The ice disappears faster on Pine Island and Thwaites in Antarctica glaciers than at any time in the last thousand years. Researchers came to this conclusion after examining ancient penguin bones and limpet shells in these places. Data point to rapidly rising ground levels – a sign of ice loss.
The news is worrying because of the role that these ice masses play. Thwaites, for example, has been nicknamed “Domeday Glacier” because of how much the sea level can rise when it melts.
The Thwaites and Pine Island glaciers are two of Antarctica's fastest shrinking. Researchers are now worried that they are not only unstable but may also undergo a rampant retreat. The researchers looked at old bones and shells reconstruct the history of glaciers. They wanted to know if these glaciers had ever been so small before.
“If the ice has been smaller in the past,” conditions today may indicate “we are not necessarily in a rampant retreat, ”explains Brenda Hall. She is a glacial geologist at the University of Maine in Orono. Unfortunately, she says, the new finding “gives us no consolation. We can not disprove the hypothesis.” That being said, this study also does not yet prove that a rampant retreat has begun.
Her team described their results on June 9 Natural geoscience.
Melting ice allowed the country to rise
Pine Island and Thwaite Glaciers are located in a wide ocean basin. It is shaped like a bowl, deeper towards the middle. This makes the ice vulnerable to warm currents of dense, salt water that surrounds the seabed. Researchers have worried that when the glaciers retreat further inland, their melting could become unstoppable. This ice loss can take place over centuries. If this happens, it may end up raising the sea level by about one meter (3.3 feet).
To understand how glaciers have changed over thousands of years, scientists turned to ancient penguin bones and seashells. Scott Braddock had collected them during a research cruise on an American icebreaker. He is a glacial geologist in Hall's lab.
One afternoon in 2019, Braddock climbed from the icebreaker to a bouncing rubber boat. He landed on Lindsey 1's barren beaches. It is one of a dozen or more rocky islands located about 100 kilometers (62 miles) from where the Pine Island Glacier meets the ocean. As Braddock climbed the slope, his boots slipped over rocks covered with pingvinguano. Annoying white feathers also dotted the rocks.
Then he came upon a series of ridges. Storm waves had piled up their rocks and pebbles thousands of years earlier. These ridges marked old shorelines.
Since the last ice age ended about 12,000 years ago, this island would have been completely submerged under the sea. But over time, nearby glaciers shed billions of tons of ice. Losing all that weight allowed the earth crust to run up. It was as if mattress springs bounced back after a heavy person got out of bed in the morning. The rising force from the earth pushed the top of Lindsey 1 and other nearby islands out of the water. Not much. At least not first. They rose only a few millimeters (maybe a tenth of an inch) each year.
When Lindsey 1 rose, a series of new shorelines formed along the island's edge. Over time, each new shoreline was elevated and created another. The older shorelines were eventually further away from the range of the waves.
How much the country moved upwards depended on how much ice the nearby glaciers lost. The ages and heights of the former coastlines told scientists how fast the islands had risen. This in turn allowed them to calculate how quickly Pine Island and Thwaite's glaciers had retreated – and if anyone had ever been smaller than it is today.
Descriptive clues to glacier retreats
The key to these analyzes was the ancient cone-shaped limpet shells and marble-sized fragments of penguin bones that Braddock broke from the rocky ridges. The rubbish he picked up had been left long ago, when the shorelines were formed.
Back in Maine, Braddock and others used radiocarbon dating to estimate the age of these shells and bones. This allowed the researchers to date almost two dozen shorelines, spread over several nearby islands.
The oldest and highest beach was formed 5,500 years ago. From that time until recent decades, the islands rose at a steady rate of about 3.5 millimeters (0.14 inches) per year. Then things changed. The land around Pine Island and Thwaites began to climb much faster in recent decades – by 20 to 40 millimeters (0.8 to 1.6 inches) per year. This indicates that the ice loss from nearby glaciers has soared. And the probable cause, they note, is the rapid emergence of global warming.
“We're entering unknown territory,” says Braddock. “We have no analogue for comparing what is happening today with what happened in the past.”
Slawek Tulaczyk is a glaciologist at the University of California, Santa Cruz. He sees the recently dated shorelines as “an important piece of information.” But he warns against making too many of the findings yet. Yes, these islands are located 100 kilometers (62 miles) from Pine Island and Thwaite Glaciers. But they are also less than 50 kilometers (31 miles) from several smaller glaciers. Changes in these closer glaciers, he says, may have masked what happened on Pine Island and Thwaites long ago.
He still suspects that Pine Island and Thwaites may have retreated to a smaller size – and then grown back to their current size. If this had happened, the retreat and regrowth would have been small, he says. Maybe just a few dozen kilometers (maybe 10 to 30 miles). When it comes to proving whether this happened or not, Tulaczyk says: “I do not think this study will determine that.”
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