Towering glacial ice cliffs may collapse in fits and starts

The world’s thickest glaciers may “calve” icebergs up to three times faster than previously predicted. However, such rapid retreat may not be sustained over long periods.

Instead, the large bodies of ice – which include Antarctica’s Thwaites glacier – may collapse in bursts, with intervening periods of stability.

The behaviour of tall ice cliffs is one of the biggest sources of uncertainty in predicting future sea level rise. Several of the world’s largest glaciers in Greenland and Antarctica extend out to sea, with their bases resting on the sea floor. Their coastal edges can form ice cliffs looming as high as 100 metres above the ocean’s surface.

For years, researchers have feared that such marine-based glaciers might be vulnerable to runaway cliff collapse. The idea here is that each time an ice cliff collapses, it exposes a new, taller – and so potentially even less stable – ice cliff further towards the glacier’s interior. The loss of ice due to runaway collapse of these ever-taller cliffs could potentially double the amount of sea level rise expected over the next 100 years, according to an Intergovernmental Panel on Climate Change report. But we don’t yet have enough empirical data to know how likely it is that runaway cliff collapse will occur, so previous studies have relied on ideal mathematical models to envision the process instead.

Caroline Needell at the Massachusetts Institute of Technology-Woods Hole Oceanographic Institute Joint Program and her colleagues used satellite data to provide the first direct, large-scale observations of how the tallest ice cliffs actually behave, offering a firmer basis for estimating the danger of runaway collapse – and the rapid sea level rise that could result.

According to the study, the risk of runaway collapse depends on two main factors: how steeply the sea floor beneath the glacial ice deepens, and how quickly the ice flows seawards. Imagine an up-ended jar of thick honey spilling onto a plate: it won’t remain in a tall, stable shape, but will thin outwards at the edges. In a similar way, if the viscous flow of ice is fast enough, it will thin out the edge of a glacier, reducing cliff height. Even if the glacier is thicker further towards its interior, sufficiently rapid ice flow appears able to compensate, lowering those ice cliffs and preventing a runaway effect.

This idea has been suggested by idealised mathematical models, but had never been systematically measured in real glaciers. “None of the glaciers we observed underwent unstable retreat, even though they did retreat much faster than our current models predict,” says Needell.

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That retreat could speed up even further in the future. Needell notes that most glaciers in the study are “buttressed” by sea-floor features. For instance, a series of underwater ridges currently help stabilise Thwaites from beneath.

“Over the next decades, it’s going to jump from one pinning ridge to another,” almost like “tripping” downhill, says Douglas Benn at the University of St Andrews in the UK. Eventually, Thwaites will reach a point where those ridges disappear. “The bed then begins to get real deep,” says Benn. At that point, which the glacier may reach within the current century, “the whole structural integrity of the ice becomes unviable”.

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That is when runaway collapse becomes a real possibility. However, those conditions may also speed up the flow of ice, thus thinning out any looming cliffs. The key question, says Needell, is whether thinning can outcompete rapid ice loss, restabilising the glacier. Her study suggests that it might – but the question remains open.

Robert Larter at the British Antarctic Survey points out that we haven’t yet seen this kind of runaway cliff collapse. One study suggests that it may have happened in the vicinity of Thwaites around 12,000 years ago. “The problem is, nobody was around to take observations at the time,” he says. “It’s a possibility we have to keep our eye on.”