A rapid shift in ocean currents could imperil the world's largest ice shelf

Antarctica’s largest ice shelf, buttressing a dozen major glaciers and slowing their flow into the ocean, may be surprisingly sensitive to warming.

Several thousand years ago, the Ross Ice Shelf and the glaciers feeding it thinned dramatically, causing sea level to rise. A new study, published April 23 in Nature Communications, suggests this was triggered by a rearrangement of ocean currents set off by a minor amount of ocean warming — just half a degree Celsius.

“This suggests that it’s actually not that hard to drive change here,” says Christina Hulbe, a glaciologist at the University of Otago in Dunedin, New Zealand, who was not involved in the study. With ocean temperatures in the region temporarily spiking as much as 0.1 to 0.2 degrees above normal in the last seven years, “we’re heading into an ocean that could do it.”

Ocean currents have kept Ross Ice Shelf stable

Even as parts of Antarctica hemorrhage ice, scientists have long believed the Ross Ice Shelf would hold steady. This slab of ice the size of Spain extends off the coastline and floats on waters south of New Zealand.

If the Ross Ice Shelf retreated today, it would open a crucial back door to the West Antarctic Ice Sheet — already losing substantial ice from Thwaites and Pine Island Glaciers — allowing warm ocean currents to melt it from two sides. If the sheet melted entirely, it would raise sea levels enough to put Miami; Newark, N.J.; Charleston, S.C.; and the Bahamas underwater at high tide.  

Currently, the Ross Ice Shelf is shielded from that melting by a natural oceanic conveyor belt. It operates just beyond the shelf’s outer, ocean-facing edge. As ocean water migrates through this area, bitter-cold air causes its surface to freeze, forming a steady procession of meter-thick sea ice.

As the sea ice forms, it ejects salt. The water beneath it becomes saltier and denser, and sinks. Roughly 30,000 cubic kilometers of this cold, dense water forms each year — more than four times the annual flow of the Amazon River. It sinks to the seafloor like a steady waterfall where the outer edge of the ice shelf meets the ocean, blocking the entry of warm water from further north.

“That’s why we see really low melt rates there today,” says Daniel Lowry, a climate scientist at GNS Science in Lower Hutt, New Zealand, who coauthored the new study. Even as some parts of West Antarctica are losing over 100 billion tons of ice per year, this region has remained stable for decades (SN: 6/9/22).

Slight warming caused a past ice shelf retreat

Many people assumed the Ross Ice Shelf had been stable since the close of the last ice age, roughly 12,000 years ago. But in 2018, scientists reported that it and the glaciers behind it had substantially thinned and retreated before eventually growing back.

A few years later, glaciologists Slawek Tulaczyk and Sarah Neuhaus of the University of California, Santa Cruz, and colleagues reported that this temporary retreat was actually caused by a slight warming from 7,000 to 1,600 years ago.

Scattered evidence of a warm spell around that time corroborates their finding. For example, scientists have found abandoned elephant seal colonies near the Ross Ice Shelf, even though the area is far too cold for this species today.

Now, Lowry and his colleagues have built on that report of a slight warming — and found worrying signs of what might come.

The past ice shelf retreat could easily happen again

The new study used computer simulations to assess how different levels of ocean warming might have impacted the amount of sea ice forming, the amount of cold salty water sinking to the seafloor, the amount of warm water intruding under the ice shelf, and how quickly the ice shelf and glaciers melted.

The researchers found that just 0.4 or 0.5 degrees of ocean warming would have greatly slowed the formation of sea ice in front of the Ross Ice Shelf. This would have weakened the cold, salty waterfall protecting the ice shelf, allowing warmer water further off the coastline to intrude under it.

If this happened today, it would commit humanity to a gradual but dramatic rise in sea level over the coming centuries: The glaciers behind the Ross Ice Shelf hold enough ice to raise sea level by roughly 2 meters.

This minor warming episode in the past “provides the closest analog to what we think may happen in the near future,” says Tulaczyk, who was not involved in the new study.

These findings come at an ominous time. Even as sea ice shrank in the Arctic, it remained stable around Antarctica for decades. But Antarctic sea ice has declined steeply since 2017, especially near the Ross Ice Shelf. Scientists recently reported that the cold, salty waterfall to the Antarctic seafloor is already starting to slow. This is “alarming,” Lowry says. We now know that the ice shelf can easily switch from cold to warm. “The question is, are we observing the switch?”