After eight months trapped in a whirlpool in the Southern Ocean, the world’s largest iceberg is on the move again – and potentially on a path to hit the island of South Georgia. The island, located between Antarctica and Argentina, is a haven for vulnerable populations of penguins, seals and seabirds.
The iceberg in question is A-23A, sometimes called A23a. It is the world’s oldest and largest – about the size of Rhode Island – and weighs nearly 1 trillion tonnes. As it moves along, the biggest concern, scientists say, is whether the iceberg or its bits could block access to parts of the island for wildlife or ships. Previous iceberg collisions have affected food foraging on the island.
The ice slab is meandering and moving parallel to South Georgia Island, oceanographer Andrew Meijers said. Currents are expected to loop A-23A back to South Georgia’s continental shelf in about two weeks.
“If it does ground on the continental shelf … this could pose a significant problem for seals and [penguins] presently feeding their chicks and pups,” said Meijers, a researcher at the British Antarctic Survey.
On a collision course with South Georgia Island
The iceberg is drifting in a current at about 1 kilometre per hour (about 0.62 miles per hour) toward South Georgia. Although its arrival could occur in weeks, Meijers said, “it is hard to be precise as wind and wave action can also be important and eddies in the current are very hard to predict.”
“The iceberg might take a turn around South Georgia like many before it” or smaller pieces broken off from the iceberg could run aground, said Jan Lieser, marine glaciologist at the Australian Antarctic Program Partnership.
South Georgia Island is a difficult place for humans to live, but it is a haven for wildlife. Its productive waters support abundant populations of krill, which serve as food for many marine predators like fur seals. In fact, more than 3.5 million fur seals live on the island, which is about 95 to 98 percent of the world’s population of the animal. Deep bays also provide homes for millions of penguins and seabirds – including the wandering albatross, which is threatened.
In 2004, an iceberg grounded on the north side of South Georgia and was associated with more seal pup and penguin chick mortalities, Meijers said. When the iceberg grounded, it likely made foraging more difficult.
“The adults must swim further to go around the berg and thus expend more energy and bring back less,” Meijers said.
In 2021, another iceberg about the size of Delaware appeared on a collision course with South Georgia but then fractured into smaller pieces around the island.
The British Antarctic Survey expects A-23A to encounter warmer water and break up into smaller icebergs and, eventually, melt.
If iceberg A-23A breaks up before hitting the island, the impact will be lessened because smaller chunks are easier for wildlife to go around. Fishing vessels, though, might have a harder time navigating around thousands of smaller chunks, Meijers said.
One chunk – measuring 80 square kilometres – broke off A-23A in late January. Leiser said the piece is large enough to be given identifying numbers and letters by the U.S. National Ice Center.
Now, A-23A is sitting in “a bit of storm” with sustained winds around 70 kilometres per hour (about 44 miles per hour) and waves as high as 6 meters (about 6½ feet), Meijers said. This storm may speed up the break up of the iceberg.
A journey down iceberg alley
The iceberg’s path isn’t unusual, nor is it the first to venture to South Georgia Island.
Iceberg A-23A broke off the Filchner-Ronne Ice Shelf to the northeast of the West Antarctic Ice Sheet in 1986. It then spent decades stuck to the seafloor. In the early 2020s, the iceberg began to loosen and by March 2023, it floated freely. One year later, as it drifted north, it became trapped in a gyre – and it would remain in the rotating system of ocean currents until it escaped several months later.
The mass mostly took the same path as other large icebergs exiting the Weddell Sea, heading down “iceberg alley,” Lieser said.
Iceberg alley starts on the northeastern side of the Antarctic Peninsula and generally heads northeast past the South Orkney Island and South Georgia Island into the South Atlantic Ocean.
“The unusual part of the iceberg’s journey was the long time it was trapped in this oceanic gyre,” said Lieser, who is also a research associate at the Institute for Marine and Antarctic Studies at the University of Tasmania.
This ocean gyre is known as a Taylor column, where rotating water above a seamount trapped the iceberg from drifting north. Imagine placing a stick in a swirling bucket of water; the water flowing around the stick forms a Taylor column.
Lieser doesn’t know why the iceberg escaped the vortex, but hypothesizes that a “random perturbation in the system” might have affected its “usual spin,” allowing the iceberg to find an exit path.
Only time will tell if iceberg A-23A will make another rare maneuver onto South Georgia and disrupt the animals.
“The exact degree of impact will depend on how closely the berg grounds, and whether or not this sits on the optimal path to [or] from the breeding beaches and feeding ground,” Meijers said. “This is pretty much impossible to predict.”