In August 2026, an iceberg the size of Manhattan broke away from the Petermann Ice Shelf in northern Greenland, marking the glacier's largest calving event since 2020.
Decoding the Planet’s First Known Ice Shelf Estuary
Long before the Manhattan-sized berg sheared off the Petermann Glacier in August 2026, CIRES scientists were tracking a unique hydrological feature. Discovered in 2021, the planet’s first known ice shelf estuary forms where salt water from the ocean mixes with fresh water from a surface meltwater river. Published in The Cryosphere, new CIRES-led research details the formation mechanics of this system.
“For the first time, we were able to measure how quickly the river cut into the ice shelf to form the estuary, which revealed more complex estuary behavior than was previously suggested,” stated Michela Savignano, a CU Boulder Geography and CIRES PhD student and lead author of the study, as reported by CIRES.
The research team calculated the elevation of the river channel above sea level during the melt season by analyzing satellite imagery. This methodology allowed them to track the exact incision rate from the surface down to the bottom of the channel.
Hydrological Stress Mechanics: How Rivers Turn Destructive
Supraglacial rivers typically flow across the surface of ice shelves and empty into the ocean. During summer months when temperatures rise above freezing, surface meltwater pools can induce structural instability, mirroring the conditions that preceded the collapse of the Larsen B ice shelf in 2002. Historically, scientists viewed flowing rivers as beneficial pressure-relief valves that drained these dangerous surface lakes.
However, the Petermann Glacier study upends that simple dichotomy. Alison Banwell, a research scientist in CIRES’ Earth Science and Observation Center and co-author of the study, noted: “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. Our new research suggests that estuaries can also be harmful.”
Over multiple seasons, the supraglacial river on Petermann incised so deeply that its mouth dropped below sea level. This shift allowed warmer, saline ocean water to mix with the fresh river water and flow backward up the channel. High-resolution satellite images from a 2021 study led by Alexandra Boghosian previously captured chunks of sea ice floating up this very channel onto the Petermann Ice Shelf.
The Trigger for the August 2026 Calving Event
According to CIRES researchers, the process of the estuary forming and reforming over multiple melt seasons compromised the structural integrity of the ice shelf.
“We think the process of 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,” explained Alison Banwell, highlighting the role of ocean water mass fluctuations inside the carved channel.
The 30-Second Verdict on Petermann’s Stability
- The Event: A Manhattan-sized iceberg calved from the Petermann Ice Shelf in August 2026, the largest since 2020.
- The Mechanism: A supraglacial river incised below sea level, creating an estuary where warm ocean water mixes and flows backward.
- The Consequence: NASA-funded research published in The Cryosphere indicates that repeated oceanic loading and unloading weakened the ice shelf prior to the August break.
This NASA-funded research builds directly upon foundational mapping efforts initiated in 2021.
