Scientists See Converging Evidence of Antarctic Ice Retreat

Drilling through 500 feet of floating ice into the Antarctic Ocean floor, climate scientists have retrieved a rare 23-million-year record of sediments that helps demonstrate why the planet’s southern ice shield could determine the fate of distant low-lying coastal areas.

The layers of rock, silt and fossils are like pages in a book of geological time, revealing how West Antarctica’s vast ice sheets and floating shelves respond rapidly to modest warming, with significant shrinking and melting in climates similar to today’s.

Along with other new modeling studies and analyses of current ice retreat, the core sample of ocean sediments affirms that human-caused warming is triggering an irreversible long-term meltdown that could submerge the southern third of Florida and other low-elevation coastal areas within two to three centuries.

The lines of evidence from paleoclimatology, as well as from modeling and observations, also converge to suggest that the average global sea level rise in the more immediate future will accelerate, reaching 3 feet by the end of the century and up to 5 feet in equatorial island regions, potentially displacing millions of people worldwide.

The landmark drilling expedition at the remote edge of the Ross Ice Shelf is part of a wide-ranging effort “to answer the question of when and under what conditions the West Antarctic Ice Sheet will disappear,” said Johann Klages, a geoscientist at the Alfred Wegener Institute in Germany and co-coordinator of the international SWAIS2C project to assess West Antarctica’s vulnerability to 2 degrees Celsius (3.6 degrees Fahrenheit) of human-caused warming above the pre-fossil fuel era baseline.

The world could reach that mark by 2050, sooner than expected, according to recent warnings from retired NASA climate scientist James Hansen. And exactly how that level of warming will affect Antarctica’s vast ice fields is the question the 29-member ice core team sought to answer when they set up camp for 10 weeks. Their equipment had to be carried by motorized vehicles across more than 600 miles of ice.

Previous attempts to drill the seabed beneath the ice had failed because of technical and logistical challenges, but succeeded this time with critical technical support from the U.S. Antarctic Program and the National Science Foundation.

Binghamton University associate professor Molly Patterson, co-chief scientist of the SWAIS2C mission, said the core’s sediment layers show how the ice advanced and retreated, but determining the exact timing requires detailed geochemical analyses that could take years to complete.

The sediment core is more than 200 meters deep, an unusual depth compared to previous sub-ice sediment cores, which rarely exceeded 10 meters, providing a continuous record of climate conditions spanning epochal swings between ice ages and warmer interglacial periods.

Reading the core is like reconstructing past environments layer by layer, Patterson said. When the ice is in contact with the seabed, it “bulldozes everything,” leaving coarse, mixed debris. But layers of finer mud, studded with larger stones that dropped from melting ice shelves, suggest floating ice. And when those layers contain fossils of light-dependent organisms like plankton, it signals open water, with no ice. Along with a chemical analysis to date the materials, the scientists can tell where the ice margin was and what the ocean temperatures were at that time.

There are other sediment samples from around the edge of the Antarctic continent, but not such a deep sediment core from the interior of an ice sheet, said Ed Gasson, a glaciologist, associate professor at the University of Exeter in England and member of the SWAIS2C team.

“This is important,” Gasson said, “because it tells us directly that this part of the ice sheet, which we think is especially vulnerable to a warming climate, retreated in the geological past, leaving behind open seawater.”

Climate Dominoes Are Falling

Two other studies published in the past month add to concerns about the potential vulnerability of Antarctica’s ice. One research team mapped how Antarctica’s ice flows through interconnected basins, showing how melting in one region can destabilize others, accelerating ice loss and sea levels rise. A separate analysis, based on measurements of West Antarctica’s Thwaites Glacier, tested the models’ accuracy and showed that the current rate of ice loss is consistent with long-term projections of significant melting.

The sediment record shows what happened yesterday, and the observations of the Thwaites Glacier provide information about recent melting. But the big question remains what will happen tomorrow, said Jonathan Donges, an Earth system researcher at the Potsdam Institute for Climate Impact Research in Germany and co-author of the paper that examined the dynamic links among the continent’s vast tracts of ice. 

By mapping how the icy basins interact, the researchers showed that melting in one area could spread and destabilize others, potentially pushing the entire system past a critical threshold that would lock in thousands of years of sea-level rise.

Such long time scales are easy to overlook, even if the consequences are profound, Donges said. People tend to focus on what happens by 2100, he added, but the real story is what we’re setting in motion over the next few decades that will play out in the coming centuries. His team looked at how the ice basins may affect one another because previous research showed that different areas have different thresholds for significant melting; the loss of even a single basin could raise sea level by 20 or 30 feet. 

The Amundsen Sea sector in West Antarctica has already been identified as one of the regions most vulnerable to current levels of warming, he said, adding that its ice will retreat for centuries even if global temperatures stabilize.

As the ice thins and pulls back, the boundary where ice meets ocean and bedrock retreats inland, exposing more open water and accelerating further melt. Over time, that retreat spreads inland, altering ice flow and destabilizing neighboring basins in a cascading process that pushes melting deeper into the continent.

The new paper on the Thwaites Glacier noted faster ice flows, thinning at the glacier’s edges, increasing structural weaknesses and the retreat of the grounding line, which anchors the ice to the seafloor. 

All the findings reinforce the growing scientific understanding that, once key parts of the ice sheet begin to weaken, feedback processes can take over, allowing retreat to continue long after the initial warming.

Donges said that even conservative estimates of about 13 feet of sea-level rise by 2300 seem massive.

“When you’re standing at the beach somewhere, it’s hard to imagine this could happen in the amount of time the United States has existed,” he said.

And it could be even worse. The projections for that amount of sea-level rise don’t include a rapid, major collapse of ice shelves, Donges said, explaining that some studies suggest feedback could cause big tracts of ice to disintegrate faster than expected. 

If there is extreme warming and melting on the surface, meltwater “pours into deep cracks and then refreezes, widening the rifts,” he said. “This cracks up the ice sheets, which can then disintegrate much faster.”