Grounding line of Greenland's glacier shifts during ocean's tidal cycles

Glaciologists have found the grounding line of Petermann Glacier in northwest Greenland to shift substantially during tidal cycles, allowing warm seawater to intrude and melt ice at an accelerated rate.

Grounding line of a glacier is where the ice detaches from the land bed and begins floating in the ocean.

The researchers from the University of California, Irvine (UCI) and NASA's Jet Propulsion Laboratory (JPL), US, said that the findings from satellite radar data from three European missions suggested that the climate community could have been vastly underestimating the magnitude of future sea level rise caused by polar ice deterioration.

Their research is published in the journal Proceedings of the National Academy of Sciences (PNAS).

"Petermann's grounding line could be more accurately described as a grounding zone, because it migrates between 2 and 6 kilometers as tides come in and out," said lead author Enrico Ciraci, UCI assistant specialist in Earth system science and NASA postdoctoral fellow. "This is an order of magnitude larger than expected for grounding lines on a rigid bed."

He said that this study replaced the traditional view of grounding lines with the knowledge that warm ocean water intruded beneath the ice through preexisting subglacial channels, with the highest melt rates occurring at the grounding zone. The traditional view held that grounding lines beneath ocean-reaching glaciers did not migrate during tidal cycles, nor did they experience ice melt.

The researchers found that as Petermann Glacier's grounding line retreated nearly 4 kilometers between 2016 and 2022, warm water carved a 200-metre-tall cavity in the underside of the glacier, and that abscess remained there for all of 2022.

"These ice-ocean interactions make the glaciers more sensitive to ocean warming," said senior co-author Eric Rignot, UCI professor of Earth system science and NASA JPL research scientist.

"These dynamics are not included in models, and if we were to include them, it would increase projections of sea level rise by up to 200 per cent - not just for Petermann but for all glaciers ending in the ocean, which is most of northern Greenland and all of Antarctica," said Rignot.

Exposure to ocean water melts the ice vigorously at the glacier front and erodes resistance to the movement of glaciers over the ground, causing the ice to slide more quickly to the sea, according to Rignot.

The Greenland ice sheet has lost billions of tons of ice to the ocean in the past few decades, the study stressed, with most of the loss caused by warming of subsurface ocean waters, a product of Earth's changing climate.