The Antarctic ice sheet is a geophysically - and in an age of growing concern about global warming, geopolitically - important portion of Earth. The composition and dynamics of the Antarctic ice sheet influence global climate patterns, global sea level and the planet's radiation budget. Recent evidence also suggests that the long term stability of portions of the ice sheet may be in jeopardy. In this thesis I use data from three Ku-band space-borne scatterometers to monitor changes in the backscatter signature of the Antarctic ice sheet from 1978 through 2007. Significant changes in backscatter, which result from geophysical changes in the ice sheet itself, are found over much of the Antarctic continent, especially in West Antarctica and along much of the coasts. Less drastic changes, including regular seasonal variations, are observed over much of the ice shelf. Possible scattering mechanisms are proposed and discussed. A secondary result is the demonstration of the stability of NASA's QuikSCAT scatterometer, data from which is used extensively in this thesis and in many other publications. It is shown that QuikSCAT's observation geometry and backscatter instrumentation have remained consistent to great precision throughout its nearly nine-year long mission.

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Recent work suggests an emerging new paradigm for the Scandinavian ice sheet (SIS); one of a dynamically fluctuating ice sheet. This doctoral research project explicitly examines the history and dynamics of the SIS at four sites within Sweden and Norway, and provides results covering different time periods of glacial history. Two relatively new dating techniques are used to constrain the ice sheet history: the optically stimulated luminescence (OSL) dating technique and the terrestrial cosmogenic nuclide (TCN) exposure dating technique. OSL dating of interstadial sediments in central Sweden and central Norway indicate ice-free conditions during times when it was previously inferred the sites were occupied by the SIS. Specifically, the SIS was absent or restricted to the mountains for at least part of Marine Isotope Stage 3 around 52 to 36 kyr ago. Inland portions of Norway were ice-free during part of the Last Glacial Maximum around 25 to 20 kyr ago. Consistent TCN exposure ages of boulders from the Vimmerby moraine in southern Sweden, and their compatibility with previous estimates for the timing of deglaciation based on radiocarbon dating and varve chronology, indicate that the southern margin of the SIS was at the Vimmerby moraine ~14 kyr ago. In central Sweden, consistent TCN ages for boulders on the summit of Mt. Åreskutan and for the earlier deglaciated highest elevation moraine related to the SIS in Sweden agree with previous estimates for the timing of deglaciation around 10 ka ago. These results indicate rapid decay of the SIS during deglaciation. Unusually old radiocarbon ages of tree remains previously studied from Mt. Åreskutan are rejected on the basis of incompatibility with consistent TCN ages for deglaciation, and incompatibility with established paleoecological and paleoglaciological reconstructions. Altogether this research conducted in different areas, covering different time periods, and using comparative geochronological methods demonstrates that the SIS was highly dynamic and sensitive to environmental change. / At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Submitted. Paper 4: Manuscript.

Geomorphological evidence of glaciations exist for the Last Glacial Maximum (about 20 kyr ago). At this time, both North America and Eurasia were covered by extensive ice sheets which are both absent today. However, the temporal and spatial evolution of the ice sheets from the previous interglacial up to the fully-glaciated conditions at LGM is still unresolved and remains a vexing question in climate dynamics. The evolution of ice sheets is essentially controlled by the prevailing climate conditions. On glacial time-scales, the climate is shaped the by the orbital variations of the Earth, but also by internal feedbacks within the climate system. In particular, the ice sheets themselves have the potential to change the climate within they evolve. This thesis focuses on the interactions between ice sheets and the time-mean atmospheric circulation (stationary waves). It is studied how the stationary waves, which are forced by the ice-sheet topography, influence ice-sheet evolution through changing the near-surface air temperature. In this thesis, it is shown that the degree of linearity of the atmospheric response controls to what extent the stationary waves can reorganise the structure of ice sheet. Provided that the response is linear, the stationary waves constitute a leading-order feedback, which serves to increase the volume and deform the shape of ice sheets. If the stationary-wave response to ice-sheet topography is nonlinear in character, the impact on the ice-sheet evolution tends to be weak. However, it is further shown that the amplitude of the nonlinear topographical response, and hence its effect on the ice-sheet evolution, can be significantly enhanced if thermal cooling over the ice sheets is taken into account. / At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 3: Submitted.

The Darwin-Hatherton glacial system (DHGS) drains from the East Antarctic Ice Sheet (EAIS) and through the Transantarctic Mountains (TAM) before entering the Ross Embayment. Large ice-free areas covered in glacial sediments surround the DHGS, and at least five glacial drift sheets mark the limits of previous ice extent. The glacier belongs to a group of slow-moving EAIS outlet glaciers which are poorly understood. Despite this, an extrapolation of a glacial drift sheet boundary has been used to determine the thickness of the EAIS and the advanced West Antarctic Ice Sheet (WAIS) during the Last Glacial Maximum (LGM). In order to accurately determine the past and present contributions of the Antarctic ice sheets to sea level changes, these uncertainties should be reduced. This study aims to examine the present and LGM ice dynamics of the DHGS by combining newly acquired field measurements with a 3-D numerical ice sheet-shelf model. The fieldwork included a ground penetrating radar survey of ice thickness and surface velocity measurements by GPS. In addition, an extensive dataset of airborne radar measurements and meteorological recordings from automatic weather stations were made available. The model setup involved nesting a high-resolution (1 km) model of the DHGS within a lower resolution (20 km) all-Antarctic simulation. The nested 3-D modelling procedure enables an examination of the impact of changes of the EAIS and WAIS on the DHGS behaviour, and accounts for a complex glacier morphology and surface mass balance within the glacial system. The findings of this study illustrate the difference in ice dynamics between the Darwin and Hatherton Glaciers. The Darwin Glacier is up to 1500 m thick, partially warm-based, has high driving stresses (~150 kPa), and measured ice velocities increase from 20-30 m yr⁻¹ in the upper parts to ~180 m yr⁻¹ in the lowermost steepest regions, where modelled flow velocities peak at 330 m yr⁻¹. In comparison, the Hatherton Glacier is relatively thin (<900 m), completely cold-based, has low driving stresses (~85 kPa), and is likely to flow with velocities <10 m yr⁻¹ in most regions. It is inferred that the slow velocities with which the DHGS flows are a result of high subglacial mountains restricting ice flow from the EAIS, large regions of frozen basal conditions, low SMB and undulating bedrock topography. The model simulation of LGM ice conditions within the DHGS implies that the ice thickness of the WAIS has been significantly overestimated in previous reconstructions. Results show that the surface of the WAIS and EAIS away from the TAM would have been elevated 600-750 and 0-80 m above present-day levels, respectively, for the DHGS to reach what was inferred to represent the LGM drift sheet limit. Ultimately, this research contributes towards a better understanding of the dynamic behaviour of slow moving TAM outlet glaciers, and provides new insight into past changes of the EAIS and WAIS. This will facilitate more accurate quantifications of contributions of the WAIS and EAIS to changes in global sea level.

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.

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This thesis consists of two papers dealing with a stabilization method for free-surface flows. The method was initially developed to stabilize mantle-convection simulations, but is in this work extended to ice-sheet- and glacier modeling. The objective of this thesis is to assess the method when applied glaciological simulations, with regards to stability and accuracy. It is shown that the method works well and increases stable time-step sizes substantially both for ice-sheet- and glacier simulations, without loss of accuracy. The increased stability properties might be useful for performing long-term simulations and increasing sea-level-rise predictions on a centennial time scale. / Denna avhandling består av två artiklar som inom ramen för ismodellering undersöker en stabiliseringsmetod för flöden med en fri yta. Metoden framtogs först för stabilisering av simuleringar av mantelkonvektion, men har i den här avhandlingen anpassats till ismodellering. Avhandlingens mål har varit att utvärdera metoden med avseende på stabiltet och noggrannhet. Från de utförda studierna framkommer det att metoden ökar längden på stabila tidssteg avsevärt, utan att nämnvärt påverka noggrannheten hos islösaren. De ökade stabilitetsegenskaperna hos metoden kan exempelvis innebära ökad nogrannheten i fastställandet av framtida havsnivåhöjning genom möjliggörandet av långtidssimuleringar på en tidsskala av flera hundra år.

Repeat track analysis is one tool that can be used to derive parameters describing elevation changes from elevation data collected from a satellite with a near-repeat groundtrack. While initially developed to study ocean topography, it was then applied to ice sheet data. This study expands upon that previous research by testing the method’s ability to estimate parameters using different amounts of data, different grid sizes and types, and different elevation models containing different parameters to be estimated. In all cases, ICESat-derived elevations were used as input data, as ICESat has a near-repeat groundtrack with extensive coverage of the Greenland and Antarctica ice sheets. Results were compared using the differences between modeled and ICESat-derived elevations, correlation of estimated parameters to known physical features, and differences between known and estimated parameter values for simulated elevation data. It was found that there should be data from at least as many distinct time periods (or, in the case of ICESat, laser campaigns) as parameters being estimated, grids centered on and aligned with the reference groundtrack should be used, and that elevation models containing a constant elevation change rate, slopes, an initial elevation at the grid center, and annual terms should be used. Crossover analysis is a different method to determine elevation change rate with elevation data and serves as an independent verification of the repeat track analysis method. It was found that the hdot values determined from crossover and repeat track analyses agreed to within 5 cm/yr in most areas of the ice sheets, with differences greater than 40 cm/yr along the coasts. While repeat track analysis provides greater coverage than crossover analysis, it is uncertain which method provides the most accurate results. / text

This thesis uses geological field data and numerical ice sheet modelling to study the Younger Dryas ice cap in Scotland. The Younger Dryas stadial is important because it represents the most recent period of high-magnitude global climate change, and was marked by the expansion of ice sheets in North America and Scandinavia, and the regrowth of glaciers in the British Isles. An integrated methodology linking field results and modelling is developed and applied here, specifically focussing on the deposits, landforms, and palaeoglaciology of Younger Dryas glaciers in western Scotland. This combined approach enables data of different scales to be compared, and connected, from local sedimentological investigations and empirically derived reconstructions, to regional ice-sheet simulations from a high-resolution numerical model. Previous geological mapping in western Scotland resulted in contradictory views of the thickness and extent of ice during the Younger Dryas, consequently leading to uncertainty about the dynamics of the former ice cap. By using a ‘landsystem’ method to characterise the terrain, it is argued here that geological evidence in the study area implies a relatively thick central ice cap that fed steep outlet glaciers around its margins. These glaciers oscillated throughout the stadial, and during deglaciation produced suites of moraines that marked successive positions of glacier retreat. Widespread preservation of superimposed landforms, and of sediment sequences pre-dating the Younger Dryas, suggest that, despite being active, the Younger Dryas ice cap was not particularly erosive in its central area and only subtly modified its bed. These geological interpretations are supported by high-resolution numerical modelling of the ice cap, which reveals clear spatial variability in the velocity structure, thermal regime, and flow mechanism of the ice cap; patterns that led to local contrasts in basal processes and diversity in the geological imprint. These model experiments also highlight the non-linear relationship between climate forcing and glacier response, identifying evidence of ice sheet hysteresis and climatically decoupled glacier oscillations – concepts as relevant to geological investigations of former ice masses as they are to the prediction of glacier response under future climate changes.