Spectacular images of what was the world's largest iceberg, A23a, show how erosion has chiselled huge arches and cave-like depressions into the colossus. — Photo: Ian Strachan/Eyos Expeditions/dpa
Once the largest iceberg in the world, A23a may soon vanish into the vastness of the ocean.
Covering about 4,000 square kilometres when it broke away, the iceberg - soberly named A23a - was once roughly twice the size of Greater London, the largest city by land mass in Western Europe.
The giant A23a broke off from Antarctica's mainland, the Filchner-Ronne Ice Shelf, in 1986 and then remained grounded on the seabed in the Weddell Sea for over 30 years.
The British Antarctic Survey (BAS) has been tracking the mega-iceberg for years and recently announced on the Bluesky platform that it was "rapidly breaking up" into several "very large chunks" after it began drifting north.
According to Andrew Meijers, a BAS oceanographer, the gigantic slab of ice has broken up into substantial sections, each comparable in scale to large icebergs.
But with the arrival of spring in the southern hemisphere, the iceberg will likely break up into pieces too small to be tracked any further, he notes, and will eventually melt.
The image, taken in 2024, shows massive arches carved into A23a, revealing spectacular erosion and cave-like formations. — Photo: Richard Sidey/Eyos Expeditions/dpa
Already lost its crown
Meijers says the original megaberg A23a has already shrunk and lost its 'largest iceberg' crown to D15A, an iceberg near an Australian research station.
"Icebergs of this size are relatively rare – there have been two other similarly sized bergs in the same region over the past five years or so and sporadically before that."
He expects the fragmentation to accelerate - since its migration north of the Weddell Sea in early 2024, it has been melting faster and is located in waters whose temperature is well above freezing.
A23a is following the fate of other mega-mountains, explains Meijers, citing icebergs with similarly cryptic names that have also disintegrated near South Georgia in recent years.
NASA's Aqua satellite shows an iceberg with the designation A23a heading towards the island of South Georgia off the coast of Antarctica on January 15, 2025. Photo: Uncredited/NASA Worldview/dpa
The beginning of the end
In March 2025, A23a ran aground near the sub-Antarctic island of South Georgia - an event experts saw as the beginning of its end.
Daniela Jansen, glaciologist at the Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research, explains: "The special thing about A23a is that it is so old."
After ice broke off in 1986 in a process known as iceberg calving, A23 settled in cold and calm water and that's why it has lasted so long, she says.
Oceanographer Meijers said after an expedition in 2023: "It looks like a high wall rising out of the sea, stretching from horizon to horizon. Some parts are quite eroded and undermined by waves and meltwater."
Geraint Tarling, also of the British Antarctic Survey, warns that the influx of cold freshwater from the melting iceberg could significantly affect sea floor ecosystems in the region.
Understanding such changes is crucial, he adds, especially if these events become more frequent with climate change.
But is this process driven by global warming - or is it part of nature's rhythm?
Iceberg calving is a normal feature of Antarctica's ice dynamics, Jansen says. Typically, icebergs break away from glaciers flowing into the sea or from floating ice shelves.
"A warming of the ocean and the atmosphere can cause ice shelves to become unstable and disintegrate completely," she says. This was observed on the Antarctic Peninsula in the late 1990s and early 2000s.
"This, in turn, can accelerate the flow of land ice into the ocean, contributing to sea-level rise," Jansen says. "When a floating iceberg melts, it doesn't change the sea level."
Satellite image of iceberg A23a in late 2024. Photo: Cover Images/Zuma Press/dpa
Melting with metre-high consequences
According to the British Antarctic Survey, mega-mountains like A23a are too rare to map systematic changes.
However, Meijers explains that we know that ice shelves have irrevocably lost around 6 trillion tons of ice since 1997 due to the increased formation of icebergs - and a similar amount due to increased melting over the same period.
Changes in ocean circulation and warmer water reaching the ice shelves are probably responsible for this.
Meijers emphasizes that the melting of these glaciers - mainly in the Antarctic - will definitely lead to a sea level rise of 2 metres.
However, the timing depends on how much greenhouse gases mankind continues to emit.
There is still great uncertainty about what the melting will mean for sea level rise, as Meijers says.
He notes that the uncertain physics of the ice shelves and possible chain reactions in the ocean mean that a rise in sea level of more than 15 metres over the coming centuries due to Antarctic melting "cannot be ruled out." – By Larissa Schwedes, dpa
