Nine thousand years ago, long before industrial smokestacks and jet engines, East Antarctica experienced a sudden and dramatic loss of ice. New research shows that this was not a slow, gentle response to a slightly warmer world, but a rapid chain reaction driven by the ocean itself — the kind of runaway process that could be reawakening under today’s human-driven warming.

The work, reported in a popular summary by ScienceDaily under the title “9,000-year-old ice melt shows how fast Antarctica can fall apart” and in a detailed article in Nature Geoscience, combines decades of fieldwork with cutting-edge modelling. Researchers collected marine sediment cores from Lützow-Holm Bay, near Japan’s Syowa Station on the Sôya Coast, during successive Japanese Antarctic Research Expeditions. These sediments, taken from the seafloor by the icebreaker Shirase over more than 40 years, preserve microscopic fossils, chemical fingerprints and isotope ratios that together act like a time machine, revealing how the ice and ocean changed after the last Ice Age.

By analysing beryllium isotope ratios (10Be/9Be) alongside sedimentological, micropalaeontological and geochemical data, the team reconstructed a sharp transition in the bay. Around 9,000 years ago, their records show a sudden intensification of warm Circumpolar Deep Water — a relatively salty, heat-rich water mass that circles Antarctica at depth. When that warm deep water spilled onto the continental shelf and into the bay, it attacked the underside of floating ice shelves that had been buttressing the inland ice sheet. Once those shelves broke up, they no longer held back the ice behind them, and glaciers accelerated toward the ocean.

This was not just a local mishap. To find out why the warm deep water suddenly intensified, the scientists turned to high-resolution climate and ocean-circulation models. Those simulations reveal that meltwater from other parts of Antarctica, including the Ross Ice Shelf region, spread across the Southern Ocean, freshening the surface layer. Fresher surface water is lighter, so it tends to sit on top of denser, saltier water. That strengthens vertical stratification — the layering of the ocean — and makes it harder for cold surface water to mix downward.

The consequence is counter-intuitive but deadly for ice: with mixing suppressed, the pool of warm deep water can move more easily towards the continental shelf without being cooled on the way. In the simulations, this process funnels heat into submarine troughs that lead straight toward East Antarctic ice shelves in Lützow-Holm Bay. Extra melt at the base of the shelves then produces even more freshwater, which maintains the strong stratification and keeps the warm water coming. The result is a self-reinforcing loop, a “cascading positive feedback” in which melting in one sector of Antarctica helps destabilise ice in others via large-scale ocean circulation.

The Nature Geoscience study shows that this loop did not just thin the floating ice; it also triggered inland ice-sheet thinning and retreat. Once the buttressing ice shelves collapsed, grounded ice retreated tens of kilometres, and sea level rose accordingly. The ability of a relatively small regional disturbance to drive continent-scale change is precisely what worries scientists today.

For decades, much of the focus has been on West Antarctica, where glaciers like Thwaites and Pine Island are already being eroded from below by warm deep water. Observations and previous modelling work have shown that Circumpolar Deep Water is increasingly intruding onto the continental shelf there, accelerating basal melt and reducing the stability of the ice sheet. What this new reconstruction adds is powerful evidence that similar ocean-driven processes can also undermine East Antarctica — the vast “sleeping giant” that stores more than half of Earth’s freshwater in its ice.

That is the core of the warning. East Antarctica has often been treated as relatively secure on human timescales, while West Antarctica has been framed as the main source of near-term sea-level risk. But the sediments from Lützow-Holm Bay, together with the models, show that once the right feedbacks are activated, warm water can carve pathways into East Antarctic embayments too. The same kind of stratified, meltwater-reinforced ocean structure that helped collapse ice shelves 9,000 years ago is now being observed and simulated again in parts of the Southern Ocean.

The early Holocene world in which this ancient collapse occurred was shaped by natural changes in Earth’s orbit, which gradually warmed the high latitudes after the last Ice Age. Today, the driver is different: human-made greenhouse gas emissions are heating the planet much faster, and pushing atmospheric carbon dioxide to levels far above anything seen in that period. But the physics connecting ice and ocean has not changed. Warm water still melts ice; meltwater still reshapes the ocean; and the resulting feedbacks can still accelerate ice loss far beyond simple, linear projections.

The implications for coastal societies are profound. The Antarctic Ice Sheet as a whole contains enough frozen water to raise global sea levels by around 58 metres if it were all to melt, and even small percentage losses translate into substantial centimetres or metres of sea-level rise. Infrastructure built for a slow, predictable rise could be overwhelmed if tipping-like behaviour in the ice-ocean system leads to bursts of rapid retreat. The study shows that such behaviour is not a remote, theoretical possibility, but something Antarctica has already done in the recent geological past.

The research is also a reminder that international collaboration is essential to understand and respond to the climate crisis. More than 30 institutions contributed to the new findings, combining field campaigns, marine geology, cosmogenic nuclide dating, and sophisticated coupled climate–ocean models. That same scale of effort will be needed to keep monitoring the Southern Ocean, improve projections of ice-sheet change and translate those projections into concrete adaptation and mitigation plans for vulnerable regions.

Above all, the 9,000-year-old meltdown is a wake-up call from deep time. It shows that once Antarctic ice begins to retreat in earnest, the ocean can help turn local melt into a continent-wide cascade. Continuing to heat the planet increases the chances of crossing thresholds after which ice loss becomes difficult or impossible to halt on human timescales. The choices made this decade about cutting emissions and protecting the cryosphere will determine whether the next great Antarctic collapse remains a story told by ancient sediments — or becomes a disaster unfolding along today’s coastlines.