Antarctic Ice Thinning - a Literature Review

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In the study by McMillan et al. (2014), the major find from the research was the increased rate of ice loss. It was found that WAIS changed in mass by 134 billion tonnes. This is an alarming 31% higher than last reported in the 2005-2010 period. Moreover, glaciers were found to have significantly depleted and contributed to rising sea levels by 0.45mm/year, which is more than double previous estimates (0.19 mm /year) determined in Shepherd et al. (2012). It appears that the melting of ice shelves has accelerated the thinning of glaciers, such as those that used to feed Larsen A ice shelf before its collapse (Pritchard et al. 2012). This suggests that the ice shelves have a critical role in restraining the flow of the inland ice, which then highlights the integral importance of ice sheets not only to the future of the whole continent but also to sea level rise.

In other WAIS glacial studies, Joughin et al. (2011) determined that the Thwaites Glacier has already entered the early stages of collapse. The projected timeframe for the complete and irreversible collapse is set to take place in the next 200 to 1000 years. Another article investigating the grounding line (the critical border between grounded ice and ocean) of glaciers in the Amundsen Sea sector of West Antarctica coastline has found the region to be severely unstable. The rapid retreat of these glaciers is particularly threatening as it will drain a large sector of West Antarctica. Being termed ‘severe’ is no exaggeration; the study found no evidence of a major bed obstacle to prevent further retreat of the grounding lines (Rignot et al. 2014). The conclusion of the study was that this ice sheet instability will contribute to rising sea levels in the coming decades to centuries.

CONCERNS FOR EAST ANTARCTICA

Despite being the largest continental ice mass on Earth, the East Antarctic Ice Sheet (EAIS) has received far less scientific attention. The majority of EAIS rests on bedrock above current sea level and thus is regarded as more stable. Supporting this is the study by McMillan et al. (2014) that found 159 billion tonnes of ice disappearing from the continent each year, with East Antarctica only contributing three billion tonnes. It was evidence like this that meant, until earlier this year, the stability of areas like the Wilkes marine ice sheet were not considered in predictions of future sea level. Air temperatures in the East Antarctic region rarely reach above freezing, but if water temperatures rise the gap between the basin and the coast will decrease. This will then speed up the melt rate, which puts the EAIS at risk. A study by Mengel and Levermann (2014) has found the EAIS to be particularly vulnerable to climatic change. The study found a small volume of ice (hereafter referred to as the ‘ice plug’) that is holding back huge glaciers in East Antarctica's Wilkes Basin. This ice plug is the determining factor for the Eastern region’s stability and if the plug disappears, the consequential melting will be irreversible. Even though the full effects are predicted to take place in over 200 years, the melting of this obstacle would lead to the global ocean rising 3 - 4m. The basin’s sensitivity to climatic disturbances indicates that, by the end of the 22nd Century, East Antarctica will most likely be a major contributor to rising sea levels.

There is conflicting evidence that the EAIS has both expanded and increased in mass, which would appear to mitigate predictions of sea level rise (Davis et al. 2005; Turner and Overland 2009). However, new research that has taken these ice gains into account has still concluded that parts EAIS are still vulnerable (Miles et al. 2014). Using measurements from 175 glaciers, it was revealed that while glaciers underwent rapid periods of advance they still had periods of significant retreat. These advance and retreat coincided with periods of cooling and warming which, for a warming climate, is concerning news. The study found that in the 1970s and 1980s, temperatures rose and many glaciers retreated; 1990s saw temperatures decrease and most glaciers advance; and in the 2000s temperatures fluctuated resulting to in a mix of retreat and advance. This new evidence suggests strong variability for the future East Antarctic climate; however, further research is urgently required. While many studies determine ice loss through observations and configurations made by climate models, examining the glaciation history and thus past climate change is another technique. A study using glacial history techniques has found that since the Early Pleistocene ice sheet thickness in East Antarctica’s Sør Rondane Mountains have decreased by 500 metres (Suganuma et al. 2014). Using past changes of ice sheet mass can be a valuable tool in interpreting present-day and future behaviour, which is needed for further East Antarctic studies.

DISCUSSION

Determining the overall contribution of Antarctic ice sheets to sea-level rise is an important but complex problem. The amount of physical processes involved in melt rates, like ice dynamics and surface climate, make it difficult to determine not only the contribution but also timescales for sea level increases. Rising sea level figures tend to predict a range between 0.52-0.98 metres before 2100, yet all model simulations depend on warming rates, which as anthropogenic disturbances continue is something the scientific community is unsure of.

The lack of complete certainty and understanding of the Antarctic climate appears to have resulted in studies that make assumptions or models that simplify the problem. As such, older studies provide reassuringly long response times to climatic change (e.g. Alley & Whillans 1984). Back then, it was expected that the Antarctic ice sheets would grow as atmospheric temperatures warmed due to increased precipitation. Unfortunately, this discrepancy of sea ice growing in a warming world was cited as proof climate change did not exist. As new technologies and studies emerge, we find this is clearly not the case and in fact climate change is both a real and concerning issue. Even with slight increases of Antarctic inland ice the effects of a simultaneous loss of ice along the coast will not be negated (Miles et al. 2014).

Recent studies and observations now find shorter time-scale changes to the Antarctic climate with the potential to affect sea level rapidly. The Amundsen Sea sector is an area of most concern due to its unique location. Here glaciers are retreating the fastest (Rignot et al. 2014) which corresponds to increased sea level rises (McMillan et al. 2014). The changes seem to be undoubtedly attributed to higher air and sea temperatures (Steig et al. 2009; Pitrichard et al.