Even with decades of melting, much of the world's water lies trapped in ice that sits on land. If Antarctic ice melted entirely, it's estimated that ocean levels would rise by roughly 60 meters—a nearly incomprehensible figure.

But a lot of it wouldn't reach the ocean by melting. Instead, large areas of the Antarctic ice sheet sit on rock that's below sea level. Were the ocean to reach these sheets, the ice would break up and float off while melting, a process that could raise sea levels relatively suddenly. Now, researchers have performed a catalog of all of the ice that empties into the ocean in Antarctica, allowing us to identify those that pose the largest threat of rapid sea-level rise.

You can view Antarctica as having four types of ice. Inland, there are large ice sheets, some of which sit above sea level, others below. Some of the ice in these sheets flows to the coast through exit glaciers, which often pass through narrow valleys on their way to the sea. At the coast, you'll find the third type: permanent floating ice shelves, which can extend for miles into the ocean. Beyond those, 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 provide resistance to the flow of material through the exit glaciers. The ice has to push against the shelves to flow, which buttresses the ice sheets, keeping them from dumping their contents into the ocean.

Thus, the stability of the ice shelves helps control the dynamics of Antarctic ice, which is rather unfortunate, since a number of them have experienced some rather dramatic breakups over the last few decades. And in keeping with our understanding, the disintegration was generally accompanied by an acceleration of the glaciers that feed into this area of the ocean.

How worried about these collapses should we be, and are there other areas of Antarctica we should be watching nervously? To answer this question, the authors built a model of ice flow that incorporated all the data we have on the structure and strains 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 an acceleration of the exit glaciers.

Where possible, they compared their model results to recent cases where we have data from ice shelf collapses to confirm that everything was working as expected.

The results suggest that there are some stark differences among the ice shelves. The Brunt/Stancomb-Wills Ice Shelf could lose more than 35 percent of its material without significantly affecting the flow of glaciers; the Shackleton Ice Shelf could afford to lose more than 25 percent. Others, like the Dotson Ice Shelf, have nearly nothing to give up before ice starts flowing faster into the ocean.

More generally, the area emptying into the Indian Ocean and along the Queen Maud Land are relatively stable. By contrast, the ones emptying into the Amundsen and Bellingshausen Seas are at the edge of instability.

This is good news for the model, since the latter are some of the areas that other researchers, looking at real-world data, have shown to already be destabilized. It's bad news in the sense that the glaciers in that area lead to ice sheets that sit on land below sea level. Invasion of that area by sea water could potentially lead to a relatively rapid rise in ocean levels of more than two meters.

Nature Climate Change, 2015. DOI: 10.1038/NCLIMATE2912 (About DOIs).