Today’s Image of the Day from the European Space Agency features the world’s largest iceberg currently in existence, known as Iceberg A-23a. It is a colossal tabular iceberg that originated from Antarctica’s Filchner-Ronne Ice Shelf. 

“The ice mass is currently lodged on the seabed 73 kilometers (45 miles) from the remote island of South Georgia, which can be seen just poking out from beneath the cloud cover,” noted ESA.

“Although the size of the island is not clearly visible in this image, it is only marginally bigger than the iceberg.” 

Largest active iceberg

This massive chunk of ice initially calved off the ice shelf in 1986, making it not only the largest active iceberg today but also among the oldest still afloat.

For decades, A-23a remained grounded in the Weddell Sea, lodged against the seafloor and relatively immobile. However, in recent years – particularly in 2020 – it began to drift again, having finally dislodged. 

Since then, it has slowly but steadily been moving northward, driven by ocean currents and winds. By late 2023, A-23a had entered the open waters of the Southern Ocean and was heading toward the region around South Georgia Island in the South Atlantic – a typical path for icebergs from the Weddell Sea. 

This drift raised concerns about possible ecological impacts, especially if the iceberg were to run aground near South Georgia, potentially disrupting local marine ecosystems and wildlife, such as seals and penguins that rely on open access to the ocean.

An enormous chunk of ice

A-23a is enormous in scale. At its peak size, it covered an area of around 3,900 square kilometers – that’s larger than the U.S. state of Rhode Island or roughly one-and-a-half times the size of Luxembourg. 

It’s around 400 meters thick, about the height of the Empire State Building, and the portion visible above water represents only a fraction of its total size, as most of the iceberg lies below the surface. 

“A23a has started to disintegrate and many smaller blocks of ice are visible in the dark blue ocean, particularly to the north of the berg. The disintegration is typical of icebergs that reach this far north and is caused by the warmer sea temperatures and weather conditions,” said ESA.

Despite these losses, however, it has retained much of its original bulk.

Significance of Iceberg A-23a 

Iceberg A-23a carries significance not just due to its size but also for what it tells scientists about climate and ocean dynamics. 

Iceberg drift patterns are closely monitored via satellite imagery to study how changing climate conditions affect calving rates, iceberg longevity, and meltwater inputs into the ocean. 

As it continues its journey, A-23a will gradually shrink, releasing vast quantities of freshwater into the sea, which can affect local salinity levels and nutrient distribution.

While icebergs this large are rare, they’re not unprecedented. In 2000, an even larger iceberg, B-15, broke away from the Ross Ice Shelf and initially covered more than 11,000 square kilometers. But that iceberg has long since broken into smaller fragments and dissipated. 

What makes A-23a remarkable is its longevity and recent reactivation, allowing researchers to witness the slow transformation of a giant frozen relic that had remained stationary for most of its existence.

Large icebergs and potential collisions

Large icebergs, particularly those calved from Antarctica’s vast ice shelves, can span hundreds to thousands of square kilometers and weigh billions of tons. When they break away, they typically drift into the Southern Ocean, carried by wind and powerful ocean currents like the Antarctic Circumpolar Current. 

These icebergs gradually melt as they travel into warmer waters, but some, like A23A or the infamous B-15, can persist for years or even decades.

As they drift, large icebergs can become hazards – primarily to the environment but occasionally to shipping lanes. Potential collisions usually involve these ice masses grounding on continental shelves or crashing into islands or seafloor ridges. 

When such collisions occur, they can damage benthic ecosystems, which are sensitive and often host slow-growing, endemic species. The scraping of a massive iceberg across the seabed disrupts sediments and organisms, potentially changing that habitat for years.

Ecological chain reactions

Additionally, when a large iceberg grounds near biologically rich areas like South Georgia Island, it can block access to feeding zones used by marine mammals and seabirds, forcing them to take longer, more exhausting foraging trips. 

This can lead to increased mortality, especially during breeding seasons when animals are tethered to colonies or nurseries. 

These disruptions highlight how icebergs, though natural parts of the cryosphere, can trigger ecological chain reactions as they interact with biologically productive and topographically complex regions.

The image was captured on April 5, 2025 by the Ocean and Land Color Instrument on Copernicus Sentinel-3.

Image Credit: ESA

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