In August 2026, an iceberg the size of Manhattan broke away from the Petermann Ice Shelf in northern Greenland. It was the glacier's largest calving event since 2020.
Long before the breakup, CIRES scientists were developing a new method to understand the planet's first known ice shelf estuary on the Petermann Ice Shelf. The estuary — where salt water from the ocean and fresh water from a surface meltwater river mix — was discovered in 2021. New CIRES-led research, published today in The Cryosphere , explains how and when the estuary formed — something no one has studied until now.
“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,” said Michela Savignano, CU Boulder Geography and CIRES PhD student and lead author of the study. “We did this using a new method for calculating the elevation of the river channel above sea level during the melt season from satellite imagery”
The novel research may help explain the August calving event.
“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,” said Alison Banwell, research scientist in CIRES’ Earth Science and Observation Center and co-author of the study.
The NASA-funded research builds on the 2021 study led by one of the study’s co-authors, Alexandra Boghosian, which confirmed the first ice shelf estuary using high-resolution satellite images. The photos revealed chunks of sea ice floating up the river channel onto the Petermann Ice Shelf. Understanding the estuary requires understanding supraglacial rivers — rivers that flow on the surface of ice shelves and empty into the ocean.
During summer months, when air temperatures are above freezing, glaciers and surface snow melt and water pools on the surface, creating instability and making them more susceptible to collapse, like the famous Larsen B ice shelf breakup in 2002. Yet rivers that flow from these pools can reduce pressure and weight by carrying water from the ice shelf into the ocean.
“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,” Banwell said. “Our new research suggests that estuaries can also be harmful.”
The supraglacial river on Petermann began like any other, but over time it cut deep into the ice, eventually dropping below sea level at its mouth. The authors measured the river’s incision rate from the surface of the river all the way down to the bottom of the channel. It formed because the river incised below sea level, enabling warmer, salty ocean water and river water to mix and flow backwards into the river.
“We’re using satellite data not to just look at where the water is and how deep it is, but also to look at elevation change over time, and how quickly the river is cutting into the ice surface,” Savignano said.
The researchers looked at satellite images of the estuary from 2013 to 2018 and concluded it formed, disappeared, and reformed repeatedly during subsequent summers. It first formed in 2014, but then it disappeared when all of the surface water on the ice shelf drained or froze. It formed again late in the 2016 melt season; this cyclical pattern continued through 2018, when a crack in the ice perpendicular to the ice flow cut off the river upstream of the estuary.
Instability caused by estuaries could lead to more calving events, an increasing challenge in the polar regions as the planet warms. Specifically in Antarctica, where about 75 percent of the continent is surrounded by ice shelves.
“We think estuaries may become more common on other ice shelves as they melt and thin, which they’re going to do at increasing rates in the future,” Banwell said. “It will be easier for a surface river to incise down to sea level.”
The research will help scientists monitor and predict when and where estuaries may show up in the future.
“Only five ice shelves remain in Greenland, including Petermann, and estuaries could make them more prone to break-up,” Savignano said. “In Antarctica, estuaries could begin forming within 30 years, weakening ice shelves that currently play a substantial role in mitigating future sea level rise. Our research will help us to understand when and where that is most likely to happen.”
The Cryosphere