The finding matters far beyond paleoclimate circles. The Miocene Climatic Optimum, abbreviated MCO and centered around fifteen million years ago, is one of the best available analogues for understanding how Earth’s climate system behaves when greenhouse gas concentrations and temperatures sit well above those of the modern era. If the MCO was warmer than previously believed, then the climate system’s long-term sensitivity to carbon dioxide, a quantity known as Earth system sensitivity, is likely higher than reconstructions based on the same interval have implied. That has direct consequences for how scientists project the planet’s response to ongoing fossil fuel emissions.

The central obstacle has long been the gap between what deep-sea sediments record and what climate models predict. The workhorse of deep-time temperature reconstruction is the oxygen isotope ratio, denoted δ18O, measured in the calcium carbonate shells of benthic foraminifera, single-celled organisms that live on or within seafloor sediments. Because foraminifera incorporate oxygen isotopes from seawater into their shells in a temperature-dependent way, the ratio preserved in fossil shells encodes information about the temperature of the deep ocean where the organisms lived. But the signal is confounded: the isotopic composition of seawater itself changes with global ice volume and with the hydrological cycle, so a single δ18O measurement cannot cleanly separate temperature from water-mass chemistry.

Earlier benthic δ18O-based reconstructions of the MCO typically handled this ambiguity with simplified assumptions, often subtracting an ice-volume correction from the isotope record and converting the remainder into temperature using linear calibrations. Those approaches suggested a Miocene world that was warm but, by some estimates, only modestly hotter than preindustrial conditions at the global mean. Surface proxy records, however, including those based on organic molecules and other temperature-sensitive indicators, frequently painted a picture of much greater warmth, particularly at high latitudes. The tension between deep-ocean and surface estimates has been one of the persistent puzzles of Miocene science.

Zhu and his coauthors, drawn from the NSF National Center for Atmospheric Research, Brown University, the University of Arizona, George Mason University, and the University of Southampton, attacked the problem by refusing to treat models and data as separate enterprises. Instead, they built a probabilistic inference framework that fuses the two. On the modeling side, they ran long simulations of the Community Earth System Model, or CESM, configured for Miocene boundary conditions and, crucially, run long enough for the deep ocean to reach full equilibrium with the imposed forcing. Deep-ocean equilibration is a technical but decisive requirement: the abyssal ocean adjusts to climate change on timescales of many thousands of years, and simulations that stop short of equilibrium can misstate deep-ocean temperatures by large margins, corrupting any subsequent comparison with benthic records.

The second modeling innovation was water isotope capability. The CESM simulations tracked the full cycle of oxygen isotopes through evaporation, atmospheric transport, precipitation, and ocean mixing, allowing the researchers to predict not just temperature fields but the isotopic composition of seawater at every location and depth. This matters because the δ18O of a benthic foraminiferal shell reflects both the temperature of the surrounding water and the δ18O of that water. By simulating both quantities explicitly, the model can generate synthetic benthic isotope values that can be compared, like for like, against the real sedimentary measurements, without relying on the simplifying assumptions that plagued earlier conversions.

On the data side, the team assembled a global compilation of benthic foraminiferal δ18O spanning the Miocene Climatic Optimum, drawing on deep-sea sediment cores from multiple ocean basins. The probabilistic framework then asked a deceptively simple question: given the ensemble of model simulations and their uncertainties, which combination of global mean surface temperature and other climate parameters is most likely to have produced the observed pattern of benthic isotope values across the globe? Because different climate states produce distinct spatial fingerprints in both deep-ocean temperature and seawater isotopes, the global pattern of measurements carries far more information than any single core, and the statistical framework exploits that structure to narrow the range of plausible climates.

The answer that emerged was striking. The maximum likelihood estimate placed MCO global mean surface temperature 7.5 degrees Celsius above preindustrial, a value significantly warmer than previous benthic δ18O-based reconstructions had delivered. The corresponding surface temperature field, the full geographic map of Miocene warmth implied by that best-fit climate state, ranked among the best agreements with independent surface temperature proxies, the very records that had previously seemed at odds with the deep-sea data. In other words, the framework did not merely split the difference between conflicting datasets; it found a climate state consistent with both, resolving the apparent contradiction by showing that earlier deep-ocean temperature estimates had been biased cold.

The implications ripple outward in two directions. First, a warmer MCO implies a higher MCO-derived estimate of Earth system sensitivity, the total warming the planet eventually delivers in response to a given change in radiative forcing once ice sheets, vegetation, and other slow feedbacks have fully adjusted. Earth system sensitivity inferred from warm paleoclimate intervals is one of the key benchmarks used to evaluate the long-term tail of future warming projections, and a higher Miocene value suggests that slow feedbacks may amplify greenhouse warming more strongly than some estimates allow. Second, the study demonstrates a methodological template. The authors emphasize that deep-ocean equilibration and proxy-model integration are both essential for accurately estimating deep-ocean and surface temperatures, and they present their inference framework as an approach applicable to improving global climate reconstructions across other time intervals, from older greenhouse episodes to the more recent glacial cycles.

The research also reflects the growing maturity of isotope-enabled climate modeling as a bridge between the geologic record and the physics of the climate system. Computing support came from the Derecho system at the NSF National Center for Atmospheric Research, and the work was funded by the U.S. National Science Foundation and the Heising-Simons Foundation, with additional support to coauthor David Evans from the Royal Society. As climate scientists confront a future that is pushing the Earth system toward states not seen in millions of years, studies like this one sharpen the only real experiments the planet has ever run on itself. Fifteen million years ago, the deep ocean was recording the signature of a world far hotter than the sediments had seemed to admit. Reading that signature correctly, with models and data speaking the same isotopic language, suggests the Miocene Climatic Optimum was warmer than we thought, and that the climate system’s memory of high carbon dioxide runs deeper than previous reconstructions allowed.

Subject of Research: Reconstructing the global warmth of the Miocene Climatic Optimum by integrating isotope-enabled climate model simulations with benthic foraminiferal oxygen isotope records

Article Title: Model-data synthesis of benthic isotopes suggests a warmer Miocene Climatic Optimum

Article References: Zhu, F., Zhu, J., Si, W., Nirenberg, J. E., Herbert, T., Tierney, J. E., Acosta, R. P., Burls, N. J., & Evans, D. (2026). Model-data synthesis of benthic isotopes suggests a warmer Miocene Climatic Optimum. Nature Communications. https://doi.org/10.1038/s41467-026-77980-5

Image Credits: AI Generated

DOI: 10.1038/s41467-026-77980-5

Keywords: Miocene Climatic Optimum, paleoclimate, benthic foraminifera, oxygen isotopes, Earth system sensitivity, climate modeling, CESM, deep-ocean equilibration, proxy-model integration, Nature Communications, global mean surface temperature, paleoceanography

The research, led by Wanxiong Zhang of Southwest Forestry University and Zhuoya Zhang, with colleagues from the Chinese Academy of Sciences, Fujian Normal University and the Gaoligongshan National Nature Reserve, focused on the cellulose locked inside the tree’s annual growth rings. Cellulose, the structural polymer of plant cell walls, is laid down each growing season and incorporates oxygen from the water the tree takes up and from the leaf water that evaporates through its stomata. Because the heavy isotope oxygen-18 behaves differently from the lighter oxygen-16 during evaporation and condensation, the ratio of these isotopes in tree-ring cellulose acts as a natural archive of the humidity conditions under which each ring formed. In humid air, less water evaporates from the leaf and less isotopic enrichment occurs; in dry air, the opposite happens. The result is a proxy that can be read like a barometer of past atmospheric moisture.

Sampling such a rare and protected species demanded extraordinary care. Working with the reserve administrations of Baoshan and Nujiang, the team obtained cores from living trees without harming them, then cross-dated the rings using standard dendrochronological quality-control procedures to assign each ring a precise calendar year. From the resulting 221-year chronology, spanning 1802 to 2022, the researchers reconstructed May-to-November relative humidity, the period covering the monsoon growing season. The calibration against instrumental records from 1960 to 2022 explained 36.3 percent of the observed variance in relative humidity, a respectable figure for a single-species isotope chronology in a topographically complex mountain environment.

The reconstructed record tells a clear story of long-term change. Wet years cluster predominantly before 1900, while the twentieth century brought a general drying trend that has continued into the present. This finding echoes results from other Himalayan isotope studies, including a 223-year tree-ring oxygen isotope chronology from Nepal that documented increasing aridity over a similar interval. What makes the new record particularly valuable is its location: the Gaoligong Mountains sit at a climatic crossroads where Indian monsoon circulation, Tibetan Plateau dynamics and local orographic effects interact, making the region both hydrologically sensitive and poorly covered by long instrumental observations.

To confirm that the chronology reflects genuine regional climate rather than local noise, the team performed spatial correlation analyses. The reconstructed relative humidity series showed coherent relationships with observed relative humidity, vapor pressure deficit, the standardized precipitation-evapotranspiration index and precipitation across southwestern China and adjacent monsoon-influenced regions. Vapor pressure deficit, the gap between how much moisture the air holds and how much it could hold at saturation, is a key driver of plant water stress, and rising deficits have been implicated in increasing tree mortality across tropical forests worldwide. The fact that the rhododendron isotopes track these regional moisture fields so consistently supports the record’s use as a regional hydroclimatic archive.

A central technical question in tree-ring isotope research is what exactly the signal represents. Oxygen isotopes in cellulose are shaped both by the isotopic composition of the source water taken up by the roots and by evaporative enrichment in the leaves, which is strongly modulated by ambient humidity. Using modeled precipitation isotope data alongside the humidity-related atmospheric variables, the researchers quantified the relative contributions of each pathway. Both made important contributions to the cellulose isotope variability, leading the team to interpret the chronology as an integrated, relative-humidity-sensitive hydroclimatic record that captures atmospheric evaporative demand and source-water isotope changes together. This dual sensitivity, they argue, is precisely what makes the proxy robust: it responds to the full moisture environment of the tree rather than to a single variable in isolation.

The record also carries the fingerprints of the planet’s great ocean-atmosphere oscillations. Spectral analysis of the reconstruction, combined with examinations of sea-surface temperatures and large-scale atmospheric circulation patterns, suggests that May-to-November relative humidity in the Gaoligong region is intermittently modulated by the El Niño-Southern Oscillation and the Walker circulation, the vast east-west overturning of air across the tropical Pacific that shifts rainfall patterns during El Niño and La Niña events. The influence of the Indian Ocean Dipole, by contrast, appears more conditional, depending on the background climate state. This kind of intermittent teleconnection is consistent with earlier work showing that ENSO events disrupt Indian summer monsoon rainfall and that dipole events have been linked to drought episodes in southwestern China, including the severe droughts of 2006 and 2011.

Perhaps the most consequential part of the study looks forward. The team analyzed climate model simulations from the sixth phase of the Coupled Model Intercomparison Project, the model ensemble underpinning the most recent IPCC assessment, under a range of future emissions scenarios. The models consistently indicate an overall decline in May-to-November relative humidity in the region during the twenty-first century, with stronger decreases under higher radiative-forcing scenarios. In other words, the drying trend that the rhododendrons have quietly recorded since 1900 is projected to intensify as greenhouse gas concentrations rise, with the magnitude of future aridification depending directly on humanity’s emissions choices.

For Rhododendron protistum var. giganteum itself, the implications are alarming. The species is already classified as endangered, restricted to fragmented habitat on these mountains, and a companion study by several of the same authors found that rising temperatures have reshaped the tree’s climate-growth relationships and increased its vulnerability. Declining relative humidity and rising vapor pressure deficit compound the physiological stress on a species whose enormous leaves, while spectacular, present a large surface area for water loss. Global syntheses have shown that tropical and subtropical tree mortality rises with atmospheric water stress, and hydraulically vulnerable species have been killed outright by catastrophic droughts elsewhere in the world’s mountains and tropics. A long-term drying trajectory, superimposed on warming, leaves this botanical giant with shrinking margins for survival.

Beyond its significance for conservation, the study demonstrates the power of oxygen isotope dendroclimatology in regions where conventional ring-width records fall short. In humid, montane forests, tree growth is often limited less by water availability than by temperature or light, muting the drought signal in ring widths. Isotope ratios, by contrast, record the evaporative environment directly, regardless of how much the tree grows in a given year. By extending this approach to a new species and a new corner of the monsoon domain, the researchers have added a crucial piece to the puzzle of Asian hydroclimate variability. The full dataset and reconstruction code have been made publicly available in the Zenodo repository, allowing other scientists to scrutinize, reuse and extend the record. As climate change accelerates across the world’s mountains, the silent testimony of these ancient rhododendrons, and the 221-year moisture history they preserve, may prove essential for anticipating what the coming century holds for the water supplies, forests and extraordinary biodiversity of southwestern China.

Subject of Research: Tree-ring oxygen isotope reconstruction of 221 years of relative humidity variability in the Gaoligong Mountains, China

Article Title: Tree-ring oxygen isotopes reveal a 221-year hydroclimatic history for the endangered Rhododendron protistum var. giganteum in the Gaoligong Mountains

Article References: Zhang, W., Ge, H., He, Y., Xu, C., Wang, J., Fang, K., An, W., & Zhang, Z. (2026). Tree-ring oxygen isotopes reveal a 221-year hydroclimatic history for the endangered Rhododendron protistum var. giganteum in the Gaoligong Mountains. Climate Dynamics, 64(11), Article 458. https://doi.org/10.1007/s00382-026-08425-4

Image Credits: AI Generated

DOI: 10.1007/s00382-026-08425-4

Keywords: tree rings, oxygen isotopes, paleoclimate, relative humidity, drought, Rhododendron protistum var. giganteum, Gaoligong Mountains, monsoon, ENSO, Indian Ocean Dipole, CMIP6, climate change

The team’s approach relied on a clever piece of geochemical detective work. Benthic foraminifera, single-celled organisms that live on the seafloor, build their calcium carbonate shells using the chemistry of the water around them. The oxygen isotope composition of these shells has long been the workhorse of palaeoclimate science, but it carries an awkward ambiguity: it records both the temperature of the deep water and the oxygen isotope composition of the seawater itself, which in turn reflects both global ice volume and local salinity. To untangle these signals, the researchers measured magnesium-to-calcium ratios in the shells alongside the oxygen isotopes, allowing them to calculate temperature independently and then subtract its influence to isolate the seawater signal.

The centrepiece of the study is a new 1.5-million-year record from IODP Site U1385 on the Iberian Margin off Portugal, a location famous among palaeoceanographers as the Shackleton Site for its exceptional sedimentation rates of roughly 11 centimetres per thousand years. The team analysed more than a thousand samples of two infaunal foraminiferal species, Uvigerina peregrina and Globobulimina affinis, at resolutions as fine as 500 years in key intervals. Rigorous quality control was essential: the researchers screened every sample for contamination from clays and from manganese-iron oxide coatings that form on shells buried in sediment, applying a correction based on the well-characterised magnesium-to-manganese ratio of such coatings to avoid biasing their temperature estimates.

With the new Atlantic data in hand, the team stacked their record with published reconstructions from the North Atlantic, the North and Southwest Pacific, and the Southern Ocean, interpolating everything to a common 3,000-year grid and weighting the basins by their true volumes of deep water. The result is one of the first globally representative pictures of how abyssal temperature and salinity evolved together across the transition. What emerged was a striking asymmetry. Before about 930,000 years ago, deep North Atlantic water was on average about 2.5 degrees Celsius warmer than the deep Pacific. Across the transition, the glacial Atlantic cooled while the Pacific barely changed, narrowing that gap to roughly 1.1 degrees.

The salinity story proved even more consequential. The oxygen isotope composition of seawater rose in both basins across the transition, as expected from the growth of continental ice sheets, which lock up isotopically light water on land and leave the ocean isotopically heavier. But the increase was significantly larger in the Pacific than in the Atlantic. Because ice volume affects the whole ocean uniformly, that asymmetry must reflect a regional change: the researchers conclude that Southern Component Water, the cold dense water formed around Antarctica that feeds the deep Pacific, became substantially saltier and therefore denser during glacial periods after about 930,000 years ago. A statistically significant step-like jump in the volume-weighted mean seawater isotope stack at 930,000 years ago marks this reorganisation.

What could have made Antarctic-sourced waters saltier? The authors propose that freshwater input to the marginal seas around Antarctica, such as the Weddell and Ross Seas, declined beginning around 930,000 years ago. Several mechanisms could have contributed: reduced melting of the Antarctic Ice Sheet and its floating ice shelves, a transition of the ice sheet toward a marine-based configuration with less terrestrial melting, increased export of freshwater away from Antarctica by icebergs and sea ice, and expanded sea ice formation, which concentrates salt in the underlying water through brine rejection. Notably, sea ice formation does not appreciably alter the oxygen isotope composition of seawater, which helps explain why the salinity signal appears in the isotope records without a corresponding freshwater isotope fingerprint.

The climatic payoff of this densification lies in the physics of ocean stratification. When the salinity and density of Southern Component Water increased, the deep ocean became more strongly layered, with dense bottom waters sitting beneath less dense waters above and mixing between them suppressed. A more stratified abyssal ocean is a more effective carbon trap: organic carbon sinking from the surface is remineralised at depth, consuming oxygen and releasing dissolved inorganic carbon that then accumulates in the isolated deep reservoir rather than being vented back to the atmosphere. The effect was amplified, the authors argue, by increased iron fertilisation of Southern Ocean productivity from wind-blown dust and by a strengthened surface halocline that reduced the deep-to-surface exchange through which the ocean releases carbon dioxide around Antarctica.

Independent lines of evidence bolster this picture. Records of neodymium isotopes and carbon isotopes from the deep Atlantic indicate a greater contribution of southern-sourced water during glacials after the transition, consistent with a denser Southern Component Water shoaling over its northern counterpart. Studies of deep-sea nutrients, carbonate ions, and oxygenation in both the Atlantic and Pacific point to enhanced carbon storage in the abyss across the same interval. Some atmospheric carbon dioxide reconstructions suggest glacial concentrations dropped modestly, by around 20 parts per million, across the transition, and lower carbon dioxide would have allowed ice sheets to grow large enough to survive modest rises in summer insolation that would previously have triggered deglaciation, thereby lengthening the cycles to the familiar 100,000-year beat.

Importantly, the study also constrains what did not change. The volume-weighted mean deep ocean temperature record shows minimal cooling across the transition, because the Pacific, which dominates the global deep ocean by volume, was already near freezing during glacials before 900,000 years ago. This finding aligns with noble gas-based estimates of mean ocean temperature from Antarctic ice cores and challenges reconstructions that infer pronounced global deep-ocean cooling, which the authors attribute to over-reliance on a single North Atlantic site. Similarly, their seawater isotope stack supports a substantial increase in glacial ice volume across the transition, in line with sea-level reconstructions and process-based estimates.

The authors are careful to frame their sequence of events as a working hypothesis rather than a settled verdict. With only five long paired magnesium-calcium and oxygen isotope records spanning the interval, the global stacks remain sparsely constrained, and the propagated uncertainties on the seawater isotope reconstructions are considerable. Testing the hypothesis will require additional records from under-sampled basins, numerical model simulations of the coupled ocean-cryosphere-carbon system, and, perhaps most eagerly awaited, high-resolution carbon dioxide and mean ocean temperature measurements from the Beyond EPICA Oldest Ice core, which extends continuously through the transition. If the abyssal stratification mechanism holds, it would mark a major step toward solving a puzzle that has occupied palaeoclimate science for half a century, and a reminder that the deepest layers of the ocean can quietly rewrite the rhythm of the ice ages.

Subject of Research: Abyssal ocean density stratification and carbon storage changes across the Middle Pleistocene Transition

Article Title: Increased abyssal ocean density stratification across the Middle Pleistocene Transition

Article References: Thomas, N. C., Ford, H. L., Greaves, M., & Hodell, D. A. (2026). Increased abyssal ocean density stratification across the Middle Pleistocene Transition. Climate of the Past, 22(8), 1559-1584. https://doi.org/10.5194/cp-22-1559-2026

Image Credits: AI Generated

DOI: 10.5194/cp-22-1559-2026

Keywords: Middle Pleistocene Transition, abyssal ocean, density stratification, Mg/Ca paleothermometry, benthic foraminifera, oxygen isotopes, deep ocean circulation, Southern Component Water, carbon storage, atmospheric CO2, ice sheets, palaeoclimate

The puzzle centers on a geochemical measurement known as the stable oxygen isotope ratio, written as δ18O. When water evaporates from the ocean, lighter isotopes of oxygen rise more readily into the vapor, and as that vapor travels and rains out along its path, the remaining moisture becomes progressively depleted in the heavy isotope oxygen-18. Stalagmites lock this isotopic signature into their calcite as they grow, because the drip water that feeds them ultimately comes from precipitation. In China, cave records stretching back more than 600,000 years show a pronounced cyclicity of roughly 21,700 years, matching the precession cycle, the slow change in the orientation of Earth’s rotational axis that shifts where and when the Northern Hemisphere receives its most intense summer sunlight. Chinese stalagmite δ18O values rise and fall by about 4 per mil across these cycles, a signal that researchers have long interpreted as a record of changing monsoon strength.

Precession matters because it modulates Northern Hemisphere summer insolation, which in turn drives the great heat engines of the Asian monsoon. When summer perihelion brings Earth closest to the Sun during the boreal summer, landmasses heat more intensely, pressure gradients steepen, and moist air surges inland from the tropical oceans. The East Asian Summer Monsoon, with its characteristic Meiyu-Baiu rainband over China and Japan and its Korean counterpart, the Changma front, is governed by the position of the subtropical front, which forms where the Westerly jet interacts with the Western North Pacific Subtropical High. A recent modeling study by Wen and colleagues identified a striking consequence of this orbital forcing: a grand dipole pattern in which precipitation and isotope signals over South Asia move in opposition to those over Japan, with the Korean Peninsula sitting almost exactly on the nodal line between the two lobes.

That theoretical prediction is precisely what the new study set out to test with real data. The team assembled oxygen isotope records from two Korean stalagmites, one from Gwaneum Cave and a previously unpublished record from Eden Cave, which together grew intermittently across several segments of the past 550,000 years. The records are well constrained by uranium-series dating, and their growth fabrics indicate that the calcite precipitated under conditions close to isotopic equilibrium, meaning the signals preserved in the stone faithfully reflect the water that formed them. Crucially, the weak covariation between the carbon and oxygen isotope records in both stalagmites suggests that kinetic effects, which can distort isotope signals during rapid degassing, are not the dominant control. The absence of a precessional cycle in the Korean oxygen isotope profiles is therefore a genuine climate signal, not a cave artifact.

To understand why the signal is so weak, the researchers turned to the isotope-enabled Community Earth System Model, iCESM1.2, a fully coupled climate model that tracks the isotopes of oxygen and hydrogen in water as it moves through the atmosphere, land, and ocean. They ran a transient simulation covering the past 130,000 years of orbital forcing, compressed by a factor of 100 so that 130,000 years of astronomical history unfolded over just 1,300 model years, an acceleration justified because the atmospheric processes of interest adjust far faster than the precessional cycle itself. They also performed equilibrium time-slice experiments under extreme precession conditions with amplified eccentricity, compared results with an ultra-high-resolution simulation of the last interglacial and the subsequent cold stage, and cross-checked their precipitation patterns against a separate three-million-year transient simulation.

The simulations reproduced the observed geography of the monsoon response with remarkable fidelity. The leading mode of annual precipitation variability over eastern China, Korea, and Japan forms a north-south dipole that tracks summer insolation at 30 degrees north, and Korea sits squarely on its nodal line. Even more tellingly, the leading mode of precipitation δ18O forms an east-west dipole: during Northern Hemisphere summer perihelion, western Asia becomes isotopically depleted while eastern Asia becomes enriched, and the Korean Peninsula again lies in the zone of transition where the two signals oppose and largely annihilate each other. The model shows no distinct precessional cyclicity in Korean precipitation isotopes, exactly matching the silence of the stalagmites.

The mechanism behind this cancellation emerged from a series of water-tagging experiments, in which the model tracked moisture from fourteen source regions to see how each contributed to Korean rainfall under high and low precession. During summer perihelion, the Western North Pacific Subtropical High strengthens and shifts northwestward, pumping additional moisture from the subtropical North Pacific toward Korea, contributing about 0.40 millimeters per day, while East Asian land sources add another 0.11 millimeters per day. At the same time, however, moisture deliveries from the northern North Pacific and the equatorial Indian Ocean decline, offsetting much of the gain. The isotopic story follows the same logic: subtropical Pacific and East Asian land moisture arrive isotopically enriched, while Indian Ocean and South Asian moisture arrives depleted, and the two effects nearly cancel. On annual timescales the compensation is even more complete, leaving a net shift of less than a tenth of a per mil, far too small to register in the cave record.

Perhaps the most exciting implication of the study is that the muted oxygen isotope signal does not mean the Korean Peninsula was climatically frozen through half a million years of orbital change. The team showed that deuterium excess, a secondary isotope parameter defined as the difference between the hydrogen isotope ratio and eight times the oxygen isotope ratio, still carries a strong precessional signal over Korea. Deuterium excess reflects conditions at the moisture source, particularly the relative humidity during evaporation, because hydrogen isotopes diffuse more readily than oxygen isotopes when water evaporates under dry, windy conditions. In the simulations, surface relative humidity over the southeast Indian Ocean and the northwest Pacific oscillates by 3 to 4 percent on precessional timescales, in antiphase between the two basins, and these changes propagate faithfully into the deuterium excess of precipitation falling over Korea, which is low when summer insolation is high and high when it is low.

Because deuterium excess can be recovered from ancient water trapped as fluid inclusions within speleothem calcite, the authors propose that such reconstructions from East Asian caves could provide a powerful new constraint on the drivers of the regional monsoon system, even in places where oxygen isotopes alone are uninformative. They also point to oxygen-17 excess, measurable directly from calcite and insensitive to temperature, as a promising complementary proxy where fluid inclusions are not preserved. A multi-proxy framework combining oxygen isotopes, deuterium excess, carbon isotopes, trace elements, and growth rates, integrated with marine and lacustrine archives, could finally reconcile the apparently conflicting hydroclimate records scattered across Asia.

The study also surfaced an unexpected curiosity: simulated Korean precipitation displays a semi-precessional rhythm, with a period of roughly 11,000 years, visible during certain marine isotope stages. The researchers suggest this may arise from combination tones, a nonlinear interaction in which precessional-scale winds multiply with precessional-scale changes in atmospheric moisture, though they caution that the mechanism remains unresolved and warrants further investigation. What is already clear is that the Korean Peninsula, far from being a boring corner of the Asian monsoon system, is a natural laboratory where competing moisture streams meet and cancel, and where the right proxy, read carefully, can still reveal the heartbeat of orbital-scale climate change pulsing beneath an apparently still surface.

Subject of Research: Orbital-scale monsoon variability and speleothem oxygen isotope records on the Korean Peninsula

Article Title: Muted orbital-scale monsoon variability over the Korean Peninsula

Article References: Muted orbital-scale monsoon variability over the Korean Peninsula. (n.d.). https://doi.org/10.5194/cp-22-1711-2026

Image Credits: AI Generated

DOI: 10.5194/cp-22-1711-2026

Keywords: Korean Peninsula, speleothems, oxygen isotopes, East Asian Summer Monsoon, precession, paleoclimate, deuterium excess, moisture sources, Western North Pacific Subtropical High, iCESM, stalagmites, hydroclimate

Surface seawater density is a deceptively powerful diagnostic. It is the combined product of temperature and salinity, and because density gradients steer ocean circulation through geostrophic balance, getting density right means getting the ocean’s engine right. Until now, model evaluations of the glacial ocean have focused almost entirely on sea surface temperature, leaving the coupled temperature-salinity signal largely unexplored. Héloïse Barathieu of the University of Bordeaux and her colleagues, working with researchers at the Laboratoire des Sciences du Climat et de l’Environnement, exploited a recently developed Bayesian calibration method that converts the oxygen isotope composition of foraminiferal shells — the microscopic calcite tests left behind by plankton that drifted through the glacial ocean — into quantitative estimates of annual surface density, complete with explicit uncertainty ranges.

The team assembled a database of 474 density reconstructions spanning all major ocean basins, each available for both the Last Glacial Maximum and the late Holocene, which serves as a stand-in for the pre-industrial baseline. Against this dataset they pitted sixteen climate model simulations from the third and fourth phases of the Paleoclimate Model Intercomparison Project, known as PMIP3 and PMIP4, the coordinated exercises that feed into the assessments of the Intergovernmental Panel on Climate Change. Two further simulations had to be discarded because of inconsistencies in their salinity data. Model outputs were regridded onto a common one-degree grid, and seawater density was computed using the thermodynamically consistent TEOS-10 framework via the Gibbs Seawater Python package, ensuring that every simulation was judged on an identical physical footing.

The headline finding is that the models get the big picture right but consistently undershoot the magnitude of the change. All sixteen simulations produce positive density anomalies between the glacial and pre-industrial oceans, agreeing on the sign of the shift, but their spatially averaged anomalies range only from about 0.7 to 1.6 kilograms per cubic metre, with a mean near 1.1. The reconstructions, by contrast, average around 1.6 kilograms per cubic metre. When the researchers examined the full statistical distributions of anomalies, using Kolmogorov–Smirnov statistics, interquartile range overlap and Monte Carlo propagation of reconstruction uncertainties, none of the simulations satisfied all three agreement criteria simultaneously. Part of the mismatch may stem from the uneven geography of the sediment cores, which cluster near coastlines where runoff and upwelling make simulation notoriously difficult.

The most striking result concerns what drives the discrepancies. Because density responds to both heat and salt, the team used a Shapley decomposition — a technique that partitions the total difference fairly between temperature and salinity regardless of the order of attribution — to separate the two contributions. Salinity emerged as the dominant culprit. The link traces back to the hydrological cycle: during the glacial period, tropical precipitation fell more sharply than evaporation, leaving surface waters saltier and denser. When the researchers plotted simulated density anomalies against simulated precipitation anomalies, a robust linear relationship appeared, with a coefficient of determination of 0.67 in the tropics and 0.64 globally. Models that best reproduced the reconstructed density signals were precisely those that simulated the strongest reductions in low-latitude rainfall.

Ranking the individual simulations revealed a clear hierarchy. Taylor diagrams, which condense spatial correlation, variability and error into a single plot, together with metrics such as root mean square error, bias, Nash–Sutcliffe efficiency and the Kling–Gupta efficiency, showed that the two iLOVECLIM experiments, MRI-CGCM3, IPSL-CM5A2 and MPI-ESM consistently performed well, with the iLOVECLIM runs achieving root mean square errors near 0.9 kilograms per cubic metre and Kling–Gupta efficiencies of 0.87. At the other end, the HadCM3 family and MIROC-ESM lagged behind, hampered by pronounced negative biases — a systematic tendency to underestimate reconstructed densities. Encouragingly, no simulation was wholly inadequate; all achieved Kling–Gupta efficiencies of at least roughly 0.5, indicating moderate to good overall skill.

That global comfort, however, masks serious regional trouble. Zooming into the Indian Ocean, the team split the basin at the Equator into monsoon-dominated northern and subtropical southern sectors. Most simulations handled the Indian Ocean reasonably well in both periods, but performance was systematically weaker in the north than in the south. The iLOVECLIM simulations, stars of the global evaluation, collapsed in the North Indian Ocean, with Kling–Gupta efficiencies approaching zero — a vivid demonstration that strong global scores can conceal deep regional failures, and that ensemble averages would have hidden the problem entirely.

The researchers then probed one of the tropical Indian Ocean’s defining features: the west–east gradient in surface properties, the mean-state backdrop to the Indian Ocean Dipole, a climate oscillation that many modern models already misrepresent. Proxy reconstructions indicate that during the Last Glacial Maximum this zonal density gradient weakened, with an anomaly close to minus one kilogram per cubic metre, reflecting stronger cooling in the east than the west and associated salinity changes. Yet only seven of the fourteen eligible simulations — exactly half — reproduced the correct sign and magnitude within the reconstruction’s 68 percent uncertainty range. HadCM3-ICE-6G_C and CESM1.2 came closest, and the authors note that the choice of ice-sheet reconstruction, which reshapes atmospheric circulation, jet structure and stationary waves, likely influences how well a model captures this gradient.

Why does any of this matter beyond the halls of paleoclimate science? Because the same models being graded here are the ones projecting how monsoons, ocean circulation and tropical rainfall will respond to ongoing warming. If a model cannot reproduce how salinity and density shifted when the planet last underwent a massive climate transition, its handling of the hydrological cycle — arguably the most consequential variable for human societies — remains open to doubt. The study’s authors argue that improving the representation of regional hydrological changes is crucial for reducing uncertainties in both paleoclimate simulations and future projections, and they call for expanding density reconstructions into the poorly sampled open ocean.

The work also signals a methodological shift. By evaluating each simulation individually rather than blending them into an ensemble mean, and by deploying a battery of complementary diagnostics across global, basin and zonal-gradient scales, the study offers a template for stricter model accountability. The authors suggest that integrating such datasets into statistical constraint frameworks and data assimilation could eventually identify which models most faithfully reproduce past climate states. For now, the message is sober but constructive: the models pass the broad test of an ice-age ocean, yet the salt in the water — and the rain that controls it — remains their hardest exam.

Subject of Research: Evaluation of PMIP3 and PMIP4 Last Glacial Maximum climate simulations using foraminiferal oxygen-isotope-based ocean surface density reconstructions

Article Title: Using ocean surface paleo-density to evaluate PMIP3 and PMIP4 Last Glacial Maximum climate simulations

Article References: Using ocean surface paleo-density to evaluate PMIP3 and PMIP4 Last Glacial Maximum climate simulations. (n.d.). https://doi.org/10.5194/cp-22-1803-2026

Image Credits: AI Generated

DOI: 10.5194/cp-22-1803-2026

Keywords: Last Glacial Maximum, paleoclimate, climate models, PMIP, ocean surface density, foraminifera, oxygen isotopes, sea surface salinity, Indian Ocean Dipole, hydrological cycle, Climate of the Past, model-data comparison

The team worked with two full-depth cores recovered from the accumulation zones of Tibetan glaciers. The first, 169.45 meters long, was drilled in November 2020 from the summit of the Weigeledangxiong Glacier on Mount Animaqing in the eastern Kunlun Mountains, at 5750 meters above sea level. The second, 77.6 meters long, came in November 2022 from the southern slope of Mount Bugyai Kangri in the eastern Tanggula Mountains, at 6180 meters. Both regions are dominated by the Asian monsoon and both glaciers have been retreating in recent decades, but they differ in a crucial way: the Animaqing site is warmer, with borehole temperatures ranging from about -0.22 to -6.84 degrees Celsius, while the Bugyai Kangri core is colder, spanning roughly -1.54 to -8.31 degrees Celsius. That temperature contrast turned out to be the key to interpreting everything the crystals had to say.

Measuring grain size continuously along an ice core is a formidable technical challenge, and the authors solved it with a Microstructure Mapping system adapted from protocols developed for polar cores. Vertical ice slices, cut with a band saw and shaved flat with a Leica microtome, were sublimated at -20 degrees Celsius for about fifteen hours until grain boundaries stood out in sharp relief. Each slice was then photographed under vertical coaxial illumination, in which air bubbles and grain boundaries appear dark against the translucent ice, producing panoramic images at a resolution of roughly eight micrometers per pixel. A U-Net neural network performed the binary segmentation of grain boundaries, and the ImageJ software package with its MorphoLibJ plugin extracted the area, position and shape of every grain. In total the researchers analyzed 1890 slices from Animaqing, covering 91 percent of the core, and 560 slices from Bugyai Kangri, covering 71 percent, a sampling density that captures centimeter-scale variations invisible to traditional discrete sampling.

The resulting profiles divide naturally into four depth zones that mirror, in modified form, the classic three-stage model established for polar ice cores. In the firn layer, extending to about 35 meters in both cores, snowflakes of roughly one square millimeter are slowly compressed as irregular pores close off; average grain areas remain small, below about two square millimeters at the surface, and grow gently with depth. Both cores reached the critical density of 0.830 grams per cubic centimeter for pore closure at approximately 35 meters, sealing the atmospheric archive in bubbles. Below that lies a ten-meter coarsening zone in which the firn becomes true ice and average grain areas rise rapidly, reaching 18.00 square millimeters in the Animaqing core and 8.06 square millimeters at Bugyai Kangri, as larger grains grow by consuming their smaller neighbors.

This early growth is governed by Normal Grain Growth, a textbook process in which grain boundaries migrate to reduce the total surface energy of the ice, and it is strongly temperature-dependent: warmer ice allows boundaries to migrate faster, so crystals grow larger. The data show this vividly. The Animaqing core, several degrees warmer than Bugyai Kangri, maintains substantially larger grains at every comparable depth even where it bears greater overburden stress. In the recrystallization zone that follows, the trend reverses. From about 45 meters down to 140 meters at Animaqing and 73 meters at Bugyai Kangri, average grain area stabilizes and declines below five square millimeters, small grains become more abundant, and micrographs fill with subgrain boundaries, the telltale signature of Rotation Recrystallization. Under accumulating stress, subgrain boundaries rotate and gradually mature into true grain boundaries, splitting large crystals into smaller ones and setting up a dynamic balance between stress-driven refinement and temperature-driven growth.

The deepest zone belongs to Strain-Induced Boundary Migration, a rapid recrystallization mechanism that flourishes when ice is warmer than about -10 degrees Celsius, a condition satisfied at the base of both cores. Here average grain areas surge, the population of large crystals grows roughly ten times faster than in the coarsening layer, and micrographs show interlocked grains with highly curved boundaries. At Animaqing the effect is spectacular: below 150 meters, where temperatures climb above -6.27 degrees Celsius, crystals balloon to more than 400 square millimeters, and near the very base they exceed 3000 square millimeters, their boundaries spilling beyond the edges of the prepared slices. There, the researchers infer, repeated freeze-thaw cycles in excessively warm ice generated a five-meter-thick bubble-free layer of refrozen ice with gigantic, nearly transparent crystals.

Between these hundred-meter trends lie two centimeter-scale troublemakers, and both are ultimately climatic in origin. The first is melt-refreezing. Because surface temperatures at Animaqing approach zero, summer meltwater percolates into the firn and refreezes, producing centimeter-scale ice layers in which liquid-mediated recrystallization drives crystal sizes upward in abrupt jumps. The colder Bugyai Kangri site rarely experiences this, following a classic cold-type ice formation without meltwater involvement. The second troublemaker is impurity. Both cores contain millimeter to centimeter bands rich in dust and soluble impurities, and within these bands crowds of small grains, typically under two square millimeters, pile into walls that drag average grain area sharply down. In the Animaqing core a dense zone of impurity layers between 120 and 140 meters contains up to 90 centimeters of impurity-bearing ice per meter of core, and impurity thickness correlates strongly and negatively with grain size, with a correlation coefficient of -0.715. The mechanism is thought to be pinning: impurity particles obstruct grain boundary migration, holding grains small in much the same way documented in polar cores from Greenland and Antarctica and in the Monte Perdido Glacier of the Pyrenees.

The decisive test of the study is whether grain size survives this barrage of confounding influences and still records temperature. To find out, the authors compared average grain area with the oxygen isotope ratio, expressed as δ18O, the workhorse paleoclimate proxy in Tibetan ice core research. In the interval between 45 and 51 meters, where vertical pressures are low enough that Rotation Recrystallization is negligible and Normal Grain Growth dominates, the Bugyai Kangri core shows a statistically significant correlation between grain area and δ18O, with r equal to -0.286, corresponding to ice dated between 1975 and 1985. The correlation at Animaqing is obscured by heavy refreezing, but in the dense impurity zone the Animaqing core instead delivers a significant positive correlation of 0.205, tying small crystals to cold, dusty periods on an inferred centennial scale. In both settings, the physical texture of the ice is keeping step with the chemical thermometer.

Comparisons with other mountain glaciers reinforce the temperature story. When the authors placed their cores alongside four others from the eastern Tibetan Plateau, the Tianshan Mountains and the European Alps, the ranking of grain sizes at 35 meters depth, where vertical pressures are broadly comparable, broadly tracked the ranking of ice temperatures. The Guliya core tops the grain size list and the Tsanfleuron core tops the temperature list, with Animaqing, Bugyai Kangri and the KCC core following in the expected order. The consistency, the authors argue, marks temperature as the dominant control on grain growth in mountain glaciers, outweighing even the greater stresses borne by deeper, warmer ice.

The implications reach well beyond crystallography. Ice cores from the polar sheets have long used grain size as a supplementary climate archive, with abrupt changes in crystal dimensions marking cold-warm transitions, but mountain glaciers were widely considered too warm, too fast-flowing and too contaminated to preserve such signals. This study demonstrates that a mountain glacier ice core can, in specific layers where stress-driven grain refinement is suppressed, hold onto temperature information despite melt layers, impurities and rapid flow. That matters for the Third Pole, where observational records are short and paleoclimate archives are scarce, and it suggests that physical properties of ice, from grain size to bubble number density, could eventually complement isotope chemistry in reconstructing how High Mountain Asia responded to past warming. As glaciers across the Tibetan Plateau retreat under modern climate change, their crystals may prove to be among the last witnesses of the climates that built them, recording in millimeter-scale geometry the temperatures of decades, centuries and perhaps millennia gone by.

Subject of Research: Temperature-dependent grain size variation and climate signal preservation in Tibetan Plateau ice cores

Article Title: Temperature dependence of grain size in Tibetan ice core

Article References: He, Z., & Xu, B. (2026). Temperature dependence of grain size in Tibetan ice core. The Cryosphere, 20(10), 5745-5759. https://doi.org/10.5194/tc-20-5745-2026

Image Credits: AI Generated

DOI: 10.5194/tc-20-5745-2026

Keywords: ice cores, Tibetan Plateau, grain size, recrystallization, paleoclimate, glaciology, oxygen isotopes, Third Pole, The Cryosphere, ice microstructure, mountain glaciers, climate change

Lead author Julia Cole, professor and chair of the University of Michigan Department of Earth and Environmental Sciences, emphasized that the comparison between past and present is strikingly unambiguous. According to Cole, there is no period in the reconstructed record where El Niños were as strong as they are today, and the strength of El Niño appears to change in parallel with the warming of global temperatures. The findings, she noted, demonstrate that the large El Niño events of the last 40 years are not normal within the context of the last thousand years. This conclusion carries significant weight because El Niño is one of the most influential sources of year-to-year climate variability on the planet, capable of reshaping weather patterns across entire continents and driving droughts, floods, wildfires, and disease outbreaks far from its tropical Pacific origins.

Understanding why the researchers turned to corals requires an appreciation of both the phenomenon itself and the remarkable natural recorders that grow in the waters surrounding the Galápagos. El Niño is a natural climate oscillation that, every few years, causes the tropical Pacific to become warmer than usual. Under normal conditions, trade winds blow steadily along the equator from east to west, pushing sun-warmed surface water from the coast of South America toward Australia and Indonesia. When these trade winds weaken, the warm water sloshes back eastward toward South America, setting an El Niño event in motion. The atmosphere responds in turn: strong rainfall shifts from the Indonesian region into the central Pacific, leaving the western Pacific in drought, which further weakens the trade winds and locks in El Niño conditions that can persist for one to two years.

Although these temperature and precipitation fluctuations originate in the tropical Pacific, their consequences ripple across the entire globe. Storm tracks over the United States shift southward, bringing more rain to the desert Southwest and less to the Pacific Northwest. Similar circulation changes around the world produce droughts and flooding that contribute to crop failure, wildfires, waterborne disease, and other crises affecting human health and well-being. The Galápagos Islands sit at a geographic sweet spot for recording these shifts, a location where El Niño exerts its largest influence. The archipelago is famous for its astounding biodiversity, a product of high nutrient availability and the mixing of cool and warm waters, and its corals grow in an environment that faithfully registers every swing of the eastern Pacific climate system.

To reconstruct the history of El Niño intensity, Cole and her colleagues sampled cores from 13 corals, drawing on both living colonies and boulders of ancient coral collected from the Galápagos. Corals grow by secreting layers of calcium carbonate at a rate of roughly one to two centimeters per year, and the chemistry of each layer preserves a record of the seawater temperature in which the coral grew. Because El Niño extremes occur every few years, the team focused on core samples spanning at least 20 years, ensuring that each record captured multiple events. Sampling the cores a millimeter at a time, the researchers measured two independent aspects of the skeleton’s chemistry, building a robust proxy archive of past ocean conditions.

The first measurement targeted the ratio of strontium to calcium in the coral skeleton, a geochemical parameter that depends directly on the temperature at which the coral formed its skeleton. The team supplemented these results with an analysis of oxygen isotope ratios, which at this particular site also serve as a reliable measure of temperature. Together, these two proxies yielded a detailed history of temperature variability in the Galápagos region. The pattern that emerged was remarkably consistent: strong El Niños appeared throughout the last 40 to 50 years, whereas the El Niños recorded prior to that period displayed a steady pattern of lower intensity. Cole described the experience of adding record after record and expecting the story to grow more complicated, only to find that it did not—the signal was that clear.

A critical next step was to rule out the possibility that natural processes alone could have produced a strengthening of this magnitude. The researchers turned to climate models that simulate the last thousand years using reconstructions of volcanic eruptions and solar variability, the two dominant natural forcings of pre-industrial climate. When the models were run with these natural drivers, they produced no large shifts in El Niño behavior that could account for the magnitude of change documented in the coral records. This absence of a natural explanation, combined with the timing of the intensification after about 1850, points toward human-driven global warming as the most plausible cause of the observed strengthening, a conclusion consistent with the finding that El Niño intensity tracks rising global temperatures.

The timing of the study’s release gives its conclusions particular urgency. As a powerful El Niño develops atop an already warmed climate system, Cole noted that the relevant question is not whether the event will happen but how severe it will become and how damaging its impacts will be. Because the event is superimposed on global warming, it is likely to supercharge the temperature increase that would normally be expected from greenhouse gases alone. Forecasts cited in connection with the study suggest global temperatures could reach as much as 1.7 or 1.8 degrees Celsius above pre-industrial levels during this period, which would be considerably higher than the current record. The research was supported by the U.S. National Science Foundation and the United Kingdom Natural Environmental Research Council, with additional support from the Galápagos National Park and the Charles Darwin Research Station.

The implications extend well beyond the immediate forecast. El Niño is a major source of climate extremes, and if the phenomenon is intensifying, then its impacts intensify with it: droughts, floods, wildfire, and food insecurity. Changes in the hydrologic cycle also translate into damage to infrastructure, including floods that destroy homes, highways, and railroads, as well as public health consequences such as outbreaks of diseases like cholera. Cole and her colleagues argue that global warming is supercharging El Niño, and if that is correct, stronger climate extremes should be expected to amplify ecological, infrastructural, and human losses. No country, she cautioned, has the resources to be fully protected from these impacts, a reality that makes the strengthening of El Niño a global concern rather than a regional one.

For the research team, the ultimate takeaway is a call for mitigation. The coral archives of the Galápagos have delivered one of the clearest long-term pictures yet of how a major climate oscillation is responding to human influence, and the message is that the recent strengthening of El Niño represents one more reason to move away from fossil fuels, which the researchers identify as the root cause of the problem. The study’s co-authors include University of Michigan researchers Kelsey Dyez, Cameron Tripp, Jonathan Overpeck, and alumnus Jake Okun; University of Arizona researchers Diane Thompson and Marcus Lofverstrom; Samantha Stevenson-Karl of the University of California, Santa Barbara; Sandy Tudhope of the University of Edinburgh; Allison Lawman of Colorado College; Jessica Conroy of the University of Illinois; Gloria Jimenez of Moody’s Risk Management Services; and R. Lawrence Edwards of the University of Minnesota. Together, their work transforms a string of tropical islands into a millennium-scale warning system, one that indicates the powerful El Niños of recent decades stand alone in a thousand years of climate history.

Subject of Research: Unprecedented strengthening of El Niño events over the last four decades reconstructed from Galápagos coral paleoclimate records

Article Title: El Niños more intense over last 40 years than previous 1,000, according to U-M study

Article References: El Niños more intense over last 40 years than previous 1,000, according to U-M study. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: El Niño, ENSO, coral proxies, paleoclimatology, Galápagos Islands, climate change, global warming, tropical Pacific, strontium-to-calcium ratio, oxygen isotopes, climate models, University of Michigan

The research centered on a careful comparison of two sets of ice cores drilled from the Guliya ice cap, one recovered in 1992 and the other in 2015. When the team analyzed the oxygen isotope patterns preserved in both samples, they found strikingly similar signatures, indicating that the environmental signals locked within the ice are consistent and reproducible across a gap of more than two decades. This kind of internal agreement is a cornerstone of paleoclimate science, because it demonstrates that the record has not been distorted by melting, refreezing, or other processes that can scramble the layered history of a glacier. Lonnie Thompson, lead author of the study, professor of earth sciences and senior research scientist at the Byrd Polar Research Center at The Ohio State University, emphasized the significance of this match, noting that reproducibility of records within a given ice cap is extremely important and that obtaining an identical record a quarter of a century later reveals a great deal about the behavior of the ice over time.

Determining the true age of ancient ice is one of the most technically demanding challenges in glaciology. Ice cores serve as vital climate archives of the locations and time periods they represent, but deciphering these flash-frozen treasure troves requires an accurate knowledge of the core’s timescale. Without reliable dating, the chemical and physical signals within the ice cannot be confidently matched to specific climatic events. To overcome this obstacle, the research team deployed advanced dating techniques capable of detecting two radioactive isotopes, beryllium and chlorine, whose half-lives make them reliable tools for capturing large-scale environmental changes across deep time. These cosmogenic isotopes are produced when cosmic rays interact with the atmosphere and are then deposited onto the ice surface, where their concentrations can serve as chronological markers for extraordinary events in Earth’s history.

The pivotal marker used in this study was the Laschamp Geomagnetic Excursion, a dramatic reversal of Earth’s magnetic field that occurred more than 41,000 years ago. During this event, the planet’s protective magnetic shield weakened substantially, allowing greater fluxes of cosmic radiation to reach the atmosphere. That surge in cosmic ray intensity produced elevated concentrations of cosmogenic isotopes such as beryllium-10 and chlorine-36, which were subsequently deposited and preserved in the accumulating snow of the Guliya ice cap. By identifying this distinctive isotopic spike within the cores and anchoring it to the well-established timing of the Laschamp event, the team was able to construct a robust chronological framework for the ice, effectively pinning a known date onto the deep layers of the glacier and allowing the ages of overlying and underlying ice to be constrained with far greater confidence.

To further validate and extend this timescale, the researchers compared their isotope-based results with oxygen-isotope records obtained from cave deposits in nearby regions of the Tibetan Plateau. Speleothems, the mineral formations that grow in caves, carry their own independent records of past climate and can be dated with high precision using uranium-series methods. The alignment between the Guliya ice core signals and these cave records led the team to estimate that the age of the Guliya record may stretch back well over 128,000 years, a span that reaches into or beyond the last interglacial period. This cross-verification between two entirely independent paleoclimate archives represents a powerful convergence of evidence, strengthening the case that the deepest ice at Guliya predates the Holocene by a wide margin.

The significance of this finding becomes especially clear when placed in a global context. Thompson pointed out that outside of the polar regions, this is the only ice core from the mountaintops that reaches back that far. He contrasted the Guliya record with that of Huascarán in the Peruvian Andes, another celebrated high-altitude drilling site where the ice record extends back only over 30,000 years rather than over 100,000. That comparison makes Guliya a very unique record, preserving a continuous or near-continuous account of climate variability across a stretch of time that no other mountain glacier on Earth has been shown to retain. For scientists seeking to reconstruct how monsoons, temperature regimes, and atmospheric circulation patterns behaved across multiple glacial cycles in Asia, the Guliya cores now stand as an irreplaceable resource.

The new evidence also settles a persistent scientific controversy. Previous work had suggested that the Guliya record stopped at the Mid-Holocene, an era that began nearly 12,000 years ago, implying that the ice cap contained only relatively young ice. If that interpretation had been correct, it would have meant that the large ice sheets of the last ice age did not persist through the early Holocene on the Tibetan Plateau. The evidence from this work lays those doubts to rest, confirming that some of the ice is in fact much older than the Holocene. As Thompson explained, if only Holocene-era ice existed in Tibet, it would indicate that the great ice masses of the last glacial period failed to survive into the current warm epoch, but the data demonstrate that they did. This confirmation reshapes scientific understanding of glacial persistence in one of the most climatically sensitive regions on the planet, often described as the water tower of Asia because its glaciers feed major river systems serving billions of people.

Guliya’s distinctive topography likely plays an important role in its extraordinary ability to preserve such ancient paleoclimate records. The ice cap sits at extreme altitude, where frigid temperatures limit surface melting, and its geometry may shield deeper layers from the deformation and basal processes that typically destroy old ice. Yet the value of constructing a more accurate timescale for the ice cap extends beyond validating its antiquity. A precise chronology opens the door to answering bigger environmental questions, such as when Earth’s intense periods of glaciation began and ended. Thompson framed the challenge in stark terms, asking when ice started forming on the planet and what the natural processes of ice loss looked like, and observing that the only way to truly demonstrate these dynamics is with accurate timescales on these records. In an era when scientists are working to distinguish natural climate variability from human-driven change, benchmarks of past glacial behavior drawn directly from ice are indispensable.

Because these results open new pathways for detailed scientific investigations of the Guliya ice cores, the team plans to use future samples to determine how other key climate and environmental indicators may change in light of this revised understanding of Earth’s geologic history. Dust concentrations, trapped atmospheric gases, microbial communities, and a host of geochemical tracers preserved in the ice can now be interpreted against a trustworthy timeline, potentially yielding insights into past atmospheric composition and ecosystem responses at altitudes above six kilometers. The international collaboration behind the study included Ohio State co-authors Mary Davis and Ellen Mosley-Thompson, along with Juerg Beer from the Swiss Federal Institute of Aquatic Science and Technology, Christof Vockenhuber and Marcus Christl from ETH Zurich in Switzerland, Tandong Yao from the Chinese Academy of Sciences, and Ninglian Wang and Ling Fang from Northwest University in China, with support from the U.S. National Science Foundation. Reflecting on the work, Thompson observed that the results tell a very consistent story, and that if the science is done right, it is consistent, adding that it is human understanding of what the ice is trying to tell us that has to catch up.

Subject of Research: Dating of ancient ice cores from the Guliya ice cap on the Tibetan Plateau

Article Title: Digging deep, researchers find high-altitude glacier endured last ice age

Article References: Digging deep, researchers find high-altitude glacier endured last ice age. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: Guliya ice cap, Tibetan Plateau, ice cores, paleoclimatology, Laschamp Geomagnetic Excursion, cosmogenic isotopes, beryllium-10, chlorine-36, oxygen isotopes, last ice age, glaciology, Science Advances

The research, led by Victoria E. Taylor of the University of Bergen together with Alison M. Piasecki, Eivind O. Straume and A. Nele Meckler, applied a technique called carbonate clumped isotope thermometry to fossil foraminifera from two of the world’s great ocean basins. Unlike conventional oxygen isotope measurements, which conflate temperature with the isotopic composition of seawater, clumped isotope thermometry exploits the tendency of heavy isotopes of carbon and oxygen to bond with one another in carbonate minerals. The abundance of these carbon-13–oxygen-18 bonds, quantified as the Δ47 value, depends on temperature alone at the time the shell formed. That independence makes it possible to reconstruct deep-sea temperatures directly, without having to guess how salty or isotopically heavy the ancient ocean was.

The team assembled temperature records spanning the Eocene–Oligocene transition from sediment cores recovered by the Integrated Ocean Drilling Program in the eastern equatorial Pacific, at Sites U1333, U1334 and 1218, and from the northwest North Atlantic, at IODP Site U1406. What they found was genuinely surprising. Although the oxygen isotope records from the two basins look remarkably similar, the actual temperatures behind them behaved in completely different ways. The deep Pacific began cooling earlier, with its temperature decline preceding the main phase of the oxygen isotope increase by roughly 100,000 years. The North Atlantic, by contrast, cooled later in the transition, even though its isotope signal shifted at much the same time as the Pacific’s.

This decoupling is the kind of result that forces a rethink of a foundational dataset. For decades, the globally coherent rise in benthic oxygen isotope values at the Eocene–Oligocene transition has been treated as evidence that the deep ocean cooled everywhere in step as the Antarctic ice sheet expanded. The new clumped isotope data reveal that the isotope signal can be misleading. Because oxygen isotope values in foraminiferal calcite depend on both temperature and the isotopic composition of seawater, which itself tracks salinity, two water masses can record identical isotope shifts while experiencing entirely different thermal histories. The authors argue that contrasting salinity gradients, required to maintain similar water densities between the basins, probably masked the divergent temperature behaviour in the traditional records.

Perhaps the most striking finding concerns the direction of the temperature gradient between the basins. Today, the deep North Atlantic is bathed in relatively warm water formed in the northern high latitudes, while the deep Pacific receives colder water sourced from the Southern Ocean. In the early Oligocene, the situation appears to have been reversed: the deep North Atlantic was, on the whole, cooler than the equatorial Pacific. In fact, between about 33.52 and 33.29 million years ago, deep northwest Atlantic temperatures dropped to values approaching those of the modern ocean, levels that, within the limits of the available records, were not reached again until the Pleistocene ice ages tens of millions of years later. That is a remarkable early glimpse of ice-age-like cold in a basin usually associated with warmth.

The explanation the researchers propose is a deep ocean organised in a fundamentally different way from today’s. Rather than a single dominant overturning circulation, the Eocene–Oligocene deep ocean may have hosted two distinct southern-sourced deep-water masses with divergent properties. The compartmentalised Southern Ocean of the time, still partially cut off by the unfinished opening of the Drake Passage and the Tasmanian Gateway, offered greater potential to form cold deep waters in its Atlantic sector than in its Pacific sector. The two basins were therefore effectively decoupled, each receiving deep water whose characteristics depended on where and how it last contacted the surface.

This picture carries a broader implication: the growing Antarctic cryosphere was not merely a passive response to cooling, but an active participant in shaping the ocean’s circulation. The spatially and temporally variable early ice sheet would have altered brine rejection, freshwater fluxes and the buoyancy of surface waters around the Antarctic margin, directly influencing the properties of the deep waters forming in adjacent sectors. The divergent behaviour of the Pacific and Atlantic basins at the transition may thus record the fingerprint of an ice sheet learning, so to speak, how to be an ice sheet, with each phase of its growth leaving a distinct imprint on the water masses it helped create.

The study builds on a growing body of work applying clumped isotope thermometry to the deep past. A 2022 analysis in Science by Meckler and colleagues established the long-term Cenozoic evolution of deep ocean temperature using the same method, and subsequent studies have documented transient deep cooling in the eastern equatorial Pacific at the transition. By extending the approach to the North Atlantic and comparing basins directly, the new work transforms what was a single global narrative into a two-basin story with genuinely different thermal trajectories. The methodological rigour is considerable: the team used long-integration dual-inlet mass spectrometry, community standardisation protocols and careful error propagation, and their Atlantic temperatures reproduce independently published measurements from nearby Expedition 342 sites.

Why does this matter beyond the Eocene? The Eocene–Oligocene transition is one of the best-studied natural experiments in how a greenhouse world tips into an icehouse one, and it is frequently used to benchmark climate models and to frame expectations about future change. If the deep ocean’s response to that tipping was spatially heterogeneous rather than uniform, then reconstructions and models that assume a globally coherent deep ocean may need revision. It also matters for how scientists interpret the oxygen isotope archives that underpin so much of Cenozoic climate science: the same isotope shift can hide different combinations of cooling and salinity change in different places, and clumped isotope thermometry offers a way to untangle them.

The deep ocean, covering most of the planet and storing the vast majority of its accessible heat, is the flywheel of the climate system. What this study reveals is that during one of Earth’s great climatic reorganisations, that flywheel was not turning as a single machine but as two, with the Pacific and Atlantic spinning along divergent paths even as the ice advanced on Antarctica. The samples and data underpinning the work, provided through the Integrated Ocean Drilling Program and archived on Zenodo, ensure that other researchers can now interrogate this pivotal interval further. As the Antarctic ice sheet assembled itself 34 million years ago, it did not simply chill the world’s oceans in unison; it reorganised them from the bottom up, leaving a signature that is only now coming into focus.

Subject of Research: Deep-sea temperature evolution in the Pacific and Atlantic oceans during the Eocene–Oligocene onset of Antarctic glaciation

Article Title: Divergent deep-sea temperatures in the Pacific and Atlantic during the onset of Antarctic glaciation

Article References: Taylor, V. E., Piasecki, A. M., Straume, E. O., & Meckler, A. N. (2026). Divergent deep-sea temperatures in the Pacific and Atlantic during the onset of Antarctic glaciation. Nature Geoscience. https://doi.org/10.1038/s41561-026-02106-x

Image Credits: AI Generated

DOI: 10.1038/s41561-026-02106-x

Keywords: Eocene–Oligocene transition, Antarctic glaciation, clumped isotope thermometry, benthic foraminifera, deep-ocean circulation, palaeoceanography, oxygen isotopes, North Atlantic Deep Water, Southern Ocean, IODP, paleoclimate, ocean gateways