4 research outputs found
The Greenland and Antarctic ice sheets under 1.5◦C global warming
Even if anthropogenic warming were constrained to less than 2°C above pre-industrial, the Greenland and Antarctic ice sheets will continue to lose mass this century, with rates similar to those observed over the last decade. However, nonlinear responses cannot be excluded, which may lead to larger rates of mass loss. Furthermore, large uncertainties in future projections still remain, pertaining to knowledge gaps in atmospheric (Greenland) and oceanic (Antarctica) forcing. On millennial timescales, both ice sheets have tipping points at or slightly above the 1.5-2.0°C threshold; for Greenland, this may lead to irreversible mass loss due to the surface mass balance elevation feedback, while for Antarctica, this could result in a collapse of major drainage basins due to ice-shelf weakening
Nature and dynamics of ice-stream beds: assessing their role in ice-sheet stability
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