Above and Below the Pine Island Glacier

A key technique to calculate mass loss, and consequential Sea Level Rise (SLR) contribution, from Antarctica is to measure the ice flow from individual outlet glaciers. The Antarctic glacier which has received the greatest scientific (and media e.g. BBC) attention is a marine-terminating glacier named the Pine Island Glacier (PIG), which drains into the Amundsen Sea (AS). PIG is located on the coast of the West Antarctic Ice Sheet (WAIS) (shown in Figure 1), in a region called the Amundsen Sea Embayment (ASE).

Figure 1: Map of Antarctica with the location of PIG labelled.

Source: RealClimate 

PIG has been studied frequently because observation suggests it is flowing at a much greater rate than other Antarctic glaciers. As it drains a large portion of the WAIS, PIG has a significant potential contribution to SLR. The exceptional retreat of the PIG can be explained by its physical characteristics. PIG is grounded below sea level, on a bedrock topography which retreats inland. This means that the PIG is especially sensitive to changes in ocean temperature.

This post will discuss two papers from 2016 – (1) Smith et al. and (2) Konrad et al. – both of which aim to reconstruct a long term historical record of the behaviour of PIG, although they use very different methods.

Smith et al. 2016

This study addresses an important weakness of the satellite observational record. Satellite records only began in 1973, hence it is insufficient to analyse long-term temporal variation of PIG .

Smith et al. took sediment cores from three bedrock locations below the PIG (labelled as A, B, and C on Figure 2). A combination of biological proxies and isotope dating are used to reconstruct ice extent over a much longer period.

Results from this analysis indicate three separate states (Figure 2) of the PIG. In its earliest state, ice from the PIG is consistently in contact with the bedrock up to the maximum of the ridge (Figure 2, panel a). However, in 1945 (± 12 years) an ocean cavity developed between the ice and the underlying bedrock (Figure 2, panel b). The authors attribute the growth of this cavity to warm ocean temperatures caused by El Niño activity. By 1970 (± 4 years) the PIG was completely detached from the bedrock ridge, in a state of continual retreat (Figure 1, panel c).

Key findings from Smith et al. (2016), enabled by greater time-period of analysis:

1.  Significant retreat of the PIG began approximately in the 1940s.
2.    This retreat has continued up to present, despite the removal of the initial forcing (El Niño).

Figure 2: ‘Processes and sedimentation beneath the PIG ice shelf’. Caption quoted from source.

Source: Smith et al. 2016, Figure 3, top 3 panels.

Konrad et al. (accepted 2016, yet to be published):

This study observes the evaluation of the PIG over a shorter time-period, from 1992 to the present. In contrast to the previous study, Konrad and colleagues combine five satellite altimetry datasets to analyse the ASE (including PIG) from above. The high spatial resolution of data in this study enables in-depth comparison between glaciers on the ASE.

Altimetry data is used specifically to measure changes in glacier elevation. In turn, this can be used to determine the net amount of ice discharged by each glacier into the ocean (equivalent to SLR contribution). The term ‘ice-dynamical imbalance’ is used to describe rate of thinning and consequent mass loss.

Results from Konrad et al. indicate that PIG has thinned continually throughout the period of observations (Figure 3). Figure 3 shows an important trend in the PIG over the satellite observational period. The rate of surface lowering has increased up the glacier over time, at an approximately linear rate. This means that the point of a given rate of surface lowering has moved higher up the glacier over time (i.e. the red and blue lines on Figure 3). In addition, the rate of surface lowering close to the grounding line has increased over time. Thinning rate has spread up the PIG at a rate of approximately 13 km yr^-1 (red and blue lines), which is approximately double the equivalent rates for the nearby Pope, Smith, and Kohler glaciers.

Key findings from Konrad et al. (2016):

1.  PIG surface-lowering rate has increased continually (linearly) from 1992 to the present.
2.  PIG thinning rate is notably greater than surrounding glaciers on the ASE.

Figure 3: ‘Temporal evolution of surface-lowering rates. Distance is taken from the grounding line’. Caption quoted from the source.

Source: Konrad et al. 2016, Figure 2, panel A.

To summarise, both studies analyse the temporal evolution of the PIG. However, they use highly contrasting forms of data – and therefore study the glacier at different time scales. Despite these varying methodologies, the pattern shown in both studies is similar – an increasing PIG contribution to SLR. The current diversity of research in the polar regions is undoubtedly beneficial to improving estimates of SLR from Antarctica.