Essay: The West Antarctic Ice Sheet

Introduction
The West Antarctic Ice Sheet (WAIS) is inherently unstable (Thomas and Bentley, 1978) as its ice shelves, which protect the WAIS from being undercut and separated from the bedrock by the ocean (Lenton et al, 2008), are vulnerable to melting from natural and anthropogenic ocean and atmosphere warming. Although its instability is generally accepted, debates arise over the existence of a threshold or tipping point which would lead to a full WAIS collapse; while some believe the WAIS will “survive beyond this century” (2011-506), Feldmann and Levermann believe a collapse is already underway due to the destabilisation of the potential ‘tipping point’ of the Amundsen Sea. Similarly, the effect of WAIS melting on sea levels is debated and ranges from 3-6m. Such discrepancies arise from the complexity of the dynamic Earth system in which the WAIS functions.
Stability
The stability of the WAIS is largely driven by climate. Ice shelves are needed to stabilise the WAIS through ‘buttressing’ so warmer ocean waters cannot undercut the ice sheet (Figure 1a). Atmospheric or ocean warming can melt these ice shelves, reducing this ‘buttressing’ as the grounding line retreats (Figure 1b). The upstream deepening of the bed could initiate runaway retreat (Pollard and DeConto, 2009) and this “threshold behaviour” (Joughin and Alley, 2011:509) could eventually lead to WAIS collapse. Calculations place this threshold at a 5oC local surface temperature increase (Lenton et al, 2008) which Pollard and DeConto also identify as the tipping point of collapse (2009). Conversely, Blankenship explores alternative factors such as “active volcanism”(1993:526) at the base of the ice sheet which could “trigger a collapse of the inland ice reservoir”(1993:526) however such localized destabilizations would be contained by topographic factors (Feldmann and Levermann, 2015) so would not massively influence stability. Thus, the WAIS’ stability is most vulnerable to climactic change.
The paleorecord can indicate the stability of the WAIS in response to previous climate changes, through the presence of diatom and beryllium-10, which suggest open-water conditions, in WAIS boreholes (Joughin and Alley, 2011). Records suggest the loss of the WAIS during interglacial warming in the Pleistocene (Pollard and DeConto, 2009). Paleorecords can also identify the likely effect on sea levels; a 6m sea level high identified in the Sangamon interglacial has been attributed to WAIS collapse (Thomas and Bentley, 1978). Similarly, paleorecord analysis can estimate the likelihood of a future WAIS collapse, as “polar warming by the year 2100 may reach levels similar to those 130,000 […] years ago” (Overpeck et al, 2006) which would destabilise the WAIS. However, this likelihood is not universally agreed upon; while some believe the “WAIS most likely will not collapse in the next few centuries” (Vaughen and Spouge, 2002), Feldmann and Levermann conclude the localized destabilisation of the Amundsen Sea region has already been initiated and the positive feedback system will result in collapse within the next millennia (2015).
The Amundsen Sea region has been identified as the ‘weak underbelly’ of the WAIS (Joughin and Alley, 2011) and Lenton identifies this as a potential climate ‘tipping point’ (2008). Although all regions of the WAIS are vulnerable to instabilities, with varying levels of sensitivity to ocean and atmospheric warming (Joughin and Alley, 2011), simulations suggest a destabilisation of the Amundsen Sea region would not be halted by topographic features and cause a full WAIS collapse (Feldmann and Levermann, 2015). Present net losses of the WAIS result mainly from the 0.6-5.5m/yr thinning of this region’s glaciers (Joughin and Alley, 2011), which are “losing 60% more ice than they are gaining” (Lenton et al, 2008:1789), supporting Feldmann and Levermann’s proposal that the Amundsen Sea region has already been destabilised and the Earth system has entered a positive feedback system in which sea levels will rise by 3m in the next millennia (2015).
Future global sea levels
The resulting projections for future global sea levels range greatly. Initially, it was estimated a collapse would produce a sea level rise of 4-6m (Mercer, 1968), which is supported by Blankenship who estimates a global sea level rise of 6m (1993) and the paleo-evidence of a 6m sea level rise in the Sangamon Interglacial (Thomas and Bentley, 1978) however “previous assessments have substaintally overestimated its likely contribution” (Bamber et al, 2009:901). Instead, “more recent analysis suggests collapse of only the unstable marine portions would produce a 3.3m eustatic sea-level rise”(Joughin and Alley, 2011:507) which would take place over several centuries.
These discrepancies reflect “both the unpredictability of the physical system and the scientific uncertainty”(Vaughen and Spouge, 2002:65) of complex interactions within the Earth system. Sea level rise is non-uniform (Bamber et al, 2009), and feedbacks produced by WAIS variations could initiate “the growth or decay of other marine ice sheets” (Thomas and Bentley, 1978:164). Therefore, although we currently accept a predicted 3.3m sea level rise, to accurately quantify the WAIS’ stability and the sea level response, an improved understanding of the Earth system and its responses is required.
Conclusion
Therefore, we can accept the current projections of a 3.3m sea level rise over several centuries if the WAIS collapses, but more complex models are required to quantify the WAIS’s stability, and the likelihood of such a collapse happening (Joughin and Alley, 2011). It is unlikely a collapse will occur in the next century (Vaughen and Spouge, 2002), however it could possibly occur in the next millennium (Lenton et al, 2008) due to the WAIS’ vulnerability to anthropogenic climate warming. If we are to accept Feldmann and Levermann’s theory that we are currently at the start of a WAIS collapse, caused by the localised destabilization of the Amundsen Sea region and maintained through positive feedbacks (2015), we need to adapt globally through implementing long-term coastal protection to mitigate damage from the predicted 3.3m sea level rise over the next several centuries. This is highly uncertain, however it is certain the WAIS is losing mass as a result of ocean/atmosphere warming, thus the development of more complex models is necessary to predict such global changes in the Earth system.