A sudden ice collapse hints at disturbing changes on the Antarctic coast

A hot spot is starting to form along the coast of East Antarctica.

An ice shelf that broke apart seemingly unprovoked a few years ago had been steadily weakening for 30 years, largely unnoticed by scientists, researchers report Dec. 3 in Nature Geoscience. The discovery, based on decades of satellite observations, raises concerns about a region of Antarctica long considered stable.

“The East Antarctic Ice Sheet holds 10 times more ice” than West Antarctica, says Mathieu Morlighem, a glaciologist at Dartmouth College who was not part of the study. West Antarctica is already bleeding ice at an alarming rate (SN: 15.2.23). But if the East Antarctic ice sheet also retreats, it could dramatically increase the rate of sea-level rise over the next few centuries.

The latest reminder of this concern is East Antarctica’s Conger Ice Shelf, a former floating glacial ice sheet with an area about 20 times the size of Manhattan. In 2022, it suddenly broke into icebergs, which then dispersed over several days.

“Nobody was thinking it was going to go,” says Catherine Walker, a glaciologist with the Woods Hole Oceanographic Institution in Massachusetts, who led the new study. “It wasn’t even melting that fast.”

Before it broke up, the Conger Ice Shelf had existed for perhaps thousands of years. It was formed by several neighboring glaciers that protruded from the coastline and floated into the ocean. It was only by chance that Walker noticed his collapse in 2022.

While reviewing satellite images to examine another nearby ice shelf, she noticed that the 1,200-square-kilometer Conger Ice Shelf was present in a photo taken on March 10 that year — but missing from another taken six days later. later.

So began a two-year quest to figure out what destroyed him.

Polar meteorologist Jonathan Wille, along with Walker and 50 other scientists, reported an important finding earlier this year: A powerful storm passed along the coast during that time, tilting the sea surface up and down a portion of a degree. As the ice shelf flexed, it broke along existing cracks. Then strong winds separated the fragments.

“We have every reason to think so [these storms] will become more intense in the future” as the Earth warms, says Wille, of the Institute for Atmospheric and Climate Sciences, ETH, in Zurich, Switzerland. These stronger storms can damage the protective ice shelves surrounding the Antarctic coastline (SN: 25.9.19).

But for the Conger Ice Shelf, the story is more complicated. The new study shows it was already in bad shape when the storm hit.

Some famous ice shelf breakups were preceded by mass melting at the upper surface at warm temperatures. But Conger was in an area of ​​generally cold air, and the melting at its bottom was driven by seawater. Looking at archival satellite measurements, Walker and colleagues found that the floating shelf had gradually thinned, from a thickness of about 200 meters in 1994 to 130 meters in 2021. Satellite radar measurements suggest that cracks cut through its thin, brittle ice – allowing salt water to seep in and weaken it further.

The Conger Ice Shelf had long been stabilized because it pressed against an island 50 kilometers offshore. But as the ice shelf thinned, it became too weak to withstand those compressive forces. The island became “a slow-motion rock going through the windshield,” says Walker. Spider fissures appeared from the point of contact of the ice shelf with the island, the new study reports. Then, on March 7, 2022, it broke away from the island, leaving it vulnerable to the approaching storm.

Conger’s fall will not significantly affect sea level because the glaciers he stabilized are small. But the fact that it happened in this supposedly stable part of Antarctica “concerns me,” says Morlighem.

Coastal waters in the area have historically been quite cold, but small changes began around 2010. Ocean currents shifted, allowing water that was 0.6 degrees Celsius warmer than before to seep toward the coastline, the researchers reported. last year. This may have accelerated the destruction of the Conger Ice Shelf.

It could also eventually destabilize a massive glacier just 130 kilometers west of Conger: Denman Glacier holds enough ice to raise global sea levels by 1.5 meters if it all slid into the ocean. It alone contains the equivalent of nearly half the ice in West Antarctica. As the Denman flows offshore, it grinds between a floating ice shelf on one side and an island on the other, slowing its progress into the ocean. But the ice that binds it to those stabilizing structures is slowly thinning and weakening. It can eventually break free and accelerate.

“This sector of the East Antarctic Ice Sheet has been very stable,” says Morlighem. Some computer simulations predicted that East Antarctica could even gain mass over the next century. But if Denman and his neighbors are destabilized, “then that completely changes the picture.”