Antarctica accounts for ~61% of all fresh water on the planet (Fretwell et al, 2013), with ~8% of this water being contained in the West Antarctic Ice Sheet (WAIS), and ~92% of Antarctic water contained within the East Antarctic Ice Sheet (EAIS). With such large volumes of water locked within the cryosphere of the continent (particularly the EAIS) any changes to the mass balance of the continent will have a large influence on global sea levels, with a maximum potential rise estimated at 58m (Fretwell et al, 2012). Coastal zone population estimates suggest in excess of 1.2 billion people currently live within the coastal zone, with this figure set to increase dramatically over the coming decades (Small and Nicholls, 2003), therefore any change in eustatic sea levels will have huge implications for coastal populations.
Harsh climatic conditions, particularly during the austral winter, combined with the remote location of the continent have made access to, and monitoring of, the ice sheets historically difficult. With the growing use of satellite observation methodologies available (Talpe et al, 2017) remote monitoring of ice flow, gravitational variability and sub-glacial topography has become easier, but data points and the resolution of data sets over such a vast area are still inadequate (Golledge et al, 2017). Remote sensing methods increase our understanding of the ice sheet dynamics, but physical observations and fieldwork is still required to assess smaller scale processes. With the huge physical size of the Antarctic ice sheet (Figure 1), relatively small areas (as a percentage of the total), can have large effects on mass balance and eustatic sea levels (Rignot et al, 2013).
Figure 1. Estimation of basal melt rates of Antarctic ice shelves. Circular graphs are proportional in area to the mass loss from that shelf in Gt/year (hatch fill – iceberg calving, solid fill – basal melting). Modified from Rignot et al, 2013.
Historically the academic consensus has been for a stable or accreting ice sheet in East Antarctica where differing conditions to West Antarctica meant that ice loss was not perceived to be a problem (Gardner et al, 2018). The West Antarctic Ice sheet grounds on bedrock which is extensively below mean sea level, and subsequently intrusion of ‘warm’ sea water below the ice sheets accelerates speed of ice flow and also melt rates (Rignot et al, 2013). East Antarctica’s topography has been thought of as being generally much higher altitude and specifically above mean sea level (Lythe and Vaughan, 2001; Vaughan et al, 1997), hence suffering limited effect of basal melting (Figure 2).
Figure 2. Topographical elevation of Antarctica created during the BEDMAP project showing East Antarctica predominantly above sea level. Modified from Lythe and Vaughan, 2001.
There has been little or no evidence for ablation rates for the WAIS being anything other than significant and seemingly increasing (Shepherd et al, 2018; Rignot et al, 2013; Gardner et al, 2018). Studies comparing East and West Antarctic ice loss recognise the variation in estimates of ice loss depending upon the method of calculation and the model used, as well as the margin of error within findings (Gardner et al, 2018; Shepherd et al, 2018; DeConto and Pollard, 2016; Cook et al, 2013). A study by Shepherd et al (2018) suggested EAIS net gain of 5 ± 46 billion tonnes per year for 1992 to 2017 being a strong example of the large error range.
A growing number of studies are proposing that processes previously unrecognised may be causing the EAIS to suffer increasing net ablation. Due to the volume of water contained within the massive Eastern Ice sheet, the impact on small changes of ablation as a percentage of the total could have huge implications (DeConto and Pollard, 2016). With improved mapping of the sub-glacial topography of the continent in the Bedmap 2 project (Fretwell et al, 2013), the area of the EAIS which grounds on bedrock below sea level has increased greatly from previous estimates (Figure 3). The implication of this are that ingress from ‘warm’ ocean water beneath the ice sheet could increase ice flow and net loss (Shen et al, 2018; Rignot et al, 2013; Rintoul et al, 2016; Pritchard et al, 2012). This effect may be further amplified by melt waters acting as a surface barrier to ocean waters, thus allowing ‘warmer’ ocean water to reach the base of the ice sheet (Silvano and Rintoul, 2018).
Figure 3. Revised topographical map of Antarctica taken from the BEDMAP2 project, showing far greater detail and a significantly lower elevation of areas of East Antarctica. Modified from De Conto and Pollard, 2016.
Geological findings demonstrate EAIS’s reaction to temperature change through geological time could mean we have underestimated its reaction to warmer sea temperatures (Gulick et al, 2017). Comparison of sea levels at the last interglacial (DeConto and Pollard, 2016), as well as atmospheric CO2 when compared to sea levels in the Pliocene (Cook et al, 2013; Fretwell et al, 2013) suggest that our understanding of the way the EAIS reacts to climate change needs revision, and that future EAIS loss may increase.
Accuracy of data within studies is highlighted by the authors of almost all papers dealing with mass balance estimates for Antarctica (Gardner et al, 2018; Shepherd et al, 2018; Rignot et al, 2013; Li et al, 2016), with margins for error cited ranging dramatically. This disparate collection of values of not only the estimated ablation/accretion rates, but also the margins for error in the estimates, highlights the spatially and temporally limited data from which the estimates have been based.
Comparing the rates of snow fall and subsequent accretion within the EAIS against the mass balance is an area of research with insufficient data to make accurate observations that demonstrate relatively small margins of error. Combining this with the dynamic nature of climatic change in the Southern Polar Region, and the annual variance in mass balance, means that the answer to the question ‘Is the East Antarctic Ice sheet stable?’ becomes a question that needs to be defined temporally. As demonstrated by Li et al (2016) mass balance for the EAIS can be cyclical, and the answer to the question of EAIS’s stability would produce very different answers for the data relating to 2000-2007 (net ablation) when compared to the years 2007-2013 (stable/increasing mass balance) (Figure 4).
Figure 4. Flow speed variance, and by inference mass balance loss/gain, from year a) 2000 to 2007, b) 2007 to 2013, and c) 2009 to 2010 on Totten Glacier, East Antarctica, with the glacial grounding line in black. Modified from Li et al (2015).
Summarising the papers examined in this study, it appears that the East Antarctic Ice Sheet has been relatively stable in the last twenty years, especially when compared to the West Antarctic Ice Sheet, but the EAIS may be approaching a tipping point when ocean temperatures combined with topographical features may cause the ablation of the EAIS to increase dramatically. Due to the extensive and catastrophic results this ablation would cause, more detailed research is urgently required to quantify the likelihood and scale of the problem.
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