CIRES researchers suspect a recurring estuary weakened Petermann before its August calving

"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 CIRES research scientist and a co-author of the study ^[9][19]. Pooled meltwater can destabilize a shelf, as it did ahead of the Larsen B breakup in 2002 ^[8]. A river helps by carrying that water off the shelf into the ocean and reducing its weight ^[8].

Petermann's river started out as the helpful kind. Then it cut so deep that its mouth dropped below sea level. Warmer, salty ocean water began mixing with the meltwater and flowing backward up the channel ^[11]. The 2021 images that confirmed the estuary, in a study led by Alexandra Boghosian, showed chunks of sea ice floating up the river onto the shelf ^[7]. "Our new research suggests that estuaries can also be harmful," Banwell said ^[10].

The new paper adds depths and dates to that process ^[3]. The team worked out the channel's height above sea level through each melt season from satellite imagery, and tracked the cut from the river's surface down to the channel bed ^[5][11]. "We're using satellite data not just to 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," said Michela Savignano, the lead author and a doctoral student at CU Boulder and CIRES ^[12][19].

Banwell's case for harm rests on the estuary switching on and off ^[13]. "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," she said ^[6].

The record behind that link stops in 2018. That year a crack running perpendicular to the ice flow cut the river off upstream of the estuary ^[13]. The calving came eight years later ^[1]. The observations cover one estuary on one shelf over six melt seasons ^[2], and the published summary reports neither the incision rates nor whether the estuary came back after 2018. I think the loading idea is plausible. The imagery described cannot separate it from whatever else happened to Petermann between 2018 and 2026.

For shelf monitoring, the concrete output is a condition that can be measured from orbit: a river mouth approaching sea level ^[5][11]. Banwell expects that condition to become easier to reach. "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," she said. "It will be easier for a surface river to incise down to sea level." ^[15][16]

Savignano looked first at Greenland. "Only five ice shelves remain in Greenland, including Petermann, and estuaries could make them more prone to breakup," she said ^[17]. About 75% of Antarctica is surrounded by ice shelves ^[14]. There, she said, "estuaries could begin forming within 30 years" ^[18].