Past Response of the Antarctic Ice Sheet to Climatic Change
Micahel Hambrey, Neil Glasser, Bryn Hubbard and Justin Taylor.
The role of the Antarctic ice sheet in influencing global change is increasingly being recognised. As the Earth's climate warms, the manner in which glaciers and ice sheets respond is of considerable significance for human civilisation. The Antarctic ice sheet stores 90% of the world's ice and 70% of its fresh water, and if this were all to melt then sea level would rise globally by an estimated 60 metres, flooding many of the world's major cities and valuable agricultural land. Although considerable progress is being made to predict the behaviour of ice sheets under a warming climate, through the development of global circulation and glaciological models, it is important to determine how ice sheets behaved in the past, as such knowledge will provide constraints on future predictions.
The Centre for Glaciology is at the forefront in developing our understanding of the response of ice sheets to climatic change. Because of his wide experience of glaciers and their sediments, Michael Hambrey has been invited on several occasions since 1986 to join the national programmes of New Zealand, Germany, Australia, the USA and the UK, as well as two international offshore drilling projects, the Ocean Drilling Program and the Cape Roberts Project. Arising from this work, major findings include: (i) the Antarctic ice sheet first developed at least 36 million years ago; (ii) the early ice sheet was subject to major fluctuations, resulting in major changes in sea level; (iii) the early ice sheet was much warmer, and associated with vegetation of the type that grows near the tree-line in Tasmania and Patagonia today.
The Antarctic continent illustrating principal areas of Centre for Glaciology research.
A particularly important phase of ice sheet evolution was in the Pliocene Epoch (3-5 m.y. ago). This was the last time the Earth's atmosphere had CO2 levels equivalent to those predicted for the later 21st century, if humans continue to pollute the atmosphere. Since CO2 levels correlate strongly with global temperatures, and therefore ice sheet growth and decay, it is important to understand what happened in Antarctica in Pliocene time. Unfortunately, there is no simple solution, and two opposing views have evolved. From work in the Transantarctic Mountains, geomorphologists (nicknamed "The Stabilists") have provided convincing evidence that the ice sheet was stable for at least 15 m.y., indicating that there were no drastic changes in the Pliocene Epoch. In contrast, geologists examining thick successions of glacial sediments have equally convincing evidence of an unstable ice sheet until as recently as 3 m.y.; if this is the case, then we have cause for concern that the ice sheet could become unstable once again, perhaps within a hundred years. This group has become known as "The Dynamicists". Hambrey, working with the Dynamicists, Prof. Peter Webb of Ohio State University and Prof. David Harwood of the University of Nebraska at the Shackleton Glacier in latitude 85°S, has gathered evidence of a much warmer climate with glaciers and abundant meltwater during Pliocene time, but the geomorphologists claim that the sediments are much older.
Shackleton Glacier flowing from the East Antarctic Ice Sheet past Bennett Platform towards the Ross Ice Shelf. Ancient glacial sediments are preserved on the flanks of this glacier. Photo: M. J. Hambrey
There are few signs that this sometimes heated debate is likely to be resolved in the near future, but a relatively new area of investigation, the Prince Charles Mountains, is yielding an analogous, but more readily datable record. This area was only discovered in 1948, and few places on Earth are more remote. Along the flanks of the world's biggest glacier system, the Lambert, several hundred metres of glacial sediments are preserved. These sediments, examined by Hambrey and Dr Barrie McKelvey of the University of New England and Dr Jason Whitehead of the University of Tasmania in Australia, indicate a series of glacial events between 30 and 3 m.y. ago, but the most important finding is that the East Antarctic ice sheet fluctuated strongly throughout this time, and that the Pliocene Epoch in particular was a period of instability. This supports the views of Dynamicists Webb and Harwood from the Transantarctic Mountains.
Drilling projects do not have the same aesthetic appeal as fieldwork, since all the work is undertaken in laboratories. However, in terms of scientific excitement they are second to none. The most recent such drilling operation was the $5 million Cape Roberts Project (1997-2000), set up on the winter sea ice in the western Ross Sea. Like earlier drilling projects, this was designed to retrieve the long-term record of glaciation, targeting especially sedimentary rocks that show the first signs of ice activity in Antarctica. The project was an international venture involving (in order of decreasing financial contributions) New Zealand, the USA, Germany, Italy, Australia and the UK. Over 50 scientists participated in the project. Hambrey was invited to join the New Zealand team by the Project leader and Science Manager, Prof. Peter Barrett of the Victoria University of Wellington, and we also worked closely with Prof. Ross Powell (University of Northern Ilinois) and Prof. Larry Krissek (Ohio State University) in preparing the results for publication. The principal conclusions from a glacial sedimentological perspective are that the ice sheet in its early history (Oligocene Epoch) was temperate, becoming successively poythermal (Miocene) and the cold (Quaternary). This matches the palaeoecological record, which shows the progressive demise of forests through tundra to zero vegetation cover.
Barrie McKelvey (University of New England, Australia) examining Miocene glacial sediments on Fisher Massif, Prince Charles Mountains. Photo: M. J. Hambrey
Drilling site at Cape Roberts, western Ross Sea, Antarctica. Photo: P. J. Barrett.
These field investigations focus primarily on the glacial geological record of the East Antarctic Ice Sheet, but peripheral areas of Antarctica are already responding to climatic warming in the form of collapsing ice shelves in the Antarctic Peninsula. Thus, in collaboration with Dr John Smellie of the British Antarctic Survey, Hambrey is investigating the Neogene to Quaternary record on James Ross Island near the northern tip of the Antarctic Peninsula. Here, glacial deposits are uniquely associated with volcanic rocks, and the products of subglacial eruptions are of particular relevance. The advantage of having an association with volcanic rocks is that glacial events can be dated precisely.
All the work on glacial sediments needs modern analogues. Elsewhere, we describe the work on glacial sedimentary processes in Alpine, Andean, Himalayan and Arctic environments. In Antarctica, Bryn Hubbard in association with Dr Wendy Lawson of the University of Canterbury (New Zealand), and Michael Hambrey with Dr Sean Fitzsimons of the University of Otago are undertaking ice deformational and glacial geological studies on modern cold glaciers in the Dry Valleys of Victoria Land, Antarctica.
Following these field-based investigations of the history of the East Antarctic ice sheet, the Centre for Glaciology is working with glaciological modellers in order to better understand the response of the ice sheet to climatic change in the past. This work has been undertaken by postdoctoral researcher, Justin Taylor in collaboration with Prof. Martin Siegert (former member of the Centre for Glaciology) and Dr Tony Payne, both of the Bristol Glaciology Centre. Our main focus is on the Lambert Glacier system, and the aim is to reconstruct the ice sheet using a range of glaciological parameters and geological data (both onshore and offshore). Dr Taylor has demonstrated how the ice sheet has changed its dynamic regime from uniform flow across a wide area to more focused ice-stream flow, and how changing bed conditions has played a crucial role in explaining long-term ice-sheet fluctuations. Interpreting the Antarctic landscape, both above and below the ice sheet, is also crucial in determining how the ice cover in Antarctica has changed through time. This aspect is being examined in collaboration with Prof. David Sugden and Dr Nick Hulton at the University of Edinburgh. Neil Glasser is focusing on interpreting landscape features in critical areas along the flanks of the Lambert Glacier.
Cartoon showing how the western margin of the Ross Sea may have looked in late Oligocene time, with vegetation on the lower slopes and glaciers reminiscent of those in the Arctic today.
In conclusion, all this work is of fundamental importance in trying to understanding the links between glaciers and climate, and how these ultimately control global sea levels. If we understand these links, then we will be in a better position to plan for future sea-level rise as global warming proceeds.
The fieldwork was funded by the New Zealand, German, American, Australian and British Antarctic programmes. NERC has provided funds for the UK-based work on data analysis and modelling.
Snowstorm over Battye Glacier, Amery Oasis, east Antarctica, with frozen Radok Lake in the foreground. This is the site of work on past (Cenozoic) glaciations in association with Australian colleagues. Photo: M. J. Hambrey.