Antarctica’s megaberg faces its final days in the South Atlantic

After nearly four decades of stubbornly clinging to life, one of the largest icebergs ever recorded is entering its final act. A23a – the trillion-ton “megaberg,” which once sprawled over an area more than twice the size of Greater London – has drifted into the warmer waters of the South Atlantic and has begun to disintegrate rapidly.

In recent weeks, satellite images show vast slabs calving away – some measuring hundreds of square miles – while a blizzard of smaller bergy bits spreads around it, posing hazards to ships as the giant wastes away.

Iceberg freed after 30 years

A23a’s story stretches back to 1986, when it calved from Antarctica’s Filchner-Ronne Ice Shelf and promptly ran aground in the Weddell Sea, pinned to the seafloor for more than 30 years.

Freed at last in 2020, it joined “iceberg alley,” the conveyor belt of the Antarctic Circumpolar Current that sweeps big bergs toward the South Atlantic.

By early 2025, it had loomed into view near South Georgia, raising fears it could block the island’s penguins and seals from their feeding grounds. The berg briefly grounded on shallow banks, then worked loose in late May and accelerated north.

Warming seas speed its demise

Heat and waves are now doing what gravity and time could not. According to an analysis of Copernicus satellite data, megaberg A23a is down to roughly 683 square miles, with a maximum width of about 37 miles – less than half its size at the start of the year.

Its drift speed has jumped too, at times topping 12 miles per day as it rides stronger currents and increasingly rough seas.

“It’s breaking up fairly dramatically,” said Andrew Meijers, a physical oceanographer at the British Antarctic Survey (BAS). “I’d say it’s very much on its way out – it’s basically rotting underneath.”

“The water is way too warm for it to maintain. It’s constantly melting…It won’t be really identifiable within a few weeks.”

What made megaberg A23a so unusual

Iceberg calving is part of the normal life cycle of polar ice, and giant bergs are not unprecedented. But A23a’s persistence and path are striking.

“Most icebergs don’t make it this far,” Meijers said. “This one’s really big, so it has lasted longer and traveled farther than others.”

When BAS researchers sailed alongside it in late 2023 aboard the RRS Sir David Attenborough, the experience was otherworldly.

“It looked like a towering wall emerging from the ocean, stretching from horizon to horizon,” Meijers said. “Some portions were quite pitted and undercut by the action of waves and melt.”

That undercutting is the giveaway. As the berg moves into warmer, saltier water and meets heavier swell, melting hollows its base while cracks propagate through the plate-like ice. Running aground – whether at South Georgia or now on patches of the continental shelf – adds extra stress that hastens the breakup.

“Large bergs have made it a long way north before – one got within 1,000 kilometers (621 miles) of Perth, Australia – but they all inevitably break up and melt quickly afterward,” Meijers said.

Danger and opportunity collide

Icebergs of this size can reshape local ecosystems as they pass. They can temporarily block straightforward routes from island rookeries to offshore feeding grounds, forcing adult penguins and seals to detour and burn energy while chicks and pups wait ashore.

“Potentially, it could interrupt their pathway to feeding sites and force the adults to expend more energy to travel around it,” Meijers said of the South Georgia colonies, which is why the grounding in March drew concern. The berg’s drift around the island reduced that immediate risk.

There may also be benefits. As megabergs like A23a melt, they stir stratified waters and leak micronutrients such as iron into the sunlit surface layer – fertilizer for phytoplankton blooms that can feed krill and fish, and ultimately seabirds and mammals.

BAS researchers sampled waters around A23a in December 2023 to gauge its biogeochemical “footprint.” They paired these results with past deployments of robotic ocean gliders around similar bergs.

Together, the data show how iceberg-borne nutrients and upwelling can boost productivity and even enhance carbon sequestration as organic matter sinks to depth.

Iceberg’s collapse tracked by satellites

A23a’s decay has unfolded in near-real time thanks to daily satellite imagery, which captures both its changing outline and the swarms of fragments it sheds.

“It is easily observed from space, and daily visual-spectrum satellite snapshots make it easy to track its progress,” Meijers said.

Ocean current maps – also derived from satellites – help project its likely path, though the precise route always depends on winds, waves, and where the berg next snags the seafloor.

For shipping, vigilance is everything. A single monster is easy to avoid; a broken field of smaller floes is not. “As it breaks up over time, the smaller bergs are much harder to track,” Meijers said. Fishing operators may temporarily write off entire regions until the ice debris disperses.

Icebergs in a warming world

Scientists are careful to distinguish between calving – a natural release valve for ice shelves – and the broader, climate-driven changes transforming Antarctica.

“Icebergs, including megabergs like this one, are a completely normal part of the lifecycle of the Antarctic (and Greenland) ice sheets,” Meijers said. But the context has shifted.

“Observations show that the ice shelves have lost around 6,000 giga (billion) tons of their mass since the year 2000.”

It is a loss “roughly matched by an increase in straight-up melt of the ice shelves” and aligned with measured mass loss from grounded Antarctic ice.

That matters for global sea level – because thinning shelves buttress glaciers less effectively – and for ocean circulation, as freshwater input alters density and flow. Scientists are especially concerned about potential “tipping points” in West Antarctica, where retreat could become self-sustaining.

Final days of a giant

No one can say exactly how A23a’s last weeks will play out, only that the megaberg’s direction is one-way. Grounding events, wave attacks, and heat from above and below guarantee a cascade of fractures until the berg is a memory and a field of bergy bits.

“Now it’s grounded, it is even more likely to break up due to the increased stresses,” Meijers noted earlier; whether or not it’s currently snagged, the outcome is the same.

For scientists, though, this isn’t just spectacle. It’s a chance to watch, measure, and model how a true giant interacts with the Southern Ocean – how it fertilizes, how it freshens, how it fragments – and to fold those insights into a bigger picture of a warming world.

As Meijers put it, icebergs provide interesting natural laboratories for studying ocean physics, chemistry, and ecosystems. A23a’s long journey is nearly over. The lessons it leaves behind won’t melt away.

Image Credit: MODAPS at NASA/GSFC

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