In 1986, a massive iceberg, almost five times larger than New York City, calved off West Antarctica’s Filchner-Ronne Ice Shelf. The iceberg, designated A23a, immediately grounded on the floor of the Weddell Sea, where it remained stuck for almost four decades.

Now, the world’s largest and among the world’s oldest icebergs is on the move, drifting into what is known as “iceberg alley” on the Antarctic Circumpolar Current. Recent satellite images reveal that the berg, weighing nearly a trillion metric tonnes, is now drifting quickly past the northern tip of the Antarctic Peninsula, aided by strong winds and currents.

So, why, after almost 40 years, is A23a on the move now?

“I asked a couple of colleagues about this, wondering if there was any possible change in shelf water temperatures that might have provoked it, but the consensus is the time had just come,” said Dr. Andrew Fleming, a remote sensing expert from the British Antarctic Survey told the BBC.

Scientists will be watching the track of A23a closely. Depending on its drift, it could be a threat to wildlife or shipping.

In 2020, an iceberg A68a, comparably sized to A23a, was drifting on a collision course with the British Overseas Territory of South Georgia. If the berg ran aground offshore of South Georgia island, it would pose a grave threat to local penguins and seals on the island as well as to creatures living on the seafloor. Fortunately, A68a broke up in the slightly warmer waters of the Southern Ocean before it could drift into the island.

World’s largest iceberg breaks free

Thanks to Alaric Bond for contributing to this post.