The world's largest iceberg drifting past the tip of the Antarctica Peninsula on November 27, 2023 (Credit: European Union/Copernicus Sentinel-3/ CC-BY-SA-2.0)

The world's largest iceberg is on the move after being stuck to the ocean floor for 37 years. A23a stretches almost 1,500 square miles (3,885 sq. km) wide, or about three times the size of New York City and its five boroughs. The massive slab of ice weighs over one trillion tons and stands 1,313 feet (400 m) tall. For comparison, the Empire State Building measures 1,250 feet (380 m) from top to bottom.

While A23a is currently the world's largest iceberg, it is not the biggest one to ever form. However, the others have all melted away, allowing the "megaberg" to reclaim the title several times.

When did A23a start to move?

A23a is one of three pieces of a larger iceberg, called A23, which broke off from Antarctica's Filchner Ice Shelf in 1986. While the other two floated away, A23a remained stuck in the Weddell Sea for over three decades. Dr. Andrew Fleming, at the British Antarctic Survey, first noticed some movement in the massive iceberg in 2020. But it has recently been gaining momentum driven by winds and currents.

A23a's movement 2011 (Credit: CC-BY-SA-2.0)

Dr. Fleming does not attribute A23a's movement to warmer water temperatures.

"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," he said."It was grounded since 1986, but eventually, it was going to decrease (in size) sufficiently to lose grip and start moving."

Where is A23a heading?

In late November 2023, A23a was seen drifting in the Southern Ocean just beyond the Antarctic Peninsula. Experts expect it to end up in "iceberg alley" in the South Atlantic and ultimately melt.

"All icebergs have that same fate, no matter how big they are," said Walter Meier, a senior research scientist at the National Snow and Ice Data Center. "The bigger ones obviously last longer and tend to make it further."

What danger could A23a pose?

Experts are carefully monitoring how close the "megaberg" gets to South Georgia, an island in the South Atlantic Ocean. If it comes too close to shore, it could trap the island's penguins and other animals, preventing them from getting food in the open water.

"It could have ramifications since there's a lot of wildlife in South Georgia, both in the ocean and on land, and that could really disrupt the ecosystem there," Meier said.

Icebergs and climate change

Icebergs are pieces of ice shelves formed from historical snow compacted into ice millions of years ago. The dust particles and air bubbles trapped inside the layers are released when icebergs melt. They provide researchers a record of how air temperatures and carbon dioxide levels have changed over time.