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 chain of causality that makes such reconstructions possible begins far beyond Earth. The Sun’s magnetic field, dragged outward by the solar wind, fills the heliosphere, the vast protective bubble surrounding the solar system. Galactic cosmic rays, high-energy particles arriving from outside, must fight their way through this magnetic shielding to reach Earth. When the Sun is magnetically active, the shielding strengthens and fewer cosmic rays penetrate; when the Sun quiets, the floodgates loosen. Upon entering the atmosphere, cosmic rays collide with nitrogen, oxygen and argon, spawning showers of secondary particles that forge rare radionuclides such as carbon-14 and beryllium-10. Carbon-14, once formed, is incorporated into carbon dioxide, absorbed by trees during photosynthesis and permanently archived in annual growth rings. Because a magnetically active Sun suppresses cosmic ray influx, the abundance of these isotopes in wood and ice is inversely correlated with solar activity, giving scientists a decipherable record stretching back thousands of years.

Deciphering it, however, is harder than it sounds. Previous reconstruction efforts relied on statistical regressions that mapped isotope-derived quantities onto sunspot numbers, and those regressions carried hidden dangers. As the new study’s authors, led by Chitradeep Saha of the University of Reading, point out, even a regression with an impressively high correlation coefficient can misfire if the data contain non-linearities, zero-level offsets, or uneven variance across amplitudes. The most notorious failure mode appears during grand minima, the extended intervals when solar activity collapses, such as the Maunder minimum of the seventeenth century. When regression equations calibrated on modern data are extrapolated to these unusually quiet conditions, they can yield sunspot numbers below zero. Values are often simply clipped to zero, but that crude fix distorts cycle averages and inflates estimates of the Sun’s total irradiance, which in turn muddies attempts to understand the Sun’s influence on past climate.

The Reading-led team, which also includes Mathew Owens, Mike Lockwood and Luke Barnard, together with colleagues at ETH Zurich, the University of Lancashire, the University of Oslo, Lund University and the University of Groningen, took a different path. Rather than inverting the physics with a regression, they ran it forward, over and over, in a Monte Carlo framework. The method, implemented in publicly released code under the name PRISM, begins by generating an ensemble of thousands of hypothetical sunspot cycles drawn from statistical priors: cycle amplitudes sampled from a log-normal distribution, cycle lengths from a Gaussian distribution centred on 10.5 years, and a random start offset for each window of time. Each trial cycle is then passed through a sequence of two semi-empirical forward models that translate sunspot number into open solar flux, the total magnetic flux threading the outer boundary of the corona, and then into the heliospheric modulation potential, a quantity describing how much energy cosmic rays lose as they traverse the heliosphere.

The forward models rest on decades of established solar physics. The first couples sunspot number to the emergence of new magnetic flux through a empirically optimised source function, balanced against a phase-dependent loss rate derived from solar cycles 13 through 24. The second model computes the modulation potential from the open flux together with the tilt and polarity of the heliospheric current sheet, following a formulation calibrated by Owens and colleagues in 2024. Crucially, the open solar flux evolves with memory: it accumulates from past sunspot activity and decays through magnetic reconnection, introducing a hysteresis that makes the inverse problem fundamentally non-unique. Multiple distinct sunspot histories can produce statistically indistinguishable modulation records, which is precisely why simple deterministic inversions break down.

To handle that non-uniqueness, the team employed Approximate Bayesian Computation, a statistical technique that sidesteps the need for an explicit likelihood function. In each sliding ten-to-fifteen-year window, ten thousand Monte Carlo realisations of sunspot cycles are propagated through the forward models and compared directly against the observed modulation potential using a weighted Euclidean distance. The best two percent of candidates, some two hundred realisations, are retained as samples from the approximate posterior distribution, and their spread provides rigorous, quantified uncertainty bounds reported as 68 percent highest-density intervals. Because the sunspot cycle amplitudes are constrained to be non-negative by construction, the resulting reconstruction can never produce the negative values that haunted earlier regression-based approaches, and it requires no post-hoc correction.

To test the method, the researchers applied it to two annual-resolution records of the modulation potential. The first, spanning 1845 to 2020, was derived from geomagnetic observations of open solar flux by Owens and colleagues. When used as the inversion target, the method recovered sunspot numbers and open flux in close agreement with the direct instrumental record maintained by SILSO, with a mean absolute error of just 19.25 megavolts, about three percent of the mean modulation potential. That success validated the technique and justified applying it to the second, far longer dataset: a radiocarbon-based modulation potential record covering 971 to 1932, reconstructed by Nicolas Brehm of ETH Zurich and colleagues from tree-ring carbon-14 measurements. Before feeding the tree-ring record into the inversion, the team cross-calibrated it against the geomagnetic record over their overlapping decades, applying an additive correction of 65.76 megavolts that statistical tests showed produced a near-perfectly symmetric, homoskedastic residual distribution.

The millennial-scale reconstruction that emerged is rich with detail. It captures the familiar grand minima and maxima of the past thousand years, including the Maunder minimum, the Spörer minimum, the Dalton minimum, and the double-peaked grand maximum the Sun passed through between 1900 and 2020. Crucially, the reconstructed open solar flux never falls to zero. Even during the deepest quiet of the Spörer minimum, the unsigned open flux dropped only to about 1.21 times ten to the fourteen webers in 1443, the lowest value in the entire record and well below anything observed in the telescopic era, yet still decisively nonzero. During the Maunder minimum the flux averaged around 2.63 times ten to the fourteen webers. This confirms that the solar dynamo never fully shuts down during grand minima but instead idles in a reduced, finite activity state, a conclusion consistent with flux transport dynamo models in which meridional plasma circulation sustains weak cycles and eventually drives recovery.

The new record also documents some striking extremes at the other end of the scale. The peak annual open solar flux of the twentieth century, reached in 1958, was about 10.96 times ten to the fourteen webers, the highest since the year 1200. Only three years in the entire millennium exceeded it, the largest peaking at roughly 14.21 times ten to the fourteen webers in 981 CE, shortly after the record begins. In other words, the era of telescopic observation has sampled a large fraction, but not all, of the Sun’s true dynamic range; deeper minima and higher maxima both occurred before instruments existed to see them. The reconstruction also flags three intervals around 993, 1052 and 1279 CE where proposed Miyake events, extreme solar particle storms recorded as abrupt radiocarbon spikes, contaminate the cosmic-ray-based record, and the authors conservatively mask these windows from their results.

Beyond its intrinsic appeal as a thousand-year diary of solar magnetism, the work has immediate practical value. Annually resolved, physically consistent sunspot numbers feed directly into reconstructions of total and spectral solar irradiance, which in turn constrain climate models exploring the Sun’s role in terrestrial temperature variability over past centuries. The uncertainty-quantified cycle amplitudes during grand minima provide empirical targets for solar dynamo theorists probing the minimum operating point of the solar cycle engine. The authors note that their forward model templates were built from modern, regular solar cycles, so reconstructions of grand minimum cycles should be treated as indicative rather than definitive, and that the additive cross-calibration between the two modulation potential datasets is itself a simplification. Even so, the framework offers what the team describes as a probabilistic ensemble of physically admissible solar histories rather than a single deterministic answer, and it opens the door to pushing the same technique further back in time as longer and older cosmogenic isotope records become available. The Sun, it turns out, kept meticulous records all along; the trick was learning to read them without breaking the laws of physics.

Subject of Research: Physics-constrained reconstruction of annually resolved sunspot numbers from millennial-scale heliospheric modulation potential records.

Article Title: Physics-Constrained Reconstructions of Sunspot Number from Millennial-Scale Annual Heliospheric Modulation Potential

Article References: Saha, C., Owens, M., Lockwood, M., Barnard, L., Brehm, N., Dalla, S., Herbst, K., Muscheler, R., & Wang, J. (2026). Physics-Constrained Reconstructions of Sunspot Number from Millennial-Scale Annual Heliospheric Modulation Potential. Solar Physics, 301(9), Article 144. https://doi.org/10.1007/s11207-026-02731-0

Image Credits: AI Generated

DOI: 10.1007/s11207-026-02731-0

Keywords: sunspot number, solar cycle, heliospheric modulation potential, cosmogenic isotopes, radiocarbon, open solar flux, Maunder minimum, Spörer minimum, approximate Bayesian computation, space climate, solar dynamo, galactic cosmic rays