Two glaciers in West Antarctica—Thwaites Glacier and Pine Island Glacier—are central to modern concerns about sea-level rise. Together, they account for a large share of Antarctica’s current ice loss. New geological evidence shows that their present instability is part of a much longer pattern. Sediments recovered from the ocean floor reveal that this region experienced repeated episodes of major ice retreat during the Pliocene, when Earth’s climate was only modestly warmer than today. The findings suggest that Antarctic glacier retreat has occurred multiple times in the past under conditions comparable to those the planet may soon face again.
The evidence comes from marine sediment cores drilled offshore in the Amundsen Sea, beyond today’s thinning ice fronts. These cores were collected during IODP Expedition 379 and preserve a layered history of how the edge of the West Antarctic Ice Sheet behaved over millions of years. The Pliocene period, spanning roughly 5.3 to 2.58 million years ago, is of particular interest because global temperatures were about 3–4°C higher than today and sea levels stood more than 15 meters higher. Geological evidence indicates that Antarctic ice contributed substantially to that rise.
Within the sediment cores, researchers identified repeating patterns that correspond to alternating cold and warm climate phases. Thick gray clay layers reflect colder glacial periods when ice expanded across the continental shelf. In contrast, thinner greenish layers indicate warmer interglacial conditions. The green coloration comes from microscopic algae, signaling open water and reduced sea ice. These warmer layers also contain iceberg-rafted debris—small rock fragments dropped onto the seafloor as icebergs melted. Between 4.65 and 3.33 million years ago, scientists identified fourteen particularly strong debris-rich intervals, each marking a major pulse of Antarctic glacier retreat and intensified iceberg calving.
To determine how extensive these retreats were, the research team analyzed isotopes of strontium, neodymium, and lead in the debris. These geochemical fingerprints vary across West Antarctica depending on bedrock type and age. Many of the samples matched rocks from deep within the continent, especially the Ellsworth–Whitmore Mountains. Because these mountains lie far inland, their presence in offshore sediments implies that ice margins retreated deep into the interior, excavating and transporting material before calving into the ocean. The data indicate that the ice sheet retreated far inland at least five separate times during the Pliocene, reinforcing the idea that Antarctic glacier retreat can be rapid and extensive under relatively moderate warming.
Crucially, the sediment record also shows that these collapses were not permanent. The ice sheet followed a recurring four-stage cycle: stable expansion during cold periods, retreat as basal melting increased, peak iceberg calving during warm intervals, and rapid regrowth as temperatures cooled. Even so, each retreat phase likely drove significant sea-level rise while it was underway. The Pliocene record therefore delivers a clear warning. As modern warming continues and grounding lines migrate inland, the Amundsen Sea sector may again approach thresholds that trigger abrupt Antarctic glacier retreat, with lasting consequences for global coastlines.
https://www.earth.com/news/west-antarctic-ice-sheet-has-collapsed-many-times-before