The vast Antarctic ice sheet contains about 30 million cubic kilometers of ice, which is 90% of the Earth’s freshwater ice. If it were all to melt, it would increase sea level by about 70 meters. Fortunately, surface temperatures across most of the continent stay well below freezing all year round so there is virtually no ice loss through surface melt. Instead, most of the ice loss is through iceberg calving and ocean-induced melting from the under-side of ice shelves.
One area that scientists are keeping a close eye on is the Amundsen Sea Embayment to the west of the Antarctic Peninsula. Here the ice is being melted from below by warm ocean waters at a greater rate than ice is added through snow accumulation, and this region is currently contributing about a tenth of current global sea level rise. A review article recently published in Reviews of Geophysics examined the complex atmospheric and oceanic factors that control the delivery of warm waters to the sub-ice region of West Antarctica and considered the potential for ice loss in the future. The editors asked two of the authors to give an overview of scientific research in this area.
What are marine-based ice sheets and why are they important?
Most of the ice across the East Antarctic rests on rock that is above sea level and is therefore relatively stable. However, the West Antarctic Ice Sheet is a marine-based ice sheet where much of the ice sits on bedrock that is below sea level. This makes it vulnerable to relatively warm waters melting the ice from below, especially in the areas where the ice floats on the ocean as ice shelves. The melting, retreat and thinning of the glaciers around the Amundsen Sea Embayment over recent decades is of great concern because the ice shelves buttress the ice in the interior of West Antarctica and there is a fear that loss of the ice shelves will accelerate the loss of ice in the future.
What controls the loss of ice from West Antarctica?
Delivery of warm water to below the ice shelves of the Amundsen Sea Embayment is strongly influenced by the winds over the Southern Ocean just to the north of the region and therefore the weather systems in this area. Storm activity here is more variable than anywhere else on Earth as a result of being affected by tropical climate variability, such as the El Niño Southern Oscillation, and the Antarctic ozone hole. Melting of the ice is therefore very variable on a year to year basis.
How is data collected about ice, ocean and atmosphere characteristics around Antarctica?
The Amundsen Sea Embayment is a very remote part of the Antarctic but summer field parties have visited this area to make surface measurements and drill down through the ice shelves to make measurements of the ocean conditions below. Satellite pictures have also been extremely important for monitoring the retreat of the ice, and photographs taken from aircraft have provided data since the 1940s. Recently, autonomous vehicles have been used to make measurements of temperature and water mass characteristics under the ice.
How can modeling contribute to a better understanding of these interactions?
Coupled atmosphere–ocean models are the main tool we have for predicting the future evolution of the Earth’s climate system. However, these models don’t currently include ocean currents penetrating under the ice shelves, nor the motion of ice streams. So, at present, a range of high resolution, limited area models of different parts of the system are run separately, such as models of individual ice streams. The eventual goal is to develop coupled atmosphere–ocean–ice modelling systems that can reproduce observed past changes and provide guidance on how the West Antarctic Ice Sheet will change in the future.
What additional data or modeling efforts are needed?
We need more information on the conditions under the ice shelves, especially near the grounding line where the ice starts to float on the ocean. Such data could be obtained from autonomous vehicles and drilling through the ice shelves to install sub-ice instrumentation. We need longer records of broadscale atmospheric and ocean conditions for the period before in-situ observations were made. Data from ice and ocean sediment cores can help in this regard. Modelling the complex interactions between the atmosphere, ocean and ice is currently not possible and we need to advance the modelling of selected elements of the Antarctic coastal environment, such as transport of warm waters under the ice shelves and ocean–ice coupling before we can develop models of the entire system.
What is the future of the West Antarctic Ice Sheet?
This is the biggest and most important question of all. The recent ice loss around the Amundsen Sea Embayment could signal the start of an unstable phase of glacier retreat in the region that will result in the collapse of much of the West Antarctic Ice Sheet, such as has happened a number of times in the past. On the other hand, the current retreat could be short-lived and the system may find a new stable state. Without improved observations of the ongoing retreat and better models of the whole system either scenario is possible.
—John Turner, British Antarctic Survey; email: [email protected]; and Hilmar Gudmundsson, British Antarctic Survey