially extensive areas, and has fundamentally improved our understanding of the distribution and structure of near-surface snow and firn, the ice thickness distribution and englacial structure of ice masses, and the topography and thermal/hydrological properties of glacier beds. Although several airborne RES surveys have been conducted over Devon Ice Cap (DIC) in the Canadian Arctic prior to this work, the data had not been utilized to investigate either the properties of the near-surface firn or the subglacial hydrological conditions. The Devon Ice Cap is one of the largest ice masses in the Canadian Arctic, and is thought to have a cold-based interior where ice is frozen to the underlying bedrock. Under recent warming conditions leading to intensified summer melt, the firn of DIC has been affected by significant melting and refreezing processes that can complicate the measurement of the ice cap's surface mass balance. Here, we use airborne RES measurements over DIC to (i) investigate whether the nature of the glacier surface reflection can be used to characterize the spatial heterogeneity of the near-surface firn, and (ii) investigate the hydrological conditions beneath the ice cap.A comparison of airborne- and ground-based RES, along with analysis of shallow firn cores, led to the development of a novel technique where the spatial heterogeneity of firn is characterized via the scattering component of surface returns from airborne RES data. This method allows for the characterization of firn over spatially extensive areas and can help to identify regions where the structure and stratigraphy of the firn layer are affected significantly by melting and refreezing processes. Investigations of the RES reflection from the base of the ice column led to the identification and detailed characterization of a hypersaline subglacial lake and provide evidence for an extensive brine-network beneath DIC. Since basal ice temperatures are well below the pressure-melting point, this water system is considered to be brine-rich, to the point that the salinity significantly depresses its freezing point. Geological evidence suggests that a salt-bearing evaporite unit outcrops beneath DIC, and is presumed to be the solute source for the brine.The hypersaline subglacial lake beneath DIC and its surrounding hydrological and geological conditions are globally unique. The subglacial lake beneath DIC is not only the first to be discovered in the Canadian Arctic, but also the only spatially isolated hypersaline subglacial lake so far identified on Earth. This subglacial lake and the surrounding brine-network may host viable microbial habitats and could thus be world-class analogs for potential microbial habitats on other icy planetary bodies, such as Europa and Mars, where it has been suggested that sub-ice brine bodies may also exist. Thus, the unique subglacial water system beneath DIC, and the hypersaline subglacial lake in particular, are compelling targets for future in-situ sampling and biogeochemical investigations. The results from this study provide crucial information for the planning of future research over the subglacial lake and surrounding brine-network, including in-situ access and sampling of the water to explore its habitability for microbial life.