In September 2017, a large iceberg roughly four times the size of Manhattan calved off of the Pine Island glacier in Antarctica (not to be confused with last July’s massive iceberg that originated from the Larcen C ice shelf further east). While the event itself isn’t that unusual — the Pine Island Glacier accounts for 25 percent of all of Antarctica’s annual ice loss — this iceberg began to break up shortly after separating from its host glacier, instead of doing so after drifting far out into the Southern Ocean, as ‘bergs from Pine Island typically do. This new rapid-breakup behavior has researchers worried for what this might mean for the stability of the glacier itself, and for sea level rise in general.

Over nearly seventy years of observation, the Pine Island Glacier has exhibited a stable pattern in the calving cycle of its 60-meter (197-foot) ice face, but the sudden breakup of one of its icebergs close to shore suggests something has changed, according to Robert Larter, a marine geophysicist with the British Antarctic Survey.

"We’re now seeing changes in the calving behavior of the ice shelf, when for 68 years we saw a pattern of advance and retreat resulting in the calving of a single large iceberg which left the ice front to approximately the same place."

The sudden breakup appears to be due to a continued thinning that the glacier has been experiencing — basically, the resulting iceberg wasn’t as thick as its predecessors, causing it to break up into nearly two dozen pieces in under two months. Currently, the Pine Island Glacier extrudes 10 billion tonnes of ice into the ocean each year, and on its own accounts for nearly a full millimeter (0.04 inches) of sea level rise every eight years.

However, this current flow rate follows a period of acceleration that saw the glacier’s speed increase by 73 percent between 1974 and 2007, with 8 percent of that increase being seen in the last 16 months of that period alone. Although no-one expects such an extreme case to occur, if the entirety of the Pine Island Glacier were to suddenly melt, it contains enough water to raise sea levels by a full half-meter, or 1.7 feet. But what researchers are more immediately concerned about is a continued acceleration of the flow of ice, including an inland retreat of the ice face.

"If new rifts continue to form progressively inland, the significance to ice shelf retreat would be high," warns glaciologist Ian Howat of Ohio State University.