Mid-latitude Mountain Research Articles

Multiple late Holocene glacier advances on the sub-Antarctic Kerguelen (49°S) islands: Evidence from a 1200 yr sediment core from a glacial threshold basin

Southern Ocean (SO) climate is rapidly changing because of global warming and regional climate feedback loops like shifts of the Southern Hemisphere (SH) westerly winds (SHW) and related Southern Annular Mode (SAM). Over the past decades, the former has been persistently positive, shifting the latter southwards: the ensuing changes in temperature and precipitation are linked to the rapid retreat of mid-latitude mountain glaciers. Beyond the short instrumental period, the long-term impact of this coupled SHW-SAM system on regional glaciers remains poorly constrained. To help close this gap, we reconstruct glacier advances from an outlet glacier on the sub-Antarctic Kerguelen (49°S, 69°E), an archipelago that is strategically located in an under-investigated region of the SHW core belt. Based on alternations between relatively organic and minerogenic mud detected using a multiproxy approach on a 1200-year-long sediment record from a glacial threshold basin, we document glacier advances between 1150 and 850, 820–620, 500–250 and 160–90 cal yr BP. Coincident glacier advances in adjacent regions like sub-Antarctic South Georgia and southern Patagonia, suggest that SO glaciers responded symmetrically to climate forcings during the past 1200 years. We attribute this synchronicity to shifting SAM-like conditions and associated temperature changes. We suggest that cold and negative SAM-like conditions, favourable for glacier growth on the Kerguelen and other SO land masses, dominated during much of this period. Furthermore, the findings presented here support the growing consensus for a two-phased regional expression of the Little Ice Age (LIA).

Underestimates of Orographic Precipitation in Idealized Simulations. Part I: Evidence from Unidirectional Warm-Sector Environments

Abstract Heavy precipitation in midlatitude mountain ranges is largely driven by the episodic passage of weather systems. Previous studies have shown a high correlation between the integrated vapor transport (IVT) in the airstream striking a mountain and the precipitation rate. Using data collected during the Olympic Mountain Experiment (OLYMPEX) project from a pair of sounding stations and a dense precipitation network, we further document the tight relation between IVT and precipitation rate and obtain results consistent with earlier work. We also survey previous studies that simulated orographic precipitation forced by unidirectional shear flows. Some of these simulations were performed using models that produce reasonably accurate rainfall totals in nested simulations of actual events driven by large-scale flows. Nevertheless, the increase in precipitation with IVT in all the simulations with unidirectional upstream flows is far lower than what would be expected based on the observationally derived correlation between IVT and precipitation rate. As a first step toward explaining this discrepancy, we conduct idealized simulations of a midlatitude cyclone striking a north–south ridge. The relationship between IVT and rainfall rate in this “Cyc+Mtn” simulation matches that which would be expected from observations. In contrast, when the conditions upstream of the ridge in the Cyc+Mtn case were used as upstream forcing in a horizontally uniform unidirectional flow with the same IVT over the same mountain ridge, far less precipitation was produced. These idealized simulations will, therefore, be used to study the discrepancy in rainfall between simulations driven by unidirectional shear flows and observations in a companion paper. Significance Statement Idealized simulations of orographic precipitation using horizontally uniform environmental forcing fail to capture the observed relationship between integrated water vapor flux impinging on the mountain and the precipitation rate. This suggests we need to improve the design of such idealized simulations.

Surface soil phytolith assemblages across an altitudinal transect in the Qilian Mountains of Northwestern China, and their implications for palaeoecologic analysis in arid alpine regions

Reconstructing changes in altitudinal vegetation belts is crucial for comprehending vegetation response to past environmental changes in arid alpine regions, however, proxy data are limited. In this study, we report 51 surface soil phytolith samples from various vegetation types across an altitudinal transect in the arid alpine region of the Qilian Mountains, to elucidate the relationships between vegetation types and phytolith assemblages. Our results show that phytolith morphotypes in surface soil samples could be categorized into 16 classes, with Gobbet, Crenate, Rondel, Elongate entire, and Acutebulbosus being the predominant classes. The characteristics of surface soil phytolith assemblages vary across an altitudinal transect, reflecting differences in local vegetation. Discriminant analysis identifies Cubic, Elongate entire, Crenate, Gobbet, and Rondel as discriminant variables that correlated with five vegetation types (desert vegetation, mountain steppe vegetation, mountain coniferous forest vegetation, subalpine shrub meadow vegetation, and alpine meadow vegetation). We establish five discriminant functions based on the relationships between vegetation types and phytolith assemblages, demonstrating a high accuracy of our method for distinguishing between different vegetation types. These discriminant functions are applicable for distinguishing between diverse vegetation types in the mountains of arid inland regions of Asia. They are also effective in identifying coniferous forest vegetation in mid-latitude mountains. Therefore, these discriminant functions hold significant potential for reconstructing changes in altitudinal vegetation belts in mountainous regions and determining the vegetative types recorded by terminal lakes in arid regions. Our study introduces a new perspective on palaeoecologic analysis in arid alpine regions, advancing our understanding of ecosystem dynamics in response to environmental fluctuations.

Are Atmospheric Models Too Cold in the Mountains? The State of Science and Insights from the SAIL Field Campaign

Abstract Mountains play an outsized role in water resource availability, and the amount and timing of water they provide depend strongly on temperature. To that end, we ask the question: How well are atmospheric models capturing mountain temperatures? We synthesize results showing that high-resolution, regionally relevant climate models produce 2-m air temperature (T2m) measurements colder than what is observed (a “cold bias”), particularly in snow-covered midlatitude mountain ranges during winter. We find common cold biases in 44 studies across global mountain ranges, including single-model and multimodel ensembles. We explore the factors driving these biases and examine the physical mechanisms, data limitations, and observational uncertainties behind T2m. Our analysis suggests that the biases are genuine and not due to observation sparsity or resolution mismatches. Cold biases occur primarily on mountain peaks and ridges, whereas valleys are often warm biased. Our literature review suggests that increasing model resolution does not clearly mitigate the bias. By analyzing data from the Surface Atmosphere Integrated Field Laboratory (SAIL) field campaign in the Colorado Rocky Mountains, we test various hypotheses related to cold biases and find that local wind circulations, longwave (LW) radiation, and surface-layer parameterizations contribute to the T2m biases in this particular location. We conclude by emphasizing the value of coordinated model evaluation and development efforts in heavily instrumented mountain locations for addressing the root cause(s) of T2m biases and improving predictive understanding of mountain climates.

Tracking rapid ice recession in a major Southern Alps valley during the last glacial termination

The last glacial termination featured a major reconfiguration of Earth's climate and cryosphere, whose underlying cause remains unresolved. To investigate this problem, we combine 10Be surface-exposure dating of moraines with former equilibrium line altitudes to determine the magnitude and timing of atmospheric temperature warming that ended the Last Glaciation in the Takapō/Tekapo valley in the central Southern Alps of New Zealand. We show mountain-valley glacier recession from a nearly full-glacial configuration to a near-interglacial configuration early in the termination between ∼18,000 and ∼17,000 yrs ago, commensurate with a net atmospheric warming of ∼3.8 °C (from −6.25 °C to −2.5 °C cooler than present). Similar recession also affected mid-latitude mountain glaciers in South America. We suggest trans-South Pacific glacier withdrawal early in the termination resulted from a decisive poleward shift of the austral westerlies that increased the proportion of warm subtropical air masses flowing over southern mid-latitude mountains, markedly raising glacier ablation rates. Farther south, the poleward-shifted westerlies drove increased ocean upwelling and surface warming, outgassing of carbon dioxide, and progressive ocean destratification, together raising atmospheric temperature over the Antarctic Ice Sheet, but at a rate slower than over mid-latitude mountain glaciers. Overall, we consider that Southern Hemisphere middle-latitude glacier recession was linked to Antarctic warming by a poleward displacement in latitude and an increase in strength of the Southern Hemisphere westerlies.

Weakened Orographic Influence on Cool‐Season Precipitation in Simulations of Future Warming Over the Western US

AbstractHigh‐resolution regional climate model (RCM) simulations of global warming consistently predict larger percentage increases in precipitation in the lee of midlatitude mountain ranges than on their windward slopes, indicating a weakening of the orographic rain shadow. This redistribution of precipitation could have profound consequences for water resources and ecosystems, but its underlying mechanisms are unknown. Here we show that rain‐shadow weakening is just one manifestation of a more general decrease in the influence of orography on precipitation under global warming. We introduce a simple model of precipitation change based on this principle, and find that it agrees well with an ensemble of high‐resolution simulations performed over the western United States. We argue that diminished orographic influence can be explained by the unique vertical structure of orographically forced ascent, which tends to maximize in the lower atmosphere where condensation is thermodynamically less sensitive to warming.

Evaluating Topographic Effects on Kilometer-Scale Satellite Downward Shortwave Radiation Products: A Case Study in Mid-Latitude Mountains

Evaluating Topographic Effects on Kilometer-Scale Satellite Downward Shortwave Radiation Products: A Case Study in Mid-Latitude Mountains

Snow accumulation and ablation measurements in a midlatitude mountain coniferous forest (Col de Porte, France, 1325 m altitude): the Snow Under Forest (SnoUF) field campaign data set

Abstract. Forests strongly modify the accumulation, metamorphism and melting of snow in midlatitude and high-latitude regions. Recently, snow routines in hydrological and land surface models were improved to incorporate more accurate representations of forest snow processes, but model intercomparison projects have identified deficiencies, partly due to incomplete knowledge of the processes controlling snow cover in forests. The Snow Under Forest (SnoUF) project was initiated to enhance knowledge of the complex interactions between snow and vegetation. Two field campaigns, during the winters 2016–2017 and 2017–2018, were conducted in a coniferous forest bordering the snow study at Col de Porte (1325 m a.s.l., French Alps) to document the snow accumulation and ablation processes. This paper presents the field site, the instrumentation and the collection and postprocessing methods. The observations include distributed forest characteristics (tree inventory, lidar measurements of forest structure, subcanopy hemispherical photographs), meteorology (automatic weather station and an array of radiometers), snow cover and depth (snow pole transect and laser scan) and snow interception by the canopy during precipitation events. The weather station installed under dense canopy during the first campaign has been maintained since then and has provided continuous measurements throughout the year since 2018. Data are publicly available from the repository of the Observatoire des Sciences de l'Univers de Grenoble (OSUG) data center at https://doi.org/10.17178/SNOUF.2022 (Sicart et al., 2022).

Homogenization and species compositional shifts in subalpine vegetation during the 60-year period

Subalpine and alpine plant communities are considered highly sensitive and hence endangered by global changes. In central Europe, the highly sensitive habitats are also influenced by human activities: land use, industrial pollution, and tourism. That is especially true for subalpine plant communities formed on mid-latitude mountains during specific postglacial development. Our study aimed to (1) document changes in cover and plant species diversity between the past (1950s and 1970s) and present (2019) and (2) reveal differences in the functional composition of the species among the studied periods. In 2019, quasi-permanent marked plots originally surveyed in the 1950s and 1970s were resurveyed at Králický Sněžník (Śnieżnik Kłodski) Mountains, the High Sudetes Mts.. We assessed temporal changes in plant species composition, species cover, functional groups, growth forms, and degree of specialization. We found homogenization of the vegetation over time and changes in the cover of specific functional groups that were attributed to environmental factors connected with the cessation of historical land use, atmospheric deposition, and climate change. Implementing a suitable combination of mowing and grazing to support diversity, and to prevent homogenization of vegetation is highly recommended.

Asymmetric glaciation, divide migration, and postglacial fluvial response times in the Qilian Shan

Abstract Glacial-interglacial cycles have repeatedly perturbed climate and topography in many midlatitude mountain ranges during the Quaternary. Glacial erosion can move drainage divides and induce fluvial adjustments downstream, yet the time scale over which these adjustments occur remains unclear. We examined landscape evolution in the northwest-southeast–trending Qilian Shan, where the contrast in solar insolation between north- and south-facing slopes has generated larger glaciers on the northern range crest. Our analyses suggest that this asymmetric glaciation has caused southward migration of the main drainage divide, prompting river channels below the extents of ice on north-facing slopes to become oversteepened for their drainage area and channels on south-facing slopes to become analogously understeepened. These changes in steepness should accelerate or slow down postglacial fluvial incision, even in the regions where topography has not been directly modified by glacial erosion. Numerical modeling suggests these discrepancies persist for millions of years, much longer than the duration of recent glacial-interglacial cycles, implying a widespread and enduring influence of intermittent glaciations on landscape evolution in glaciated mountain ranges during the Quaternary.

Snow sensitivity to temperature and precipitation change during compound cold–hot and wet–dry seasons in the Pyrenees

Abstract. The Mediterranean Basin has experienced one of the highest warming rates on earth during the last few decades, and climate projections predict water scarcity in the future. Mid-latitude Mediterranean mountain areas, such as the Pyrenees, play a key role in the hydrological resources for the highly populated lowland areas. However, there are still large uncertainties about the impact of climate change on snowpack in the high mountain ranges of this region. Here, we perform a snow sensitivity to temperature and precipitation change analysis of the Pyrenean snowpack (1980–2019 period) using five key snow–climatological indicators. We analyzed snow sensitivity to temperature and precipitation during four different compound weather conditions (cold–dry (CD), cold–wet (CW), warm–dry (WD), and warm–wet (WW)) at low elevations (1500 m), mid elevations (1800 m), and high elevations (2400 m) in the Pyrenees. In particular, we forced a physically based energy and mass balance snow model (FSM2), with validation by ground-truth data, and applied this model to the entire range, with forcing of perturbed reanalysis climate data for the period 1980 to 2019 as the baseline. The FSM2 model results successfully reproduced the observed snow depth (HS) values (R2>0.8), with relative root mean square error and mean absolute error values less than 10 % of the observed HS values. Overall, the snow sensitivity to temperature and precipitation change decreased with elevation and increased towards the eastern Pyrenees. When the temperature increased progressively at 1 ∘C intervals, the largest seasonal HS decreases from the baseline were at +1 ∘C. A 10 % increase in precipitation counterbalanced the temperature increases (≤1 ∘C) at high elevations during the coldest months because temperature was far from the isothermal 0 ∘C conditions. The maximal seasonal HS and peak HS max reductions were during WW seasons, and the minimal reductions were during CD seasons. During WW (CD) seasons, the seasonal HS decline per degree Celsius was 37 % (28 %) at low elevations, 34 % (30 %) at mid elevations, and 27 % (22 %) at high elevations. Further, the peak HS date was on average anticipated for 2, 3, and 8 d at low, mid, and high elevation, respectively. Results suggest snow sensitivity to temperature and precipitation change will be similar at other mid-latitude mountain areas, where snowpack reductions will have major consequences for the nearby ecological and socioeconomic systems.

Decline in Seasonal Snow during a Projected 20-Year Dry Spell

Snowpack loss in midlatitude mountains is ubiquitously projected by Earth system models, though the magnitudes, persistence, and time horizons of decline vary. Using daily downscaled hydroclimate and snow projections, we examine changes in snow seasonality across the U.S. Pacific Southwest region during a simulated severe 20-year dry spell in the 21st century (2051–2070) developed as part of the 4th California Climate Change Assessment to provide a “stress test” for water resources. Across California’s mountains, substantial declines (30–100% loss) in median peak annual snow water equivalent accompany changes in snow seasonality throughout the region compared to the historic period. We find that 80% of historic seasonal snowpacks transition to ephemeral conditions. Subsetting empirical-statistical wildfire projections for California by snow seasonality transition regions indicates a two-to-four-fold increase in the area burned, consistent with recent observations of high elevation wildfires following extended drought conditions. By analyzing six of the major California snow-fed river systems, we demonstrate snowpack reductions and seasonality transitions result in concomitant declines in annual runoff (47–58% of historical values). The negative impacts to statewide water supply reliability by the projected dry spell will likely be magnified by changes in snowpack seasonality and increased wildfire activity.

Near Invariance of Poleward Atmospheric Heat Transport in Response to Midlatitude Orography

Abstract Total poleward atmospheric heat transport (AHT) is similar in both magnitude and latitudinal structure between the Northern and Southern Hemispheres. These similarities occur despite more major mountain ranges in the Northern Hemisphere, which help create substantial stationary eddy AHT that is largely absent in the Southern Hemisphere. However, this hemispheric difference in stationary eddy AHT is compensated by hemispheric differences in other dynamic components of AHT so that total AHT is similar between hemispheres. To understand how AHT compensation occurs, we add midlatitude mountain ranges in two different general circulation models that are otherwise configured as aquaplanets. Even when midlatitude mountains are introduced, total AHT is nearly invariant. We explore the near invariance of total AHT in response to orography through dynamic, energetic, and diffusive perspectives. Dynamically, orographically induced changes to stationary eddy AHT are compensated by changes in both transient eddy and mean meridional circulation AHT. This creates an AHT system with three interconnected components that resist large changes to total AHT. Energetically, the total AHT can only change if the top-of-the-atmosphere net radiation changes at the equator-to-pole scale. Midlatitude orography does not create large-enough changes in the equator-to-pole temperature gradient to alter outgoing longwave radiation enough to substantially change total AHT. In the zonal mean, changes to absorbed shortwave radiation also often compensate for changes in outgoing longwave radiation. Diffusively, the atmosphere smooths anomalies in temperature and humidity created by the addition of midlatitude orography, such that total AHT is relatively invariant. Significance Statement The purpose of this study is to better understand how orography influences heat transport in the atmosphere. Enhancing our understanding of how atmospheric heat transport works is important, as heat transport helps moderate Earth’s surface temperatures and influences precipitation patterns. We find that the total amount of atmospheric heat transport does not change in the presence of mountains in the midlatitudes. Different pieces of the heat transport change, but they change in compensatory ways, such that the total heat transport remains roughly constant.

Rock walls distribution and Holocene evolution in a mid-latitude mountain range (the Romanian Carpathians)

Rock walls in high mountain areas are the expression of long–term slopes response (103–105 years) to tectonics, weathering and denudation and a major source of sediment and hazard. Controls of mountain rock walls (RW) distribution and the response to post-glacial evolution are rarely discussed in the literature at the scale of mountain ranges. Using a database of 791 RW mapped in the Romanian Carpathians, we present their distribution and morphometry in respect to lithological classes, structural features and topography and relate their exposure to post–Younger Dryas (Holocene) rock slope failure chronology. Statistical analysis results show the high significance of structural and tectonic control on RW distribution, which prevails in sedimentary units where it imposed the predominance of West and North orientations and led to the formation of RW with dimensions up to a degree higher compared to other lithologies. Morphometric data indicate that metamorphic and igneous RW (linked to a great extent to glacial valleys and cirques headwalls) are usually restricted to the highest sectors of the mountain slopes, being characterized by reduced relative heights, asymmetrically distributed, common on the North-exposed slopes and extremely rare on the South. Based on 38 in-situ produced 10Be surface exposure ages obtained on meter-sized boulders from the Southern and Eastern Carpathians, we hypothesise that metamorphic and igneous RW in the formerly glaciated Carpathian valleys were significantly shaped during Early Holocene (before 9 ka) by rock slope failures events that followed the deglaciation of the highest cirques and by intense RW permafrost degradation, which also affected some of the highest sedimentary units. We associate the long–term imprints of frost weathering to the significant North/South RW and rock glaciers distribution asymmetry, also identified in other mid-latitude mountain sites with similar topographic constraints.

Increase of the energy available for snow ablation in the Pyrenees (1959–2020) and its relation to atmospheric circulation

Mid-latitude mountain snowpacks are highly vulnerable to climate warming. Past and future hydroclimate changes require a thorougout knowledge of snow ablation physical processes and the associate climate drivers. In this work we provide the first spatio-temporal characteritzation of the energy available for snow ablation (Qm) in the Pyrenees for the period 1959–2020, during the main ablation season (March to June) for low (1200 m), mid (1800 m) and high (2400 m) elevations. We analyze the role of the the main Circulation Weather Types (CTs) in the Qm components for the entire mountain range. Finally, we train and tune a machine learning algorithm, the Random Forest, with atmospheric data (Surface Level Pressure and 500 hPa Geopotential Height) as an independent variable and Qm as the dependent one, in order to determine how much of the observed changes in Qm can be related with atmospheric circulation variability. The largest contribution of Qm is Net Radiation (Rn), increasing with elevation. Qm has shown a statistically significant increase since the 1980s. The comparison between the period 1959–1980 and the 2000–2020 revealed that positive Qm fluxes have been anticipated 22 and 12 days at mid and high elevations, respectively, showing evidence of an advance in the timing of the ablation season and faster snow ablation in high-elevation areas of the Pyrenees. The Qm is principally driven by Rn during the prevailing antyciclonic situations, characterized by the extension of high-pressure systems, low-barometric gradients and the SE advection of hot and dry air masses. The positive frequency of these anticyclonic CTs explains the majority (75%) of the Qm variability, the upward Qm trends and the earlier snow ablation onset since the 1980s.

Low variability runoff inhibits coupling of climate, tectonics, and topography in the Greater Caucasus

Hypothesized feedbacks between climate and tectonics are mediated by the relationship between topography and long-term erosion rates. While many studies show monotonic relationships between channel steepness and erosion rates, the degree of nonlinearity in this relationship varies by landscape. Mechanistically explaining controls on this relationship in natural settings is critical because highly nonlinear relationships imply low sensitivity between climate and tectonics. To this end, we present a coordinated analysis of cosmogenic 10Be concentrations in river sands paired with topographic, hydroclimatic, and tectonic data for the Greater Caucasus Mountains where topography is invariant along-strike despite large gradients in modern precipitation and convergence rates. We show that spatial patterns in erosion rates largely reflect regional tectonics with little sensitivity to mean precipitation or runoff. The nonlinearity in the erosion rate – steepness relationship may arise from very low runoff variability, which we attribute to the large contribution from snowmelt. Transitioning from rainfall- to snowmelt-driven runoff as mean elevation increases is common to many mid-latitude mountain ranges. The associated decrease in runoff variability may represent important, unrecognized dynamics inhibiting the sensitivity of tectonics to climate more broadly.

Glacier response to Holocene warmth inferred from in situ <sup>10</sup>Be and <sup>14</sup>C bedrock analyses in Steingletscher's forefield (central Swiss Alps)

Abstract. Mid-latitude mountain glaciers are sensitive to local summer temperature changes. Chronologies of past glacier fluctuations based on the investigation of glacial landforms therefore allow for a better understanding of natural climate variability at local scale, which is relevant for the assessment of the ongoing anthropogenic climate warming. In this study, we focus on the Holocene, the current interglacial of the last 11 700 years, which remains a matter of dispute regarding its temperature evolution and underlying driving mechanisms. In particular, the nature and significance of the transition from the early to mid-Holocene and of the Holocene Thermal Maximum (HTM) are still debated. Here, we apply an emerging approach by combining in situ cosmogenic 10Be moraine and 10Be–14C bedrock dating from the same site, the forefield of Steingletscher (European Alps), and reconstruct the glacier's millennial recession and advance periods. The results suggest that, subsequent to the final deglaciation at ∼10 ka, the glacier was similar to or smaller than its 2000 CE extent for ∼7 kyr. At ∼3 ka, Steingletscher advanced to an extent slightly outside the maximum Little Ice Age (LIA) position and until the 19th century experienced sizes that were mainly confined between the LIA and 2000 CE extents. These findings agree with existing Holocene glacier chronologies and proxy records of summer temperatures in the Alps, suggesting that glaciers throughout the region were similar to or even smaller than their 2000 CE extent for most of the early and mid-Holocene. Although glaciers in the Alps are currently far from equilibrium with the accelerating anthropogenic warming, thus hindering a simple comparison of summer temperatures associated with modern and paleo-glacier sizes, our findings imply that the summer temperatures during most of the Holocene, including the HTM, were similar to those at the end of the 20th century. Further investigations are necessary to refine the magnitude of warming and the potential HTM seasonality.

Sensitivity of forest–snow interactions to climate forcing: Local variability in a Pyrenean valley

Mountain forests affect spatial and temporal variability of snow processes through snow interception and by modifying the energy balance of snowpack. The high sensitivity of snow cover to seasonal temperatures in mid–latitude mountains is well known and is of particular interest with regard to a future warmer climate. The snowpack in the Pyrenees is expected to be the most impacted by climate change in the Mediterranean mountains, where future climate trends project rising temperatures and decreasing precipitation. This study analyzes how changes in temperature and precipitation can affect current forest–snow interactions in four forests, located near each other but under contrasting topographic settings, in the Spanish Pyrenees. This understanding will allow us to anticipate the future hydrological responses of Pyrenean forested mountain basins. The research was accomplished by performing a sensitivity analysis using simulations from the Cold Regions Hydrological Model (CRHM) and by comparing forest canopy sites (F) vs. openings (O). The CRHM platform focuses on the incorporation of physically based descriptions of snow–dominated regions hydrological processes. It was found that forest cover induced different snowpack sensibility to climatic change conditions in the studied forests. Delayed onset of snow accumulation (F: 13 days·°C−1; O: 5 days·°C−1) and reduced snowpack duration (F: 28 %·°C−1; O: 23 %·°C−1) under warmer temperatures were more intense in areas beneath the forest canopy compared to openings. A lower annual peak of snow water equivalent (SWE) (F: 81 mm·°C−1; O: 129 mm·°C−1), earlier melt-out date (F: 8 days·°C−1; O: 10 days·°C−1) and slower melting rates (F: 0.4 mm·day−1·°C−1; O: 0.5 mm·day−1·°C−1) with increasing temperatures were more intense in forest openings. The forest–driven reduction in snowpack duration (40%) was significantly enhanced with warming (10% per °C). Lower precipitation (20% precipitation reduction) could increase the response of this forest effect to warming (32%), while higher precipitation (20% precipitation increment) could reduce it (−26%). There was relevant topographic variability in the forest−snow interactions in response to climate change among the study stands, despite their proximity.

Glaciers Control the Hydrogeochemistry of Proglacial Streams During Late Summer in the Wind River Range, Wyoming, United States

Glaciers alter the geochemistry of sensitive alpine streams by exposing freshly weathered bedrock and releasing atmospherically deposited trace metals from melting ice. Changes in the timing and quantity of glacial melt also affect discharge and temperature of alpine streams. To investigate the effects of glacier meltwater on the geochemistry and hydrology of proglacial streams in the western US, we sampled supraglacial meltwaters and proglacial streams in the Dinwoody Creek watershed in the Wind River Range, Wyoming during a one-week period in 2015. The upper watershed contains Gannett Glacier ( ∼ 5 km2) and Dinwoody Glacier ( ∼ 4 km2) at elevations between 3,300–4,000 m asl. Samples were collected during late summer (27 August–4 September) when the contributions of glacier meltwater were highest. Supraglacial meltwater was enriched in a suite of trace metals (Cd, Co, Cu, Hg, Mn, Pb, Zn) relative to proglacial streams, suggesting an atmospheric source of metals to the glaciers. Concentrations of major ions and the remaining 30+ analyzed trace elements were enriched in proglacial streams relative to supraglacial meltwater, reflecting weathering of granite and gneiss bedrock. To evaluate the diurnal effects of glacier meltwater inputs, we deployed loggers to monitor water levels, temperature, and specific conductance at 15 min intervals over a one-week period and collected hourly water samples from Dinwoody Creek for a 24 h period. The influx of glacial meltwater during the daytime diluted major ion and rare earth element concentrations and caused increased concentrations for a subset of trace metals. Stable water isotopes (δD and δ18O) in Dinwoody Creek were more depleted during peak flow relative to baseflow due to contributions from isotopically depleted meltwater. The combination of multiple hydrologic tracers (solute concentrations, high frequency logger data, water isotopes) shows strong potential to improve estimates of glacier meltwater contributions to proglacial streams. Changes in water chemistry and discharge need to be monitored as glaciers recede across the Wind River Range and other midlatitude mountain ranges for mitigating negative impacts on alpine ecosystems and downstream water resources.

Rapid deglaciation during the Bølling-Allerød Interstadial in the Central Pyrenees and associated glacial and periglacial landforms

The Central Pyrenees hosted a large ice cap during the Late Pleistocene. The cirques under relatively low-altitude peaks (2200–2800 m) include the greatest variety of glacial landforms (moraines, fossil debris-covered glaciers and rock glaciers), but their age and formation process are poorly known. Here, we focus on the headwaters of the Garonne River, namely on the low-altitude Bacivèr Cirque (highest peaks at ~2600 m), with widespread erosive and depositional glacial and periglacial landforms. We reconstruct the pattern of deglaciation from geomorphological observations and a 17-sample dataset of 10Be Cosmic-Ray Exposure (CRE) ages. Ice thickness in the Bacivèr Cirque must have reached ~200 m during the maximum ice extent of the last glacial cycle, when it flowed down towards the Garonne paleoglacier. However, by ~15 ka, during the Bølling-Allerød (B-A) Interstadial, the mouth of the cirque was deglaciated as the tributary glacier shrank and disconnected from the Garonne paleoglacier. Glacial retreat was rapid, and the whole cirque was likely to have been deglaciated in only a few centuries, while paraglacial processes accelerated, leading to the transformation of debris-free glaciers into debris-covered and rock glaciers in their final stages. Climate conditions prevailing at the transition between the B-A and the Younger Dryas (YD) favored glacial growth and the likely development of small moraines within the slopes of the cirque walls by ~12.9 ka, but the dating uncertainties make it impossible to state whether these moraines formed during the B-A or the YD. The melting of these glaciers favored paraglacial dynamics, which promoted the development of rock glaciers as well as debris-covered glaciers. These remained active throughout the Early Holocene until at least ~7 ka. Since then, the landscape of the Bacivèr Cirque has seen a period of relative stability. A similar chronological sequence of deglaciation has been also detected in other cirques of the Pyrenees below 3000 m. As in other mid-latitude mountain regions, the B-A triggered the complete deglaciation of the Garonne paleoglacier and promoted the development of the wide variety of glacial and periglacial landforms existing in the Bacivèr cirque.