In 2016, a study found that adding a few new processes to a model of Antarctic ice sheets made them much more vulnerable to melting, greatly increasing global sea level rise – both this century and in the coming centuries. It was an alarming result, to be sure, but also a bit of speculation. The researchers had no way of assessing how realistically the new processes were modeled, so they saw their paper as calling up a question that deserves attention instead of giving an answer.
The new processes were the collapse of ice cliffs above a certain height (a theoretical limitation, but not what we have seen happen) and hydrofracturing. The latter is a propagation of a surface fracture in the ice clean through to the bottom of the ice sheet as the crack fills with water. Hydrofracturing is a well-known commodity – it was probably the dominant process in the sudden collapse of Larsen B’s Antarctic ice sheet in 2002. The question here is instead how vulnerable the rest of Antarctica is to this process?
A new study led by Ching-Yao Lai at Columbia University’s Lamont-Doherty Earth Observatory has attempted to answer this question by mapping fractures and calculating where hydrofracturing should be possible.
On the shelves
The work focuses on ice planks, which are the floating tongues of ice that exist where a glacier slides into the sea. In many places, ice planes attach to the land along the sides of an embayment, providing some anchorage that slows their forward movement. This one ek slows the flow of the rest of the glacier on land – a critical feature called ‘support’. If a supporting ice plank breaks, the glacier will more or less not coagulate, and will accelerate the current in the sea.
To assess the relevance of hydrofracturing for future Antarctic ice loss, the researchers looked for a combination of three things: the presence of fractures, the conditions that make fractures susceptible to hydrofracturing, and the supporting area of ice boards.
First, they want to map fractures. Instead of working thousands of hours away to find and mark all fractures by hand, the researchers turned to a machine-learning technique. After training the algorithm in a small area with hand-marked fractions, the researchers turned it over to satellite images around Antarctica, identifying nearly 32,000 of them.
To determine whether fractures are vulnerable to hydrofracturing, the researchers looked at measurements and models of ice flow and temperature. Hydrofracturing only works if there is enough tension in the ice perpendicular to the fracture. Otherwise, filling the fracture with water is not enough to force that fracture wider and deeper to the bottom. It will just fill with water and sit there.
The vast majority of fractures appear to be in stable conditions where flooding of surface meltwater would not cause hydrofracturing. However, this still leaves a significant number of unstable fractures behind.
Finally, the map of vulnerable fractures is compared to the map of ice planks that support the land ice behind it. And here the stats are not comforting. Roughly 60 percent of the area of ice sheet support around Antarctica should be vulnerable to hydrofracturing, the researchers estimate. And their fracture map shows that some are present in all of these areas.
I’m melting
The recipe for hydrofracturing obviously requires the “hydro” part – meltwater on the ice surface. That’s pretty rare today. Using a recent map of surface meltwater ponds on the East Antarctic ice sheet, which reaches lower (warmer) latitudes, the researchers find that only 0.6 percent of the ice shelf area there are both vulnerable. en currently hosting meltwater. In the few notable areas with significant amounts of meltwater, fractures must be stable.
The problem is that projected warming of air temperatures could lead to much larger amounts of surface meltwater on the ice by the end of this century. And given the apparent vulnerability to hydrofracturing, this could mean more collapsed ice sheets like the Larsen B. There is also much more land ice behind these vulnerable sites, with greater commitment to sea level rise.
To date, the biggest problem for Antarctic ice warmer seawater is eating the ice shelves from below. This study points to a spherical attack method that needs to be studied in detail to better anticipate Antarctica’s response to climate change.
Nature, 2020. DOI: 10.1038 / s41586-020-2627-8 (About DOIs).
