The technique builds on a long tradition of sediment fingerprinting, the idea that every source area imprints a characteristic signature on the material it sheds. Researchers have traced sediment origins using geochemistry, radionuclides, isotopes, magnetic susceptibility, organic carbon, pollen, environmental DNA and even colour. The trouble is that many of these tracers are costly and slow to measure, which limits how widely they can be applied. X-ray diffraction, by contrast, is a workhorse technique: powdered samples are bombarded with X-rays, and constructive interference occurs only at specific angles determined by the spacing of atomic planes in the crystals present. The resulting diffraction pattern is a mineralogical fingerprint that can be collected quickly and cheaply for large numbers of samples.

The team’s workflow transforms raw diffractograms into usable fingerprints through a series of processing steps. Baseline drift is corrected and background noise removed with a peak-finding algorithm, and the patterns are aligned to a pure quartz reference to eliminate instrumental misalignments. Diagnostic two-theta windows for each target mineral are selected from reference spectra in the RRUFF database, and the total peak area within each window is averaged and normalised, yielding relative mineral concentrations on a scale from zero to one. Crucially, averaging peak areas across a window rather than relying on individual peaks mitigates the overlap that occurs when different minerals diffract at similar angles. The researchers focused on 21 primary rock-forming minerals, from quartz and feldspars to pyroxenes, amphiboles, micas and garnet, deliberately excluding the clay minerals produced by chemical weathering.

Turning fingerprints into erosion rates requires an inversion. The forward model is conceptually straightforward: each source area, typically a geological unit, carries its own mineral signature, erodes at some rate, and contributes sediment in proportion to that rate. The mineral concentrations measured in suspended sediment downstream are therefore a weighted average of the source signatures, with the weights being the erosion rates. Because erosion rates must be positive, the team solved the problem in log-space, making the mixing model non-linear. The inverse problem is underdetermined, since the number of tracers is far smaller than the number of pixels for which erosion rates are desired, so many different maps could explain the same data.

To tame this ambiguity, the authors adopted a Bayesian maximum a posteriori approach. They sought the most probable erosion map given the observed detrital data and prior knowledge encoded in a model covariance that favours spatially smooth solutions, with the smoothing distance and prior variance acting as tunable hyperparameters. Two iterative optimisation schemes were tested: steepest descent and a quasi-Newton method that uses curvature information to accelerate convergence. In synthetic forward-inverse tests, the quasi-Newton approach reconstructed a known erosion pattern with a localised hotspot far more accurately, while the steepest descent scheme proved unstable unless its gradient was normalised, which smoothed out sharp features and required many more iterations. The quasi-Newton method was therefore adopted for all subsequent experiments.

The sensitivity tests revealed which ingredients matter most. When the researchers progressively blended the source fingerprints together, making different lithologies mineralogically similar, the inversion deteriorated sharply: once a similarity metric called the average Jensen-Shannon distance fell below about 0.55, both errors and posterior uncertainty rose steeply, and in the most blended cases the inversion failed to converge at all. Uncertainty in the geological map, relevant in glaciated terrain where bedrock cannot be directly observed, was the second most influential factor. Reducing the number of tracer minerals had comparatively little effect, provided the most informative ones were retained: seven tracers produced essentially the same posterior solution as the full set of 21. Parameter sweeps showed the method is robust across a wide range of smoothing distances, standard deviations and step sizes, with convergence essentially complete after roughly 115 iterations.

The real-world validation took place in the Gornergletscher catchment in the Swiss Alps, an ideal testing ground because it combines multi-year suspended sediment records with strongly heterogeneous bedrock. The catchment drains lithologies from both oceanic and continental domains of the Pennine Alps, including serpentinites and eclogites of the Zermatt-Saas Fee ophiolites, calcareous mica schists and quartzites of the Penninic Mesozoic sediments, and the granites and garnet-mica schists of the Monte Rosa nappe. The XRD fingerprints derived from bedrock samples aligned well with established geological knowledge of these units, confirming that the cleaning and binning procedure preserves meaningful mineralogical information.

Two natural experiments put the method through its paces. In the first, the team exploited a natural analogue of a known-mixture test: sediment collected at the Gorner-Grenzgletscher confluence, where a subcatchment drains only part of the full study area. When the inversion was run using the geology of the entire catchment, it correctly attributed erosion only to lithologies actually present within the subcatchment, predicting rates close to zero for the Zermatt-Saas Fee sediment unit, which has no outcrops there. In the second, the XRD approach was benchmarked against zircon U-Pb age fingerprinting on the same detrital sample. The zircon-only inversion was highly unstable, producing implausible erosion rates above 8000 millimetres per year and high uncertainty, partly because two lithological units lacked zircon data altogether. The XRD-based solution was markedly more stable, and concatenating both datasets improved the distinguishability of source fingerprints and stabilised the posterior further, suggesting the two tracers are complementary rather than competing.

The authors are careful about the method’s limits. Grain-size effects, mineral fractionation during transport and post-depositional alteration can all bias fingerprinting, but several lines of evidence suggest these are modest in the Gorner setting: the suspended load is dominated by silt-sized glacial flour produced by physical comminution, only primary rock-forming minerals were used as tracers, and pump samples and depth-integrated samples yielded nearly identical mineralogies. The inversion also assumes that sediment is exported promptly rather than stored, so catchments with large sediment reservoirs are poor candidates. With those caveats, the recipe is appealingly simple: binned XRD data from sediments and source rocks, a map of potential source areas, and an estimate of total annual suspended sediment export. Because XRD is fast and affordable, the approach could open up provenance analysis to far more catchments, and the framework is flexible enough to accept other tracers, from zircon ages to geochemistry, or source maps based on land use rather than geology. For glacierised basins, where ice conceals the very landscapes doing the eroding, a cheap X-ray scan of river mud may now be enough to reveal where the ground is giving way.

Subject of Research: X-ray diffraction sediment fingerprinting and non-linear inversion to map erosion rates in glacierised catchments

Article Title: From XRD signal to erosion rate maps

Article References: From XRD signal to erosion rate maps. (n.d.). https://doi.org/10.5194/esurf-14-443-2026

Image Credits: AI Generated

DOI: 10.5194/esurf-14-443-2026

Keywords: sediment fingerprinting, X-ray diffraction, erosion rates, provenance analysis, Bayesian inversion, glaciers, Gornergletscher, suspended sediment, mineralogy, Swiss Alps, Earth Surface Dynamics, inverse problems

The physics of why a glacier can be simultaneously frozen and thawing is subtle. Englacial temperature reflects a tug-of-war between several heat sources and sinks: the surface energy balance that drives the winter cold wave downward, strain heating produced as ice deforms under its own weight, the modest geothermal heat flux from below, and latent heat released when meltwater percolates into the ice and refreezes. A glacier is classified as temperate when all of its ice sits at the pressure melting point, cold when it remains entirely below that threshold, and polythermal when both regimes coexist. The boundary between the two, known as the cold-temperate transition surface, or CTS, is a critical feature because it controls where water can move freely and where it becomes imprisoned.

Until now, almost all englacial temperature measurements in the Alps came from high-elevation accumulation zones above 3800 meters, in the Monte Rosa, Mont Blanc and Bernese Alps massifs, where cold firn has historically dominated. The ablation zones at lower elevations, where ice is actively melting away, were essentially unmeasured, with only a handful of exceptions dating back to pioneering nineteenth-century observations and a few studies from the 1970s. The new study targeted precisely this gap. The team selected six glaciers smaller than half a square kilometer, chosen for their minimal strain heating, limited crevassing, and, at two sites, the availability of historical borehole data for comparison. Between the summers of 2024 and 2025 they drilled 23 boreholes, using a portable steam drill for shallow holes and a hot-water drill system for deeper ones reaching up to 58.3 meters down to the glacier bed.

The measurement campaign was technically demanding. Tinytag loggers with an accuracy of plus or minus 0.2 degrees Celsius were installed in shallow boreholes, while multi-sensor Geoprecision thermistor chains, accurate to 0.05 degrees Celsius, were deployed in the deep holes after calibration in an ice bath and a temperature-controlled alcohol bath. Sensors were corrected for surface melt that thinned the ice above them during the year of deployment. To extend the point measurements into a spatial picture, the team ran ground-penetrating radar surveys along glacier centerlines and, at one site, flew an uncrewed aerial vehicle carrying an 80-megahertz radar antenna in a grid pattern with four-meter line spacing. The radar exploits a simple contrast: cold ice, lacking liquid water, is nearly transparent to radar waves, whereas temperate ice containing water at grain boundaries scatters the signal diffusely.

The results confirmed polythermal conditions at three glaciers: Alphubel South, Chessjengletscher and Hohsaasgletscher. In each case the cold-temperate transition surface lay between 17 and 38 meters below the surface, and ice temperatures ranged from temperate at depth down to minus 2.1 degrees Celsius near frozen termini. A consistent spatial pattern emerged across all three sites: temperate ice occupied the higher reaches of each glacier, while the tongues were fully or partially cold-based. At Alphubel, the only site with direct evidence of a completely frozen terminus, temperatures at the bed dropped as low as minus 2.1 degrees Celsius, and the vertical structure was inverted in an unusual way, with a temperate core sandwiched between a cold surface layer and a cold bed. At Chessjen, a borehole that reached temperate basal ice in early August had cooled to minus 0.43 degrees Celsius by mid-December, revealing a seasonally evolving basal regime in which meltwater sustains warmth through summer and refreezing sets in once melt ceases.

The radar surveys broadly corroborated the borehole thermometry, with enhanced scattering marking temperate zones and clean, transparent signal characterizing cold ice. At Chessjen and Hohsaas, near-bed scattering hinted that temperate basal conditions extend beyond what the borehole network could resolve. At Alphubel, the radar profile was unexpectedly clean even where boreholes confirmed temperate ice, likely because the survey took place in mid-May, when liquid water content at 3800 meters is at its seasonal minimum. The team also compared their data with historical measurements at two sites. At Glacier du Sex Rouge, where a polythermal structure with a roughly 20-meter cold surface layer was documented in 2013 to 2015, the new data suggest that structure likely persists, with about 20 meters of surface ice lost to ablation since then complicating direct comparison. At Vadret dal Corvatsch, which registered around minus 4 degrees Celsius at 13 meters depth in 1999 and 2000, the glacier has thinned so dramatically that its entire ice column, now just 9.3 meters thick, is subject to seasonal fluctuations, yet it remains below freezing year-round, aided by its exposed ridge geometry that promotes lateral heat loss.

The most consequential insight of the study concerns firn, the multi-year snowpack that blankets glacier accumulation zones. Firn acts as a thermal reservoir: meltwater percolates into its porous structure and refreezes at depth, releasing latent heat that warms the underlying ice. When firn cover is lost, that heat source vanishes. Meltwater instead runs off over impermeable ice or refreezes at the surface, where its heat escapes quickly to the atmosphere. The team reconstructed firn cover evolution at all six sites from 1970 to 2025 using a distributed mass balance model calibrated with stake readings and snow-probing data from the GLAMOS monitoring network. The correlation was unmistakable: sites that lost their firn earliest, such as Sex Rouge, Tortin and Corvatsch, which have been firn-free for more than three decades, show the most advanced cooling, while Alphubel, firn-free for less than a decade at the borehole locations, retains temperate ice beneath its upper reaches. At Chessjen and Hohsaas, the depth of the transition surface tracks the local history of firn loss, deepest where firn disappeared first.

Counterintuitively, this means that warming air temperatures can drive glaciers to cool from the inside out. The loss of retained latent heat from refreezing meltwater can outweigh other heat inputs, allowing the winter cold wave to penetrate ever deeper. Conductive cooling of a temperate layer tens of meters thick proceeds over decades, and the observed transition depths of 17 to 20 meters align well with the roughly 20 to 25 meters of cooling expected over the firn-free period. Residual pore water refreezing ahead of the descending cold front, along with meltwater still draining from remnant firn patches through crevasses and bergschrunds, slows the process and sustains temperate cores longer than pure conduction would predict. In the 1980s, the authors argue, these glaciers were likely entirely temperate, warmed by extensive firn, vigorous crevassing that channeled water inward, and faster flow that generated more strain heating. Sustained warming has since dismantled each of those heat sources in turn.

The hazard implications are sobering. Polythermal glaciers are not inherently dangerous, but a configuration combining a frozen terminus with a temperate upper glacier is known to promote meltwater accumulation beyond a glacier’s drainage capacity, because cold ice inhibits the development of efficient drainage systems. The 2022 collapse of the Marmolada Glacier in the Dolomites, which killed eleven people, has been linked to just such a configuration, in which cold basal ice likely prevented meltwater from draining, leading to a frontal ice break-off. During drilling at Alphubel, the team witnessed a dramatic illustration of the mechanism: pressurized water escaped from a borehole at about 15 meters depth, producing a fountain three to five meters high, evidence that water pressure locally exceeded the ice overburden and that meltwater had been trapped with no escape route.

How many other glaciers share this configuration? Filtering the Swiss Glacier Inventory 2023 for glaciers smaller than 0.5 square kilometers between 2900 and 3800 meters yields approximately 515 candidates in Switzerland alone, a figure the authors consider an underestimate because small hanging glaciers are often mapped as parts of larger accumulation areas. The team’s borehole records have been added to the global glenglat database, and the researchers call for systematic regional-scale thermal observations and modeling to assess which of these glaciers may be developing hazardous polythermal structures. As the Alps continue to warm and firn lines climb ever higher, the invisible thermal architecture inside the smallest glaciers may prove to be one of the most consequential, and least expected, legacies of climate change.

Subject of Research: Borehole thermometry revealing polythermal ice conditions in small Swiss Alpine glaciers and their link to firn cover loss

Article Title: Polythermal conditions in small glaciers in the Swiss Alps

Article References: Beer, J., Jacquemart, M., Huss, M., Santin, I., Clara Racz, G., Ogier, C., Gindraux, S., Hösli, L., Moser, R., Irving, J., Fischer, M., & Farinotti, D. (2026). Polythermal conditions in small glaciers in the Swiss Alps. The Cryosphere, 20(9), 5453-5473. https://doi.org/10.5194/tc-20-5453-2026

Image Credits: AI Generated

DOI: 10.5194/tc-20-5453-2026

Keywords: glaciology, polythermal glaciers, Swiss Alps, firn cover, borehole thermometry, ground-penetrating radar, englacial temperature, glacier hazards, climate change, cold-temperate transition surface, Marmolada, The Cryosphere

The discovery is the work of Christian Schlüchter of the University of Bern and colleagues from ETH Zurich, the Swiss Federal Institute for Forest, Snow and Landscape Research, the University of Innsbruck, and the University of Copenhagen, published as an express report in the E&G Quaternary Science Journal. Their findings carry a startling implication: during the early Preboreal, the opening phase of the current interglacial, the Morteratschgletscher did not cover the sampling site at all. The glacier terminus at that time lay far back up the valley, possibly as far as the confluence of the Pers and Morteratsch glaciers, at a much higher elevation than the present-day ice front. In other words, ten millennia ago the ice retreated further up the Bernina Massif than it has in any year of the instrumental record, before later advances buried the forested ground under hundreds of metres of moving ice.

The story of how subfossil wood comes to lie in glacier forefields is itself a tale of glacial mechanics. Over the past three decades, hundreds of samples of organic detritus have been collected from the margins of Alpine glaciers, ranging from insect remains to disc-shaped clasts of compressed peat and splintered conifer logs several metres long and up to sixty centimetres in diameter. Most of these remnants were not found where they grew. They were transported subglacially and englacially, dragged along the glacier bed or frozen into its interior, and released at the terminus through the meltwater system, often during dramatic meltwater outbursts. The surge-like advance of many Alpine glaciers between 1970 and 1990, with its strong initial push, appears to have triggered subglacial erosion wherever the ice base rested on soft, deformable sediments from earlier, warmer times. Deformed wood structures and intensely compressed peat attest to this violent journey beneath the ice.

That transport history has always been the Achilles heel of glacier-forefield palaeoecology. If a log has been carried an unknown distance from its growth site, its dating tells you when and where a forest stood somewhere up-valley, but not precisely where the treeline lay or how far the ice had retreated. One adventurous researcher even travelled underneath the Glacier de Mont Miné in search of in situ wood, yet the uncertainty about true growing positions persisted. The missing link, long sought, was a subfossil tree found in growing position, ideally together with a fossil soil. Lateral moraines have yielded trees in growth position before, but a subglacial example emerging at a receding ice margin remained elusive, a matter of time as melting accelerated. The Morteratsch find now supplies exactly that link.

The specimen, catalogued as MORT-25-01, came to light during the annual autumn survey of the ice margin, a monitoring tradition that first recorded a small log at the site in 2014. Since 2015 the glacier has melted back into a basin, exposing a dominant cross-valley bedrock ridge that once acted as a mechanical obstacle to ice flow. This ridge generated a compressive flow regime in which most debris carried at the glacier base was lodged against it. The exposed bedrock landscape is a textbook of glacial morphology: roches moutonnées sculpted by ice abrasion, alignments of boulders, fluted till surfaces, glacial striae, and irregular patches of till smoothing the underlying topography. Within one of these till patches, a tree trunk of about sixty centimetres visible length lay embedded, sheared off at the top and tilted roughly forty-five degrees in the direction of ice flow.

When the team first examined and excavated the trunk on 24 June 2025, the diagnostic details were unambiguous. Roots remained attached, part of the trunk itself, while roots on the buried underside were deformed and partially sheared off by the dragging force of the glacier. The wood surface bore the compressive-pressure-induced drying cracks typical of long burial and was extremely hard, unlike the smoother surfaces of wood melted out weeks or months before discovery. The preservation of the root system indicates the tree was not dragged far, perhaps only a few metres to a few tens of metres, and it may even have been overrun and lodged at its original growth site before being sheared a short distance to its place of excavation. The chaotic sediments with weathering horizons around the roots make that latter scenario entirely plausible.

Dendrochronological analysis of the sample counted 118 tree rings, with an estimated pith offset of ten years, meaning the visible record represents only part of the tree’s life. Radiocarbon dating at the Laboratory of Ion Beam Physics at ETH Zurich returned an uncalibrated age of 9126 plus or minus 29 years before present. Calibration places the tree’s final growth rings at 8350 BCE, with an uncertainty of twenty to twenty-five years, corresponding to roughly 10,300 years before present, the youngest possible end date for its life. A second sample taken from the same trunk, MORT-25-H2, yielded a consistent radiocarbon age of 9194 plus or minus 28 years BP, and neighbouring wood fragments reinforced the picture. Samples H1, H2, and H4 from the immediate vicinity produced a combined mean of 9216 plus or minus 17 radiocarbon years BP, a statistically coherent age cluster testifying to a forest that stood at this elevation in the early Holocene.

One fragment stands out even among this ancient company. Sample MORT-25-H5, dated to 9437 plus or minus 28 radiocarbon years BP, is some two hundred radiocarbon years older than its neighbours, calibrating to between 8792 and 8627 BCE. The researchers suggest it may represent the oldest Preboreal tree yet found at Morteratsch, and possibly the oldest by radiocarbon age anywhere in the Alps. An earlier discovery in the same forefield, the trunk MORT-19-06 excavated in August 2019 about five hundred metres closer to the bedrock ridge and some fifteen metres higher, adds crucial depth to the record. That tree contained 337 counted rings, preserved bark around relict root suckers, and till-containing soil fragments, and it too had been displaced by no more than fifteen metres. Its radiocarbon age of 9198 plus or minus 29 years BP and dendrochronological end year of approximately 8350 BCE match the younger tree almost exactly.

The combined evidence permits a remarkably detailed reconstruction. The older tree, MORT-19-06, began growing around 10,800 years ago, in the early Preboreal, its seed having been blown up to the Morteratsch valley. That germination occurred only about 800 years after the end of the Younger Dryas, the last cold snap of the deglaciation, which cosmogenic nuclide dating has fixed at Julierpass roughly sixteen kilometres to the north. When that seed took root, the Morteratschgletscher front still formed end moraines near Pontresina, but the retreat from that Younger Dryas position to the ice front of 2025 and beyond was fast. Both trees stopped growing around 10,350 years ago, presumably overwhelmed by a renewed glacier advance that buried and sheared the forest floor. The timing aligns with evidence from Nicolussi and colleagues for the first Holocene glacier minimum in the Alps followed by a pronounced advance at the end of the Preboreal, and it resonates with a recent study from northwestern Greenland documenting early Holocene warming of comparable character, underscoring that the dramatic early interglacial warmth was a hemispheric phenomenon.

For a glacier that has become one of the most visited and most photographed casualties of modern climate change in the Alps, the message from its own forefield is sobering and strangely hopeful in equal measure. The ice has been here before, advancing over a forest that had flourished for centuries at 2150 metres, and it has retreated further before than any satellite has recorded. The larch trunk with its roots still clutching ancient soil is both a warning and a baseline: today’s rapid melt is exposing landscapes that the early Holocene sun last saw ten millennia ago, and every additional metre of retreat may yet yield more trees in growing position, each one a calendar page from the turbulent dawn of our current world.

Subject of Research: Early Holocene tree growth and glacier retreat history at the Morteratschgletscher forefield, Switzerland

Article Title: Tree growth at the beginning of the Holocene at an elevation of 2150 m in the Engadin Valley, Switzerland

Article References: Schlüchter, C., Hösli, L., Nicolussi, K., Korte, C., & Hajdas, I. (2026). Tree growth at the beginning of the Holocene at an elevation of 2150 m in the Engadin Valley, Switzerland. E&G Quaternary Science Journal, 75(2), 163-167. https://doi.org/10.5194/egqsj-75-163-2026

Image Credits: AI Generated

DOI: 10.5194/egqsj-75-163-2026

Keywords: Morteratschgletscher, Holocene, Larix decidua, radiocarbon dating, dendrochronology, glacier forefield, Younger Dryas, Preboreal, Engadin, palaeoclimate, subfossil wood, Swiss Alps

The physics of why suncups darken snow rests on two distinct optical mechanisms. The first is the effective-angle effect: on a rough surface, some facets tilt toward the sun and receive light at a steeper incidence angle than a flat surface would, absorbing more energy, while other facets fall into shadow or catch only grazing illumination and contribute little to the reflected flux. The second is multiple reflections: photons that strike the inside of a concave hollow tend to bounce between its walls rather than escape, and with each collision the probability of absorption rises. This photon-trapping effect is strongest where snow albedo hovers near 0.5, typically in the near-infrared part of the spectrum, and it operates under both direct sunshine and diffuse overcast light. Together, these mechanisms mean that a rough, suncup-covered snowfield reflects measurably less solar radiation than a flat snowfield with identical grain properties.

But suncups do not act alone. As snow melts, light-absorbing particles such as black carbon and mineral dust are retained at the surface because percolating meltwater is inefficient at flushing them out, a process called melt amplification that can multiply surface concentrations by up to a factor of five during extreme events. In some years, blooms of the red snow alga Sanguina nivaloides add further darkening. The central difficulty, and the scientific puzzle the new study set out to address, is that suncup growth and impurity enrichment co-evolve: the same meltwater that deepens the hollows also washes dark particles into them, concentrating dirt in the concavities while leaving ridges relatively clean. Separating the optical signature of geometry from that of impurities on a natural snowfield has therefore remained one of the stubborn problems of snow optics.

The observational setup was deceptively simple in concept but demanding in execution. A fixed FARO Focus3D S120 laser scanner monitored roughly 300 square meters of snow surface adjacent to the Weissfluhjoch station, at 2,536 meters above sea level, scanning every hour through three melt seasons. After filtering out noise from snowfall and blowing snow, the team interpolated the point clouds into digital surface models on a five-millimeter grid, yielding nearly ten thousand surface reconstructions. From these they derived two complementary families of roughness descriptors: vertical measures anchored in the aerodynamic roughness length, the height at which the logarithmic wind profile extrapolates to zero, and horizontal measures based on the correlation length of surface height fluctuations, whose ratio reveals whether the relief is directionally organized or isotropic.

The time series revealed two sharply distinct roughness regimes. During the smooth phase, the horizontal index swung widely, often in sync with wind gusts, while the vertical roughness length barely moved above its one-millimeter baseline, indicating that wind-driven redistribution of dry snow was reshaping the surface without building vertical relief. The rough phase began on 11 May 2022, 22 May 2023, and 3 June 2024, and its onset required three concurrent conditions: sustained daytime surface melting, wet-bulb temperatures consistently above freezing, and reduced wind speeds. In every year, the transition occurred when the snow surface sat at zero degrees Celsius for roughly twenty hours per day and the snowpack held about two percent liquid water by volume. Because these thresholds can be detected with ordinary weather stations and snow models, the authors propose them as a practical diagnostic for flagging the shift from flat to rough surface conditions in energy balance schemes.

Suncups formed in all three seasons, but their geometry varied dramatically with the weather. In 2022, the most radiatively favorable year, with the highest net shortwave gains, the warmest wet-bulb temperatures, and the weakest winds, the hollows grew deep, small, circular, and evenly distributed, reaching an average depth of seven centimeters in a compressed seventeen-day window. In 2023, cooler and clearer conditions with remarkably steady northwesterly winds produced shallower but markedly elongated suncups aligned with the prevailing wind. In 2024, frequent cloud cover suppressed solar input and strong, highly variable winds favored turbulent ablation, yet suncups still formed, likely aided by exceptional Saharan dust deposition and a red algal bloom that insulated the surface locally. Deepening tracked the radiative energy balance, while orientation and elongation followed wind speed and directional constancy.

To quantify the optical consequences, the team compared measured broadband albedo from pyranometers against simulations from the TARTES radiative transfer model, forced with snow properties from the SNOWPACK model and run for a perfectly flat surface. After correcting for a systematic wet-snow bias in the simulations, the residual between measurement and flat-surface prediction isolated the combined signature of suncup roughness and impurities. That signature amounted to a broadband albedo reduction of 0.03 to 0.1, depending on illumination geometry and assumed impurity load, consistent with earlier controlled experiments and field campaigns in the Sierra Nevada and Finnish Lapland. Notably, the residual was roughly twice as large under diffuse overcast light as under direct sun, an asymmetry that suncup geometry alone cannot explain, since the effective-angle effect requires direct beam illumination and photon trapping peaks in the near-infrared where the overcast spectrum carries little weight. The excess instead points to visible-range absorption by impurities and algae, preferentially weighted under diffuse conditions.

Webcam imagery added a vivid clue to how the two darkening agents interact. Over early-stage suncups, impurities appeared spread homogeneously across the surface; as the hollows deepened, dark particles concentrated visibly into the concavities, leaving the exposed surface apparently cleaner despite likely higher total loading. The researchers propose that this even early distribution is particularly effective at accelerating albedo loss, because uniformly scattered absorbers raise the probability of absorption at every photon collision, whereas later clustering into hollows makes the loading spatially heterogeneous and may dampen its radiative impact. Aeolian transport contributed as well: dark rock debris blown from steep northwestern slopes dusted the upwind edge of the study plot, while years with southeasterly winds, which open onto the valley, showed far less debris.

The practical payoff of the study is a robust logarithmic relationship between broadband albedo and the aerodynamic roughness length, with a correlation strong enough to serve as a proxy for the combined roughness-impurity darkening regardless of their relative contributions. This matters because the roughness length evolves by a full order of magnitude through the ablation season, from about one millimeter to roughly fifteen millimeters, yet nearly all snow models hold it constant. Treating it as static misrepresents both the turbulent heat fluxes it governs directly and, through its link to albedo, the radiative fluxes it now demonstrably influences. The authors argue for an explicit, time-varying roughness length in physics-based snow models, and they point to a promising observational route: C-band radar backscatter from satellites such as Sentinel-1 is highly sensitive to early suncup development on wet snow, meaning a single remotely sensed roughness signal could constrain both the turbulent and radiative components of the surface energy balance precisely during the phase when each changes fastest.

As warming shortens the snow season across the world’s mountains, every feedback that hastens melt carries weight far beyond a single research plot. Seasonal snow can cover up to half of the Northern Hemisphere in winter, and even marginal albedo reductions ripple through the planetary energy budget. By showing that the humble, photogenic suncup is not merely a symptom of melting but an active amplifier of it, and by offering a single measurable quantity that captures its darkening effect, the Weissfluhjoch record gives snow scientists and modelers a new handle on one of the cryosphere’s most intricate feedback loops. The next step, the authors acknowledge, will require instruments that can track both surface geometry and impurity distribution across wavelengths at the same fine scale, a challenge that remains open, but the path toward roughness-aware snow models now has a clear empirical foundation.

Subject of Research: Seasonal evolution of suncup surface roughness and its control on broadband albedo decay of melting alpine snow

Article Title: Seasonal evolution of suncup roughness describes broadband albedo decay on alpine snow

Article References: Carletti, F., Helbig, N., Wever, N., Brouet, L., Bavay, M., Walter, B., & Lehning, M. (2026). Seasonal evolution of suncup roughness describes broadband albedo decay on alpine snow. The Cryosphere, 20(10), 5653-5673. https://doi.org/10.5194/tc-20-5653-2026

Image Credits: AI Generated

DOI: 10.5194/tc-20-5653-2026

Keywords: suncups, snow albedo, surface roughness, aerodynamic roughness length, Swiss Alps, light-absorbing particles, black carbon, mineral dust, snow algae, LiDAR, SNOWPACK, TARTES

The research was triggered by real events in the Swiss Alps. In June 2022, in the Gruobenwald forest near Klosters, a storm with heavy rain and wind felled a barkless, branchless dead tree standing on a slope of roughly 35 degrees. The snag, about 30 metres long with a diameter at breast height of 75 centimetres, slid approximately 120 metres downhill and came to rest on a road of national importance. Two days later, another log of similar dimensions slid down and lodged in a wooden palisade next to a house. During a subsequent helicopter operation to remove dangerous snags, two more logs were mobilised and slid onto the road and a hiking trail. In February 2022, near Rothenbrunnen, a tree slid some 200 metres in altitude, blocked both a road and a railway line, and apparently launched off a small terrace to jump clean over a 3.5-metre-high rockfall barrier.

Until now, no modelling framework existed to describe how mobilised deadwood moves on steep slopes. The team recognised that sliding logs share key mechanical characteristics with rockfall, including gravity-driven acceleration, impact, sliding, and deflection by obstacles, and therefore adapted the RAMMS::Rockfall simulation software. Their approach rests on nonsmooth rigid-body dynamics, combining hard contact laws with Coulomb friction. Each log is modelled as a rigid, convex polytope shaped like a truncated cone, defined by its diameter at breast height, stem length, and wood density, with the top diameter approximated at half the base diameter. The log’s position and orientation are tracked with generalised coordinates and unit quaternions, while its motion obeys the balance of linear momentum and Euler’s equations for rotation.

Contact between the log and the terrain is detected whenever a vertex of the log’s point-cloud geometry touches or penetrates the digital elevation model. At each contact point, the Signorini condition governs the normal force, and a spatial Coulomb friction law determines whether the log sticks or slips. Impacts are handled with a Newtonian impact law using a normal restitution coefficient of zero, consistent with the underlying rockfall model. Standing trees, whose locations and dimensions were extracted from airborne laser scanning data using single-tree detection, are represented as rigid truncated cones that block or deflect sliding logs. Rockfall barriers are included as thin, rigid, platy obstacles. The equations of motion are solved iteratively with Moreau’s time-stepping scheme and Gauss-Seidel iteration on a high-resolution 0.5-metre digital elevation model.

Calibrating the model against the documented events yielded a remarkably narrow range for the effective sliding friction coefficient, between 0.2 and 0.3, which the authors interpret as the typical contact friction between wet terrain and a wet, barkless log. With this value, the simulations reproduced the observed deposition patterns at both sites. In the Klosters case, 20,000 simulated logs were released and 1,829, or 9.1 percent, reached the national road, with average velocities of 14 metres per second and maxima of 30 metres per second. At Rothenbrunnen, the model confirmed that jumping the barrier was physically plausible: fitting a flight parabola between the take-off and landing traces gave a take-off velocity of 24 metres per second, and 178 of 20,000 simulated logs, or 0.9 percent, cleared the barrier, a small but realistic frequency given that such an event had never before been recorded there.

The simulations distilled three prerequisites for sliding deadwood hazards. First, friction must be low, which occurs when terrain or the log itself is wet, whether from rain, wet snow, or foliage cover. Second, slopes must be very steep, with all documented events occurring on gradients of 35 degrees or more; although the calibrated friction coefficient suggests mobilisation is theoretically possible above about 18 degrees, the macroscopic roughness captured by the terrain model raises the practical threshold. Third, and perhaps most intriguingly, the wood must be in a narrow decay-stage window: late stage II or early stage III in the standard classification, when bark and branches have been lost, slashing sliding friction, but the stem still retains enough structural strength to survive impacts without shattering. For spruce killed by bark beetles, this critical window opens roughly two to five years after death, while beech decays significantly faster.

Forest structure emerged as a decisive factor in the hazard equation. When the researchers removed all standing trees from the Klosters simulation, the fraction of logs reaching the road exploded from 9.1 percent to 74.5 percent; at Rothenbrunnen it rose from 23.3 percent to a full 100 percent. The forest, in other words, is a highly effective shield against hazards created by its own deadwood. Yet trees cut both ways: while they arrest logs, they also deflect them, increasing lateral spread. In the Klosters simulations, lateral spread shrank from values of 40 to 60 degrees with the real forest to 18 to 31 degrees without it, meaning that deadwood can traverse slopes far more widely than rocks and strike infrastructure that rockfall models would deem safe. Notably, logs slid further when the simulation used real laser-scanned tree positions rather than a randomly generated forest of identical density, because natural gullies, which are often sparsely vegetated, form preferential sliding corridors that random tree placement would artificially block.

The findings carry pointed implications for forest management, especially as deadwood volumes in Swiss mountain forests have doubled over three decades, from 17 to 35 cubic metres per hectare, and climate-driven disturbances promise still more standing and lying dead timber. The study reveals a genuine trade-off over time: young deadwood is strong and protective against rockfall and avalanches, mid-stage deadwood in the two-to-five-year window is both still protective and maximally prone to sliding, and old, rotten wood loses its protective value while its ecological worth as habitat and seedling substrate keeps climbing. Spatially, the same gullies that channel sliding logs are also prime locations for intercepting rockfall and stabilising snow. The authors argue that targeted interventions, such as felling and securing snags across the slope on wire-cable-supported stumps, or prioritising removal in the lower, steepest sections of forested slopes where logs have little forest left to traverse, can minimise the sliding hazard while preserving most of deadwood’s protective and ecological benefits.

The model is not without limits, and the authors are candid about them. Logs are treated as rigid bodies that never fracture, standing trees never break or dissipate energy by swaying, and the single effective friction coefficient is tied to the 0.5-metre terrain resolution, rising to an 18 percent reach frequency when a coarser 5-metre model was tested. Initial conditions are idealised, releasing logs upright within a 10-metre release polygon rather than simulating the actual toppling process. Yet because the framework is explicitly conditioned on a prior assessment that mobilisation is possible, these simplifications yield deliberately conservative, worst-case estimates of runout and impact frequency. The researchers also note that slender logs concentrate impact loads on small frontal areas, a concern for ring-net barriers with large mesh openings, and that logs left uncleared in nets can shorten the braking distance available for future rockfall. With well-documented events still rare, the team calls for systematic documentation of future incidents, but their message is already clear: sliding deadwood is a real, quantifiable, and increasingly relevant mountain hazard, and for the first time it can be simulated, mapped, and managed.

Subject of Research: Physics-based simulation of sliding deadwood logs as a natural hazard in steep mountain forests

Article Title: Simulation of sliding deadwood logs in mountain forests: towards a quantitative hazard assessment

Article References: Borner, J., Bebi, P., Bührle, L. J., Ringenbach, A., & Leine, R. I. (2026). Simulation of sliding deadwood logs in mountain forests: towards a quantitative hazard assessment. Natural Hazards and Earth System Sciences, 26(10), 4843-4859. https://doi.org/10.5194/nhess-26-4843-2026

Image Credits: AI Generated

DOI: 10.5194/nhess-26-4843-2026

Keywords: deadwood, mountain forests, natural hazards, rockfall simulation, Coulomb friction, nonsmooth dynamics, forest management, wood decay, Swiss Alps, airborne laser scanning, hazard assessment, bark beetle

Alpine catchments are often described as water towers for humanity, and the description is literal. Snow that accumulates through winter is released during spring and summer melt, sustaining rivers across entire downstream valleys during the dry months when lowland agriculture, hydropower and drinking water systems need it most. But warming temperatures are dismantling this arrangement. Snow cover durations are shortening, melt is starting earlier, and a rising share of winter precipitation falls as rain instead of snow. Previous work cited in the study suggests that snow storage in the Swiss Alps may decline by roughly fifty percent even at elevations above 3000 meters by the end of the century, while at lower elevations snow may barely accumulate at all. The consequences ripple downstream: winter flows rise, spring and summer flows fall, and the annual runoff peak arrives earlier in the year.

What has been missing from most projections, the authors argue, is an explicit treatment of geology. Groundwater has long been under-represented in mountain hydrology, partly because alpine aquifers are heterogeneous and hard to observe, and partly because modeling them requires solving coupled surface and subsurface flow equations at high spatial resolution. Yet recent field studies have shown that groundwater can be a major contributor to mountain streamflow, stored disproportionately in unconsolidated Quaternary deposits such as moraines, talus slopes, alluvial fans and rock glacier sediments. The critical open question has been where, precisely, this water resides and to what extent it can cushion the blow of a warming climate on streamflow.

To answer it, the research team built a spatially explicit model of an 11-square-kilometer catchment in the Pennine domain of the Valais region, ranging from 2150 to 3148 meters in elevation. The valley’s geology is typical of alpine glacial landscapes: quartzite, gneiss and calcschist bedrock carved by glaciers into a staircase of rock basins separated by riegels, with the lower basins filled by moraine, alluvium and peat. Talus deposits and rock glacier sediments mantle the slopes. The model domain extends 285 meters below the surface and discretizes the catchment into more than 75,000 triangular prism elements, with fine vertical resolution near the ground surface to capture infiltration and unsaturated flow. Quaternary deposit thickness was mapped from geophysical surveys and sediment cores, reaching roughly 20 meters in the flat valley bottoms and thinning systematically on steeper slopes.

The simulations were performed with HydroGeoSphere, a physically based integrated model that simultaneously solves Richards’ equation for three-dimensional variably saturated subsurface flow and the depth-averaged Saint–Venant equations for surface flow, allowing water to exchange between stream and aquifer at every node. Snow accumulation and melt were handled with a degree-day approach calibrated against a sophisticated assimilated snowpack product built from more than 300 Swiss monitoring stations. The model was tuned against three years of observed discharge, hourly water levels in three piezometers, and catchment-averaged snow water equivalent, achieving a Nash–Sutcliffe efficiency of 0.63 for discharge during calibration and 0.73 during an independent validation period. Importantly, the authors acknowledge that equifinality, the problem of multiple parameter sets producing similar behavior, cannot be fully resolved with the sparse data available in alpine terrain, so the analysis focuses on storage changes and relative contributions rather than absolute values.

The model was then driven with two RCP 8.5 climate scenarios drawn from the Swiss Climate Change Scenarios 2018 dataset, both selected for their pronounced spring warming to maximize the effect on snowmelt. The projections imply mean annual temperature increases of 4.8 to 6.4 degrees Celsius by the 2085 period. Under these extreme conditions, the simulations show snow water equivalent collapsing to below 60 millimeters on average by the end of the century, with the seasonal maximum arriving one to two months earlier than today. Evapotranspiration rises steadily by about 45 millimeters per year. Mean annual groundwater storage across the whole catchment declines by roughly 110 to 130 millimeters, driven overwhelmingly by a slow, continuous depletion of water stored in the low-permeability bedrock, while storage in the Quaternary deposits remains comparatively stable on an annual basis.

The seasonal picture is more nuanced and, in some respects, encouraging. Groundwater storage in Quaternary deposits increases in winter and decreases in summer under future conditions, and by 2085 summer discharge drops sharply, by roughly 38 to 45 percent in August and 45 to 55 percent in September depending on the scenario. Yet the proportionality matters: in those same months, groundwater storage in the Quaternary deposits declines by only about 10 to 14 percent. The asymmetry reveals the buffering function. The valley-fill aquifers are recharged by precipitation, snowmelt, stream infiltration during high flows, and upwelling water from bedrock, then release it gradually during dry periods. In effect, the sediments act as a reservoir that smooths out the violent seasonal swings imposed by the loss of the snowpack, so that streamflow responds more dramatically to climate change than the groundwater store that sustains it.

Perhaps the most consequential finding concerns the timing of low flows. Today, the driest period in such catchments occurs in winter, when the landscape is frozen and snowmelt has yet to begin. The simulations show that the annual minimum seven-day discharge progressively migrates out of winter and into late summer, alternating between the two seasons between 2062 and 2091 and settling firmly into summer after 2092. Strikingly, the future summer low flows remain higher than the current winter low flows even under these extreme projections, although after sequences of very dry summers, such as the hypothetical consecutive drought years embedded in one scenario, the two converge uncomfortably. The authors caution that because mean annual precipitation does not decline in their projections, prolonged multi-month precipitation deficits could exhaust the buffering capacity quickly, since the shallow Quaternary aquifers react rapidly to changes in recharge.

A systematic sensitivity analysis sharpened the understanding of which geological units do the heavy lifting. When hydraulic conductivity in the moraine and talus units was lowered by an order of magnitude, groundwater storage in those units increased and low flows rose, demonstrating that these coarse, heterogeneous deposits primarily regulate the catchment’s buffering capacity. Conversely, raising bedrock conductivity drained deep storage and released more water to streams, showing that bedrock controls the slow release of water during dry periods even though its seasonal storage fluctuations are small. Alluvium, with its limited spatial extent, played a minor role in peak flows, and bedrock barely influenced high-flow dynamics at all. Together, the sensitivity experiments establish a complementary division of labor: Quaternary deposits set the buffer, bedrock supplies the deep reserve.

The study’s authors are careful to frame their model as a process-based exploration of a representative small alpine catchment rather than a deterministic forecast for one specific valley. Because Quaternary landforms such as moraines and talus recur across alpine regions worldwide, the qualitative conclusions likely generalize to other non-glacierized mountain catchments, though site-specific geology will always modulate the details. The practical stakes are considerable. Reduced summer flows combined with growing downstream demand could produce water shortages later this century, and the findings make a clear case that low-flow projections must incorporate geological structure and groundwater dynamics, elements routinely omitted from mountain water resource assessments. As snowpacks shrink and the alpine water cycle reorganizes, the invisible reservoirs scattered through rock and sediment beneath the peaks may prove to be the last line of defense for the rivers that half of humanity depends upon.

Subject of Research: The buffering role of groundwater storage in Quaternary deposits and bedrock on alpine catchment discharge under climate change

Article Title: Groundwater storage in Quaternary deposits and bedrock buffers the discharge response of a small alpine catchment to climate change

Article References: Arnoux, M., Cochand, F., Brunner, P., Schaefli, B., Winstral, A., & Hunkeler, D. (2026). Groundwater storage in Quaternary deposits and bedrock buffers the discharge response of a small alpine catchment to climate change. Hydrogeology Journal. https://doi.org/10.1007/s10040-026-03157-5

Image Credits: AI Generated

DOI: 10.1007/s10040-026-03157-5

Keywords: alpine hydrology, groundwater storage, climate change, snowmelt, Quaternary deposits, bedrock, low flows, HydroGeoSphere, Swiss Alps, integrated surface-subsurface modeling, streamflow, moraine