◆Research findings by a group led by Professor Masao Iwai of the International Research Institute for Marine Core Science have been published in the U.S. scientific journal *Proceedings of the National Academy of Sciences of the United States of America*
Release Date:
The melting status and scale of the West Antarctic Ice Sheet during the Pliocene Warm Period, which is often compared to future climates
Elucidating the melting conditions and extent of the West Antarctic Ice Sheet
A research group led by Professor Masao Iwai of the International Institute for Marine Core Research at Kochi University, in collaboration with researchers from Toyama University, Nagoya University, and other institutions, has conducted isotopic ratio analyses of seafloor sediment samples collected from the Amundsen Sea in West Antarctica during International Ocean Discovery Program (IODP) *1)Expedition 379*2) (conducted January–March 2019), revealed that during the Pliocene Warm Period, the West Antarctic Ice Sheet*3) underwent a complete melt (equivalent to a 3-meter rise in sea level) extending far inland.
The Pliocene Warm Period, which occurred 3–5 million years ago, is characterized by global average temperatures 3–4°C higher than today and is attracting attention as a model for future warming. However, definitive geological evidence regarding the extent of Antarctic ice sheet melting during that period has been lacking until now.In this study, we conducted isotopic ratio analyses of sediment samples from the Amundsen region of West Antarctica and obtained clear geochemical evidence indicating that the West Antarctic Ice Sheet had retreated significantly far inland at that time. These findings suggest that ongoing global warming could cause large-scale melting of the West Antarctic Ice Sheet, making a sea-level rise of several meters a realistic risk in the future.
The findings of this study were published in the U.S. scientific journal *Proceedings of the National Academy of Sciences of the United States of America* (PNAS) on Tuesday, December 23, 2025 (Japan Standard Time).
[Glossary]
※1) International Ocean Discovery Program (IODP)
A multinational scientific research collaboration project launched in October 2013.By drilling the deep seabed using research vessels provided by Japan (the deep-sea drilling vessel "Chikyu"), the United States (the Joides Resolution), and Europe (mission-specific drilling vessels), the project promotes research aimed at elucidating global environmental changes, the Earth's internal structure, and the biosphere within the crust.
*2) International Ocean Discovery Program (IODP) Expedition 379
This research expedition aims to drill into the continental shelf and the seafloor near the continental shelf in the Amundsen region of the West Antarctic Ice Sheet—where rapid ice loss is currently being observed—and, through analysis of the sediment core samples collected, to elucidate the relationship between past warm periods and the growth and retreat of the West Antarctic Ice Sheet.For details, please refer to the link below.
Announcement of the Start of the International Ocean Discovery Program (IODP) Expedition 379: Exploring the Dynamics of the West Antarctic Ice Sheet through Drilling at the Ice Sheet Margin in the Amundsen Sea
※3) West Antarctic Ice Sheet
The West Antarctic Ice Sheet refers to the ice sheet located on the western side of the Transantarctic Mountains (0–180°W). Since the bedrock of most of this ice sheet lies below sea level, the ice sheet is in direct contact with seawater; consequently, as sea temperatures rise, melting tends to progress more easily from the base of the ice sheet.In the Amundsen Region of the West Antarctic Ice Sheet, melting is currently progressing rapidly due to the influence of this relatively warm seawater.
[Paper Details]
Paper Title:
Repeated major inland retreat of Thwaites and Pine Island glaciers (West Antarctica) during the Pliocene
Authors:
, Keiji Horikawa*, Masao Iwai, Claus-Dieter Hillenbrand, Christine S. Siddoway, Anna Ruth Halberstadt, Ellen A. Cowan, Michelle L. Penkrot, Karsten Gohl, Julia S. Wellner, Yoshihiro Asahara, Ki-Cheol Shin, Masahiro Noda, Miyu Fujimoto, and the Expedition 379 Science Party
*Corresponding Author:
Journal:
Proceedings of the National Academy of Sciences of the United States of America (PNAS)
DOI: 10.1073/pnas.2508341122