Clues from the past reveal the West Antarctic Ice Sheet’s vulnerability to warming
Ancient sediment records show the ice sheet retreated at least five times during warmer periods millions of years ago
University of Toyama
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By studying Pliocene sediments deposited when Earth was warmer than today, the researchers found that the West Antarctic Ice Sheet retreated far inland at least five times. These findings provide critical insight into how the ice sheet may respond to ongoing climate warming and the potential scale of future sea-level rise.
view moreCredit: Professor Keiji Horikawa from the University of Toyama, Japan
The Thwaites and Pine Island glaciers, located in the Amundsen Sea sector of the West Antarctic Ice Sheet (WAIS), are among the fastest-melting glaciers on Earth. Together, they are losing ice more rapidly than any other part of Antarctica, raising serious concerns about the long-term stability of the ice sheet and its contribution to future sea-level rise.
To better understand the risks that warmer conditions pose to the WAIS, researchers are looking back to the Pliocene Epoch (5.3–2.58 million years ago), when global temperatures were about 3–4 °C higher than today and sea levels stood more than 15 meters higher, with melted ice from Antarctica contributing to much of that rise.
Now, examining a deep-sea sediment from this region, researchers from the IODP Exp379 Scientists, found that the WAIS margin retreated far inland at least five times during the Pliocene period.
The study was led by Professor Keiji Horikawa from the Faculty of Science, University of Toyama, Japan, and included Masao Iwai (Kochi University), Claus-Dieter Hillenbrand (British Antarctic Survey), Christine S. Siddoway (Colorado College), and Anna Ruth Halberstadt (University of Texas at Austin). The findings, made available online on December 22, 2025, and published in Vol. 123 of the journal PNAS on January 6, 2026, highlight the vulnerability of the WAIS to future warming.
“We wanted to investigate whether the WAIS fully disintegrated during the Pliocene, how often such events occurred, and what triggered them,” says Prof. Horikawa.
The team analyzed marine sediments collected during the IODP Expedition 379. The sediments recovered from the Site U1532 on the Amundsen Sea continental rise act as a historical archive, recording changes in ice sheets and ocean conditions over millions of years.
They identified two distinct sediment layers reflecting alternating cold and warm climate phases: thick, gray, and finely laminated clays from cold glacial periods, when ice extended across much of the continental shelf; and thinner, greenish layers formed during warmer interglacial periods. The green color comes from the microscopic algae, indicating open, ice-free ocean waters. Crucially, these warm-period layers also contain iceberg-rafted debris (IRD), small rock fragments carried by icebergs, that broke off from the Antarctic continent. As these icebergs drifted across the Amundsen Sea and melted, they released this debris onto the seafloor.
The team identified 14 prominent IRD-rich intervals between 4.65 and 3.33 million years ago, each interpreted as a major melt event when the WAIS partially retreated.
To determine how far inland the ice had retreated, the researchers analyzed the chemical “fingerprints” of the sediments. They measured isotopes of strontium, neodymium, and lead, which vary depending on the age and type of the source rock. By comparing these signatures with those of modern seafloor sediments and bedrock samples from across West Antarctica, the team traced much of the debris to the continental interior, particularly the Ellsworth-Whitmore Mountains.
The sediment record reveals a consistent four-stage cycle of warming and cooling. During cold glacial periods, the ice sheet was extensive and stable, covering the continent. As the climate warmed, during the early interglacial stage, basal melting began, leading to the inland retreat of the ice sheet. At peak warmth, during the peak interglacial stage, large icebergs calved from the retreating ice margin and transported sediment from the Antarctic interior across the Amundsen Sea. As temperatures cooled again, during the glacial-onset stage, the ice sheet rapidly regrew, pushing previously deposited sediments toward the shelf edge and transporting them further downslope into deeper waters.
“Our data and model results suggest that the Amundsen Sea sector of the WAIS persisted on the shelf throughout the Pliocene, punctuated by episodic but rapid retreat into the Byrd Subglacial Basin or farther inland, rather than undergoing permanent collapse,” says Prof. Horikawa
The findings indicate that the WAIS has undergone retreats far beyond its current extent, underscoring its extreme vulnerability to future warming and its potential to drive substantial sea-level rise.
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Reference
DOI: 10.1073/pnas.2508341122
About University of Toyama, Japan
University of Toyama is a leading national university located in Toyama Prefecture, Japan, with campuses in Toyama City and Takaoka City. Formed in 2005 through the integration of three former national institutions, the university brings together a broad spectrum of disciplines across its 9 undergraduate schools, 8 graduate schools, and a range of specialized institutes. With more than 9,000 students, including a growing international cohort, the university is dedicated to high-quality education, cutting-edge research, and meaningful social contribution. Guided by the mission to cultivate individuals with creativity, ethical awareness, and a strong sense of purpose, the University of Toyama fosters learning that integrates the humanities, social sciences, natural sciences, and life sciences. The university emphasizes a global standard of education while remaining deeply engaged with the local community.
Website: https://www.u-toyama.ac.jp/en/
About Professor Keiji Horikawa from the University of Toyama, Japan
Keiji Horikawa is a Professor in the Faculty of Science at the University of Toyama, Japan, and a geochemist specializing in paleoceanography and paleoclimate research. His work focuses on reconstructing past climate and ocean conditions through geochemical analyses of marine sediment cores. He participated in the International Ocean Discovery Program Expedition 379 to the Amundsen Sea in 2019 and studies the response of the West Antarctic Ice Sheet to warm Pliocene climates. He heads the Horikawa Lab for Paleoceanography and Geochemistry, which aims to improve understanding of Earth’s climate system.
Funding information
This work was supported by JSPS KAKENHI Grant Numbers JP21H04924 and JP25H01181 and JP21H03590, JP23K21746, and JP25K03252 and was conducted by the support of Joint Research Grant for the Environmental Isotope Study of Research Institute for Humanity and Nature, and partly carried out under the Joint Research Program of the Institute of Low Temperature Science, Hokkaido University (23G056).
C. Siddoway’s contributions were supported by U.S. NSF awards 1917176 and 1939146 was funded through the Natural Environment Research Council (NERC) UK IODP grant NE/T010975/1. E.A. Cowan was supported by a postexpedition award from the U.S. Science Support Program of IODP.
Journal
Proceedings of the National Academy of Sciences
Method of Research
Observational study
Subject of Research
Not applicable
Article Title
Repeated major inland retreat of Thwaites and Pine Island glaciers (West Antarctica) during the Pliocene
Article Publication Date
6-Jan-2026
Binghamton University
When we think of global warming, what first comes to mind is the air: crushing heatwaves that are felt rather than seen, except through the haziness of humid air. But when it comes to melting ice sheets, rising ocean temperatures may play more of a role — with the worst effects experienced on the other side of the globe.
A new paper in Nature Geoscience, “Spatially variable response of Antarctica’s ice sheets to orbital forcing during the Pliocene,” explores the complicated dynamics.
While Binghamton University Associate Professor of Earth Sciences Molly Patterson is the first author, the 43 co-authors include several Binghamton alumni, such as Christiana Rosenberg, MS ’20; Harold Jones ’18; and William Arnuk, PhD ’24. The study’s results directly address one of the main goals of the International Ocean Drilling Program (IODP) Expedition 374: to identify the sensitivity of the Antarctic ice sheet to Earth’s orbital configuration under a variety of climate boundary conditions. Because of this, all shipboard science team members are included as co-authors because of their contributions to the data sets used in the article, Patterson explained.
Their research considers the Antarctic ice sheet during the Late Pliocene period, from 3.3 to 2.6 million years ago. From 3.2 to 2.8 million years ago, the global average temperatures were around 2 to 3° Celsius higher than pre-industrial values, in line with the “middle of the road” scenario for climate change, in which temperatures are expected to rise around 2.7°C by 2100.
“Thus, Pliocene records are considered to be useful analogues for understanding what a future with this level of warming might be like,” Patterson explained.
Climate forcing refers to any external factor that causes a change in Earth’s energy balance —incoming versus outgoing heat — and ultimately leads to warming or cooling in the Earth system.
Non-human factors that can affect this energy balance include tectonic changes, volcanic eruptions and shifts in the sun’s energy output, such as sunspot cycles that happen every 11 years. Another factor is “orbital forcing,” or changes in Earth’s orbit around the sun; this has typically driven glacial and interglacial cycles, which have lasted around 100,000 years — at least for the last 800,000 years or so.
The non-human factors that affect the Earth’s climate occur on different time scales, Patterson said.
“Here we are using geological archives to test how these important components of the climate system respond naturally to warmer climates,” she said.
Antarctica is primarily divided into two sectors: West Antarctica, where the ice sheet sits in the ocean, and East Antarctica, where the ice sheet primarily sits on land. During the warm periods of the Pliocene, large parts of West Antarctica and some low-lying areas of East Antarctica experienced significant ice-melt, contributing to a 3- to 10-foot rise in global sea levels.
One of the study’s main conclusions: Under warming conditions similar to the Pliocene, the part of West Antarctica located adjacent to the Pacific Ocean will see its ice disappear at a faster rate. Over the long term, however, both oceanic and atmospheric warming will contribute to rising global sea levels.
You can think of it as an equation of sorts: A warmer climate leads to less sea ice around Antarctica, which then causes the ocean to heat up. Due to the warmer water, the parts of the ice sheet sitting on the ocean melt first. Over time, as the climate continues to warm, the ice sitting on land will also retreat.
“In other words, it’s a one–two punch on the system with a consequence of raising sea levels globally,” Patterson said.
What you may not realize: Because of gravitational effects similar to ocean tides, the loss of ice in the Southern Hemisphere actually has a greater impact on coastlines in the Northern Hemisphere. Conversely, when ice sheets lose mass in the Northern Hemisphere, Southern Hemisphere coastlines are affected more.
With that in mind, New York would be more affected by a 7-meter rise in sea levels from the loss of Antarctic ice than a similar rise from melting ice sheets in Greenland, Patterson pointed out.
Geological archives and modeling experiments provide the long-term context needed to evaluate current changes and help scientists identify the mechanisms that drive the climate system. Ultimately, this research may help us formulate more accurate predictions about our climate change future.
“Basically, geological archives serve as a vital tool for testing the accuracy of climate models used to project future scenarios,” Patterson said.
About Binghamton University
Binghamton University, State University of New York, is the #1 public university in New York and a top-100 institution nationally. Founded in 1946, Binghamton combines a liberal arts foundation with professional and graduate programs, offering more than 130 academic undergraduate majors, minors, certificates, concentrations, emphases, tracks and specializations, plus more than 90 master's, 40 doctoral and 50 graduate certificate programs. The University is home to nearly 18,000 students and more than 150,000 alumni worldwide. Binghamton's commitment to academic excellence, innovative research, and student success has earned it recognition as a Public Ivy and one of the best values in American higher education.
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