Terrestrial glacial geomorphology has long been used to evaluate the extent, chronology and dynamics of former glaciers and ice sheets. New marine geophysical methods provide an opportunity to study the glacial submarine morphology of modern continental shelves and fjord systems. This makes it possible to study landform assemblages in the submarine settings that are often better preserved than their terrestrial counterparts. This study focuses mainly on the recent surge history of the tidewater glacier Tunabreen, which calves into Tempelfjorden in Western Spitsbergen. Tunabreen is a small outlet glacier of the Lomonosovfonna ice cap and has experienced severalsurges and terminal retreats during the last century. The multiple surge events havemost likely removed or reworked landform assemblages created by earlier surges,resulting in a complex geomorphological imprint on the bed of Tempelfjorden. Tunabreen has left a specific morphological imprint on the sea floor, consisting of iceflow‐parallel lineations and generally flow‐transverse retreat moraines. Comparisonof retreat moraines mapped from high resolution multibeam bathymetric data andglacier terminal positions, established using remote sensing imagery suggest that themoraines in the inner part of Tempelfjorden are annually formed recessionalmoraines, formed during winter still stands of the glacier margin or during its minorreadvances. Although detailed reconstruction of glacier surge dynamics based solelyon the landform distribution is challenging, it is evident that Tunabreen hasexperienced fast flow during surges and semiannual retreat of the margin after thesurges. The main achievements of this study are a spatial reconstruction of the dynamics ofTunabreen, which has experienced three surges during the last hundred years.Together with the Little Ice Age surge of the adjacent von Postbreen, four recentsurges have been recorded in Tempelfjorden since 1870, which distinguishes thestudy area from earlier studied Svalbard tidewater surge glacier settigs, where theglaciers have been known to surge only once or twice. However a detailedunderstanding of surge triggering mechanisms and their role in controlling thedynamics of the tidewater glaciers in Svalbard is still poor and requires furtherinvestigations. Svalbard, where most of the small outlet glaciers are believed to be ofsurge type, is an excellent natural laboratory for such investigations.

Tidewater glaciers form a significant drainage catchment of glacierised areas, directly transporting meltwater from the terrestrial to the marine environment. Surface melt of glaciers in the Arctic is increasing in response to warmer atmospheric temperatures, whilst tidewater glaciers are also exposed to warmer ocean temperatures, stimulating submarine melt. Increased freshwater discharge not only freshens fjord waters, but also plays a key role in glacimarine sedimentary processes, transporting sediment to glacial fjords. Despite this, the temporal evolution of meltwater production, storage and release from tidewater glacier systems at seasonal and interannual time scales is poorly understood. This leaves large uncertainties in the predictions for future sea level rise, ocean circulation and the impacts on the marine ecosystem. This study focuses on Kronebreen, a tidewater glacier which flows into the head of Kongsfjorden, north west Svalbard. Surface melt produces freshwater runoff, which is discharged from the grounding line as a buoyant, sediment laden plume, which spreads laterally across the surface water. This supraglacial melt is the dominant freshwater source, contributing an order of magnitude more freshwater to Kongsfjorden, than direct submarine melting of the ice face. Calibration of MODIS band 1 satellite imagery with in situ measurements of Total Suspended Solids and spectral reflectance, provides a method to quantify meltwater and sediment discharge. Plume extent has been determined for each cloud free day, from June to September, 2002 - 2013. Analysis of plume extent with atmospheric temperature and modeled surface runoff, gives a source to sea insight to meltwater production, storage and discharge. The extent of the plume changes in response to meltwater; larger plumes form when discharge increases. These results reveal that meltwater discharge into Kongsfjorden lags atmospheric temperature, the primary driver of meltwater production, by over a week during June and July. This is reduced to only 1 - 2 days in August and September, indicating a decline in meltwater storage as the ablation season progresses, and the development of more efficient glacial drainage. Sediment plumes respond to meltwater production, making them a valuable tool for quantifying meltwater discharge from a tidewater glacier. Insights to glacier hydrology can also be obtained when surface processes are also considered. This furthers the understanding of tidewater glacier hydrology, which is valuable for improving the accuracy of sea level rise predictions.

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The recent thinning, acceleration and retreat of tidewater glaciers around Greenland suggests that these systems are highly sensitive to a change in climate. Tidewater glacier dynamics have already had a significant impact on global sea level, and, given projected future climate warming, will likely continue to do so over the coming century. Understanding of the processes connecting climatic change to tidewater glacier response is, however, at an early stage. Current leading thinking links tidewater glacier change to ocean warming by submarine melting of glacier calving fronts, yet the process of submarine melting remains poorly understood. This thesis combines modelling and field data to investigate submarine melting at tidewater glaciers, ultimately seeking to constrain the sensitivity of the Greenland Ice Sheet to climate change. Submarine melting is thought to be enhanced where subglacial runoff enters the ocean and drives energetic ice-marginal plumes. In this thesis, two contrasting models are used to examine the dynamics of these plumes; the Massachusetts Institute of Technology general circulation model (MITgcm) and the simpler buoyant plume theory (BPT). The first result of this thesis, obtained with the MITgcm, is that the spatial distribution of subglacial runoff at the grounding line of a tidewater glacier is a key control on the rate and spatial distribution of submarine melting. Focussed subglacial runoff induces rapid but localised melting, while diffuse runoff induces slower but spatially homogeneous melting. Furthermore, for the same subglacial runoff, total ablation by submarine melting from diffuse runoff exceeds that from focussed runoff by at least a factor of five. BPT is then used to examine the relationship between plume-induced submarine melting and key physical parameters, such as plume geometry, fjord stratification, and the magnitude of subglacial runoff. It is shown that submarine melt rate is proportional to the magnitude of subglacial runoff raised to the exponent of 1/3, regardless of plume geometry, provided runoff lies below a critical threshold and the fjord is weakly stratified. Above the runoff threshold and for strongly stratified fjords, the exponent respectively decreases and increases. The obtained relationships are combined into a single parameterisation thereby providing a useful first-order estimate of submarine melt rate with potential for incorporation into predictive ice flow models. Having investigated many of the factors affecting submarine melt rate, this thesis turns to the effect of melting on tidewater glacier dynamics and calving processes. Specifically, feedbacks between submarine melting and calving front shape are evaluated by coupling BPT to a dynamic ice-ocean boundary which evolves according to modelled submarine melt rates. In agreement with observations, the model shows calving fronts becoming undercut by submarine melting, but hints at a critical role for subglacial channels in this process. The total ablation by submarine melting increases with the degree of undercutting due to increased ice-ocean surface area. It is suggested that the relative pace of undercutting versus ice velocity may define the dominant calving style at a tidewater glacier. Finally, comparison of plumes modelled in both MITgcm and BPT with those observed at Kangiata Nunata Sermia (KNS), a large tidewater glacier in south-west Greenland, suggests that subglacial runoff at KNS is often diffuse in nature. In addition to the above implications for submarine melting, diffuse drainage may enhance basal sliding during warmer summers, thereby providing a potential link between increasing atmospheric temperature and tidewater glacier acceleration which does not invoke the role of the ocean. This thesis provides a comprehensive investigation and quantification of the factors affecting submarine melting at tidewater glaciers, a complex process that is believed to be one of the key influences on the current and future stability of the Greenland Ice Sheet. Based on the magnitude of modelled melt rates, and their effect on calving front shape, the process of submarine melting is a likely driver of retreat at slower-flowing tidewater glaciers in Greenland. For melting to influence the largest and fastest-flowing glaciers requires invoking a sensitive coupling between melting and calving which is as yet obscure. It should however be noted that modelled melt rates depend critically on parameters which are poorly constrained. The results and parameterisations developed in this thesis should now be taken forward through testing against field observations - which are currently rare - and, from a modelling perspective, coupling with ice flow models to provide a more complete picture of the interaction of the Greenland Ice Sheet with the ocean.

Most of the glacier-caused sea level rise to date has been sourced from melt and icebergs from marine-terminating glaciers. Marine-terminating glaciers drain nearly all the Greenland and Antarctic ice sheets and many polar ice caps, ice fields, and mountain glaciers. However, we previously did not know how much solid mass, or frontal ablation, was lost by these glaciers, a key component of glacier mass balance. This thesis quantifies the area change and mass loss of marine-terminating glaciers in the Northern Hemisphere from 2000 to 2020 by quantifying glacier retreat, advance, and frontal ablation. In total, the 1704 marine-terminating glaciers in the Northern Hemisphere lost an average of 389.7 ± 1.6 km² a⁻¹ of their terminus from 2000 to 2020, for a total of 7527 ± 31 km², with 123 glaciers ceasing to be marine-terminating over this period. Overall, 85.3% of glaciers retreated, 2.5% advanced, and the remaining 12.3% did not change outside of uncertainty limits. Frontal ablation of marine-terminating glaciers, not including the Greenland Ice Sheet, contributed an average of 44.47 ± 6.23 Gt a⁻¹ of ice to the ocean from 2000 to 2010, and 51.98 ± 4.62 Gt a⁻¹ from 2010 to 2020. Ice discharge from 2000 to 2020 was equivalent to 2.10 ± 0.22 mm of sea-level rise and comprised approximately 79% of frontal ablation, with the remainder from terminus retreat. In Greenland, frontal ablation totaled 522.00 ± 17.38 Gt a⁻¹ for 2000-2010 and 559.05 ± 12.59 Gt a⁻¹ for 2010-2020. Ice discharge comprised ~90% of frontal ablation during both periods, while volume loss due to terminus retreat comprised the remainder. In total, Greenland accounted for 90% of northern hemisphere frontal ablation from 2000 to 2020. When combined with climatic-basal mass balance estimates this allows for the first estimate of complete Northern Hemisphere glacier mass budgets, which shows that Arctic Russia, Greenland, and Svalbard have positive climatic-basal balances. For the first time, this thesis provides complete frontal ablation estimates for the entire Northern Hemisphere of 522.0 ± 17.4 Gt a⁻¹ for 2000-2010 and 559.1 ± 12.6 Gt a⁻¹ for 2010-2020.