Controls on ice dynamics of the Greenland ice sheet
In recent decades, the Greenland Ice Sheet has lost mass at an accelerating rate, such that it is now the largest individual cryospheric contributor to global sea level rise. This mass loss occurs as a combination of an increase in the melting of ice at the ice sheet surface as well as increased submarine melting and iceberg calving at marine-terminating outlet glaciers, with these processes driven by increasing atmospheric and oceanic temperatures. Understanding the dynamic processes by which ice is discharged into the oceans or moved from higher to lower elevations where air temperatures are warmer is crucial, as the acceleration of ice flow in either case would lead to enhanced ice loss from the ice sheet. This thesis produces considerable new observations and investigates the processes controlling the ice dynamics of the Greenland Ice Sheet within three distinct regions of the ice sheet: the southwest land-terminating margin, the interior accumulation zone, and Sermilik Fjord in southeast Greenland which is characterised by the presence of fast-flowing marine-terminating outlet glaciers. This thesis first investigates the dynamic response of the south-western land terminating sector of the Greenland Ice Sheet to multi-annual variability in surface meltwater forcing. Within this land-terminating region, the drainage of surface meltwater to the bed and its impact upon the subglacial hydrological system exerts a critical control on ice motion. This link between subglacial hydrology and ice dynamics is termed ‘hydro-dynamic coupling’ and is of fundamental importance to land-terminating regions of the ice sheet as it controls the transfer of ice from cooler, high elevation regions to warmer, low elevation regions where surface melting, and thus mass loss, is greater. Previous work has shown that over multi-decadal timescales, a persistent increase in the production of surface meltwater has driven a long-term slowdown. However, following a period of sustained high melt peaking with the record melt year experienced in 2012, the regional climate has cooled. This thesis aims to investigate the net impact of this reversal in climate forcing on ice dynamics through its impact upon hydro-dynamic coupling. To address this, feature tracking is applied to the entire Landsat archive covering the southwest Greenland land-terminating sector in order to quantify changes in annual ice velocity across a 10,600 km² sector of the Greenland Ice Sheet during the period 1992-2019, and in particular during the recent cooler 2013-2019 period. Between the early-2000s and 2012, a slowdown in ice motion is observed, consistent with previous results. However, from 2013 to 2019, an acceleration in ice motion occurred, coincident with atmospheric cooling and a 15 % reduction in mean surface melt production relative to the period 2003-2012. This acceleration is strongest nearer the ice margin and is strongly related to ice thickness. This thesis hypothesises that under thinner ice, increases in basal water pressure offset a greater proportion of the ice overburden pressure, leading to a greater acceleration when compared to thicker ice further inland. These findings indicate that hydro-dynamic coupling provides the major control on ‘short-term’ variations in ice motion within the ablation zone of Greenland’s land-terminating margins over multi-annual timescales. Furthermore, these findings confirm that these regions of the ice sheet are resilient to the dynamic impacts associated with increases in surface meltwater production. The thesis next explores multi-decadal changes in ice velocity within the interior accumulation zone of the Greenland Ice Sheet, an area across which our understanding of recent dynamic change and the processes driving any change is extremely limited. Changes in the ice dynamics of the ice sheet interior are important as the greater ice thickness means that any increase in ice motion will result in a much larger increase in mass flux when compared to marginal regions, and this long-term response of inland regions of the ice sheet to dynamic perturbations originating at the terminus is hypothesised to represent a major component of Greenland’s sea level rise contribution. Here, recent satellite-derived annual ice velocity measurements are combined with GPS measurements from the mid-1990s at sites spanning the 2000 m (a.s.l.) elevation contour to quantify the multi-decadal ice velocity change within the ice sheet interior. The results show a complex pattern of long-term velocity change, with inland acceleration almost ubiquitous along the west coast, reaching 28.1 ± 7.8 m yr-1 inland of Jakobshavn Isbrae, contrasting with very limited evidence of acceleration inland of tidewater glaciers on the east coast, despite similar acceleration and retreat observed at the glacier termini. It is hypothesised that this may be related to the underlying subglacial topography whereby glaciers with extensive subglacial troughs, extending considerable distances inland, are generally characterised by inland acceleration. In contrast, sites where inland acceleration is not observed are almost all upstream of a sharp rise in bed topography, consistent with recent research suggesting that such steep bedrock slopes act as to limit the propagation of any perturbation originating at the terminus. These results indicate that acceleration and thinning perturbations at tidewater glacier termini can propagate for distances >100 km inland, accelerating mass loss by increasing the draw-down of greater volumes of thicker ice from the interior toward the lower elevation margins. However, in regions where inland acceleration is not observed, the local bed topography likely limits the extent to which change at the terminus can propagate inland, such that they will be more resilient to future changes in outlet glacier dynamics. The final results chapter turns to Sermilik Fjord in southeast Greenland, in which a range of remotely-sensed data is combined to investigate and compare the dynamic response of a group of tidewater glaciers to a common climate forcing. The glaciers studied include Helheim Glacier, one of the largest and fastest-flowing outlet glaciers draining the Greenland Ice Sheet, and any dynamic change at Helheim Glacier would thus represent a significant change in the flux of mass from the ice sheet to the ocean. Moreover, by comparing the response of neighbouring glaciers to a common climate forcing, the importance of local geometrical factors such as bed and fjord geometry upon ice dynamics, and thus the transfer of mass to the ocean, can be better understood. Since 2014, the near-terminus region at Helheim has accelerated by 2.5-3 km yr-1 and retreated by >4 km, with retreat in both 2017 and 2019 extending further inland than during its recent pronounced maximum retreat position in 2005. Mass balance data shows that it has continually lost mass such that the near-terminus region is now 20-100 m thinner than it was in 2005, consistent with a long-term increase in observed air temperatures and modelled submarine melt rates. The front 5 km of the glacier is now within 25-50 m of flotation, indicating that the glacier is in a much more unstable configuration currently than during its previous maximum retreat in 2005. This concern regarding instability is supported by applying the same analyses to the neighbouring Fenris Glacier and Midgard Glacier, which have been subject to essentially equivalent climate forcing and have also undergone consistent mass loss since at least the early-2000s. Both of these glaciers thinned to flotation, after which they underwent dramatic retreat and acceleration, although the timing of this dynamic change differed between glaciers, likely reflecting the different morphological settings and characteristics of the individual glaciers. These results therefore suggest that differences in local fjord morphology and glacier geometry have affected the timing of the response of the glaciers studied to increased air and ocean temperatures, although the fundamental mechanism, the thinning of the near-terminus region towards flotation, remains the same. As a consequence, this thesis argues that Helheim Glacier is poised for a dramatic retreat under continued atmospheric and oceanic warming, which would be unprecedented in at least the last 120 years. This thesis thus presents several advances in our understanding of the processes controlling dynamic change across the Greenland Ice Sheet, with observations spanning land- and marine-terminating margins as well as the interior accumulation zone. The thesis concludes by discussing the implications of these advances in the context of informing future projects of dynamic mass loss from both land and marine-terminating margins of the ice sheet under future climate change.