The Antarctic Ice Sheet Balance: A Net Loss or Gain?
Introduction
In multiple studies it is indicated that anthropogenically driven climate change is causing a reduction in the mass of ice sheets across the globe (Vizcaino et al., 2015). However, this does not equate to all ice sheets consistently loosing mass and the publication of some papers indicating an average gain in mass has resulted in a wider public debate regarding the ice sheet impacts of climate change (Kirilenko and Stepchenkova, 2012). An example of a paper where an ice sheet has been determined to be gaining mass is that which was published by Zwally et al., (2015) whereby it was reported that the mass gains of the Antarctic ice sheet exceeded mass losses in recent years and thus is not a contributory factor in recent sea level rise. However, other studies have suggested that publications such as that by Zwally et al., (2015) do not provide realistic representations of the process that are occurring. For example, Shepherd et al., (2019) report that there is a trend of thinning in the Antarctic ice sheet that is directly contributing to global sea level rise.
The disagreement between studies means that discussions of how ice sheet mass losses contribute to sea level rise under climate change could be contentious; even more so in the public sphere where the nuances of scientific studies may not be clearly understood. In the following essay the ice sheet mass balance of Antarctic is considered in detail to explore whether there is current a net loss or gain of ice mass. A secondary aspect of this essay is to develop an understanding of how it is possible to selectively represent data to support preconceived opinions and thereby present an argument for the necessity of examining the larger scale of an environmental issue rather than solely considering the finer details.
The Antarctic Ice Sheet
The mass balance of the Antarctic ice sheet is determined by the accumulation of snowfall in the interior of the ice sheet and the losses from surface ablation and ice discharges from the ice sheet edges (Lenaerts et al., 2016). When the Antarctic ice sheet is in a state of mass equilibrium then the rate of accumulation should equal the rate of losses. It has been suggested in multiple recent studies that there is an identifiable trend of increases ice losses at the periphery of the ice sheet that are not being matched by increases in interior snowfall accumulation (Groh et al., 2014; Rignot et al., 2019). In figure 1 a diagrammatic representation of the calculated total changes in mass of the major basins of the Antarctic ice sheet between 1979 and 2017 from a report published by Rignot et al., (2019) is shown. An examination of the diagram indicated that whilst there are many areas where the mass loss over the studied time period has been identified as substantial (red circles) there are notable groupings where the circles are blue indicating mass gains (see figure 1). Thus, it would be possible to present a selective research publication that would appear to indicate that the Antarctic ice sheet is actually gaining mass if observations were solely focused on the areas indicated by the blue circles in figure 1. For example, Martin-Espanol et el., (2016) observed that in East Antarctica there was a significant ice mass gain between 2003 – 2013 of approximately 56 ± 18 Gt yr-1. However, it cannot be denied that there are significant ice mass losses across the Antarctic ice sheet, which the majority of researchers agree equates to an overall ice mass loss in recent decades (Shepherd et al., 2018; Seroussi et al., 2020).
Figure 1. The calculated total mass changes of the major basins of the Antarctic ice sheet between 1979 and 2017 with losses indicated in red and gains indicated in blue. The circle radii are proportional to the absolute mass balance (Rignot et al., 2019 p.1097).
Why are there Differences in Ice Mass Loss or Gain on the Antarctic Ice Sheet?
It has been suggested that the primary reason for the significant ice mass loss observed in the West Antarctic ice sheet is the intrusion of warm, salty, circumpolar deep water (CDW) that does not extend to the East Antarctic ice sheet (Rignot et al., 2019). The CDW is due to the Antarctic Circumpolar Current (ACC) whereby warm water is transported via advection currents. However, due to the influences of climate change on ocean currents and temperatures, the CDW introduces heat to the underside of the ice shelves thereby resulting in increased melting and a higher likelihood of basal carving (Martinson and McKee, 2012; Ribeiro et al., 2021). As the melting of ice shelves is enhanced by the presence of the CDW it can subsequently result in accelerated glacier flow towards the ocean as the buttressing effect of the outer ice shelves is removed (Morrison et al., 2020). In figure 2 the variations in ocean temperature at a depth of 310m around the Antarctic ice sheet are shown. When this is compared to figure 1 detailing the extent of ice mass loss between 1979 and 2017 it can be seen that the areas where the greatest mass losses have occurred correspond with the areas in figure 2 where the ocean temperature is indicated to be greater than 0.6°C (McKee et al., 2019).
Figure 2. The ocean temperature at 310m depth around the Antarctic Ice sheet (Rignot et al., 2019 p.1097).
It is likely that future climate change will exacerbate the impact of the CDW on the basal melt of the Antarctic ice sheet and thus contribute to the continued loss of ice sheet mass in the coming decades (Pritchard et al., 2012). The impact of the CDW has also been suggested as likely to be enhanced by eastward wind anomalies increasing the magnitude of the import of warm CDW (Holland et al., 2019). It is predicted that the eastward wind anomalies will become more prevalent under future climate change, thereby contributing to the increasing of warm ocean anomalies that will enhance basal melting and ice sheet mass loss (Holland et al., 2019).
Conclusion
The scenario of ice sheet mass loss and gains in the Antarctic illustrates that it is necessary to consider the entire large scale situation when attempting to identify trends in the environment. It is evident that there are some areas of the Antarctic ice sheet that are experiencing ice mass gains, however these are outweighed by the more significant ice mass losses. The mechanism that is driving the Antarctic ice sheet mass loss is suggested to be the warm waters that are carried by the CDW and direct by eastward wind anomalies to intersect with the West Antarctic ice sheet driving the process of basal melting. The basal melting at the ice sheet edge facilitates accelerated glacier flow in an oceanward direction ultimately resulting in ice sheet mass loss. Under the current climate change predictions, it is likely that there will continue to be the environmental conditions necessary for warm waters to intersect with the Antarctic ice sheet that will result in increased ice sheet mass losses in the decades to come.
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