Will an iceberg twice the size of NYC break off Antarctica this year?

Originally published January 20, 2021, by NASA Earth Observatory.

Two years after Antarctica’s Brunt Ice Shelf seemed poised to produce a berg twice the size of New York City, the ice is still hanging on. But the calving of one, maybe two, large icebergs is inevitable. The question is: when? Ice scientists are watching to see if a rapidly accelerating crack will cause the shelf to rip apart before the sunlit summer season ends.

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The breaking, or “calving,” of icebergs from ice shelves is part of a natural, cyclical process of growth and decay at the limits of Earth’s ice sheets. As glacial ice flows from land and spreads out over the sea, shelf areas farthest from shore grow thinner. These areas are stretched thin, and can be melted from above or below, making them more prone to forming rifts and eventually breaking away. The Brunt Ice Shelf appears to be in a period of instability, with cracks spreading across its surface.

The major rifts are visible in the wide view at the top of this page. In late October 2016, the “Halloween crack” appeared and rapidly extended eastward. In early 2019, Chasm 1 extended northward as fast as 2.5 miles (4 km) per year. Now, a new crack is zippering across the shelf north of the Halloween crack, far faster than the fissure to its south.

The rift shows up in satellite images as early as September 2019, when it had grown just over 1.25 miles (2 km) longer during the austral winter. But the biggest growth just occurred recently. Between November 18 and December 22, 2020, the rift grew in length by about 12 miles (20 km). Then it jogged toward the north and grew an additional 5 miles (8 km) by January 12, 2021.

Christopher Shuman is a University of Maryland glaciologist based at NASA’s Goddard Space Flight Center. He said:

It is impossible to know exactly what caused this new rift to extend so quickly. It’s likely that fracture dynamics near the McDonald Ice Rumples played a role, as they did in the quick propagation of the ‘Halloween Crack’ in 2016. The unusual mix of ice blocks and mélange in this part of the Brunt Ice Shelf ‘system’ is another factor.

The rumples are the result of ice that flows over an underwater formation, where the bedrock rises high enough to reach into the underside of the floating ice shelf. This rocky formation impedes the flow of ice and causes pressure waves, crevasses, and rifts to form at the surface.

All of these cracks, combined with a recent speed up at the leading edge of the ice shelf, point to an instability that is likely to spawn a new iceberg or two. The exact timing is uncertain, but until the break occurs and the shelf has been re-formed, Halley Research Station is being kept minimally staffed for safety reasons. In 2016-2017, the Halley VI station was relocated to a safer location (Halley VIa) upstream of the then-growing Chasm 1. Shuman said:

I think we are going to see big changes here. And with more than two months left of sunlight, changes should be visible in natural-color satellite images for a while longer before the onset of winter darkness.

Bottom line: Will a huge iceberg crack off Antarctica’s Brunt Ice Shelf in 2021?