A23a: why world's biggest iceberg is on the move

The planet's biggest iceberg is on the move after three decades stuck to the ocean floor.

The uprooting of the colossal chunk of ice, which is slowly moving northward into the Southern Ocean, has been a "long time coming", said Cosmos, but some experts are baffled as to why it is suddenly moving now.

What is it?

Known as A23a, the iceberg split from the Antarctic coastline in 1986 and soon became grounded in the Weddell Sea, "becoming, essentially, an ice island", said the BBC.

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At almost 4,000 square km (1,500 square miles) in area, it is more than twice the size of Greater London and more than four times as big as New York. It is a "true colossus", said the broadcaster, and "it's not just its width that impresses" – the iceberg is 400 metres (1,300ft) thick.

While this makes it the "largest iceberg now bobbing in the world's oceans", it is "not the largest on record", said New Scientist. "That behemoth, known as A-76, measured 4320 square km when it broke off from West Antarctica in 2021."

Why is A23a moving now?

"It is not clear why it is making a run for it now," said The Guardian. But British Antarctic Survey glaciologist Oliver Marsh told the paper that "over time, it's probably just thinned slightly and got that little bit of extra buoyancy that's allowed it to lift off the ocean floor and get pushed by ocean currents".

Dr Andrew Fleming, a remote sensing expert from the British Antarctic Survey, told the BBC that he had "asked a couple of colleagues" whether there was "any possible change in shelf water temperatures that might have provoked" the change. But "the consensus is the time had just come", he said.

Cosmos also reported that despite "growing concerns about the behaviour of ice in Antarctica amid record global temperatures", the "escape" of A23a is "not considered climate change related".

However, Chad Greene, from Nasa’s Jet Propulsion Laboratory in California, told New Scientist that icebergs are breaking off Antarctica at a faster rate than snow is adding mass to the ice, "meaning climate change is causing the Antarctic Ice Sheet to lose mass at a significant rate".

What might happen next?

As it "gains steam", the "colossal iceberg will probably be launched into the Antarctic Circumpolar Current", said The Guardian, and "this will funnel it toward the Southern Ocean on a path known as 'iceberg alley' where others of its kind can be found bobbing in dark waters".

But it's possible it could again become grounded at South Georgia island, which would "pose a problem for Antarctica’s wildlife", added the paper, as "millions of seals, penguins, and seabirds breed on the island and forage in the surrounding waters". There is also the danger that it could break apart and create thousands of smaller icebergs that become a danger to ships, as well as blocking access to islands for animals and humans.

It's "not all bad news for nature though", said Cosmos. Within the ice are snap-frozen nutrients, which will be "injected back into the cold Southern Ocean waters as the berg continues its journey north". This development "benefits these complex ecosystems and tiny plankton and other organisms living within them".

Or it could move even further away. An iceberg "of this scale has the potential to survive for quite a long time in the Southern Ocean", said Marsh, "even though it's much warmer, and it could make its way farther north up toward South Africa where it can disrupt shipping".

Meanwhile, scientists can track the iceberg's path using "sophisticated satellite imaging and radar data", said Cosmos, a practice known as remote sensing.