West Antarctica’s Thwaites and Pine Island Glaciers are losing ice at a rate not seen in the past 5,500 years. This is the conclusion of a new study that carbon-dated seashells and penguin bones collected from neighboring islands, reported in Nature Geoscience.
Thwaites and Pine Island Glaciers flow from the heart of the Western Antarctic Ice Sheet into the Amundsen Sea along a 250-kilometer front. They rest below sea level against a backward slope and are melting rapidly. Scientists worry that if the glaciers were to retreat from their current grounding lines, there aren’t any known topographic highs that could stabilize them, setting the stage for rapid collapse. If they disappear, they could potentially take with them the entire Western Antarctic Ice Sheet, which holds enough water to raise the global sea level by 3.4 meters over 1 or 2 centuries, authors of the new paper report.
Scientists have long wondered whether these glaciers might have experienced a similar retreat process in the past and have somehow recovered. If this happened, there is hope that the glaciers will not totally disappear and could make a comeback over time.
Answering this question is one of the goals of the International Thwaites Glacier Collaboration (ITGC), a British-American research initiative launched in 2017 aimed at studying the glacier and learning more about its current ailments.
To learn about the two glaciers’ pasts, a group of scientists from the ITGC looked at the evolution of the relative sea level in islands near both Thwaites and Pine Island Glaciers.
These islands, like most parts of Antarctica, are still bouncing up after the last glaciation when they were buried under the weight of mighty ice sheets. As a result of this postglacial rebound, beaches that formed at the shoreline thousands of years ago now stand several meters above sea level. By dating those beaches, the researchers sought to understand how sea level evolved over the millennia. If during that time the glaciers had significantly increased their size, the uplift rate should have slowed down or even reversed.
Buried Seashells and Penguin Bones
Scott Braddock and Meghan Spoth, two graduate students at the University of Maine, visited five islands in the Lindsey, Schaefer, and Edwards archipelagos to find out when some of the paleobeaches formed. The islands lie just a few kilometers from the glaciers and were likely beneath them until 8,000 or 9,000 years ago.
In early 2019, after a 2-month cruise on the Nathaniel B. Palmer icebreaker, both researchers hopped on a Zodiac for a few day trips to collect samples. They were looking for biological remains that they could carbon-date later in the lab. Bits of limpet shells and penguin bones were the most interesting findings for their purposes.
“The beaches were pretty frozen, so we had to use shovels and pickaxes, and we could probably get down to about a half a meter at the most,” Braddock said.
“The shells are fantastic because they are most likely to give us the actual age of the beach when it formed,” he continued. Although the mollusks were likely living offshore and became buried when the beaches formed, penguin bones accumulated after the beaches stabilized and the birds made their nests, giving researchers upper limits to determine the age of the beaches. “We have these two different materials that we could date, helping us really constrain the age of each beach.”
The samples, collected from several beaches at different altitudes, showed that the local sea level has been falling steadily at a rate of 3 to 4 millimeters a year for the past 5,500 years. Although there’s not a one-to-one correlation between the glaciers’ rate of retreat and bedrock uplift, the authors think that the safest interpretation is that the glaciers have remained relatively stable.
“It appears from our study that there isn’t any support for this idea that maybe Thwaites can get a lot smaller and then recover,” said Brenda Hall, a professor in the School of Earth and Climate Sciences at the University of Maine and coauthor of the new study. “That’s not to say that it can’t do it, it’s just we don’t have any evidence that it did it.”
“We sort of expected we might find evidence of a major readvance, and so it was a little bit surprising that we didn’t,” Hall said. “That hypothesis has been out there as sort of a hope for the future, I guess—that if it did it once, it can do it again sort of thing. We don’t have that hope at the moment.”
Although the new data don’t fundamentally change our understanding of the potential for rapid retreat of Thwaites, they “provide targets for model testing and insights for model improvement,” said Richard Alley, a professor of geosciences at Pennsylvania State University who wasn’t involved in the study. “These are the sort of data needed to reduce uncertainties and increase confidence in results and form part of the framework that will support better projections of sea level rise,” he added. “This is really great science.”
Thwaites’s past stability is in stark contrast to the present scenario. Recent estimates show that Thwaites and Pine Island Glaciers contribute about 1.5 millimeters to global sea level every year, losing ice at a rate of nearly 90 billion metric tons annually. In response, the bedrock below them is lifting at one of the fastest rates recorded on Earth, 41 millimeters every year, as shown by GPS measurements dating from 2014.
“There’s a huge difference there, so that’s suggesting the changes going on today are dramatic and much different than any changes that our record shows in the past 5,000 years,” Braddock said.
—Javier Barbuzano (@javibarbuzano), Science Writer