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