The Secret Life of Melting Ice

An iceberg larger than the state of Delaware broke free of its moorings in Antarctica in 2017, floating in the South Atlantic for a time before it melted into fragments and vanished.

Dramatic calving events like this are becoming more frequent – and familiar – as Earth’s oceans warm and polar ice shrinks. And researchers are using a new set of precision simulations to reveal what’s hidden below: The calving of these giant bergs allows warm ocean water to infiltrate beneath ice shelves – the ice that remains attached to land – accelerating the melting in a potentially unstoppable cycle.

“More melting weakens the entire structure,” said Mattia Poinelli of the University of California, Irvine, the lead author of the study. “Eventually, you get more and more icebergs.”

Eric Rignot, an ice researcher at UCI and NASA’s Jet Propulsion Laboratory in Southern California, said Poinelli’s study shows that the very shape of the ice can influence how melting progresses, and how the ice and ocean interact.

The study, published in Geophysical Research Letters, gives a peek under the hood of melting ice shelves – processes that are mostly invisible from above.

Poinelli created mathematical models of the ice shelves and surrounding sea-ice and ocean water. He fed in the physical parameters and data, then set in motion models that offer a detailed picture of how the melting evolves through time.

The data included observations from temperature and salinity probes lowered into coastal ice-drilling sites and from satellite observations. He also relied on a dataset known as “Bedmachine,” which includes seismic data that maps the bumpy topography of the ocean bottom.

Simulating Larsen C

In this case, the simulations were set up to replicate the real-world ice loss on the Larsen C Ice Shelf, a frozen expanse on the east coast of the Antarctic Peninsula that includes a complex of ice shelves that has seen significant collapse and rifting over recent decades. These vast chunks of ice remain attached to land but float on the ocean.

“What we were seeing in the real world was well reproduced in the model,” Poinelli said, including seasonal variability where the ice front meets the ocean and loss of elevation thickness in the ice due to melting. “High-resolution models help us understand how water is brought into contact with a deep section of the ice, where ice is detached from the bedrock.”

That point of attachment is known as the “grounding line,” and infusions of denser, more saline water can alter its position to form a “grounding zone.” Knowing where and how this occurs is the key to making more reliable projections of future breakdowns in the ice shelves – as well as potential effects on sea level rise.

The repeated computer simulations ran along two tracks: “control” runs, with no iceberg breakage, and “iceberg” runs simulating ocean conditions after the 2017 calving event. By comparing the two tracks, Poinelli and his team found that calving allows warmer water to penetrate far more deeply and rapidly beneath the ice shelf and toward the grounding line. The intrusion penetrated 50% farther, with a 30% increase in heat transported toward the grounding line. That resulted in a melt-rate 73% higher for the ice shelf in the iceberg run than in the control run.

The modeling also yielded deeper, more ominous insights. Sustained thinning in specific areas of the ice shelf can lead to ungrounding of the shelf, which can reactivate dormant rifts, or breaks, in the ice. The possible result: accelerating and increasing calving that could lead to collapse.

“After a certain point, the retreat triggers more warm water to come in contact with the grounding line in areas where it is most vulnerable,” Poinelli said. “That could trigger a feedback event, bringing more intrusion, more warm water, more retreat.”

Revealing such hidden processes could improve estimates of ice-shelf evolution as the planet warms and ice diminishes.

“Mattia conducted a rigorous study, and he is now applying the same approach to study the impact of the icescape on bigger, mightier glaciers,” Rignot said. The same study methods will be used to explore changes in even larger bodies of ice in Antarctica’s Amundsen Sea.