The ice meets the sea.
Matthias Braun, University of Erlangen-Nuremberg, Germany
Even with decades of melting, much of the world’s water is trapped in ice that sits on land. If the ice in Antarctica melted completely, it is estimated that the sea level would rise by about 60 meters – an almost incomprehensible figure.
But much of it wouldn’t reach the ocean by melting. Instead, large parts of the Antarctic ice sheet sit on rocks that lie below sea level. If the ocean reached these plates, the ice would break up and float away as it melts, a process that could cause sea levels to rise relatively suddenly. Now researchers have created a catalog of all the ice that drains into the ocean in Antarctica, allowing us to identify those that pose the greatest threat to rapid sea level rise.
You can think of Antarctica as four types of ice. Inland there are large ice caps, some above sea level, others below. Some of the ice in these plates flows to the coast through outgoing glaciers, which often pass through narrow valleys on their way to the sea. On the coast you will find the third type: permanently floating ice shelves, which can extend for miles into the ocean. In addition, you will find seasonal ice, which expands in the southern winter, but contracts again when summer arrives.
The ice shelves play a key role in the dynamics of Antarctic ice because they resist material flow through the glaciers leaving. The ice has to push against the shelves to flow, which supports the ice sheets and prevents them from dumping their contents into the ocean.
For example, the stability of the ice shelves helps control the dynamics of the Antarctic ice, which is rather unfortunate given that a number of them have experienced some rather dramatic ruptures in recent decades. And consistent with our understanding, the breakup was generally accompanied by an acceleration of the glaciers flowing into this part of the ocean.
How concerned should we be about these collapses, and are there other parts of Antarctica that we should be nervously watching? To answer this question, the authors built a model of ice flow that incorporates all the data we have about the structure and stresses on Antarctic ice shelves. With the ice in place, they then modeled the decay of the ice shelves by breaking off icebergs from the edges and determining whether the missing ice allowed for an acceleration of the outgoing glaciers.
Where possible, they compared their model results with recent cases where we have ice shelf collapse data to confirm that everything worked as expected.
The results suggest that there are some major differences between the ice shelves. The Brunt/Stancomb-Wills Ice Shelf could lose more than 35 percent of its material without significantly affecting glacial flow; the Shackleton Ice Shelf could afford to lose more than 25 percent. Others, like the Dotson Ice Shelf, have almost nothing to give up before the ice begins to accelerate into the ocean.
More generally, the area that flows into the Indian Ocean and along the Queen Maud Land is relatively stable. In contrast, those that flow into the Amundsen and Bellingshausen Seas are on the verge of instability.
This is good news for the model, as the latter are some of the areas that other researchers, looking at real-world data, have shown are already destabilized. It’s bad news in that the glaciers in that area are leading to ice caps lying on land below sea level. An invasion of that area by seawater could lead to a relatively rapid rise in ocean levels of more than two metres.
Nature climate change2015. DOI: 10.1038/NCLIMATE2912 (About DOIs).