A one trillion tonne iceberg thought to be on a collision course with the sub-Antarctic island of South Georgia has finally come to a standstill, having run aground some 50 miles off its coast.

The iceberg, which is about twice the size of Greater London, appears now to be stuck and, while little is readily predictable when it comes to icebergs, is anticipated to start breaking up on the island’s south-west shores. 

This is the latest chapter in the long history of the giant ‘megaberg’ which calved from Antarctica’s Filchner Ice Shelf back in 1986 before running aground on the seabed in the Weddell Sea for some 30 years.

After spending some time caught in an ocean vortex, the iceberg finally broke free late last year, drifting with the currents of the Southern Ocean and put on a collision course with the British island of South Georgia towards the end of 2024.

At the time, some fears were expressed that the arrival of the mega iceberg at the shores of the island would present an insurmountable obstacle for the island’s populations of macaroni penguins and seals, forcing them to circumnavigate the looming ice walls of the trillion tonne behemoth to find sources of food.

Those fears have been, largely, laid to rest for now with experts indicating that should the iceberg remain where it is currently grounded, it “isn’t expected to significantly impact the wildlife local to South Georgia.”

Dr Andrew Meijers, an oceanographer at the British Antarctic Survey, who co-leads the Ocean:Ice projects and its aim to better understand how the giant ice sheet affects the ocean, said: “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 happens now.”

Scientists are keen to observe how the grounding of the iceberg some 50 miles off the coast of South Georgia will affect the local ecosystem. It’s possible that nutrients stirred up by its grounding and those dispersed as it melts could “boost food availability” for the whole regional ecosystem, including for its penguins and seals.

“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,” said Dr Meijers. “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.” 

Over the course of the iceberg’s history, its 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. 

According to the latest satellite imagery of the iceberg, it is so far maintaining its structure. While icebergs of this magnitude are a relatively rare occurrence, scientists are keen to understand how iceberg calving has increased in recent years due to human induced climate change as well as how the freshwater they add to the ocean may impact circulation and the wider climate.

Now grounded, the iceberg faces a greater likelihood of breaking up. While this is difficult to predict, it would follow the pattern of large icebergs of the past. While it remains in its current structure, the berg is easily avoided by Southern Ocean shipping operators.

As it breaks up, however, the smaller bergs it disperses into will be 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,” said Dr Meijers.

Icebergs – including megabergs like the A23a – are a normal part of the lifecycle of the Antarctic and Greenland ice sheets. Their formation begins as they are ‘pushed’ out into the ocean by the weight of continental ice behind them, where they begin to float as ice shelves. Eventually, and due to a combination of flexure by winds, waves, tides, and melting ice, they break off as icebergs.

While this is all part of the natural cycle, Meijers noted that since the year 2000, ice shelves have lost around 6,000 giga (billion) tonnes of their mass.

“This is roughly matched by an increase in straight up melt of the ice shelves and aligns with measured mass loss of the grounded ice over Antarctica – attributed to anthropogenic climate change,” said Dr Meijers. 

“This loss of ice shelf mass has significant implications for ocean circulation due to the addition of freshwater, acceleration of sea level rise, and possible irreversible ‘tipping points’, particularly in the vulnerable west Antarctic.

“These are pressing and active areas of research at the British Antarctic Survey and elsewhere.”

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