The Cosmic Watchdog

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

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

“The East Antarctic Ice Sheet holds 10 times as much ice” as West Antarctica, says Mathieu Morlighem, a glaciologist at Dartmouth College who was not part of the study. West Antarctica is already hemorrhaging ice at an alarming rate (SN: 2/15/23). But if the East Antarctic Ice Sheet also retreats, it could dramatically increase the rate of sea level rise over the next several centuries.

The latest reminder of this concern is East Antarctica’s Conger ice shelf, a former slab of floating glacial ice with an area about 20 times that of Manhattan. In 2022, it suddenly fragmented into icebergs, which then drifted apart over the course of 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 rapidly.”

Before disintegrating, the Conger ice shelf had probably existed for thousands of years. It was formed by several neighboring glaciers that oozed off the coastline and floated on the ocean. It was only by chance that Walker noticed its collapse in 2022.

While perusing 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 March 10 of that year— but missing in another one taken six days later.

So began a two-year effort to understand what destroyed it.

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

“We have every reason to think that [these storms] will become more intense in the future” as Earth warms, says Wille, of the Institute for Atmospheric and Climate Science, ETH, in Zurich, Switzerland. Those stronger storms could damage the protective ice shelves that fringe Antarctica’s coastline (SN: 9/25/19).

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 disintegrations were preceded by massive melting on the upper surface in warm temperatures. But Conger was in an area with generally cold air, and melting on its underside was driven by seawater. Looking through archival satellite measurements, Walker and collaborators 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 permeated its thin, brittle ice — allowing salty ocean water to seep in and further weaken it.

The Conger ice shelf had long been stabilized because it pressed against an island 50 kilometers off the coast. But as the ice shelf thinned, it became too weak to withstand those compressive forces. The island became “a slow-motion rock coming through a windshield,” Walker says. Cracks spiderwebbed out from the ice shelf’s point of contact with the island, the new study reports. Then, on March 7, 2022, it broke free of the island, leaving it unsupported in the face of the approaching storm.

Conger’s collapse won’t noticeably impact sea level, because the glaciers it had stabilized are small. But the fact that it happened in this supposedly stable part of Antarctica “worries me,” Morlighem says.

The coastal waters in this 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 intrude toward the coastline, researchers reported last year. This may have hastened the Conger ice shelf’s demise.

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

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