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Obituary | A polar wonder
A23a had more adventures than most
The world’s biggest iceberg has finally melted away, aged 40 or so
Nothing would remain but water. That is the way of ice. Fresh water poured into salt water, and each absorbed the other. Some observers thought they saw a bluer tinge in the ocean round about. In its dying months, A23a carried parallel blue lines of meltwater on its scarred surface. Each represented a crack that would soon gape wider. It drifted, at about 30km a day, in runnels of its own blood.
Intermittently, as others grew or decayed, it was the biggest iceberg in the world. It took up 3,500-3,900 square kilometres: more than two Greater Londons, Rhode Island or St Petersburg, depending on your vantage point. It weighed, at a brave estimate, about a trillion tonnes. Forty metres of it stood above the surface, around 400 below, where its keel had a habit of dragging on the seabed. It was not a towering, jagged monster, but a table of snow: its great flat whiteness filled the horizon from east to west. Magnificence and majesty could be horizontal, too.
It was not only, on and off, the biggest iceberg, but also the oldest, with the most eventful history. It calved in 1986 from the Filchner Ice Shelf, along with two siblings, A22 and A24. They were born ancient, full of the leavings of the glaciers that had carved out the Antarctic continent. All three, huge as they were, sank at once to the bottom of the Weddell Sea. A23a was stuck so fast that it did not move for more than 30 years. The siblings escaped, went north and died in the tepid South Atlantic; A23a stayed where it was, in safely near-freezing waters. It was not until it began to wobble, melting very slightly, loosening the grip of the bottom-muds, that its further adventures began.
That happened in 2020. Unlike its siblings, it took its time going north-east towards South Georgia, where the shallow continental shelf was a graveyard for its kind. Yet the usual route, Iceberg Alley (used in 1916 by Ernest Shackleton, escaping the sea ice that had crushed Endurance) inexorably led that way, prodding A23a on with fierce west winds and currents. By April 2024 the iceberg had entered the Antarctic Circumpolar Current. But there it got stuck again, seized by a vortex of swirling water, a Taylor column, where the ocean currents smashed up against an underwater mountain. Every day for eight months, anticlockwise, its immense mass was turned through 15 degrees. It escaped more battered than ever, with huge cracks and arches along its edges, like cloisters, and with caverns in which whole ships could disappear. Almost every wave that hit, in those tumultuous seas, brought down crashing cascades of its own crumbling ice.
By then, being the most famous iceberg of its time, it had long been regularly visited and spied on. There was nothing quiet or isolated about its berth in the Southern Ocean. By accident, when it calved it had taken with it Druzhnaya 1, a Soviet summer field station that had sprawled like a city at the edge of the ice-sheet, with 170 people housed in prefab buildings. They were off the site at the time, but their stuff was inside: scientific equipment, food supplies, their skis. The station was abandoned and soon buried deep in snow, but in one spot along A23a’s perimeter rusty fuel tanks still oozed their kerosene down the pure white cliffs.
Later visitors were less obtrusive, but no less curious. While A23a was still in the Weddell Sea, not going anywhere, its daily drift was measured by microwave radiometers. In 2023 the research ship RRS Sir David Attenborough chanced by, not especially meaning to, and took samples of meltwater to test them for nutrients and chemicals. By 2025 the Copernicus Sentinel-3 Ocean and Land Colour Instrument was taking daily images of it, and the Sea and Land Surface Temperature Radiometer was making regular readings, as if it was a hospital patient. In response it vexed researchers with its own home-made weather, cloud and fog. They were not put off. Alerted by reports of its beauty, especially the intense and mortal blue of its melt-pools, more drones flew over, and NASA’s Terra satellite kept a close watch. They confirmed that A23a was still keeping its distinct shape, like a tooth. But it was now only half its size at calving.
What would its dissolution mean? Opinion was divided. On the one hand far smaller icebergs, breaking up near South Georgia, had threatened ships. They had blocked fishermen’s access to their fishing-grounds and the feeding grounds of fur seals and penguins. So much fresh water, spilling out into the sea, reduced the krill and plankton that liked things saltier. A vast mass of ice and rock, grinding to land on the shore, could destroy a host of small creatures: sea slugs, coral, sponges. The whole Antarctic food chain could be disrupted.
On the other hand, disasters predicted in the past had not occurred. The fur seals and penguins had suffered sharp declines, but temporarily. Iceberg deaths were simply part of a polar life-cycle that had been going on for generations. Besides, A23a was so vast, so prone to scrape the sea bed and so packed with long-frozen nutrients, that its break-up would be like a bomb of nourishment exploding in the Southern Ocean. The nutrient release would even be visible, as a green cloud of blooming phytoplankton staining the sea around it. Far from depriving local creatures, this broken iceberg would bring life to them. It might even fertilise the seas of the wider world.
There were mixed feelings, therefore, about its disappearance. Although it had calved long ago, in iceberg terms, it was still an emblem of the warming seas of climate change. It was not the fact of its mortality that worried people, as much as the speed of its breaking-up and the long-term fate of the continent it had belonged to. Tributes were paid to how long it had lasted, against the odds, as if it refused to die. Yet it moved from the Weddell Sea as if its time had come, for death was inevitable. Ice returned to water, water to the ocean. That is the way of ice. ■
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