A23a broke off from the Filchner-Ronne Ice Shelf in Antarctica 40 years ago as the largest iceberg on Earth – twice the size of the metropolitan London, roughly 4,000 square kilometres.
It has been grounded in the Weddell Sea for 30 years, losing about 25% of its area. But in 2020, A23a began moving again, carried northward by the Antarctic Circumpolar Current into warmer waters that accelerated its melt and breakup.
A23a was last seen in March as it drifted northeast of South Georgia in the South Atlantic. It has been breaking apart rapidly for about 3 months and has now shrunk below the 20-square-kilometre threshold that researchers use for active tracking. Even so, Meijers still finds himself checking NASA's satellite images every so often.
“It’s not well. It's now gone, effectively. Really, it's only got like weeks if anything, or even days,” he said. “When icebergs break, they tend to go slowly, then all at once.”
Icebergs melt. That's nothing new. But A23a offers a glimpse of something larger: the future of the Antarctic ice sheet, which holds nearly 60 metres of potential sea‑level rise – by far the highest future potential for sea level rise of any ice-covered region.
Ice shelves are crucial to the sheet's stability. They act as barriers, slowing the flow of ice into the ocean. But in a warming climate, it’s unclear how quickly these shelves might weaken or collapse, and that uncertainty makes future sea‑level rise far harder to predict. What's clear, however, is that Antarctica has raised the global sea level by about 7.4 millimetres over 30 years.
There are various thresholds beyond which ice shelves become unsustainable as global temperatures rise. For some regions, that point has already been reached.
“No matter what we do, those ones are gonna melt, and they hold back a large amount of ice,” Meijers said. “What we can influence is how long it takes before that happens. Lower carbon emissions will mean that it will take longer. Higher emissions mean it will happen sooner.”
According to Meijers, projections for sea‑level rise from Antarctic melt range from about two metres by 2100 to 15 metres by 2300. “So we know two meters is guaranteed, 15 meters is possible. And all that bit in between is very uncertain, which is where we need a lot more research, especially in collaboration with engineers and coastal planners.”
Even two meters of sea-level rise would be tough for many countries. Low‑lying nations such as the Netherlands and Denmark are exposed. But others could also face unexpected impacts if ice shelves deteriorate and collapse over decades rather than centuries – just as A23a has.
“I think that has to be the takeaway for why we should care about this iceberg. It symbolises massive potential consequences for society as a whole.”
Photo: A23a edge wasting, April 2024. Emily Broadwell, British Antarctic Survey