PREVIEW

The world’s largest and oldest iceberg, A23a, is once again on the move after decades of being grounded on the seafloor and months spinning in ocean currents.

The colossal iceberg, which is double the size of Greater London and weighs nearly a trillion tonnes, has been spinning on the spot near the South Orkney Islands for months but is now drifting into the Southern Ocean.

A23a calved from Antarctica’s Filchner Ice Shelf in 1986 and remained grounded in the Weddell Sea for over 30 years. It began its gradual northward journey in 2020 but was delayed when it became trapped in a Taylor Column—a phenomenon in which water currents above a seamount rotate, holding objects in place. This dynamic caused the iceberg to spin on the spot for months, further postponing its drift.

It is now anticipated that A23a will follow the Antarctic Circumpolar Current into the Southern Ocean, where it is likely to head towards South Georgia. As it reaches warmer waters, scientists expect the iceberg to break apart into smaller pieces and eventually melt.

Dr Andrew Meijers, an oceanographer at the British Antarctic Survey (BAS) and co-leader of the OCEAN:ICE project, explained the importance of tracking the iceberg’s movements:

“It’s exciting to see A23a on the move again after being stuck for so long. We’re keen to find out if it will follow the same path as other large Antarctic icebergs, and more importantly, what impact it will have on local ecosystems.”

A year ago, researchers aboard the BAS research vessel RRS Sir David Attenborough observed and studied A23a while on a mission in the Weddell Sea for the BIOPOLE project. This expedition collected crucial data on how Antarctic ecosystems and sea ice influence global cycles of carbon and nutrients.

Laura Taylor, a biogeochemist on the BIOPOLE cruise, highlighted the scientific significance of their encounter:

“We know that these giant icebergs can enrich the waters they pass through, creating thriving ecosystems in otherwise less productive areas. What we don’t yet understand is how the size, origin, and scale of a specific iceberg like A23a affects this process.”

Scientists took water samples from different locations around the iceberg to assess how it influences local marine life and its role in the ocean’s carbon balance.

As A23a continues its journey, researchers are eager to monitor its effects on marine ecosystems. Giant icebergs like A23a are known to release nutrients into the ocean, potentially fuelling blooms of phytoplankton, which play a key role in absorbing carbon dioxide from the atmosphere.