Ice shelf estuary discovered as culprit in Greenland's Manhattan-sized iceberg breakaway

On August 4, 2026, an iceberg the size of Manhattan broke away from the Petermann Ice Shelf in northern Greenland. It was the largest piece of floating ice the shelf had shed since 2012, and the biggest calving event in six years. Scientists have now identified what triggered it: a body of brackish water where river and ocean mix—an estuary that had never been documented on an ice shelf before.

The Petermann Ice Shelf sits at the edge of the Greenland Ice Sheet, where it extends into the Arctic Ocean. In 2021, five years after the previous major calving event, a NASA-funded research team spotted something unusual in high-resolution satellite imagery: a freshwater river flowing across the ice shelf had carved a channel so deep that it dropped below sea level. At its mouth, where the river met the ocean, fresh and salt water began to mix. The discovery was striking enough to warrant a full study, published that year in Nature Geoscience, suggesting that such estuaries could promote fractures and calving. But the mechanics remained unclear.

A new investigation, led by scientists from the Cooperative Institute for Research in Environmental Sciences and published recently in The Cryosphere, has now measured exactly how the estuary behaved and how it may have weakened the ice shelf. Using satellite imagery and a new method for calculating the elevation of the river channel above sea level during the melt season, the team traced the estuary's history from 2013 to 2018. It first formed in 2014, then vanished when surface water drained or froze. It returned during the 2016 melt season and repeated this cycle—forming and disappearing—until 2018, when a crack in the ice cut off the river upstream. Each time the estuary formed and reformed, it loaded and unloaded the ice shelf with ocean water, creating stress that accumulated over multiple seasons.

The mechanism is counterintuitive. Rivers, it turns out, can actually benefit ice shelves by carrying water away from the surface and into the ocean, reducing the weight and pressure of pooled water. Estuaries do the opposite. As the river cut deeper into the ice over time, warmer, saltier ocean water flowed backward into the freshwater channel, destabilizing the shelf from within. "We've known for a couple of decades that lakes can be harmful to the overall health of an ice shelf, whereas rivers can actually be beneficial," said Alison Banwell, a co-author of the study. "Our new research suggests that estuaries can also be harmful."

The August 2026 calving event appears to be the consequence of this repeated weakening. "We think the process of the estuary forming and reforming over multiple melt seasons weakened the ice shelf due to the loading and unloading of ocean water, and potentially contributed to the calving event that happened in August of this year," Banwell explained. The Manhattan-sized iceberg is now drifting through the Nares Strait, having survived a collision with Joe Island. As it moves into warmer waters and narrower channels, it is expected to fracture into smaller pieces before melting away entirely.

The implications extend far beyond Greenland. Antarctica is surrounded by ice shelves on roughly 75 percent of its coastline, and those shelves play a critical role in slowing the flow of glaciers into the ocean—a process that directly affects global sea level rise. As warming accelerates and ice shelves thin and melt, estuaries could begin forming on Antarctic ice shelves within the next 30 years. "In Antarctica, estuaries could begin forming within 30 years, weakening ice shelves that currently play a substantial role in mitigating future sea level rise," said Michela Savignano, the lead author. The research provides a framework for predicting where and when these estuaries are most likely to emerge, but it also reveals a mechanism of ice shelf collapse that scientists are only now beginning to understand.