What lies beneath
Australian Antarctic Division Senior Principal Research Scientist Dr Tas van Ommen describes new insights into the bedrock structure beneath the East Antarctic ice sheet that could influence future increases in sea level.
In 2007 my colleague Donald Blankenship, from the University of Texas, got in touch with a no-brainer proposal to map one of the last great unsurveyed tracts of Antarctica. He and UK glaciologist Martin Siegert had put together two-thirds of a great plan to fly radar, laser, geomagnetic and gravity instruments over an area the size of New South Wales, inland of Australia’s Casey station. They needed Australian support and participation for the project to fly.
At that time there were huge gaps in our maps of the bedrock under the ice. But we knew that in places the ice was over four kilometres thick, making it some of the deepest ice on the continent. We also held a view that East Antarctica was probably not a dynamic place. While we needed data, we expected to find a stable ice sheet that was unlikely to be affected by changes in a warming planet. This view would soon change when the ICECAP (International Collaboration for Exploration of the Cryosphere through Aerogeophysical Profiling) project began.
In 2011 I participated in several ICECAP survey flights from Casey. Each flight lasted about seven and a half hours and covered some 2000 km at a time. To date the project has surveyed over 150 000 km in East Antarctica.
It did not take long for the value of the data to be appreciated. Even as the project progressed, satellite monitoring was drawing the attention of global researchers to a hot-spot right beside our Casey hub; the Totten Glacier. This glacier is the largest in East Antarctica. It drains most of the area of our survey; every year discharging over 70 cubic kilometres of water into the ocean. As it does so it carves a deep trench over two kilometres below sea level, through which the ice emerges and begins to float. During ICECAP, satellite measurements showed that just around the point where the ice begins floating, the Totten is thinning and the surface height is lowering by about two metres per year. ICECAP set about measuring the Totten Glacier outlet so that we could understand what is happening.
What ICECAP has revealed is really a connecting story in two parts: how our view of East Antarctica has changed dramatically; and what this means for changes in the Totten Glacier itself. Previously we thought that, aside from a poorly mapped valley far inland of Casey known as Aurora Basin, most of the ice was resting on bedrock hills and mountains above sea level. It turns out that Aurora Basin is very deep and much larger than we thought. More seriously, the basin is connected to the coast by terrain that is extensively below sea level. This makes it much more like West Antarctica, where there is concern that gradual, but irreversible ice loss is underway. The prospect that such a pattern could also impact East Antarctica is a new one, and suggests that changes to the Totten Glacier might be the first stages of such accelerating loss in East Antarctica.
Outlet glaciers like the Totten meet the ocean in floating ice shelves where they calve icebergs and also melt in the relatively warmer ocean. These ice shelves are buttressed against the coast on the sides, and in some cases, like the Totten, are very congested at the calving front. This provides an impediment to the ice flow that can be likened to a cork in a bottle. Any loss of the ice shelf and retreat of the calving front reduces this impediment and is typically followed by acceleration of the feeding glacier, which leads to rising sea level.
A second issue arises when the glacier rests below sea level on a bed that deepens towards the interior. If the ice retreats, a process of accelerating flow and further retreat is unstoppable until the ice reaches a point where the bed begins to rise. In the case of the Totten Glacier, such a retreat all the way into the deep Aurora Basin would lead to a the sea level contribution of at least 3.5 metres. Such a change would take several centuries and is equal to, or a little larger than, the potential contribution from all of West Antarctica.
Interestingly, this potential sea level rise from East Antarctica could be the answer to a mystery from the last global warm period, some 120 000 years ago. At this time the Earth was around two degrees warmer than present (portentously, around where current projections are headed) and evidence suggests sea levels were six to nine metres higher than present. Earth scientists have struggled to explain the source of this rise using only Greenland and West Antarctic melt, but the presence of a significant source of melt from East Antarctica could solve the puzzle.
So where is this work leading? An oceanographic voyage to the front of the Totten in early 2015 (see Totten Glacier melt-down) has discovered that relatively warm deep ocean water is present on the continental shelf in front of the Totten Glacier. This is warmer than the colder water typical of the coastal continental shelf zone and it has potential to produce accelerated ice melt. Secondly, the ICECAP flights have been able to identify at least two deep channels reaching back under the glacier front (see Hidden oceanic gateway beneath Totten Glacier). These channels may provide a way for warm water to reach deep under the glacier and could explain the observed thinning and lowering. The airborne measurements have provided key evidence. Airborne radar measures the ice thickness, but cannot penetrate the ocean, while gravity measurements reveal contrasts between rock and water that can be used to map the shape of the ocean cavity itself. Other clues, like the brightness of radar reflection from the ice-water surface can tell us about the strength of melting that is taking place.
Ideally, what is needed are direct ocean measurements to augment the airborne evidence, and plans are being considered for the use of remotely controlled underwater vehicles and robot floats dropped from the air.
Also key to predicting the future is matching observations with computer model simulations of the ice-ocean system. This effort is technically demanding and relies heavily on observations from ICECAP to provide accurate bedrock and ocean cavity measurements. Oceanographic observations are needed to feed information about currents and heat flow, and this ice-ocean model must be coupled with climate models to project the changes in the system as a whole.
For now, ICECAP has a considerable task ahead to refine the basic mapping beneath the ice to monitor the changes. Plans are well advanced to increase the efficiency and reach of the airborne survey work with light, long-range drone aircraft, in coming seasons. These will allow for targeted studies using a reduced suite of instruments to match payload limits. For much of the work, however, our retrofitted DC3 workhorse will continue surveying East Antarctica for many years to come.
Tas van Ommen
Program Leader, Antarctica and the Global System, Australian Antarctic Division
An edited version of this article first appeared in The Conversation on 30 April 2015.