The ice sheets covering Antarctica contain enough water to raise sea levels by 57 meters—enough to cause flooding in low-lying coastal communities around the globe.
A recent study published in Nature Geoscience showed that the West Antarctic Ice Sheet went through a period of rapid melting around 8,000 years ago—and questioned whether this turn of events in the past could be a sign of what’s to come in the future.
The melting of the West Antarctic Ice Sheet would raise the sea level by about 5 meters, said Eric Wolff, a study coauthor and climatologist at the University of Cambridge. And it sits on bedrock that is below sea level, making it particularly vulnerable to a warming ocean, he explained. As sea levels rise, water could seep underneath the ice sheet and unground it, causing it to retreat. The new research indicated that that’s probably happened before.
Getting to the Core of It
To study glacial conditions, researchers drill into the ice and withdraw cylindrical ice cores. The layers of glacial ice revealed by the cores form as snow is compacted. When snow falls, pockets of air get trapped by subsequent flurries, preserving the atmosphere from when they were buried in nearly pristine condition.
Using ice cores, “we can literally analyze the atmosphere from the past,” said Isobel Rowell, a postdoctoral researcher at the British Antarctic Survey and a study coauthor.
In 2019, scientists and engineers spent 45 days drilling 651 ice cores at the Skytrain Ice Rise, a large mound of ice on the edge of the Ronne Ice Shelf, a section of the West Antarctic Ice Sheet that floats in the Weddell Sea. Skytrain itself sits on a dome of bedrock that rises above the rest of the seabed. Its ice has “never [been] overridden by ice from somewhere else,” Wolff said, so the cores collected could reveal a detailed climate history of the hilly location.
Pressure, Isotopes, Sodium
Researchers wanted to measure how Skytrain’s elevation had changed over time. They did this by assessing the pressure, isotopes, and sodium trapped in bubbles at different layers in the ice.
The air pressure of bubbles trapped in the cores is a good proxy for both temperature and elevation, Wolff explained. Lower pressure can often indicate lower temperatures and higher elevations—just think of thinner air on icy mountaintops, he said.
Scientists also measured water isotopes as proxies for past temperatures. Isotopes indicate the temperature at the time the snow fell, and in ice dating to 8,000 years ago, researchers found an anomaly. “There was what looked almost like a discontinuity in the water isotopes—where the temperature rapidly increased by several degrees over a couple of hundred years,” Wolff said.
There was no significant rise in temperature across other parts of Antarctica at the time, so the researchers inferred that the isotope and temperature jumps at Skytrain were connected to a loss in its elevation.
Finally, researchers noted the sodium content of the ice cores. Sodium could come from either the open ocean or the sea ice surface, both of which are salty. A change in sodium content “tells us as a first-order effect that the ocean got nearer to us. In this case, higher sodium concentration says that the ice shelf edge retreated so that Skytrain was nearer the sea,” said Wolff.
Using the pressure, isotope, and sodium data, researchers surmised that toward the end of the last ice age, warmer ocean water must have flowed underneath the West Antarctic Ice Sheet. Part of the ice sheet detached from the bedrock below and floated, forming what is now the Ronne Ice Shelf.
The area where Skytrain is located was no longer restrained by grounded ice, which caused the ice to shrink. Within just 200 years, the ice at Skytrain thinned by 450 meters—about the height of the Empire State Building. “Once you unground the ice, it really is going to retreat very fast,” Wolff said.
Learning from the Ice
Ice sheet models predict that the West Antarctic Ice Sheet could melt in the next few centuries, but there is still uncertainty about when, where, or how quickly the melting will occur. Ice cores such as those from the new study can help make model predictions more accurate by offering insight into how the ice sheet behaved during previous periods of warming.
“That ice has got to go somewhere. It’s going into the ocean, it’s melting, and that’s causing a rise in sea level.”
“The new work provides really strong evidence of past changes in an important part of the ice sheet, involving processes that are likely to be important in the future,” said Richard Alley, a geologist at Pennsylvania State University.
“We have known for a while that the Antarctic ice advanced during the Ice Age, often to near the edge of the continental shelf, and that as the Ice Age ended, the warming ocean and rising sea level forced a major retreat. That [research] offers a wonderful target for model testing and improvement.”
Understanding the parameters of how ice melted in Antarctica in the past, Rowell said, may help give some context to what it might do in the future—and what that might mean for sea level rise. “That ice has got to go somewhere. It’s going into the ocean, it’s melting, and that’s causing a rise in sea level. We’re not saying that this exact same thing is going to happen, but it helps us to understand the mechanisms of how things have changed in the past.”
—Rebecca Owen (@beccapox), Science Writer