A fast-growing crack in Antarctica’s Larsen C ice shelf has captured public attention, a response that puzzles some NASA sea level scientists.
While the break in the ice has yielded dramatic images, it’s not especially unusual or, as yet, an indication of significant change, though it might portend eventual trouble for the ice shelf’s stability.
“It’s not necessarily a sign of enhanced change or melting or anything like that,” said Helen Fricker, a geophysics professor at the Scripps Institution of Oceanography and a member of the NASA Sea Level Change Team. “Rifts propagate and icebergs calve off ice shelves. This is a process by which ice shelves lose the mass they have gained through snowfall. It’s not a big deal.”
But even as headlines warned of ice-shelf collapse, several scientists on NASA’s sea level team pondered the ice rift’s larger meaning – the effect of such rapid, hard-to-predict events on our understanding of ice sheet dynamics, their relationship to flowing glaciers, and their role in a world of dwindling ice and rising seas.
“We have to look at the big picture of the evolution of Larsen C,” said glaciologist Eric Rignot, a professor at the University of California, Irvine, a senior scientist at NASA’s Jet Propulsion Laboratory and also a member of the sea level team. “To be modeling these cracks is extremely challenging. It’s a little bit like being able to predict the next earthquake.”
Modeling the fractures
Ice sheet modelers, who devise computer simulations of real world effects, traditionally have set aside “fracture mechanics” – the short-term breakage patterns in ice sheets, cliffs and floating shelves. Instead, they simplify the models so that mechanical fractures average out over time, with little influence on long-term projections of large-scale behavior.
But to understand the nuances – just where, when and how an ice shelf might disintegrate – some kind of fracture modeling will be needed.
“To try to do something realistic, we’re having to come up with a calving law or a set of calving laws,” Rignot said. “We have not yet figured that out with sufficient detail.”
Press coverage of the Larsen C crack increased at the start of 2017, when British researchers revealed that a jagged split in the ice shelf, on the northeastern Antarctic Peninsula, grew by 27 kilometers (17 miles) in only two months’ time.
For some, it was a powerful reminder of the disintegration of the Larsen A and B shelves, also on the Antarctic Peninsula but to the north of Larsen C.
Larsen B vanished almost entirely in 2002 after a catastrophic breakup in a little over a month, losing 3,250 square kilometers (1,250 square miles) of ice. The breakup of Larsen A’s northern section in 1995 also was rapid but less dramatic, pulverizing some 1,500 square kilometers (579 square miles) of ice.
They joined several other Antarctic breakups documented over decades.
Determining whether the latest Larsen C crack is a prelude to another collapse, perhaps driven by a warming climate, will have to await more data and observations.
Ice shelf demise, or business as usual?
The crack could be the start of a period of sustained retreat, similar to what happened to Larsen B, said Ala Khazendar, a JPL scientist who has investigated both Larsen B and Larsen C. An increasingly weakened ice shelf allows glaciers to speed their flow into the ocean, and the shelf, unable to recover its former bulk and solidity, disintegrates.
Or, this could turn out to be a normal calving episode.
“We have no way yet of knowing whether Larsen C is doing what Larsen C has been doing for thousands of years, or whether we are witnessing the beginning of the end of Larsen C,” he said.
Larsen C probably has a long way to go before experiencing a catastrophic collapse, Rignot said. Calving off an iceberg, even a sizeable one, would not mean disintegration of the entire ice shelf is imminent.
“Icebergs detach every once in a while,” he said. “It doesn’t mean it’s climate change.”
A large iceberg’s separation from the shelf would have little or no discernible effect on sea levels. The ice of the shelf is already afloat on the ocean, so setting it adrift would not change ocean mass or volume.
Further retreat, however, would likely lead one day to total collapse of the ice sheet.
“It’s not going to melt away,” Rignot said. “It’s going to fracture. It’s going to reach a limit beyond which it is not stable. It’s going to fall apart like an eggshell that became too thin.”
"The demise of Larsen B and the evolution of Larsen C are large scale natural experiments, taking place under the gaze of our airborne and spaceborne instruments, that are giving us valuable insights into the behavior of ice shelves"
If the retreating shelf eventually pushes past a point called the “compressive arch,” the structural components that hold it together would be undermined.
“A crack upstream of that compressive arch is usually fatal for that ice shelf,” said Eric Larour, a JPL cryosphere scientist, project manager for the NASA Ice Sheet System Model (ISSM), and a member of the sea level change team.
“It will blow up,” he said – that is, fragment into many separate chunks of ice.
Yet even if the whole ice shelf were to break up, Fricker said, the resulting sea level rise would be minimal. The glaciers held back by the shelf are not so imposing.
“The Larsen C ice shelf only holds back about one centimeter of global sea level rise,” she said.
Still, the crack in Larsen C could be a bellwether for ice shelves elsewhere on the continent, Rignot said.
“What we are seeing on Larsen C has implications for the big ice shelves farther south that hold considerable (sea level) potential,” he said. The loss of these larger ice shelves and the resulting acceleration of glacial calving could amount to meters of sea level rise in the decades and centuries to come.
For now, the most immediate scientific significance of ice sheet cracking might lie in its value to modelers.
Like cracks in concrete
Larour described one successful approach used in the ISSM ice sheet model, developed by Chris Borstad from the University of Svalbard. Called a damage mechanics formulation, it assigns fractional values between zero and one. Pristine, uncracked ice has a damage of zero; fully compromised ice shelves so riddled with cracks they are no longer structurally coherent earn a damage value of one.
Applying a range of values across simulated ice shelves can allow models to predict some real-world behavior.
“It’s quite a powerful approach, borne out of the experience of crack propagation in materials such as snow or concrete,” Larour said.
Watching the development of cracks like Larsen C’s could help refine such models so they better reproduce real-world effects.
“This big event could sort of help validate their model(s) if they see the same effects for the ice shelf,” said Catherine Walker, a postdoctoral researcher at JPL who monitors changes in ice sheets and glaciers by comparing satellite images taken over decades.
Fricker also thinks modeling accuracy will improve as more observations accumulate.
“We haven’t observed that many ice-shelf disintegration events,” Fricker said. “The iceberg calving cycle is typically several decades, so we haven’t really got that much data to show how ice shelves calve and how ice shelves disintegrate. We are definitely making progress toward understanding calving well enough to put it into models, but I wouldn’t say we’re completely there yet.”