A languishing ice cream blend turns out to be the culprit. And with that, Antarctic ice shelves are a threat richer.

In July 2017, the Larsen C ice shelf gave birth to an iceberg about the size of the province of Gelderland. This iceberg – which went down in the books as the largest iceberg we’ve ever seen – was named A68 and floated around Larsen C for about two years before taking to the sea. At the beginning of 2019, the iceberg finally let itself be carried away by the current. In addition, more and more pieces of ice broke off and in the spring of 2021 the iceberg completely disintegrated and A68 was officially a thing of the past.

Creation

And now, six months after the demise of the A68, the iceberg manages to attract attention again. Scientists have discovered that it came about in a completely different way than was thought in recent years. The culprit is not the melting of the Larsen C ice shelf, but the languishing of the ice blend—a mixture of windblown snow, chunks of ice and frozen seawater.

New Threat

And with that, researchers have found another way in which ice shelves can go down in ignorance. “The thinning of the ice mix that glues large areas of floating ice shelves together is another way climate change could cause Antarctic ice shelves to shrink rapidly,” said researcher Erik Rignot. “With that in mind, we may need to revise our estimates of when and to what extent sea levels will rise from polar ice loss. It can happen much sooner and faster than expected.”

Larsen C Ice Shelf

Back to the Larsen C ice shelf, where huge cracks had formed long before A68 was born. In July 2017, some of those fissures came together and the 6,000 square kilometer iceberg A68 was born. The fact that many large icebergs have formed in Antarctica in recent years is generally attributed to the warming of the air and water caused by anthropogenic climate change. That warmer air attacks the ice shelf from above, creating meltwater lakes. The meltwater seeps down through small cracks in the ice shelf. When it gets colder and this water freezes again, it expands and the cracks also get bigger. At the same time, the water on which the ice shelf rests is also getting warmer. And that warm water attacks the ice shelf from below, making it thinner. “A lot of people intuitively think that if you thin the ice shelf, it becomes much more fragile and it breaks,” said researcher Eric Larour. But in the case of A68, something else is going on, say Larour and colleagues. It was not the thinning of the ice shelf, but the thinning of the ice mix that gave birth to A68.

Research

The scientists based this conclusion on an extensive study in which they first looked at satellite images of Larsen C. Those images reveal hundreds of ice-blend-filled trenches in the ice shelf. Some were large, others small. Some ran right through the ice shelf. Others were a bit more superficial. The researchers selected eleven that ran straight through the ice shelf and could therefore really grow into cracks. They then modeled how these trenches fare when the ice shelf thinned, when the ice blend thinned, or when both the ice shelf and ice blend thinned.

Different scenarios

The research indicates that a scenario in which the ice shelf thins, but the ice blend remains unaffected, does not directly promote crack formation. The bravely enduring ice blend still manages to repair the trenches and prevent them from cracking open. And also in a scenario in which both the ice shelf and the ice mix are thinning, the ice mix still manages to slow down the widening of the channels. But when the researchers simulated a situation where only the ice blend thinned, there was no stopping it; the channels were rapidly widening and cracking was imminent. “The ice blend is thinner than the ice right from the start,” explains Larour. “If the blend is only 10 to 15 meters thick, it’s actually just like water. The gullies then become wider and begin to rupture.”

Heart of winter

The idea that the languishing ice blend is behind the birth of A68 is further supported by the fact that A68 was born in the middle of winter. “The prevailing theory of the increase in calving large icebergs around the Antarctic Peninsula is hydraulic fracturing, where meltwater seeps from the surface into cracks in the ice shelf,” Rignot said. “When the water freezes again, it expands and the cracks also get bigger. But that theory cannot explain how iceberg A68 in the middle of winter – when there were no meltwater lakes – could break free from the Larsen C ice shelf.” The languishing ice blend may help explain that. Because when the channels run straight through the iceberg and thus reach the water below, warm ocean water can also reach the ice mix in these channels in winter and affect and promote cracking. “We’ve finally started to look for an explanation for the fact that ice shelves are already shrinking and already – decades before hydraulic fracturing can affect them – are facing conditions that lead to instability,” said Rignot.

The disintegration of ice shelves does not directly lead to sea level rise, because the ice that makes up them already rests on the water. But indirectly, the disintegration of ice shelves can lead to sea level rise. The enormous masses of ice resting off the coast of Antarctica hold back the land-based glaciers behind it. As the ice masses shrink, they are less able to counterbalance the glaciers, allowing them to flow faster and deposit more ice in the sea. That is also the reason that researchers are keeping a close eye on the ice shelves – and in particular the Larsen C ice shelf, which lost 10 percent of its surface with A68 in July 2017 with the A68.