On 26 February 2021 a slab of ice of about 1,270 square kilometres, close to the size of Greater London, broke away from the Brunt Ice Shelf on the Weddell Sea coast of Antarctica. The new iceberg, named A-74, measured roughly 56 by 33 kilometres. Scientists from the British Antarctic Survey (BAS), whose Halley VI research station stands on the same ice shelf, had been expecting it for years, and the break posed no danger to the station or its staff.
Here is how the calving was detected, why it happened, why scientists say it was not a direct sign of climate change, what became of the iceberg, and how it differs from the even larger Brunt breakaway of January 2023.
Where and how the iceberg broke away
The Brunt Ice Shelf is a floating extension of the Antarctic ice sheet along the coast of Coats Land, on the eastern side of the Weddell Sea. Ice flows off the continent, spreads out over the ocean and, from time to time, pieces at the outer edge break off. That process is called calving.
A-74 separated from the northern part of the shelf. Cracks had opened near an area called the McDonald Ice Rumples in September 2019, where the shelf presses against a submerged rise in the seabed, and they spread quickly through the southern summer of 2020–21. On the morning of 26 February 2021, GPS instruments that BAS keeps on the ice suddenly moved about 100 metres within an hour, compared with their usual drift of around 8 metres a day. The next day the US National Ice Center confirmed the break in Sentinel-1 satellite images and gave the iceberg its official name.
It was the largest iceberg to calve from that part of the Brunt shelf since 1971.
Was it caused by climate change?
No, according to the scientists who monitor the shelf. Calving is how floating ice shelves naturally shed mass, and the Brunt had been growing seaward for decades before it broke. Glaciologist Adrian Luckman of Swansea University told the BBC that breakaways are an entirely normal part of how they work
, while noting that events this large are rare.
That distinction matters, because a dramatic photo of a giant iceberg is often shared as instant proof of global warming. Some Antarctic ice shelves, especially on the Antarctic Peninsula, have thinned and collapsed in ways that are linked to warming air and ocean water. The Brunt is different: its behaviour is driven by its own structure, the way it is pinned to the seabed and the cracks that grow across it. The eastern Weddell Sea region where it sits has also been less affected by warming than the western side.
Understanding why some cracks lead to calving and others do not is one of the reasons the Brunt is studied so closely. It helps scientists predict how ice shelves elsewhere, including those that are changing because of warming, are likely to behave.
How scientists watched it coming
BAS had already moved its Halley VI station to a safer site further inland in 2016–17, after a long-dormant crack known as Chasm-1 reawakened in 2012 and a new one, nicknamed the Halloween Crack, was spotted in 2016. The station is built on hydraulic legs and skis precisely so that it can be relocated, and since the move it has mostly been staffed only during the southern summer.
To keep track of the ice through the dark winter months, BAS relies on a network of high-precision GPS units around the station, along with satellite radar from the European Space Agency, NASA and Germany's TerraSAR-X. The data are sent back to Cambridge daily. Jane Francis, then director of BAS, said the teams had been prepared for the calving of an iceberg from the Brunt Ice Shelf for years
.
When A-74 broke away, the station lay roughly 20 kilometres from the new edge of the ice, and BAS said it was not at risk.
What happened to A-74
For a while A-74 stayed close to the shelf, and scientists worried that if it bumped against the remaining ice it could trigger further breaks along Chasm-1. Instead it drifted out. In March 2021 the German research icebreaker Polarstern sailed all the way around it, at one point through a channel less than 500 metres wide.
The crew took the chance to photograph the seabed that had been hidden under ice for about 50 years. In more than 1,000 images and hours of video they recorded filter-feeding animals, sea cucumbers, brittle stars, molluscs, two kinds of octopus and five species of fish, evidence that ocean currents carry enough food to support life even tens of kilometres from open water.
By June 2022 the iceberg had drifted about 170 kilometres west and split into two pieces, A-74A and A-74B, which were expected to follow the slow circular current of the Weddell Gyre.
Not to be confused with the 2023 iceberg
Two years later the Brunt calved again, and the two events are often mixed up. On 22 January 2023, during a spring tide, Chasm-1 finally cut all the way across the shelf and released an iceberg of about 1,550 square kilometres, around 150 metres thick. That berg, expected to be named A-81, came from the western part of the shelf, closer to Halley VI, which was again unaffected.
For scale, both Brunt icebergs were far smaller than A-68, which broke from the Larsen C Ice Shelf on the Antarctic Peninsula in July 2017 and covered about 5,800 square kilometres.
Why icebergs like A-74 are worth following
Large icebergs matter long after they break away. They are tracked because they can affect shipping lanes and research vessels, they can ground on the seabed and disrupt local ecosystems or penguin colonies, and as they melt they release fresh water and nutrients into the ocean.
For anyone curious to follow the next one, the US National Ice Center publishes positions of named Antarctic icebergs, and ESA and NASA regularly release satellite images of major calving events. The Brunt shelf, with its cracks still under watch, is likely to produce more news in the years ahead.
Sources: Quotes as reported by BBC News. Wikipedia, Iceberg A-74; Brunt Ice Shelf; ESA: Giant iceberg breaks away from Antarctic ice shelf.
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