In 2016, a study found that adding a few new processes to a model of the Antarctic ice sheets made them much more vulnerable to melting, dramatically increasing global sea level rise — both this century and centuries to come. It was certainly an alarming result, but also a bit of guesswork. The researchers had no way of assessing how realistically the new processes had been modeled, so they viewed their paper as a question worthy of attention rather than providing an answer.
The new processes were the collapse of ice cliffs above a certain height (a theoretical limitation, but not something we’ve seen happening) and hydraulic fracturing. The latter is a propagation of a surface fracture in the ice that clears through to the bottom of the ice sheet as the crack fills with water. Hydraulic fracturing is a well-known good — it was probably the dominant process in the sudden collapse of Antarctica’s Larsen B Ice Shelf in 2002. The question here, though, is how vulnerable is the rest of Antarctica to this process?
A new study led by Ching-Yao Lai of Columbia University’s Lamont-Doherty Earth Observatory has attempted to answer that question by mapping fractures and calculating where hydraulic fracturing should be possible.
on the shelf
The work focuses on ice shelves, the floating tongues of ice that exist where a glacier slides into the sea. In many places, ice shelves cling to land along the sides of an embayment, providing some anchorage that slows their forward motion. This one also slows the flow of the rest of the glacier onto land — a critical feature called “buttresses.” If a supporting ice shelf breaks, the glacier is more or less uncorked and the flow to the sea will accelerate.
So to assess the relevance of hydraulic fracturing to future Antarctic ice loss, the researchers looked for a combination of three things: the presence of fractures, the conditions that predispose fractures to hydraulic fracturing, and the support area of ice shelves.
First they wanted to map fractions. Rather than toil for thousands of hours to find and mark all the fractures by hand, the researchers turned to a machine learning technique. After training the algorithm in a small area of hand-marked fractures, the researchers ran it on satellite images around Antarctica and identified nearly 32,000 of them.
To find out whether fractures are vulnerable to hydraulic fracturing, the researchers turned to measurements and models of ice flow and temperature. Hydraulic fracturing only works if there is enough stress in the ice perpendicular to the fracture. Otherwise, filling the fracture with water will not be sufficient to force the fracture wider and deeper to the bottom. It will just fill with water and sit there.
The vast majority of the fractures appear to be in stable conditions where flooding from the surface meltwater would not cause hydraulic fracturing. However, that still leaves a significant number of unstable fractures.
Finally, the map of vulnerable faults is compared with the map of ice shelves supporting the underlying land ice. And here the statistics are not reassuring. About 60 percent of the area of supporting ice shelves around Antarctica should be vulnerable to hydraulic fracturing, the researchers estimate. And their fraction map shows that some are present in all of these areas.
The hydraulic fracturing recipe, of course, calls for the “hydro” part — melt water on the ice surface. That is quite rare these days. 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 there is vulnerable. and which currently houses meltwater. In the few notable areas with significant amounts of meltwater, fractures should be stable.
The problem is that the expected warming of the air by the end of this century could lead to much larger amounts of meltwater at the surface on the ice. And given its apparent vulnerability to hydraulic fracturing, that could mean more collapsed ice shelves like the Larsen B. There is also much more land ice behind these vulnerable sites, with greater stakes for sea level rise.
Until now, the biggest problem for the Antarctic ice has been that the warmer seawater is eating away at the ice shelves from below. This study highlights an atmospheric 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).