End of an Era: Former World’s Largest Iceberg A23a Completes Final Disintegration

Following nearly four decades of drift and decay, iceberg A23a—once the world’s largest—has completed its final disintegration.

Origin & Accelerated decay

Iceberg A23a calved from the Filchner–Ronne Ice Shelf margin in 1986 with an initial area of 4,170 km² and a thickness of nearly 400 m. It remained grounded on the Weddell Sea seabed for 34 years until 2020, when oceanic currents, tidal forces and temperature‑driven basal melting freed it and initiated its northward migration. By early 2023 its area was still 4,035 km², earning a Guinness World Record as the largest iceberg, but it lost 3% of its area by the end of that year and a further 8% in 2024. In 2025, catastrophic fragmentation occurred: between June and September the area shrank by 60%, reducing the main body to approximately 1,400 km². By January 2026 only 503 km² remained, and in early April 2026 the largest fragment measured just 35.2 km²—below the 68.6 km² threshold for named icebergs—leading to the formal decommissioning of the A23a designation.

Impacts on the Earth system

The disintegration has released roughly one trillion tonnes of fresh water into the South Atlantic, perturbing surface salinity and potentially influencing thermohaline circulation. Since late 2025, satellite imagery has revealed a progressive “greening” of the fragmented ice zone, driven by phytoplankton blooms fed by long‑entrained nutrients released from the melting ice, which may disrupt local food webs and impede foraging routes for penguins and seals. Additionally, numerous large iceberg fragments pose ongoing hazards to shipping and fisheries in the region.

The rapid demise of A23a serves as a sensitive indicator of Southern Ocean warming, consistent with record‑high ocean heat content reported in the State of the Global Climate 2025 (WMO). This case underscores the urgent need to address accelerating Antarctic ice loss within the broader context of global climate change.