Australia: The Land Where Time Began

A biography of the Australian continent 

Antarctic Ice Sheet – the Prescience of Palaeoclimatology and its Future

“The emerging view that West Antarctic Ice Sheet is in the early stages of collapse owes as much to paleoclimatology as to contemporary observations” (Steig & Neff, 2018).

The central question in polar science is the stability of the Antarctic Ice Sheet in the face of anthropogenic climate change. It is well documented (Scambos, Hulbe  & Fahnestock, 2003) that the retreat of glaciers and the loss of ice shelves on the Antarctic Peninsula is underway, though the processes are distinct from those that are affecting the West Antarctic and East Antarctic Ice Sheets (WAIS and EAIS respectively) that are both much larger than those of the Antarctic Peninsula. Advances in remote sensing and dedicated field campaigns, which have provided an unprecedented view of the surface, interior and bed, as well as well as the conditions that prevail in the surrounding oceans and atmosphere, have allowed knowledge of the ice sheets to grow rapidly. In the mid-1990s it was discovered that beneath the ice shelf of the Pine Island Glacier (Jacobs, Hellmer & Jenkins, 1996) which is one of the major glaciers draining WAIS into the ocean, rapid melting is taking place. A short time later it was shown by observations that Pine Island Glacier, as well as other glaciers, were retreating and thinning (Rignot, 1998; Shepherd, Wigham & Mansley, 2002). It has now become clear that most of the Margin of the West Antarctic ice, as well as parts of East Antarctica, is undergoing rapid retreat (Pritchard et al., 2012), and that the proximal cause is the erosion of the ice shelves by warm ocean water (Jenkins et al., 2010), which is amplified by intrinsic instabilities in glacier dynamics (Christianson et al., 2016).

Contemporary understanding of the state of the WAIS is often traced to the prescience of Mercer (Mercer, 1978), who suggested 40 years ago that the ice shelves would be destabilised by anthropogenic climate warming, which would lead to the collapse of the ice sheets. Mercer’s understanding of the crucial role of the ice shelves is certainly supported by recent observations. In spite of this a solicitation of the views of researchers that was conducted as recently as 2013 concluded that expert opinion was both “uncertain and undecided” on whether the recent trends in the behaviour of Antarctic ice sheets are the result of natural variability, or are a response to climate change (Bamber & Aspinall, 2013). According to Steig & Neff the reason for this ambiguity is simple: the record of direct observations of changes in Antarctic ice sheets is too short. The earliest application (Goldstein et al., 1993) of satellite radio interferometry being used to measure flow velocities of the Antarctic ice sheet took place in 1993, and even more recently for other observational products, such as satellite altimetry which is critical for evaluating thinning. Therefore modern physical observations of the ice sheet comprise a period less than the standard textbook requirement for a representative climatology – 30 years – against which trends can be evaluated. Surely this is too short a time to have been observing the Antarctic ice sheet, which is characterised by intrinsic variability on timescales that are even longer than the atmosphere, and expect definitive answers. Steig & Neff suggest that unless people are willing to wait for another few decades, or more, by which time there might be a clear answer, though by then the answer may well be of less use, the Information from Palaeoclimatology should be incorporated.

Emerging palaeoclimate evidence motivated Mercer’s insight that the WAIS is vulnerable. It had been established by deep sea sediment cores (Shackleton, 1969) that sea level was several metres higher than present levels during the last interglacial period (about 132-116 thousand years ago - ka), and it was soon realised that the Antarctic ice sheet must have been considerably smaller (Hughes, 1975). It has since been shown by sediment cores drilled beneath the ice sheet of the present that the WAIS collapsed at least once in the last 1 million years (Scherer et al., 1998). During the last interglacial the sea level rose rapidly to between 5 and 9 m higher the present levels, which is difficult to explain without a major contribution from Antarctica (Dutton et al., 2015). Such evidence remains the strongest indication that the ice sheet is sensitive to climate change.

Palaeoclimate evidence also informs modern numerical ice sheet modelling. Future behaviour simulations of the ice sheet rely on estimates of configurations of ice sheets of the past and climate forcing of the past, i.e., of temperature and rates of accumulation of snow, for initialisation and validation. The implementation of new model physics: a new mechanism for the calving of ice shelves that is more efficient than had been assumed conventionally (Pollard, DeConto & Alley, 2015), had been demanded by difficulties with the matching of model simulation outputs to geological constraints on the configuration of palaeo-ice sheets  and sea level. The collapse of the WAIS during the last interglacial, providing the assumption of strong warming in the Southern Ocean as a boundary condition (DeConto & Pollard, 2016), were caused by simulations with this model physics. Evidence of such warming can be found in the palaeoclimate record (Capron et al., 2014). Under projected anthropogenic climate forcing, the same model physics leads to rapid deglaciation of West Antarctica (DeConto & Pollard, 2016). Therefore, understanding of the future of West Antarctica has been informed by evidence of the response of the ice sheet in the past.

Context for changes in the ice sheet that are observed at present is also provided by palaeoclimate data. Increased warm ocean water delivery to the ice sheet margin, which is implicated in the current phase of retreat, is a result of changes in the circulation of the ocean, driven by atmospheric charges. The investigation of atmosphere-ocean forcing heavily relies in the reanalysis of climate data, which in the Antarctic region, only since 1979; in West Antarctica Marie Byrd Station is the only weather station that provided continuous meteorological observations back to an earlier time, which only began in the 1957 International Geophysical Year. On the other hand ices cores provide information that stretches back thousands of years. It has been shown by annually-resolved ice cores that the major El Niño event of 1939-1942 had a significant impact on the climate (Schneider & Steig, 2008) of West Antarctica which led to the conclusion that this event was important in the forcing of glacier retreat in the Amundsen Sea (Steig et al., 2012). It has since been shown by sediment cores that were collected from beneath the Pine Island Glacier ice shelf that the flooding of the continental shelf by warm Circumpolar Deep Water began around the 1940s. In the determination of when, and if, changes in the climate of Antarctica reach the point where “a dangerous trend is under way”, as it was put by Mercer, data like these will be critical. A definitive answer to date is available only for the Antarctica Peninsula, though the result from an ice core in that region is dramatic: summer melt at present exceeds anything that has occurred in the past 1,000 years (Abram et al., 2013).

There are, of course, limitations to palaeoclimate data compared to instrumental climate observations and modern and ice sheet structure imaging by modern geophysical technology. Yet, far greater spatial coverage of ice cores is obtained than that of weather stations, and there is potential to use such records even more effectively, by way of blending of proxy data with the output of climate models to produce longer-term climate reanalysis products (Steiger et al., 2016). In order for such techniques to yield information that is reliable about the conditions that are most relevant to ice dynamics, additional records will be needed from regions that are closer to the centre of action. It would be particularly valuable in this respect to have cores from many ice domes along the coasts of the Amundsen Sea and the Bellingshausen Sea in West Antarctica, though logistical difficulties of working in this remote region have prevented, so far, initiatives to obtain such cores from moving forwards.

Whether the WAIS collapsed during the last interglacial period still remains unresolved. An important challenge presented by this uncertainty that can only be answered definitively with the help of palaeoclimate and palaeoglaciological data. According to Steig & Neff one approach is to obtain spatial reconstructions of climate by the use of deep ice core records. If the ice sheet was significantly smaller it would be associated with changes in weather patterns which should be detectable in ice cores from sites which are strategically located such as the Hercules Dome, in East Antarctica inland from the Transantarctic Mountains, adjacent to the ice sheet of West Antarctica. It is suggested by the results of climate models that the climate at Hercules Dome would be especially sensitive to charges in atmospheric circulation that were associated with the collapse of the ice sheet (Steig et al., 2015). The use of borehole drilling to obtain samples of bedrock beneath the current ice sheet, and then use cosmogenic nuclide measurements to determine the exposure history of the bedrock, which would yield information about times when the ice sheet was smaller (Schaefer et al., 2016), is another promising idea.

It has been concluded by some researchers that the WAIS is already in the early stages of an irreversible collapse (Rignot et al., 2015; Joughin, Smith & Medley, 2014). Certainly, the retreat of the ice margins of the Antarctic ice sheet that is occurring at the present is remarkable, and Steig & Neff suggest expert opinion on whether such trends are a cause for worry has probably moved somewhat from “uncertain” to “likely” in the last few years. A powerful role has been played by observations of palaeoclimate in this emerging view, by providing critical boundary conditions for use in ice sheet models and in elucidating conditions under which ice sheets may have collapsed in the past. Palaeoclimate data also provide an important role in balancing the interpretation of modern observations against the long-term context in which ongoing ice sheet changes is occurring. As the ice continues to change in a warming world, such complementary information will continue to be necessary.

Steig, E. J. and P. D. Neff (2018). "The prescience of paleoclimatology and the future of the Antarctic ice sheet." Nature Communications 9(1): 2730.