Antarctica’s Thwaites Glacier is currently losing significant mass, contributing to around 4% of all global sea level rise. Now, new research suggests that the start of Thwaites’s current retreat aligns with that of the nearby Pine Island Glacier, which is also losing mass rapidly.
“It’s really great confirmation to see the two glaciers come in with such similar age models.”
The findings, published in the Proceedings of the National Academy of Sciences of the United States of America, indicate that the mass loss was more likely spurred by regional conditions, such as an El Niño event, rather than dynamics unique to the glaciers themselves.
Clues in Cores
Thwaites and its neighbor, Pine Island Glacier, are part of the West Antarctic Ice Sheet, the area of the continental ice sheet that is retreating most quickly. If Thwaites continues to retreat at current rates, it will contribute several centimeters to global sea level rise by 2100 and has thus been nicknamed the “Doomsday Glacier.”
Scientists have observed accelerating ice loss from Thwaites since the 1970s, mostly via satellite data. But satellite data are “really not enough of a record to understand what controls such a big system,” said Julia Wellner, a glacial geologist at the University of Houston and an author on the paper. Understanding the glacier’s presatellite past helps scientists know “what the ice is capable of,” she said. “How fast can it really change? And what mechanisms drive that change?”
In the new study, researchers used marine sediment cores collected from near the Thwaites Glacier in 2019. The team dated the sediment in the cores using lead-210, a radioactive isotope that’s present in the ocean and binds to sediment as it settles onto the seafloor. The ratio of lead-210 in sediment compared to its radiogenic daughter products can tell scientists when the sediment was deposited.
The cores also showed a visible transition from sediment deposited beneath a glacier to marine sediment, marking when the glacier retreated from its foothold on the ocean floor. The analysis is “amazing, quality scientific work,” said David Holland, a physical climate scientist at New York University who was not involved in the new research.
The analysis showed that Thwaites likely began to retreat around the 1940s, coinciding with the beginning of a retreat phase at neighboring Pine Island Glacier that had been determined by previous research.
The results are in line with evidence provided by a series of parallel ridges in the seafloor near Thwaites, which researchers say indicates a period of rapid retreat that likely occurred in the 1940s.
Evidence that both glaciers began retreating around the same time indicates that the glacial mass loss is driven not by factors unique to the glaciers themselves, such as their shapes or the structures of their internal plumbing, but by external factors such as region-wide shifts in climate.
In particular, a prolonged El Niño that occurred from 1939 to 1942 could have spurred the retreat of both glaciers, according to the authors. El Niño events tend to bring warmer-than-average temperatures to the Southern Ocean and cause warm water to flow onto the continental shelf upon which the Thwaites Glacier sits, according to the authors.
A Shifting System
The radiocarbon dating done by the researchers showed that the edge of the Thwaites Glacier was near its current position around 9,400 years ago and was relatively stable until modern retreat started around the 1940s.
The study implies that even a short-term change in regional climate, such as the 1940s El Niño, can cause long-term glacial retreat, said study coauthor James Smith, a marine geologist at the British Antarctic Survey, in a statement.
It makes “perfect sense” that an El Niño event would kick off the retreat, Holland said. Distant changes in the tropics, such as an event in the El Niño–Southern Oscillation weather phenomenon, can create wind patterns and ocean conditions near Thwaites that would lead to glacial retreat, he said.
“Once the system is kicked out of balance, the retreat is ongoing.”
Why the glaciers did not quickly recover from the 1940s perturbations is an open question, according to Wellner. She hypothesized that multiple destabilizing factors in addition to an El Niño event could have combined to weaken the glacier. Additionally, both Thwaites and Pine Island Glaciers are grounded in very deep water, meaning that once their footholds are lost, it’s very difficult for them to gain back any lost mass.
“Once the system is kicked out of balance, the retreat is ongoing,” Wellner said in a statement.
Attributing the start of the glacial retreat to the 1940s El Niño event or to other causes such as human-caused climate change is a task for a future analysis, said Wellner. “Because we know these two glaciers are retreating in conjunction with each other, we are looking for external drivers. And the external drivers that happen around the right time are increased anthropogenic warming,” she said. But directly pinpointing the cause of the retreat is a “step farther” than what the new paper shows, she said.
—Grace van Deelen (@GVD__), Staff Writer