The Antarctic ice sheet contains enough ice that, if melted, could raise the global sea levels by tens of metres. A new study published in the journal Nature shows that floating ice walls offer some protection to the ice sheet. Floating ice walls do this by deflecting warm ocean currents that would otherwise penetrate cavities beneath the floating portions of the ice sheet.
Warming seas and sea level rise
A research team led by the University of Gothenburg, Sweden is exploring the physics behind the warm ocean currents that surround the Antarctic coast.
Our Centre for Southern Hemisphere Oceans Research researcher Laura Herraiz-Borreguero is a co-author of the study.
“The Antarctic ice sheet reaches the ocean through ice shelves, which are the floating edges of the ice sheet,” Laura said.
“Just like a dam wall, these ice shelves slow down the rate at which grounded ice is discharged to the ocean, where it melts and contributes to sea level rise.
We’ve witnessed ice shelves rapidly thinning in areas where warm ocean water on the continental shelf accelerates the movement of grounded ice from the ice sheet to the ocean.”
Improving our understanding of the stability of the Antarctic ice sheet – and the processes that could slow or speed its rate of melt – is of huge importance when it comes to understanding sea level rise.
“The fate of the Antarctic ice sheet is the greatest remaining uncertainty when predicting future sea levels,” Laura said.
Warm ocean water encroaching on the continental shelf is rapidly thinning the ice shelves. The removal of this ice allows the flow of the continental ice sheet towards the ocean to accelerate.
“We do know that one important control on ice loss from Antarctica is what happens where the ice sheet meets the ocean. It is here where a large amount of ice melts.”
Ice adds to the ice shelf by glaciers flowing off the continent and by snowfall that compresses to form ice. Ice is lost when icebergs break off the ice front. This also occurs from melting, as warm water flows into the ocean cavity under the ice shelf. Under some ice shelves, cold and fresh meltwater rises. It does so till a point where it refreezes onto the ice shelf. Helen Amanda Fricker, Professor, Scripps Institution of Oceanography, UC San Diego. Shrinking of Antarctic ice shelves is accelerating, The Conversation, March 27, 2015.
Risk to the Getz glacier
The question of how warm ocean currents make their way to the ice sheet has long been unanswered. Recent studies in front of the Getz glacier have revealed more than we’ve ever known before.
The data comes from instruments Laura and her colleagues placed in the ocean in front of Getz glacier ice shelf, located in West Antarctica. The Getz glacier culminates in a vertical edge. Essentially, it is a floating wall of ice that continues 300-400 metres down into the ocean. Warm ocean currents flow beneath this edge, towards the deeper ice.
“West Antarctica holds up to 3.5 metres of sea level equivalent (that is, if the ice was to melt). Moreover, this region is also losing grounded ice very rapidly,” Laura said.
“We found that the floating ice edge is blocking warm ocean currents. This limits the extent to which the warm ocean can reach the grounded ice.
“Now we understand that only about one-third of the thermal energy that travels up towards the Antarctic from the deep Southern Ocean reaches the Antarctic ice sheet.”
Getz ice shelf front. Photo provided by Johan Rolandsson.
Impact on future research
The results of the study have provided researchers with a greater understanding of how these glacier areas work.
“Our work highlights the importance of floating ice shelves. And, specifically, these ice fronts as key areas that should be closely monitored,” Laura said.
“If the ice front walls thin and disappear, a much greater portion of ocean heat would be delivered towards the grounded Antarctic ice.”
These recent findings could enable significantly better models to be able to predict future sea level rise.
“Now we know how the current flows beneath the ice shelf, we can start to explore some of the other underlying drivers for ice shelf melt. For example, the shape of the ice shelf fronts controls how much warm water is able to pass the ice wall,” Laura said.
“We can apply our results to more accurately predict how ice shelves will change into future.”
Read the full paper in Nature.