The patterns and drivers of recent outlet glacier change in East Antarctica

Abstract

West Antarctica and Greenland have made substantial contributions to global sea level rise over the past two decades. In contrast, the East Antarctic Ice Sheet (EAIS) has largely been in balance or slightly gaining mass over the past two decades. This is consistent with the long-standing view that the EAIS is relatively immune to global warming. However, several recent reports have highlighted instabilities in the EAIS in the past, and some numerical models now predict near-future sea level contributions from the ice sheet, albeit with large uncertainties surrounding the rates of mass loss. Using primarily remote sensing methods, this thesis aims to determine spatial and temporal patterns of outlet glacier change in the EAIS and assess the drivers and mechanisms of any changes in their dynamics. In doing so, it will also explore the wider debate surrounding the potential vulnerability of the ice sheet in the coming decades to centuries.
Pan-ice sheet terminus mapping in 1974, 1990, 2000 and 2012 reveals significant decadal variability in the behaviour of the EAIS. The majority of outlet glaciers retreated between 1974 and 1990, before switching to a dominant advance phase from 1990-2000. This trend of outlet glacier advance largely continued between 2000 and 2012, with the exception of Wilkes Land, where 74% of glaciers retreated. It is hypothesized that this anomalous retreat is linked to a reduction in sea ice and associated impacts on ocean stratification.
A more detailed examination of six glaciers in Porpoise Bay, Wilkes Land, reveals that large simultaneous calving events in January 2007 and March 2016, totalling ~2,900 km2 and 2,200 km2, were driven by the break-up of the multi-year landfast sea ice which usually occupies Porpoise Bay. However, these break-up events were driven by contrasting mechanisms. The 2007 break-up event is linked to an exceptionally warm December 2005 weakening the band of multi-year sea ice prior to its eventual break-up in the following summer. Whereas, the 2016 event is linked to the terminus advance of Holmes (West) Glacier pushing the multi-year sea ice further into the open ocean, making it more vulnerable to break-up.
In order to examine how changes at the terminus of glaciers might impact on their inland velocity, this thesis then analyses Cook Glacier, which is a major outlet glacier which drains a large proportion of the Wilkes Subglacial Basin. Analysis of ice-front positon change from 1947-2017, glacier velocity from 1973-2017 and ice shelf thickness from 1994-2012, reveals dynamic instability in the recent past. Cook West Ice Shelf retreated to its grounding line between 1973 and 1989, resulting the doubling of its velocity. Cook East Ice Shelf did not show a similar retreat pattern, but its ice shelf thinned rapidly between 1998 and 2002, which coincided with an increase of its velocity of ~10%. This rapid thinning is linked to periodic intrusions of warm mCDW. If these intrusions become more persistent in the future, Cook Glacier has the potential to contribute to sea level rise in the future.
In a wider context the results from this thesis highlight some key issues which need to be considered when predicting the response of the EAIS to future climate warming: i) The sensitivity of the EAIS to decadal variations in climate. ii) The potential for future changes in the location and persistency of landfast sea ice to alter outlet glacier dynamics. iii) The potential for rapid thinning of those ice shelves with a low steady-state basal melt rate.