What are ice shelves?
Ice shelves are the floating extension of the Antarctic ice sheet as it pushes out over the ocean. As snow accumulates on the continent, its weight compresses to form glacial ice, which slowly pushes the ice sheet seawards. Ice shelves may be hundreds of kilometres across and hundreds of metres thick. They melt into the ocean and calve icebergs from their front.
Why do we study them?
Because ice shelves are in contact with the atmosphere above and the ocean below, they are the most vulnerable component of the Antarctic cryosphere to environmental change.
As ice shelves consist of ice that is already afloat, iceberg calving does not significantly affect sea level. However ice shelves have a buttressing effect, slowing the discharge of inland ice off the continent (via glaciers). As a result, changes in the shape and size of the ice shelf can affect the flow of their associated glaciers from the interior of the continent, outwards.
How does this affect me?
The collapse or retreat of an ice shelf can lead to more rapid discharge of continental ice, which contributes to increased sea level. This has already been shown to be the case with the loss of ice shelves in the Antarctic Peninsula region (such as the Larsen Ice Shelf). As a large proportion of the world population is located in near-coastal environments, rising sea level has the potential for major economic and social impact.
Water mass modification beneath ice shelves may also have a role in ‘Antarctic bottom water’ formation. This cold, dense, salty water is created when sea ice forms in winter in special ‘ice factory’ areas or ‘polynyas’. As sea ice forms, it rejects salt, making the water below saltier and denser. This sinks to the deep ocean and results in powerful currents of cold water that drive global ocean mixing and regulate atmospheric temperatures.
Antarctic bottom water, as well as the freshwater distribution from the melting of ice shelves and icebergs, has a profound influence on ocean circulation patterns. Changes in these patterns, as a result of changes in bottom water formation and ice shelf melting, have a major influence on regional climate and an impact on important oceanic ecosystems and fish stocks.
The Amery Ice Shelf
The massive inland Lambert Glacier Basin, which occupies nearly 16% of the East Antarctic ice sheet area, channels its flow into the Amery Ice Shelf, whose calving front occupies 2% of the total East Antarctic coastline.
The 550 km long, 60,000 km2 Amery Ice Shelf is the major embayed shelf in East Antarctica. More than 50% of the ice flowing into the Amery is lost through basal melting processes (melting beneath the ice shelf where it interacts with the ocean) within the first few hundred kilometres of it beginning to float. Some of this meltwater refreezes to the base, as marine ice, further north.
The remaining loss (necessary to maintain a state of near equilibrium of the ice sheet) calves as icebergs from the front. This loss occurs slowly at first, by continual shedding of small icebergs and ice cliff collapse from the shelf front. Major iceberg (30 km x 30 km) discharge occurs sporadically into Prydz Bay as the ice shelf front grows, spreads, thins and weakens.
In the 1990s a ‘loose tooth’ formation appeared along the north central calving front of the Amery, as rifts opened and widened over the preceding decade. This tooth could become the next major iceberg (the last such calving event was mid-1960s).
How are we studying the Amery Ice Shelf?
By combining surface measurements with sub-ice oceanography on ice shelves and associated glaciers around Antarctica, scientists hope to understand ice shelves’ stability (ice gain versus loss) and the fundamental processes involved in their response to changes in atmospheric and ocean temperatures and circulation.
Research on the Amery Ice Shelf, for example, aims to directly measure ocean characteristics, seawater circulation and the melt-freeze processes occurring at the base of the shelf, using boreholes through the ice into the underlying ocean cavity. These boreholes have been created using a purpose-built hot water drill system to melt its way right through the shelf.
Since 2010 the Amery Ice Shelf Ocean Research team has been monitoring a range of instruments deployed into two of six boreholes (AM05 and AM06), and on the surface of the ice shelf in 2015 (see map).
Instruments deployed into the boreholes include distributed temperature sensors that measure ice and ocean temperature through the ice shelf and into the ocean, and acoustic doppler current profilers, which measure the speed of water flowing beneath the ice shelf and ice shelf thinning due to basal melting.
On the surface of the ice shelf are GPS to measure ice flow speeds and surface elevation changes, and two autonomous phase sensitive radio echo sounders (ApRES). The ApRES measure, with millimetre accuracy, thickness changes of the ice shelf due to melting and thinning.
What are we finding?
In the past decade scientists have found that a significant cause of ice shelf thinning around Antarctica is through the interaction of the base of the ice shelf with the ocean (basal melting).
Research on the Amery Ice Shelf and other ice shelves have provided scientists with a crude idea of the drivers of basal melting and refreezing, which is reflected in current ice shelf-ocean models.
However there are many details that still need to be understood to get a more accurate picture of how the interaction of the ocean with the base of an ice shelf drives melting or refreezing.
What happens next?
Research on the Amery Ice Shelf is part of broader international research on Antarctic ice shelves and glaciers. For example, data collected by the ApRES instruments deployed on the Amery will contribute to an international project known by its acronym NECKLACE. This project aims to deploy ApRES instruments on all the major ice shelves around Antarctica (like a necklace). This will allow scientists to detect seasonal variability in basal melt rates and over time provide longer-term melt trends around the continent.
In 2016, ApRES, GPS, current profilers, temperature sensors, and other oceanographic instruments will be deployed on and beneath the Totten and Sørsdal glaciers and the Amery and McMurdo ice shelves. In coming years these studies will provide good data on the fundamental processes that drive basal melting, as well as information about the bathymetry beneath ice shelves, ocean temperatures and the general state of the East Antarctic region
Data collected will feed into a much higher resolution numerical model (to replace existing models) looking at the role of ocean mixing on the basal melting of ice shelves. This information will allow scientists to provide a risk assessment of the likelihood of the collapse of ice shelves susceptible to rapid deglaciation.