The research team, led by Jiaqi Zhang and Zhenchuan Niu of the Institute of Earth Environment at the Chinese Academy of Sciences, sampled air at an urban site in Xi’an’s High-Tech Zone every day from December 23, 2021, to January 23, 2022, a window covering the city’s strict pandemic confinement. Rather than relying on total carbon dioxide readings, which mix natural and human sources, the researchers turned to isotopic forensics. Fossil fuels contain no carbon-14, the radioactive isotope of carbon that decays over tens of thousands of years, so carbon derived from coal, oil, and natural gas is distinctly radiocarbon-free. By measuring the carbon-14 composition of atmospheric carbon dioxide, the team could isolate the fossil component with precision, a technique long championed for monitoring megacity emissions.

The isotopic measurements were complemented by analysis of carbon-13, the stable heavy isotope of carbon. Different carbon sources carry different carbon-13 signatures: vehicle exhaust, coal combustion, and natural gas each leave a characteristic fingerprint in the carbon dioxide they release. By combining the two isotope systems, the researchers could not only quantify how much fossil carbon dioxide was in the air but also apportion it among its sources. This dual-isotope approach, normalized against a ventilation coefficient, formed the analytical backbone of the study and allowed the team to separate genuine changes in emissions from changes in how the atmosphere diluted them.

That ventilation correction proved to be the crux of the work. The ventilation coefficient, a classical concept in air pollution meteorology dating back to the work of Holzworth in the 1960s, combines the depth of the atmospheric mixing layer with the near-surface wind speed. When the mixing layer is shallow and winds are calm, pollutants emitted at the surface accumulate rapidly; when the boundary layer is deep and winds are brisk, the same emissions are swept away. Winter haze episodes in northern Chinese cities are typically triggered by exactly such stagnant conditions, with temperature inversions capping the boundary layer and trapping everything from fine particulate matter to greenhouse gases near the ground.

The Xi’an data captured this dynamic in dramatic fashion. During haze periods within the lockdown, fossil carbon dioxide concentrations were 159 percent, give or take 48 percent, higher than during non-haze periods, despite the fact that human activity, and presumably emissions, were suppressed throughout. In other words, the atmosphere itself, not the sources, was responsible for a large share of the variation observed at the sampling site. Without accounting for this meteorological amplification, an observer relying on concentration data alone would have dramatically misjudged how emissions were changing during the lockdown.

Once the team normalized the measurements with the ventilation coefficient, the true emissions signal emerged. Compared with the period after the lockdown ended, fossil carbon dioxide concentrations during well-ventilated, non-haze lockdown days fell by 63.0 percent, with an uncertainty of 10.6 percent. Averaged across the entire lockdown period, including hazy days, the ventilation-normalized decline was 48.8 percent, plus or minus 12.2 percent. These figures align with the substantial reductions in traffic and economic activity that characterized the confinement, and they demonstrate how powerful the isotope-based method can be when meteorological variability is properly handled.

The carbon-13 source apportionment added a second layer of insight into which activities drove the decline. Vehicle emissions dropped by 84.6 percent, with a relatively tight uncertainty of 4.7 percent, reflecting the near-standstill of urban traffic during confinement. Coal burning emissions, by contrast, decreased by only 31.2 percent, though with a much larger uncertainty of 20.3 percent. The asymmetry makes physical sense: private and commercial transport can be switched off almost entirely by lockdown orders, whereas residential and industrial coal combustion for heating continues through the frigid Shaanxi winter regardless of pandemic restrictions. Xi’an, like much of northern China, still depends significantly on coal for winter heating, and that dependence cushioned the overall emissions decline.

The study’s implications reach well beyond one city or one lockdown. Since the pandemic began, researchers have used atmospheric observations to estimate emissions changes from the COVID-19 confinement in locations from California to Hungary, and global datasets such as the Carbon Monitor have tracked near-real-time emission shifts worldwide. Many of these assessments rely on top-down approaches, in which measured atmospheric concentrations are converted into emission estimates using atmospheric transport models. The Xi’an results show that meteorological effects, particularly haze-induced stagnation, can substantially bias such top-down assessments if not explicitly corrected. A city might appear to have slashed its emissions when in fact stagnant air was simply concentrating them, or vice versa.

The methodological lesson is that concentration is not emission, and the gap between the two is governed by the vagaries of weather. For urban carbon monitoring networks being built to verify climate pledges, the Xi’an study suggests that radiocarbon measurements, paired with careful boundary-layer and wind observations, offer a robust path forward. The ventilation coefficient normalization used here is simple enough to be applied broadly, yet it transformed a potentially misleading haze signal into a clean measurement of a 63 percent emissions drop. As cities worldwide deploy increasingly dense greenhouse gas sensing networks, incorporating such meteorological corrections will be essential to distinguish real mitigation progress from atmospheric illusion.

There is also a human story embedded in the dataset. The authors acknowledge an e-bike courier who assisted with sample collection during the pandemic, a reminder that the daily air samples underpinning these isotope measurements were gathered under difficult confinement conditions. The work was supported by the National Natural Science Foundation of China, the Natural Science Basic Research Program of Shaanxi, and the Strategic Priority Research Program of the Chinese Academy of Sciences. Published on September 29, 2026, the study stands as both a detailed case study of a locked-down megacity and a methodological caution: when the air stagnates, the atmosphere can tell a story about emissions that is dramatically at odds with reality, and only isotope forensics combined with meteorological insight can set the record straight.

Subject of Research: Isotopic quantification of fossil fuel carbon dioxide emissions and meteorological haze effects during the COVID-19 lockdown in Xi'an

Article Title: Substantial increase of fossil carbon dioxide concentrations in haze time periods versus ventilated non-haze periods during the coronavirus lockdown in Xi’an

Article References: Zhang, J., Liang, D., Niu, Z., Zhou, W., Wang, P., Feng, X., Wu, S., Wang, G., Lyu, M., Lu, X., & Kong, X. (2026). Substantial increase of fossil carbon dioxide concentrations in haze time periods versus ventilated non-haze periods during the coronavirus lockdown in Xi’an. Environmental Chemistry Letters. https://doi.org/10.1007/s10311-026-01927-x

Image Credits: AI Generated

DOI: 10.1007/s10311-026-01927-x

Keywords: COVID-19 lockdown, fossil fuel CO2, carbon-14, carbon-13, haze, ventilation coefficient, source apportionment, urban emissions, Xi'an, meteorology, radiocarbon, air pollution

The challenge the study addresses is stark. In subarctic and Arctic Canada, ice cover on lakes and rivers can penetrate the entire water column for up to eight months, cutting communities off from their most obvious water reservoirs during the long winter. At the same time, permafrost in many northern settlements is so thick—up to several hundred meters—that drilling down to subpermafrost groundwater is technically or economically unfeasible. River and lake taliks, which remain unfrozen because the overlying water body moderates ground temperatures, have long been proposed as a promising alternative. If a talik is large enough and hydraulically connected to permeable sediments, it can store and transmit groundwater of sufficient quantity and quality to meet a community’s needs, without the enormous expense of drilling through deep frozen ground.

Salluit sits in a narrow, glacially carved valley about two kilometers long and five hundred meters wide, flanked by bedrock slopes rising 360 to 450 meters above sea level. The community lies squarely within the continuous permafrost zone, where average annual air temperatures hovered around minus 6.2 degrees Celsius between 2003 and 2017. After deglaciation roughly 8,600 to 8,700 years ago, the valley was flooded by the d’Iberville Sea, which blanketed glaciofluvial and till deposits with fine-grained marine sediments. These marine deposits are frost-susceptible and ice-rich, with low hydraulic conductivity that limits vertical groundwater movement. Yet beneath the Kuuguluk River corridor, a perennial talik extends through these marine deposits into shallow fractured bedrock, developing mainly within permeable sandy-silty shallow-marine sediments that form the region’s principal potential aquifer.

Earlier work by researchers at Université Laval, including Liu and colleagues, had used electrical resistivity tomography and three-dimensional cryo-hydrogeological modeling to map the geometry of this talik system. During winter, ground freezing disconnects the talik from surface water inputs, building pressure until groundwater periodically discharges through ice fractures and forms layered icings on the floodplain. What remained unknown was the origin, recharge history and residence time of the water inside the talik aquifer—critical questions for a community that already draws drinking water from an artesian well drilled into the fractured rock beneath the river.

To answer these questions, the team established three monitoring well sites along the Kuuguluk River in October 2024, installing wells above and within the talik using a direct push and rotary percussion drilling system adapted for cold regions. Real-time drilling sensor data allowed them to reconstruct the stratigraphy: two to nearly five meters of gravelly sandy alluvium overlying one to almost four meters of marine sediments, followed by glacial deposits and diamicton. The permafrost table was encountered at roughly eight to nine meters depth. Hydraulic head measurements revealed an upward gradient from the deeper, semi-confined aquifer toward the shallow zone and the river itself, consistent with groundwater discharging through the talik into the Kuuguluk River.

The chemical results painted a picture of youthful, actively circulating water. Both surface water and groundwater samples showed a calcium–bicarbonate composition, with generally low mineral saturation indices indicating minimal water–rock interaction. Stable water isotopes—deuterium and oxygen-18 ratios—plotted slightly below the Global Meteoric Water Line, suggesting modest evaporative enrichment before sampling. Most striking were the tritium concentrations, which ranged from 8.48 to 11.52 tritium units across all samples. These values closely match recent precipitation measured and modeled at Churchill, Manitoba, the nearest community at similar latitude with tritium data, confirming that the system is dominated by modern meteoric recharge rather than ancient, isolated water.

Beneath that youthful surface, however, the isotopes told a deeper story. While tritium indicated recharge within the past few decades, radiocarbon signatures of dissolved inorganic and organic carbon were significantly depleted, particularly in the deeper semi-confined aquifer at well S1-P2. There, the fraction of modern radiocarbon in dissolved inorganic carbon dropped to 0.487, and dissolved organic carbon fell to 0.405—values far below the roughly 1.0 expected for water in equilibrium with today’s atmosphere. The researchers interpret this radiocarbon-depleted carbon as evidence of interaction with aged organic matter, potentially locked in permafrost for centuries or millennia and only recently mobilized as thaw deepens the active layer. The deeper groundwater also carried the highest solute loads, the highest electrical conductivity at 147 microsiemens per centimeter, the lowest oxidation–reduction potential, and the most depleted stable isotope values, all consistent with longer residence times and more extensive geochemical evolution along deeper flowpaths.

Dissolved radon-222 provided an independent line of evidence about where that groundwater is escaping to the surface. Because radon is produced by the radioactive decay of radium in sediments and decays with a half-life of just 3.8 days, elevated concentrations in river water signal nearby groundwater inputs. Groundwater samples ranged from about 4,900 to 7,500 becquerels per cubic meter, while surface water samples—normally near zero where no groundwater enters—measured between roughly 1,200 and 2,200 becquerels per cubic meter. The highest surface value appeared at the most downstream site, where the talik is thought to narrow and concentrate upward flow, matching both the measured upward hydraulic gradient and the predictions of earlier numerical modeling. The finding marks the first combined use of radon, tritium and stable water isotopes to assess surface–groundwater interaction in a continuous permafrost river talik system in Nunavik.

The implications stretch well beyond a single Arctic river. Under continued climate warming, permafrost degradation is expected to drive vertical and lateral expansion of the talik, enlarging the unfrozen aquifer and strengthening connectivity between groundwater and the river. But the researchers caution that the response will not be one-directional: enhanced connectivity could deepen flowpaths and redistribute storage, potentially reducing near-surface water availability even as total groundwater discharge grows. Shifts in snow cover, vegetation and evapotranspiration may also reshape the seasonal timing of recharge, even if annual volumes remain similar. Meanwhile, ongoing permafrost thaw could continue releasing old organic carbon and associated solutes into the aquifer, making long-term water quality monitoring essential if the talik is to serve as a municipal supply.

For the people of Salluit, the study transforms a promising hypothesis into a chemically grounded reality: the water beneath the Kuuguluk River is young, recharged by modern precipitation, and hydraulically connected to the river in ways that models had predicted but field data had never before confirmed. The work, funded by the GEM-GeoNorth program of the Geological Survey of Canada and carried out with support from the community and the Qaqqalik Landholding Corporation, will continue with sustained monitoring of hydraulic heads and temperatures, followed by three-dimensional modeling of recharge dynamics and the impacts of groundwater withdrawal. If those efforts confirm the system’s resilience, the Kuuguluk talik aquifer could become a template for how circumpolar communities secure safe, year-round drinking water on top of the warming permafrost.

Subject of Research: Hydrogeochemical dynamics of a river talik aquifer beneath the Kuuguluk River in continuous permafrost at Salluit, Nunavik, Canada.

Article Title: Hydrogeochemical dynamics of a potential talik aquifer beneath the Kuuguluk River, Salluit, Nunavik (Québec, Canada)

Article References: Faucher, B., Benoit, N., Gammon, P. R., & Fortier, R. (2026). Hydrogeochemical dynamics of a potential talik aquifer beneath the Kuuguluk River, Salluit, Nunavik (Québec, Canada). Hydrogeology Journal. https://doi.org/10.1007/s10040-026-03140-0

Image Credits: AI Generated

DOI: 10.1007/s10040-026-03140-0

Keywords: permafrost, river talik, groundwater, hydrochemistry, isotopes, tritium, radiocarbon, radon-222, Nunavik, drinking water, Arctic hydrogeology, climate change

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