The scale of sea ice’s fingerprints on the Southern Ocean is staggering. The annual freeze-melt cycle moves more freshwater than precipitation minus evaporation and glacial melt combined, and it does so across a zone where the atmosphere and deep ocean exchange heat, salt and carbon. During the Last Glacial Maximum, roughly 19,000 to 23,000 years ago, winter sea ice expanded to approximately twice its modern extent while sea surface temperatures ran about 3.9 degrees Celsius colder than today. Conversely, during the last interglacial around 130,000 to 116,000 years ago, ice core records suggest winter sea ice contracted by as much as 36 percent relative to the Holocene, with models indicating reductions of 40 to 60 percent. These two climate states — one much icier, one much warmer — provide natural experiments for testing how sea ice couples to the rest of the planet.

The first and perhaps most fundamental coupling involves ocean circulation. Deep water from the world’s basins, collectively known as Circumpolar Deep Water, upwells in the Southern Ocean and splits into two branches. The northern branch is freshened by sea-ice melt and subducts to form Antarctic Intermediate Water, while the southern branch loses buoyancy through cooling and brine rejection during ice formation, ultimately producing the dense Antarctic Bottom Water that ventilates the abyss. Proxy evidence shows that during glacial periods Antarctic Intermediate Water shoaled in the southwest Pacific, consistent with expanded sea ice, while the deep Atlantic was filled to a greater volume by poorly ventilated southern-sourced water. Intriguingly, manganese peaks in sediments offshore major bottom-water production sites suggest that the rate of Antarctic Bottom Water formation may actually have been lower during the Last Glacial Maximum, even as its volume expanded — a paradox that highlights how much remains unknown.

Sea ice also acts as a physical guardian of the Antarctic ice sheet itself. A sea-ice barrier dampens ocean swell that would otherwise batter ice shelf fronts, and landfast ice can bond the debris-laden melange that stabilises calving glacier fronts. The disintegration of ice shelves over recent decades, driven by warm currents and rising air temperatures, has drawn attention to this protective role because ice shelves buttress the grounded glaciers behind them and thus regulate sea level rise. The review documents a darker feedback loop: declining summer sea ice exposes ocean surfaces to solar heating, and that warmed water can be advected beneath ice shelves, accelerating basal melt. Reduced winter ice formation likewise weakens stratification in ways that allow warm Circumpolar Deep Water to intrude onto the shelf. In the paleo record, a mid-Holocene expansion of sea ice off Wilkes Land has even been linked to the retreat of the Ross Sea grounding line, whose meltwater outflow cooled downstream waters and promoted ice growth downstream.

The atmosphere is not spared. Sea ice sits directly beneath the polar frontal zone, so even modest shifts in its edge can reorganise atmospheric circulation. Most modelling studies find that shrinking sea ice weakens and shifts the Southern Hemisphere westerly winds equatorward, because a warmer polar region flattens the temperature gradient that drives the jet. This sea-ice effect runs opposite to the poleward wind shift expected from greenhouse warming itself, meaning the two influences partially offset each other. So-called ghost flux experiments, which inject artificial heat fluxes to isolate sea-ice impacts, reveal responses that ripple pole to pole — warming the Antarctic interior, altering katabatic winds, slowing the Antarctic Circumpolar Current, warming equatorial Pacific sea surface temperatures and even thinning Arctic sea ice.

Yet the paleo evidence complicates this tidy picture. Reconstructions of past winds suggest that during the last deglaciation the westerlies shifted poleward by more than four degrees of latitude — more than any model predicts — implying that the sea-ice effect on winds may be overcompensating in simulations. Even more strikingly, millennial-scale Antarctic warming events recorded in ice cores show abrupt sea-ice declines that were not matched by corresponding wind shifts; instead, the winds responded to Northern Hemisphere warming. The review concludes that over millennial timescales, the direct influence of sea ice on atmospheric circulation appears to be a second-order process compared with other climate feedbacks — a humbling result for a variable often assumed to be climatically dominant.

Biology adds another layer of complexity. Sea ice structures Southern Ocean ecosystems into distinct zones, seeding phytoplankton blooms at the retreating ice edge and releasing a springtime pulse of iron that triggers algal growth. Sea ice hosts its own microbial communities, and its melt releases dimethyl sulphide, a sulphur compound that seeds cloud formation and influences albedo. But the glacial record delivers conflicting messages. Sediment cores south of the Antarctic Polar Front show lower productivity during ice ages, which might seem like straightforward light limitation under expanded ice. Yet nitrogen isotope data reveal more complete nutrient consumption during glacials — the opposite of what light limitation would produce. The proxies instead point to reduced upwelling of nitrate-rich deep water, meaning sea ice influenced productivity indirectly through stratification and circulation rather than simply by blocking sunlight.

The carbon cycle is where the stakes are highest. Sea ice caps the ocean, suppressing the release of carbon dioxide from carbon-rich deep waters that upwell in the Southern Ocean. Early box models suggested this capping alone could explain the roughly 80 parts per million drop in glacial atmospheric CO2, but subsequent work showed the mechanism requires implausibly permanent ice cover and is offset by reduced biological uptake. The emerging consensus is subtler: sea ice matters most when it acts in concert with circulation changes. Idealised models show that atmospheric cooling can lower CO2 by about 40 parts per million, but only because expanded ice forces upwelling carbon-rich water to reach the surface beneath the ice, where gas exchange is suppressed — a strengthening of the so-called disequilibrium pump. Proxy timing supports this coupling: during the glacial inception around 115,000 years ago, Antarctic temperatures and sea ice shifted roughly 3,000 years before CO2 began to fall, suggesting that sea ice alone cannot drive atmospheric carbon change without accompanying circulation reorganisation.

The review’s most sobering conclusion is that the relationships are bidirectional, state-dependent and incompletely captured by models. Most global ocean models cannot resolve the coastal polynyas where dense shelf water forms today, instead representing bottom-water formation as an open-ocean process. Summer sea-ice extent during glacial periods remains poorly constrained, and no sediment records exist south of the current summer ice limit. If the net effect of Antarctic sea ice is to enhance ocean carbon storage, its ongoing decline could weaken the ocean’s sink for anthropogenic carbon and shrink humanity’s remaining carbon budget. The past 130,000 years show that sea ice has repeatedly coordinated sweeping changes across the cryosphere, atmosphere, ocean and biosphere — and that understanding its demise requires reading the deep archive of Earth’s climate history with far greater resolution than we currently possess.

Subject of Research: The role of Antarctic sea ice in ocean circulation, ice shelves, atmospheric circulation, marine productivity and the carbon cycle over the past 130,000 years

Article Title: Antarctic sea ice over the past 130 000 years – Part 2: A review of its role in the Earth system

Article References: Antarctic sea ice over the past 130 000 years – Part 2: A review of its role in the Earth system. (n.d.). https://doi.org/10.5194/cp-22-1881-2026

Image Credits: AI Generated

DOI: 10.5194/cp-22-1881-2026

Keywords: Antarctic sea ice, Southern Ocean, paleoclimate, Last Glacial Maximum, last interglacial, ocean circulation, Antarctic Bottom Water, ice shelves, carbon cycle, atmospheric CO2, westerly winds, climate models

The research, led by Fang Qian of Tulane University and the Woods Hole Oceanographic Institution together with Yi Wang, Kassandra Costa, Sarah Shackleton, Sophia Hines, Ingrid Hendy and Sune Nielsen, hinges on an elegant but underused geochemical detective tool: thallium isotopes. Thallium is a trace metal whose two stable isotopes are sorted, ever so slightly differently, by chemical reactions in seawater. When oxygen-rich conditions prevail, manganese oxides form on the seafloor and preferentially scavenge the heavy isotope, thallium-205, leaving the water enriched in the lighter isotope. When oxygen dwindles, manganese oxide burial falters, and the seawater isotopic signature shifts in the opposite direction. Because manganese oxide burial happens across the entire ocean, the thallium isotopic composition recorded in sediments acts as a globally integrated gauge of marine oxygenation rather than a purely local thermometer of one basin’s health.

To capture that global signal, the team analyzed two sediment cores from opposite sides of the planet: core TN041-8JPC from the Arabian Sea, one of the ocean’s most pronounced oxygen minimum zones, and Ocean Drilling Program Site 1017E and its companion Site 1017B on the Southern California Margin. Both locations sit beneath waters that are sensitive to changes in oxygen supply, and both archives delivered what the authors describe as high-resolution records, with average temporal resolutions of roughly 300 to 400 years. That is fine enough to resolve changes that unfold over centuries to millennia, a pace directly relevant to how today’s ocean is responding to warming. The two independently derived records track each other closely, with a Pearson correlation of r = 0.73 and no time lag between them, a strong indication that they are recording a shared, planet-wide signal rather than regional noise.

The story the isotopes tell begins during Marine Isotope Stage 6, the penultimate glacial period that preceded the interglacial. During that icy stretch, global oceanic oxygen was substantially lower than today, consistent with a long-standing picture in which glacial oceans, cut off from vigorous ventilation and loaded with respired carbon, stagnated and lost oxygen. Then came Termination II, the abrupt deglaciation that ended the ice age. As the planet warmed and ice sheets retreated, oceanic oxygen climbed, broadly in step with rising atmospheric carbon dioxide recorded in Antarctic ice cores. By the early part of Marine Isotope Stage 5e, the reconstructed oxygen content of the global ocean peaked at levels close to what we measure today.

Here is where the record takes its surprising turn. Rather than settling into a stable plateau for the remainder of the interglacial, as a simple temperature-driven view of oxygen solubility would predict, the thallium isotope curves show a clear and sustained decline after that early peak. The ocean’s oxygen content dropped even though the climate remained warm, indicating that the interglacial state was transient rather than steady. The decline parallels reconstructed variability in Antarctic temperature and sea-ice extent, pointing the finger at the Southern Hemisphere as the dynamic control knob. When Antarctic sea ice expanded and Southern Ocean ventilation weakened, the deep ocean’s connection to the atmosphere was throttled back, and the global oxygen budget sagged in response.

The Southern Ocean’s outsized role is reinforced by a comparison the authors carried out across the two most recent deglaciations, Termination II and the more recent Termination I that ended the last ice age about 18,000 years ago. In both cases, the timing and shape of the global oxygen recovery line up with changes in Southern Ocean ventilation, suggesting that this high-latitude region, where deep waters rise, exchange gases with the atmosphere and are transformed by sea ice, is the primary driver of both global oceanic oxygen and atmospheric carbon dioxide swings. This builds on the team’s earlier work published in Science Advances in 2024, which reached a similar conclusion for the most recent deglaciation and now appears to be a repeatable feature of Earth’s climate system.

The mechanics behind that control are worth unpacking. The Southern Ocean is the main window through which the ocean’s deep interior communicates with the atmosphere. Around Antarctica, sea ice forms and melts, brine rejection and buoyancy fluxes drive the transformation of surface waters into deep and intermediate waters, and the westerly winds modulate how much old, oxygen-poor water is upwelled to the surface. When sea ice caps the region, that window effectively closes: the deep ocean becomes isolated, accumulates respired carbon dioxide, and loses its oxygen resupply. When the cap retreats, ventilation resumes, carbon dioxide escapes to the atmosphere, and oxygen floods back into the abyss. Ice core records of carbon dioxide and Antarctic temperature, together with sedimentary proxies for sea ice and productivity, all shift in concert with the thallium isotope signal, weaving a coherent mechanistic picture across two glacial cycles.

Not every candidate explanation survives scrutiny in the new data. Changes in the export of organic carbon from the surface ocean, which consumes oxygen as it decays at depth, have long been proposed as a driver of glacial-interglacial oxygen changes. But the compiled productivity records from the Southern Ocean and the deep sea show divergent trends during Marine Isotope Stage 5e compared with the preceding glacial, leaving the net effect of export productivity on global oxygenation ambiguous. The authors also ran a transient, non-steady-state thallium mass balance model, validated with 100,000 Monte Carlo simulations, to convert the isotope record into quantitative estimates of manganese oxide burial and oxygen concentration. The model confirms that the observed isotope shifts require real, time-dependent changes in global redox conditions, not artifacts of sedimentation or local diagenesis, and it brackets the magnitude of the oxygen swing from the glacial minimum toward the early interglacial peak.

Why should a warming world care about an interglacial that ended 115,000 years ago? Because Marine Isotope Stage 5e is one of the best natural analogues for the near-term future: global temperatures and sea levels were higher than preindustrial values, and atmospheric carbon dioxide, while lower than today’s anthropogenic levels, was rising out of a glacial baseline. The new record demonstrates that in such a warm climate, ocean oxygen does not simply follow temperature-dependent solubility, the factor most emphasized in current projections of ocean deoxygenation. Circulation and ventilation, particularly in the Southern Ocean, can dominate the global budget and produce oxygen losses even under warm, high-carbon-dioxide conditions. Since the 1960s, the ocean has already lost measurable oxygen, and oxygen minimum zones have expanded, squeezing the habitat of tropical pelagic fishes and threatening marine ecosystems worldwide.

The implications for climate models are direct. Earth system models that project ocean deoxygenation through the end of this century and beyond need to capture Southern Ocean ventilation processes, including sea ice dynamics, westerly wind shifts and deep water formation, with fidelity, because those processes, not solubility alone, appear to set the pace of global oxygen change. The study’s data and modeling code are openly available through Zenodo, and the authors acknowledge support from the U.S. National Science Foundation. What the ancient sediments ultimately whisper is a caution: the ocean’s oxygen supply is a living, breathing system with a hair trigger, and the last time Earth ran this warm experiment naturally, the breathing faltered mid-interglacial. Whether the modern ocean follows the same script, with humanity’s carbon emissions as the new variable, is now one of the most consequential open questions in climate science.

Subject of Research: Reconstruction of transient global oceanic oxygen variability during the Last Interglacial Period using sedimentary thallium isotopes

Article Title: Transient variability in global oceanic oxygen during the Last Interglacial Period

Article References: Qian, F., Wang, Y., Costa, K. M., Shackleton, S., Hines, S. K. V., Hendy, I. L., & Nielsen, S. G. (2026). Transient variability in global oceanic oxygen during the Last Interglacial Period. Nature Geoscience. https://doi.org/10.1038/s41561-026-02120-z

Image Credits: AI Generated

DOI: 10.1038/s41561-026-02120-z

Keywords: ocean deoxygenation, thallium isotopes, Last Interglacial, Marine Isotope Stage 5e, Southern Ocean ventilation, paleoceanography, manganese oxide burial, Antarctic sea ice, atmospheric CO2, glacial-interglacial cycles, Nature Geoscience, climate change

For decades, climate scientists have tended to characterise the last ice age in Australia as a period of relatively simple, less variable rainfall, with the assumption that rainfall patterns became patchier, less reliable and more complex only during the past 12,000 years, the epoch known as the Holocene. That framing has been used to explain changes in human behaviour, flora, fauna and fire regimes, particularly around the transition from the Late Pleistocene into the Holocene. The new analysis suggests that this apparent shift in complexity may not reflect what the climate was actually doing, but rather what the sediment and fossil records are capable of revealing about it.

The study examined 69 natural paleoenvironmental archives from tropical Australia spanning the past 130,000 years. These archives, which include sediments and fossils preserved in lakes, wetlands and other deposits, are among the main pillars of our understanding of how the climate works. Yet the team identified a striking gap in the record: there are no published hydrological records of sub-decadal resolution from the region dating to before 1900 years ago. In other words, the finest-grained evidence about short-term rainfall variability simply does not exist for most of the period in question.

That absence, Dr Wall argues, may have shaped scientific interpretation in a subtle but profound way. Reconstructions from further back in time tend to be of poorer quality, and when lower-resolution records are compared with the higher-resolution archives available for recent millennia, the older past can appear smoother and simpler than it really was. The research team found no support for the previous hypothesis that the Holocene is uniquely hydrologically heterogeneous. Instead, they suggest that the trend of reduced resolution in older records could be mistaken for greater simplicity, creating an illusion of a climate that became more erratic only recently.

“Sediment and fossil records are a main pillar for our understanding of how the climate works – but climate scientists have got something wrong about Australia’s ice age,” says Dr Wall, lead author of the paper. “Experts tend to think of the last ice age as having less complex rainfall patterns than today. Our new research suggests that narrative may have arisen from what the past can’t tell us, rather than what it can.” The distinction is more than academic. Characterising the Holocene as a period of uniquely erratic rainfall has significant implications for interpreting how past human populations and ecosystems responded to climate change, and any interpretation built on that assumption may need to be re-examined.

The terminology of ice ages can be counterintuitive, and Dr Wall is careful to place the findings in their proper temporal frame. “Technically, we’re still in the ice age, just a relatively warm and wet period called an interglacial,” she explains. The Quaternary ice age encompasses both the cold glacial periods and the warmer interludes between them, including the one humanity inhabits now. The last time the Earth was wetter than today was a period from 130,000 to 115,000 years ago, known as the Last Interglacial. That interval is of great interest to climate scientists because it is also the last time the Earth was warmer than today, which makes it a valuable analogue for understanding where a warming world might be headed.

One of the most consequential findings of the study concerns how that Last Interglacial compares with the present. When the team looked at the bigger picture across their 130,000-year window, they found that rainfall during the Last Interglacial and rainfall today look pretty indistinguishable. If the warm, wet conditions of that ancient interval produced rainfall patterns similar to those of the modern era, then the variability and complexity we experience today are not a recent novelty of the Holocene. The same kinds of hydrological behaviour were likely operating more than a hundred millennia ago, even if the surviving archives cannot capture their fine detail.

That conclusion carries an uncomfortable implication for climate forecasting. “Unfortunately, that may mean that a warming climate might be even less predictable than we had assumed,” Dr Wall warns. If past warm periods exhibited rainfall complexity comparable to today’s, then the range of natural hydroclimate behaviour under warming conditions may be broader than models and narratives built on a supposedly simple ice age would suggest. For a continent like Australia, where droughts, floods and fire seasons hinge on rainfall reliability, the predictability of water availability is among the most consequential questions climate science can address.

The methodological lesson the authors draw extends well beyond tropical Australia. “To understand how any region fits into the greater climate system, Quaternary science must be explicit about what can and cannot be determined from the data available,” says Dr Wall. Paleoenvironmental archives are inherently uneven: some periods and places preserve rich, finely layered records, while others yield only coarse snapshots. When scientists compare records of different resolutions without accounting for that difference, they risk mistaking gaps in evidence for gaps in variability. Making those limitations explicit, the study argues, should become standard practice in reconstructions of past hydroclimate.

The research, titled “Hydroclimate complexity across the last glacial cycle in tropical Australia: reassessing the uniqueness of the Holocene”, was conducted by Alexander Wall, Emma Rehn, Haidee Cadd, Vanessa Mailhammer, Will Reynolds, Brian Jones, Zenobia Jacobs, Serayah Steele and Tim Cohen, and has been published in Quaternary Science Reviews. By reassessing 69 archives across a full glacial cycle, the team has opened a new conversation about how complexity is defined and detected in the deep past. The broader message is one of humility toward the archive: the silence of ancient sediments about short-term rainfall swings does not mean those swings were absent, and the climates that shaped Australia’s earliest environments may have been every bit as intricate as the one we live in today.

Subject of Research: Hydroclimate variability and rainfall complexity in tropical Australia across the last glacial cycle

Article Title: Australia’s ancient rainfall patterns more complex than previously believed

Article References: Australia’s ancient rainfall patterns more complex than previously believed. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: paleoclimate, Australia, rainfall, Holocene, Last Interglacial, Quaternary Science Reviews, paleoenvironmental archives, hydroclimate, Flinders University, ice age, water availability, climate reconstruction