ANTARCTICA: The world's largest and oldest iceberg A23a has finally come to a standstill as it appears to have run aground near the sub-Antarctic Island of South Georgia.
The giant A23a, which weighs nearly a trillion tonnes, calved from Antarctica's Filchner Ice Shelf in 1986 and then remained grounded on the seabed in the Weddell Sea for over 30 years.
The mega-iceberg, which is twice the size of Greater London, has since been drifting with the currents of the Southern Ocean towards South Georgia since 2020.
Warmer waters, combined with the action of waves and tides, will lead the huge section of ice to break up into smaller icebergs and eventually melt.
Dr Andrew Meijers, an oceanographer at British Antarctic Survey, who co-leads the OCEAN:ICE project that aims to understand how the icesheet affects the ocean, says:
"If the iceberg stays grounded, we don't expect it to significantly affect the local wildlife of South Georgia. In the last few decades, the many icebergs that end up taking this route through the Southern Ocean soon break up, disperse and melt. Commercial fisheries have been disrupted in the past however, and as the berg breaks into smaller pieces, this might make fishing operations in the area both more difficult and potentially hazardous.
"It will be interesting to see what will happen now. From a scientific perspective we are keen to see how the iceberg will affect the local ecosystem. Nutrients stirred up by the grounding and from its melt may boost food availability for the whole regional ecosystem, including for charismatic penguins and seals. We have several ongoing studies looking at exactly how 'megabergs' influence the ocean circulation, its chemistry, and the ecosystems they support."
The iceberg's journey has been marked by intriguing scientific events. For months in late 2024, the iceberg was trapped in a Taylor Column, an oceanographic phenomenon where rotating water above a seamount traps objects in place. This dynamic kept A23a spinning in one spot delaying its expected rapid drift north.
Answers to the questions we have been asked – provided by BAS oceanographer Dr Andrew Meijers
Where is the iceberg now?
The iceberg is grounded around on the continental shelf of the sub-Antarctic island of South Georgia, about 90 km from land.
What is currently happening with the iceberg?
The iceberg, at least in satellite images, appears to be maintaining its structure and has not yet broken up into smaller chunks, as previous 'megabergs' have done. It was meandering in the currents before grounding near South Georgia.
What does A23a look like?
In late 2023, the research ship RRS Sir David Attenborough sailed along A23a, taking almost the whole day. It looks like towering wall emerging from the ocean, stretching from horizon to horizon. Some portions were quite pitted and undercut by the action of waves and melt.
Are icebergs of this size rare?
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.
Is the A23a iceberg particularly well known to scientists?
A23a has been watched closely by British Antarctic Survey since it started moving again back in 2020, and especially since 2023 when it emerged from the Weddell Sea into the wider Southern Ocean. Whilst icebergs aren't particularly the focus of much of our research, they do provide interesting natural laboratories with which to study Southern Ocean physics, biogeochemistry and ecosystems. Notably the role these play in fertilising the upper ocean with nutrients and micronutrients, both by stirring up deeper waters and via melt deposits. These can provide valuable clues on the controls of Southern Ocean biological productivity and carbon sequestration. On a larger scale we are interested in how icebergs calving has increased due to human induced climate change, and how the freshwater these are adding to the ocean may impact circulation and the wider climate.
What measurements and observations have been made by British Antarctic Survey?
British Antarctic Survey took samples around A23a to investigate the impact of the passage of the iceberg on the biogeochemistry of the water it was passing through back in December 2023. This includes determining what contribution the iceberg melting made to the water at the ocean surface, what the nutrients concentrations are around the iceberg, and what the phytoplankton community around the iceberg looks like. This will help us build an understanding of how different these areas are from what you would usually expect without an iceberg present. Increased phytoplankton growth due to supply of nutrients from a giant iceberg can be a responsible for increasing carbon sequestration from the surface ocean to the deep, but we need more research to determine the magnitude of contribution this could make.
BAS has also deployed robotic ocean gliders around and underneath similar bergs in the past to measure how they impact the ocean surface, both through direct melt of freshwater and the stirring up of deeper ocean waters into the surface layer. The delivery of important micronutrients such as iron to the ocean via melt is one very active area of research, as is the stirring up of deep nutrient rich waters. Both are important components of generating ocean blooms of phytoplankton, which can support the higher food chain as well as potentially sequester atmospheric carbon into the ocean. It is a very active area of research and icebergs are a very interesting aspect of this.
How do researchers track A23a?
It is easily observed from space, daily visual spectrum satellite snapshots make it easy to track its progress. So, we know where it has been, and how its shape is changing. We can project its path based on ocean currents, also estimated from satellite data. Example currents can be seen here. Whilst we can't be certain it will follow these particular currents, it does align with previous berg tracks.
What is the icebergs fate?
Now it's grounded it is even more likely to break up due to the increased stresses, but this is practically impossible to predict. It has happened in the past that large bergs have made it a long way north – one got within 1000 km of Perth Australia once, but they all inevitably break up and melt quickly after.
What damage can it do to wildlife? Why could it harm South Georgia seals and penguins?
It is unlikely that South Georgia's populations of seals and penguins are impacted by the iceberg. It could potentially interrupt their pathway to feeding sites and force them to expend more energy for adults to travel around it. This will reduce the amount of food coming back to pups and chicks on the island and so increase mortality. This has happened before in 2004, but in the Ross Sea area and not at South Georgia. There could be an upside too, if the berg is stimulating ocean productivity, this could boost local predator populations.
Are there risks to people?
Whilst the berg is large it is easily avoided by Southern Ocean shipping operators, who are well aware of the dangers bergs pose – and the location of A23a. However, as it breaks up over time the smaller bergs are much harder to track. Discussions with fishing operators suggests that past large bergs have made some regions more or less off limits for fishing operations for some time due to the number of smaller – yet often more dangerous – bergy bits.
Is the history of this iceberg linked in any way to global warming?
Icebergs, including megabergs like this one, are a completely normal part of the lifecycle of the Antarctic (and Greenland) ice sheets. They basically are pushed out into the ocean by the weight of continental ice behind, begin to float as ice shelves, and eventually break off as icebergs due to a combination of flexure by winds, waves, tides and ocean melting. However, observations show that the ice shelves have lost around 6000 giga (billion) tonnes of their mass since the year 2000. This is roughly matched by an increase in straight up melt of the ice shelves and aligns with a measured mass loss of the grounded ice over Antarctica – attributed to anthropogenic climate change. This loss of ice shelf mass has significant implications for ocean circulation due to the addition of freshwater, acceleration of sea level rise (due to a loss of buttressing of continental ice and faster consequent flow into the ocean), and possible irreversible 'tipping points', particularly in the vulnerable west Antarctic. These are pressing and active areas of research at BAS and elsewhere.
