Increase In Active Layer Thickness Research Articles

Thermal photogrammetry on a permafrost rock wall for the active layer monitoring

Permafrost and active layer models often cannot explain the high spatial variability, especially in heterogeneous environments like the mountainous regions due to their scarce resolution, paucity of climatic data and topographic details. In this study, we want to introduce a new application of the unmanned aerial vehicle (UAV) in thermal photogrammetry to model the active layer thickness (ALT) of an alpine rock wall through the computation of the thermal inertia and compare the results with a widespread ALT model. On the Gran Zebrù South rock wall, 8 thermal UAV surveys has been conducted in 4 different summer days during 2021-2022 in order to have two 3D thermal models per day at different solar radiation inputs. By analyzing topographic data, visible imagery and the thermal models, the apparent thermal inertias (ATIs) have been converted into heat transfer coefficients (HTCs) and then into ALT of 2021 and 2022. These maps have been validated through the placement of thermistors at different elevations and with variable depths (2, 15 and 40 cm from the rock surface).The resulting ALT has been compared with the Stefan's solution and the alpine permafrost index map (APIM), which showed large underestimations and a noncorrespondence with permafrost occurrence. The average ALT increase of 29.3 cm from 2021 to 2022 has been discussed regarding permafrost formation/degradation future trend under the climatic change and potential risks of alpine areas.

Widespread deepening of the active layer in northern permafrost regions from 2003 to 2020

The changing thermal state of permafrost is an important indicator of climate change in northern high latitude ecosystems. The seasonally thawed soil active layer thickness (ALT) overlying permafrost may be deepening as a consequence of enhanced polar warming and widespread permafrost thaw in northern permafrost regions (NPRs). The associated increase in ALT may have cascading effects on ecological and hydrological processes that impact climate feedback. However, past NPR studies have only provided a limited understanding of the spatially continuous patterns and trends of ALT due to a lack of long-term high spatial resolution ALT data across the NPR. Using a suite of observational biophysical variables and machine learning (ML) techniques trained with available in situ ALT network measurements (n = 2966 site-years), we produced annual estimates of ALT at 1 km resolution over the NPR from 2003 to 2020. Our ML-derived ALT dataset showed high accuracy (R 2 = 0.97) and low bias when compared with in situ ALT observations. We found the ALT distribution to be most strongly affected by local soil properties, followed by topographic elevation and land surface temperatures. Pair-wise site-level evaluation between our data-driven ALT with Circumpolar Active Layer Monitoring data indicated that about 80% of sites had a deepening ALT trend from 2003 to 2020. Based on our long-term gridded ALT data, about 65% of the NPR showed a deepening ALT trend, while the entire NPR showed a mean deepening trend of 0.11 ± 0.35 cm yr−1 [25%–75% quantile: (−0.035, 0.204) cm yr−1]. The estimated ALT trends were also sensitive to fire disturbance. Our new gridded ALT product provides an observationally constrained, updated understanding of the progression of thawing and the thermal state of permafrost in the NPR, as well as the underlying environmental drivers of these trends.

Permafrost on the Tibetan Plateau is degrading: Historical and projected trends

Permafrost degradation on the Tibetan Plateau (TP) will significantly affect local water cycle processes, downstream water ecology, and water security. In this study, we evaluate the long-term interannual dynamics of permafrost distribution and active layer thickness (ALT) on the TP based on historical data from Climatic Research Unit gridded Time Series (CRU TS) downscaling and projected data under four shared socio-economic pathways (SSPs) in Scenario Model Intercomparison Project (ScenarioMIP) of the Coupled Model Intercomparison Project Phase 6 (CMIP 6). To achieve this, we employ a data-driven scheme at 1 km resolution for both historical and future periods (1901–2100) that compares the performance of four machine learning algorithms to select the optimal algorithm for permafrost distribution and ALT simulations. Our results indicate that the permafrost on the TP has been undergoing degradation in both historical and future periods, with a decrease in permafrost area and an increase in ALT. The changing rates of permafrost area and regionally averaged ALT during the historical period (1901–2020) are −1.05 × 104 km2 decade–1 and 0.012 m decade–1, while an accelerated degradation is observed after the 1970 s (with changing rates of permafrost area and regionally average ALT of −3.62 × 104 km2 decade–1 and 0.055 m decade–1). Our results also suggested that permafrost degradation on the TP will continue in the future under the four SSP scenarios. The individual global climate models (GCMs) exhibit a consistent degradation trend but great uncertainty in degradation speed. The ensemble mean of simulations across 15 selected GCMs showed that the degradation percentage of permafrost area on the TP under scenarios SSP1-2.6, SSP2-4.5, SSP3-7.0, and SSP5-8.5 was 26.0 ± 6.8 %, 50.4 ± 5.6 %, 79.2 ± 4.5 %, and 89.0 ± 4.0 % by 2100, and the regionally average ALT increased by 0.301 ± 0.112 m, 0.628 ± 0.113 m, 1.204 ± 0.119 m, and 1.486 ± 0.125 m, respectively. We also analyze permafrost stability and elevation-dependent changes of ALT on the TP. The permafrost stability increases with elevation and latitude, and ALT changes more intensely with increasing elevation. This study will provide valuable data for hydrological and ecological studies related to permafrost on the TP.

Main results of permafrost monitoring in the French Alps through the PermaFrance network over the period 2010–2022

AbstractThis study presents data from the first years of permafrost monitoring in boreholes in the French Alps that started at the end of 2009 in the framework of the PermaFrance network. Nine boreholes are instrumented, among which six monitored permafrost temperature and active layer thickness (ALT) over >10 years. Ice‐poor and cold permafrost in high‐elevation north‐facing rock walls has warmed by up to >1°C at 10 m depth over the reference decade (2011–2020), whereas ice‐rich permafrost (rock glacier) temperatures remained stable. ALT has increased at four of the five boreholes for which decadal data are available. Summer 2015 marks a turning point in ALT regime and greatest ALT values were observed in 2022 (available for six boreholes), but thawing intensity did not show an obvious change. At one site with a layer of coarse blocks about 2 m thick, ALT was stable over 2018–2022 and response to the hottest years was dampened. Linear trends suggest an ALT increase of 2 m per decade for some ice‐poor rock walls, independently of their thermal state. The data reveal a variety of permafrost patterns and evolution with significant intraregional and local differences. Snow modulates the response to air temperature signal in various ways, with an important effect on near‐surface temperature trends and ALT: early snow melting in spring favors an ALT increase in rock walls. Maintaining these monitoring systems and understanding the physical processes controlling heterogeneous responses to climate signals is crucial to better assess permafrost dynamics and to adapt to its consequences.

Divergent responses of permafrost degradation to precipitation increases at different seasons on the eastern Qinghai–Tibet Plateau based on modeling approach

The Qinghai–Tibet Plateau (QTP) has responded to remarkable climate warming with dramatic permafrost degradation over the past few decades. Previous studies have mostly focused on permafrost responses to rising air temperature, while the effects of accompanying increases in precipitation remain contentious and largely unknown. In this study, a distributed process-based model was applied to quantify the impacts of increased precipitation on permafrost thermal regimes in a warming climate by employing model experiments in the source region of Yellow River (SRYR) on the eastern QTP. The results showed that the active layer thickness (ALT) of permafrost increased by 0.25 m during 2010–2019 compared to 2000 across the SRYR, which was primarily driven by climate warming. In contrast, the increased annual precipitation played a relatively limited role and just slightly mitigated active layer thickening by 0.03 m. Intriguingly, increased precipitation in the cold and warm seasons exerted opposite effects on permafrost across the SRYR. The increased precipitation in the cold season mainly promoted ALT increases, while the increased precipitation in the warm season mitigated ALT increases. In ∼81.0% of the permafrost across the SRYR, the cooling effects of warm season wetting outweighed the warming effects of cold season wetting; while at the transition zone where permafrost was unstable and degrading to seasonally frozen ground, the warming effects of cold season wetting played a relatively larger role which contributed to permafrost degradation. This study explored the physical mechanisms of permafrost thermal responses to climate wetting, thus providing a better understanding of permafrost change in a warmer and wetter climate on the QTP.

High potential for pile-bearing capacity loss and ground subsidence over permafrost regions across the Northern Hemisphere

Rapid atmospheric warming changes the thermal conditions of permafrost over the Northern Hemisphere (NH), including ground temperature warming and ground ice thawing. This warming and thawing of ice-rich permafrost damages existing infrastructure and poses a threat to sustainable development. Bearing capacity (BC) loss and ground subsidence (GS) due to permafrost thawing are two major risks to the infrastructure and key indexes for risk assessment. However, current information on the BC and GS is too coarse, restricted to the Arctic, and scarce for future periods. The aim of this study was to address these gaps by presenting spatial data on the BC and GS for current and future periods across the NH at a resolution of 1 km. A machine learning-based approach was developed to simulate permafrost thermal dynamics under four climate scenarios (SSPs 1–2.6, 2–4.5, 3–7.0, and 5–8.5). The associated changes in the BC and GS were estimated based on changes in the permafrost temperature at or near the depth of zero annual amplitude (MAGT) and active-layer thickness (ALT). The results indicate a continuous increase in MAGT and ALT by 2.3 °C (SSPs1–2.6) to 7.6 °C (SSPs5–8.5) and 16.0 cm (SSPs1–2.6) to 51.0 cm (SSPs5–8.5), respectively, at the end of the 21ts century. This permafrost degradation will lead to a high potential BC loss of 37.8% (SSPs1–2.6) to 40.2% (SSPs5–8.5) on average over 2041–2060, and up to 60.5% (SSPs1–2.6) to 92.2% (SSPs5–8.5) in 2081–2100. The produced average GS is approximately 1.0 cm in 2021–2040, and further up to 1.5 cm (SSPs1–2.6) to 4.7 cm (SSPs5–8.5) in 2081–2100, with notable variations across the permafrost region. These forecasts provide new opportunities to assess future permafrost changes and associated risks and costs with climate warming.

Drying of tundra landscapes will limit subsidence-induced acceleration of permafrost thaw

We used a model for permafrost hydrology informed by detailed measurements of soil ice content to better understand the potential risk of abrupt permafrost thaw triggered by melting ground ice, a key open question associated with permafrost response to a warming Arctic. Our spatially resolved simulations of a well-characterized site in polygonal tundra near Utqiaġvik, Alaska, agree well with multiple types of observations in the current climate. Projections indicate 63cm of bulk subsidence from 2006 to 2100 in the strong-warming RepresentativeConcentration Pathway 8.5 climate. Permafrost thaw as measured by the increase in active layer thickness (ALT)-the thickness of the soil layer that thaws each summer-is accelerated by subsidence, but the effect is relatively small. The ALT increases from the current-day value of approximately 50cm to approximately 180cm by 2100 when subsidence is included compared to about 160cm when it is neglected. In these simulations, previously identified positive feedbacks between subsidence and thaw are self-limiting on decadal time frames because landscape runoff and increasing evapotranspiration result in drier tundra with weaker surface/atmosphere coupling. These results for a tundra site that is representative of large swathes of the Alaska North Slope suggest that subsidence is unlikely to lead to abrupt thaw over large areas. However, subsidence does have significant effects on the hydrology of polygonal tundra. Specifically, subsidence increases landscape runoff, which helps maintain streamflow in the face of increased evapotranspiration but also causes drier tundra conditions that could have deleterious effects on sensitive Arctic wetland ecosystems.

Insights into runoff changes in the source region of Yellow River under frozen ground degradation

The source region of the Yellow River (SRYR) in the northeastern Tibetan Plateau is critical for supplying water resources to downstream areas. However, streamflow in the SRYR declined despite a slight increase in precipitation during the past few decades. The SRYR experienced significant frozen ground degradation with climate warming, but how frozen ground degradation influences runoff remains unclear. This study investigated the changes of the precipitation-runoff relationship using the double-mass curve method and examined the impact of long-term spatiotemporal changes in frozen ground on the water balance components using the geomorphology-based eco-hydrological model (GBEHM). The results showed that the precipitation-runoff relationship changed significantly since 1989 in the SRYR from 1960 to 2019. In the same period, the areal mean value of the maximum thickness of seasonally frozen ground (MTSFG) decreased by 0.10 m/10a and the areal mean active layer thickness (ALT) of permafrost increased by 0.06 m/10a. Besides, 21.0 % of the entire SRYR has degraded from permafrost to seasonally frozen ground (SFG). Runoff decreased mainly in the region with elevation below 4200 m, where the evapotranspiration increase exceeded the precipitation increase. Frozen ground degradation significantly altered the hydrological processes, which is reflected by the increased subsurface runoff and the decreased surface runoff. The total water storage increased by 2.9 mm/a in the permafrost region due to the increase in active layer thickness and by 5.7 mm/a in the degradation region where permafrost completely thawed during 1960–2019. The runoff seasonality was also altered, being indicated as an increase in winter runoff. These findings help provide a better understanding of the runoff change under climate warming in permafrost-affected regions and provide insights into future water resources management in the Yellow River basin under the climate warming.

Emerging solute-induced mineralization in Arctic rivers under climate warming

Permafrost degradation under a warming climate is accelerating the hydrological processes in Arctic river basins. However, corresponding changes in river mineralization, riverine solute exports and their potential influencing factors are not fully understood. In this study, we selected six major Arctic rivers (Ob, Yenisei, Lena, Kolyma, Yukon and Mackenzie Rivers) with different permafrost extents, meteorological conditions and hydrological regimes to reveal the changes in river mineralization and riverine solute exports using ArcticGRO sampling data from 2003 to 2019. Our results indicate that solute-induced river mineralization has already been observed in the Lena, Yukon and Mackenzie Rivers during 2003–2019. The annual flux of total dissolved solids (TDS; a key parameter of drinking water quality), calculated by the Load Estimator (LOADEST) program, from these six rivers was approximately 295.24 ± 12.50 Tg, with the Ob, Kolyma and Yukon Rivers exhibiting significant increasing trends (p < 0.05) at rates of 4.38 Tg/10 yr, 1.62 Tg/10 yr and 3.03 Tg/10 yr, respectively. Climate-induced changes in hydrological regimes regulate riverine solute exports, with relatively higher TDS concentrations in the groundwater-dominated winter low-flow season and lower TDS concentrations under the dilution of groundwater by snowmelt spring floods and summer precipitation events. The riverine solute fluxes with higher TDS concentrations (e.g., those of the Yukon and Mackenzie Rivers) increased more rapidly (~0.14 Tg/km3) with changes in river discharge; however, the TDS concentrations were more sensitive to climate warming in continuous permafrost-dominated colder basins (i.e., the Kolyma and Lena River basins) than in other relatively warmer basins. Our results suggest that riverine solute exports are likely affected by permafrost thaw-induced changes in hydrogeological processes, which are tightly associated with increases in active layer thickness and enhanced groundwater discharge to rivers. Under a warming climate, riverine solute exports in Arctic rivers are expected to increase with intensifying groundwater–surface water exchanges.

Understanding the Effects of Cold and Warm Season Air Warming on the Permafrost Hydrology Changes in the Source Region of the Lancang River, the Qinghai‐Tibetan Plateau

AbstractCold season air warming was more rapid than warm season air warming on the Qinghai‐Tibetan Plateau (QTP). However, the effect of this asymmetrical seasonal air warming on permafrost hydrological changes has not been fully understood. This study applied a distributed cryospheric hydrological model to evaluate the effects of different seasonal air warming on the changes in frozen soil and hydrological processes in a typical catchment, the source region of the Lancang River on the eastern QTP. The results show that the area of permafrost reduced by 14.0%. The maximum frozen depth of seasonally frozen ground (MFDSFG) decreased at 5.0 cm decade−1, and the active layer thickness (ALT) of permafrost increased by 3.3 cm decade−1. Controlled experiments illustrate that cold season air warming dominated the reduction in MFDSFG which caused the liquid soil moisture increase in seasonally frozen ground, and warm season air warming primarily determined the increase in ALT which enhanced the liquid soil moisture in permafrost. Cold season air warming had a greater effect on runoff than warm season air warming because it dominated the permafrost degradation into seasonally frozen ground. In the region where permafrost degraded into seasonally frozen ground, both the cold and warm season air warming contributed to the soil liquid water increase, and the cold season warming had a greater effect due to its more important role in thermal degradation of permafrost. The findings of this study reveal different complex impacts of cold and warm season air warming on permafrost hydrological changes on the QTP.

Active Layer Thickness in the Northern Hemisphere: Changes From 2000 to 2018 and Future Simulations

AbstractActive layer dynamics are basic information for understanding permafrost degradation, but there are gaps in our knowledge of the active layer thickness (ALT) variations during the past two decades and their future changes; this is especially true for data with high spatial resolution. Here, based on permafrost monitoring data and ERA5 (the 5th generation reanalysis project from the European Centre for Medium‐Range Weather Forecasts)‐Land temperature data, we simulated spatial changes in ALT in the Northern Hemisphere during 2000–2018 at a spatial resolution of 1 km2 using the Stefan model. We also simulated future changes in the active layer based on the CMIP6 temperature data. The results showed that ALTs less than 50 cm were mainly distributed in northeastern Siberia in Russia, Alaska and Greenland. The areas with ALTs greater than 600 cm were mainly in southern Norway, and the Mongolian Plateau. The mean ALT of permafrost in the Northern Hemisphere from 2000 to 2018 increased from 127.19 to 145.37 cm at a linear rate of 0.65 cm year−1 (R2 = 0.37, p &lt; 0.05). During 2000–2018, the ALT increased in 71.17% of the total permafrost area, and only 27.23% of the permafrost area showed a decreasing trend. Under different shared socioeconomic pathways (SSP1‐2.6, SSP2‐4.5, SSP3‐7.0, and SSP5‐8.5), the ALT will increase considerably by 2,100. There is a greater increase in ALT in low‐ and mid‐latitude permafrost areas such as the Mongolian Plateau and Qinghai‐Tibet Plateau, with the ALT increasing from 50 to 500 cm. Our results present an overall picture of contemporary distribution as well as future changes of ALT in the Northern Hemisphere.

Influence of the Thickness of the Seasonally Thawed Layer of Permafrost in the Eastern Siberia Catchments on the Content of Organic Matter in River Waters

In the context of global climate change, a significant increase in the active layer thickness (ALT) of permafrost is expected in the current century. This process has been observed by researchers over the past few decades. If the current climate trend continues, an increase in ALT may have a significant impact on the concentration of organic matter in Arctic river waters. The relationship between the thickness of the seasonally thawed layer of permafrost and the concentration of dissolved organic matter in river waters has been explored using clustering, one-way ANOVA, and cross-tabulation analysis. The data set for analysis included original details on the content of organic matter in the rivers of Eastern Siberia (in terms of COD, BOD5, and the color of the water), phytoplankton abundance and biomass, and data on the permafrost active layer thickness (ALT) in the catchments. It was revealed that in the areas of catchments where the ALT is deeper, the content of organic matter in the rivers is lower than in areas with a shallow, seasonally thawed permafrost. Our results are consistent with the existing conceptual model of the influence of ALT on the chemistry of river waters in the cryolithozone. This knowledge is important for predicting the chemical composition of the Arctic rivers, eutrophication, and the rate of inflow of dissolved solids into the Arctic Ocean under the current conditions of ALT deepening.

Regulation of Crystallinity and Vertical Phase Separation Enables High‐Efficiency Thick Organic Solar Cells

AbstractThick film organic solar cells have great potential for preparing large‐area devices in the future. However, unsuitable vertical donor/acceptor distribution, poor crystallinity of materials, and voltage losses remain unclear, limiting the performance of thick film devices. Here, a target ternary strategy by introducing a nematic liquid crystalline small molecule donor BTR‐Cl into the PM6:Y6 host system is used to reduce the performance degradation caused by the increase of active layer thickness. By incorporating 10% BTR‐Cl into the blend film, the voltage loss is minimized in both thin and thick ternary devices. Moreover, the power conversion efficiency of the ternary device is maintained at 15.28% compared to 12.94% of binary devices with an active layer thickness of 300 nm. It is benefited from the optimized crystallization behavior and more suitable vertical phase distribution driven by differences in miscibility between components. While ensuring sufficient phase separation, the charge transport is improved, it also prompts the balance of charge transport and reduces the non‐radiative recombination. This work demonstrates that the use of the third component to enhance the crystallization of the active layer and improve the vertical phase distribution is an effective method to build thick films for future large‐scale printing techniques.

Investigation of conventional and inverted organic solar cell from optical aspect

As an interesting research topic, investigation on organic solar cells is increasing day by day. In this study, the performance of conventional P3HT: PCBM based organic solar cell (OSC) structure and inverted organic solar cell (IOSC) structure has been investigated through optical simulation using GPVDM software. As an active layer material, P3HT: PCBM is used. Al, ITO, PEDOT: PSS and TiO2 is used as cathode, anode, hole transporting layer (HTL) and electron transporting layer (ETL) respectively for OSC structure. In case of IOSC, Ag has been used as an anode. Investigations have been made to study the effect of active layer thickness on the exciton generation rate. It has been found that the exciton generation rate is oscillatory due to optical interference effect for both OSC and IOSC structure. Photon absorption reduces due to increase in active layer thickness. However, exciton generation increases due to creation of more peak into the active layer.

New insights into the drainage of inundated ice-wedge polygons using fundamental hydrologic principles

Abstract. The pathways and timing of drainage from the inundated centers of ice-wedge polygons in a warming climate have important implications for carbon flushing, advective heat transport, and transitions from methane to carbon dioxide dominated emissions. Here, we expand on previous research using a recently developed analytical model of drainage from a low-centered polygon. Specifically, we perform (1) a calibration to field data identifying necessary model refinements and (2) a rigorous model sensitivity analysis that expands on previously published indications of polygon drainage characteristics. This research provides intuition on inundated polygon drainage by presenting the first in-depth analysis of drainage within a polygon based on hydrogeological first principles. We verify a recently developed analytical solution of polygon drainage through a calibration to a season of field measurements. Due to the parsimony of the model, providing the potential that it could fail, we identify the minimum necessary refinements that allow the model to match water levels measured in a low-centered polygon. We find that (1) the measured precipitation must be increased by a factor of around 2.2, and (2) the vertical soil hydraulic conductivity must decrease with increasing thaw depth. Model refinement (1) accounts for runoff from rims into the ice-wedge polygon pond during precipitation events and possible rain gauge undercatch, while refinement (2) accounts for the decreasing permeability of deeper soil layers. The calibration to field measurements supports the validity of the model, indicating that it is able to represent ice-wedge polygon drainage dynamics. We then use the analytical solution in non-dimensional form to provide a baseline for the effects of polygon aspect ratios (radius to thaw depth) and coefficient of hydraulic conductivity anisotropy (horizontal to vertical hydraulic conductivity) on drainage pathways and temporal depletion of ponded water from inundated ice-wedge polygon centers. By varying the polygon aspect ratio, we evaluate the relative effect of polygon size (width), inter-annual increases in active-layer thickness, and seasonal increases in thaw depth on drainage. The results of our sensitivity analysis rigorously confirm a previous analysis indicating that most drainage through the active layer occurs along an annular region of the polygon center near the rims. This has important implications for transport of nutrients (such as dissolved organic carbon) and advection of heat towards ice-wedge tops. We also provide a comprehensive investigation of the effect of polygon aspect ratio and anisotropy on drainage timing and patterns, expanding on previously published research. Our results indicate that polygons with large aspect ratios and high anisotropy will have the most distributed drainage, while polygons with large aspect ratios and low anisotropy will have their drainage most focused near their periphery and will drain most slowly. Polygons with small aspect ratios and high anisotropy will drain most quickly. These results, based on parametric investigation of idealized scenarios, provide a baseline for further research considering the geometric and hydraulic complexities of ice-wedge polygons.

Improved Simulation of Arctic Circumpolar Land Area Snow Properties and Soil Temperatures

The impact of high latitude climate warming on Arctic snow cover and its insulating properties has key implications for the surface and soil energy balance. Few studies have investigated specific trends in Arctic snowpack properties because there is a lack of long-term in situ observations and current detailed snow models fail to represent the main traits of Arctic snowpacks. This results in high uncertainty in modeling snow feedbacks on ground thermal regime due to induced changes in snow insulation. To better simulate Arctic snow structure and snow thermal properties, we implemented new parameterizations of several snow physical processes—including the effect of Arctic low vegetation and wind on snowpack—in the Crocus detailed snowpack model. Significant improvements compared to standard Crocus snow simulations and ERA-Interim (ERAi) reanalysis snow outputs were observed for a large set of in-situ snow data over Siberia and North America. Arctic Crocus simulations produced improved Arctic snow density profiles over the initial Crocus version, leading to a soil surface temperature bias of −0.5 K with RMSE of 2.5 K. We performed Crocus simulations over the past 39 years (1979–2018) for circumpolar taiga (open forest) and pan-Arctic areas at a resolution of 0.5°, driven by ERAi meteorological data. Snowpack properties over that period feature significant increase in spring snow bulk density (mainly in May and June), a downward trend in snow cover duration and an upward trend in wet snow (mainly in spring and fall). The pan-Arctic maximum snow water equivalent shows a decrease of −0.33 cm dec−1. With the ERAi air temperature trend of +0.84 K dec−1 featuring Arctic winter warming, these snow property changes have led to an upward trend in soil surface temperature (Tss) at a rate of +0.41 K dec−1 in winter. We show that the implemented snowpack property changes increased the Tss trend by 36% compared to the standard simulation. Winter induced changes in Tss led to a significant increase of 16% (+4 cm dec−1) in the estimated active layer thickness (ALT) over the past 39 years. An increase in ALT could have a significant impact on permafrost evolution, Arctic erosion and hydrology.

Active layer thickness as a function of soil water content

Active layer thickness (ALT) is a critical metric for monitoring permafrost. How soil moisture influences ALT depends on two competing hypotheses: (a) increased soil moisture increases the latent heat of fusion for thaw, resulting in shallower active layers, and (b) increased soil moisture increases soil thermal conductivity, resulting in deeper active layers. To investigate their relative influence on thaw depth, we analyzed the Field Measurements of Soil Moisture and Active Layer Thickness (SMALT) in Alaska and Canada dataset, consisting of thousands of measurements of thaw depth and soil moisture collected at dozens of sites across Alaska and Canada as part of NASA’s Arctic Boreal Vulnerability Experiment (ABoVE). As bulk volumetric water content (VWC) integrated over the entire active layer increases, ALT decreases, supporting the latent heat hypothesis. However, as VWC in the top 12 cm of soil increases, ALT increases, supporting the thermal conductivity hypothesis. Regional temperature variations determine the baseline thaw depth while precipitation may influence the sensitivity of ALT to changes in VWC. Soil latent heat dominates over thermal conductivity in determining ALT, and the effect of bulk VWC on ALT appears consistent across sites.

Permafrost thaw and ground settlement considering long-term climate impact in northern Alaska

Alaska’s North Slope is predicted to experience twice the warming expected globally. When summers are longer and winters are shortened, ground surface conditions in the Arctic are expected to change considerably. This is significant for Arctic Alaska, a region that supports surface infrastructure such as energy extraction and transport assets (pipelines), buildings, roadways, and bridges. Climatic change at the ground surface has been shown to impact soil layers beneath through the harmonic fluctuation of the active layer, and warmer air temperature can result in progressive permafrost thaw, leading to a deeper active layer. This study attempts to assess climate change based on the climate model data from the fifth phase of the Coupled Model Intercomparison Project and its impact on a permafrost environment in Northern Alaska. The predicted air temperature data are analyzed to evaluate how the freezing and thawing indices will change due to climate warming. A thermal model was developed that incorporated a ground surface condition defined by either undisturbed intact tundra or a gravel fill surface and applied climate model predicted air temperatures. Results indicate similar fluctuation in active layer thickness and values that fall within the range of minimum and maximum readings for the last quarter-century. It is found that the active layer thickness increases, with the amount depending on climate model predictions and ground surface conditions. These variations in active layer thickness are then analyzed by considering the near-surface frozen soil ice content. Analysis of results indicates that thaw strain is most significant in the near-surface layers, indicating that settlement would be concurrent with annual thaw penetration. Moreover, ice content is a major factor in the settlement prediction. This assessment methodology, after improvement, and the results can help enhance the resilience of the existing and future new infrastructure in a changing Arctic environment.

Colloidal organic carbon and trace elements in peat porewaters across a permafrost gradient in Western Siberia

The majority of organic carbon (OC), nutrients, and dissolved trace elements in soil porewaters are present in the form of colloids which determine element transport, bioavailability, and overall impact on ecosystems. Climate warming and permafrost thaw in high latitudes will primarily affect the soil liquid phase thereby modifying delivery of colloids to the hydrological network and their role in C transport and emission. Here we studied colloids in peat porewaters across a natural gradient of sporadic, isolated, discontinuous and continuous permafrost zone in the Western Siberian Lowland (WSL), the largest peatland in the world. The depth of sampling and the microrelief (mounds and hollows) had a generally weak impact on the proportion of colloidal forms (3 kDa − 0.45 µm) of OC, major (Fe, Al, P, alkali and alkaline-earth metals) and trace elements (TE) including micronutrients (Zn, Mn, Ni, Co, Cu), toxicants (Sb, As, Cd, Pb) and geochemical tracers (trivalent and tetravalent cations). Considering all micro-landscapes together, there was no sizable change in the proportion of colloidal fraction of OC, Fe, Al, P, micronutrients and toxicants across the permafrost zones. The majority of colloidal forms of all elements of these groups were represented by a size fraction between 3 and 30 kDa and were essentially Fe-Al-organic compounds with an average Fe:Al:OC molar ratio of 1.9:1:308. Overall, the degree of impact from environmental factors on OC and metal distribution among various colloidal fractions can be classified as depth ≤ permafrost type < microlandscape. Applying a substitution “space for time scenario” for the climate warming and permafrost thaw in Western Siberia, we do not expect sizable changes in C and element colloidal status during active layer thickness increase and permafrost boundary shift northward. Future studies of colloids in peat ice (below the active layer) are needed to assess possible changes in delivery of C and metals from soil to rivers and onward into the Arctic Ocean under massive permafrost thawing in the WSL.

Mapping Frozen Ground in the Qilian Mountains in 2004–2019 Using Google Earth Engine Cloud Computing

The permafrost in the Qilian Mountains (QLMs), the northeastern margin of the Qinghai–Tibet Plateau, changed dramatically in the context of climate warming and increasing anthropogenic activities, which poses significant influences on the stability of the ecosystem, water resources, and greenhouse gas cycles. Yet, the characteristics of the frozen ground in the QLMs are largely unclear regarding the spatial distribution of active layer thickness (ALT), the maximum frozen soil depth (MFSD), and the temperature at the top of the permafrost or the bottom of the MFSD (TTOP). In this study, we simulated the dynamics of the ALT, TTOP, and MFSD in the QLMs in 2004–2019 in the Google Earth Engine (GEE) platform. The widely-adopted Stefan Equation and TTOP model were modified to integrate with the moderate-resolution imaging spectroradiometer (MODIS) land surface temperature (LST) in GEE. The N-factors, the ratio of near-surface air to ground surface freezing and thawing indices, were assigned to the freezing and thawing indices derived with MODIS LST in considerations of the fractional vegetation cover derived from MODIS normalized difference vegetation index (NDVI). The results showed that the GEE platform and remote sensing imagery stored in Google cloud could be quickly and effectively applied to obtain the spatial and temporal variation of permafrost distribution. The area with TTOP &lt; 0 °C is 8.4 × 104 km2 (excluding glaciers and lakes) and accounts for 46.6% of the whole QLMs, the regional mean ALT is 2.43 ± 0.44 m, while the regional mean MFSD is 2.54 ± 0.45 m. The TTOP and ALT increase with the decrease of elevation from the sources of the sub-watersheds to middle and lower reaches. There is a strong correlation between TTOP and elevation (slope = −1.76 °C km−1, p &lt; 0.001). During 2004–2019, the area of permafrost decreased by 20% at an average rate of 0.074 × 104 km2·yr−1. The regional mean MFSD decreased by 0.1 m at a rate of 0.63 cm·yr−1, while the regional mean ALT showed an exception of a decreasing trend from 2.61 ± 0.45 m during 2004–2005 to 2.49 ± 0.4 m during 2011–2015. Permafrost loss in the QLMs in 2004–2019 was accelerated in comparison with that in the past several decades. Compared with published permafrost maps, this study shows better calculation results of frozen ground in the QLMs.