Permafrost Regions Research Articles

Application of a concrete thermal pile in cooling the warming permafrost under climate change

Permafrost degradation caused by climate warming is posing a serious threat to the stability of cast-in-place pile foundations in warm permafrost regions. Ambient cold energy can be effectively utilized by two-phase closed thermosyphons (TPCTs) to cool the permafrost. Therefore, we installed TPCTs in a cast-in-place pile foundation to create a unique structure called a thermal pile, which effectively utilizes the TPCTs to regulate ground temperature. And we conducted a case study and numerical simulation to exhibit the cooling performance, and optimize the structure of the thermal pile. The purpose of this study is to promote the application of thermal piles in unstable permafrost regions. Based on the findings, the thermal pile operated for approximately 53% of the entire year and effectively reduced the deep ground temperature at a rate of at least −0.1 °C per year. Additionally, it successfully raised the permafrost table that is 0.35 m shallower than the natural ground level. These characteristics prove highly beneficial in mitigating the adverse effects of permafrost degradation and enhancing infrastructure safety. Expanding the length of the condenser section and the diameter of the TPCT in a suitable manner can effectively enhance the cooling capability of the thermal pile and ensure the long-term mechanical stability of the pile foundation even under climate warming.

Environmental Characteristics of High Ice-Content Permafrost on the Qinghai–Tibetan Plateau

Permafrost areas are sensitive to climate change and have a significant impact on energy and water cycles. Ground ice is a crucial component on the Qinghai–Tibetan Plateau (QTP). Understanding the environmental characteristics of ground ice is vital for accurately modeling its distribution and evolution. In this study, we analyzed 15 environmental characteristics of high ice-content permafrost sites. These attributes were extracted from 400 high ice-content permafrost datasets including 300 drilling boreholes and 100 thaw slumps collected throughout the entire plateau using large-scale remote sensing data and their products. The results are as follows: The mean annual air temperature in areas where high ice-content permafrost exists ranges from −5 to −3.5 °C, with an average warming rate of 0.08 °C/a. Additionally, there was an average increase in precipitation of about 25 mm/10a and an increase in soil moisture of about 4%/10a. Geomorphology strongly influences the occurrence of high ice content permafrost, with 85% of high ice-content permafrost development at altitudes between 4400 and 5100 m. Approximately 86% of high ice-content permafrost were developed in lowland or relatively flat areas, preferably in gently sloping and shady slope regions. Soils exhibit less variability in clay particles and more variability in silt and sand. Key indicators in the high ice content permafrost region include warming rate, active layer thickness, elevation, bulk density, soil thickness, clay content, precipitation, soil moisture, and NDVI. High ice-content permafrost is the result of a combination of environmental factors and is expected to undergo significant changes in the future. This study provides a foundation for comprehending the environmental changes in the high ice-content permafrost areas and modeling the distribution of ground ice. It underscores the urgent need to address the significant environmental changes faced by high ice-content permafrost regions.

Qualitative evaluation of thaw settlement potential in permafrost regions of Canada

Permafrost thaw negatively impacts the built environment in circumpolar regions. Thaw-induced settlement causes various engineering problems such as differential settlement, sinkholes, culvert failures, and pipeline deformations. Large-scale pre-construction evaluation reduces the risk of building infrastructure in thaw settlement-prone areas. This evaluation enables the comparison and screening of multiple route/location alternatives, the identification of vulnerable sections of a route, and the development of more effective site investigation plans. This study presents a systematic approach for evaluating thaw settlement potential in various regions across Canadian permafrost. In developing the approach, we considered three main factors: ground ice content, surficial deposit unit, and bedrock type. These factors were prioritized based on their relative importance in defining settlement behaviour, and a weighting factor was determined for each. In this study, the weighting factors of 0.78, 0.15, and 0.07 were suggested for ground ice content, surficial deposit type and bedrock type, respectively. We subsequently developed a Thaw Settlement Potential Index (TSPI), which can be calculated for any given location using publicly available Canada-wide resources. We calculated TSPI for several reported cases of thaw settlement issues across Canada and compared the results with site observations. A comparison was also made with a more detailed assessment conducted on the Northern Alaska Highway using a different approach. While the TSPI assessment required minimal time, effort, and resources, the obtained results generally matched those of finer-scale assessments with more detailed information. Thus, our methodology simplifies early identification of thaw-unstable regions in projects with large footprints and helps allocate resources more efficiently for subsequent site investigations.

The role of root zone soil moisture return on vegetation green-up: A comparison of two degrading permafrost sites

The effects of permafrost degradation on ecological changes have attracted more and more attention, but the underlying interactive mechanisms are still not well understood. From a unique angle of view, this study focuses on the linkage between the freezing-induced root zone soil moisture return (SMR) and vegetation green-up along with climate warming and permafrost degradation. Using 7-year soil-weather monitoring data in the high-altitude and high-latitude permafrost regions, we investigated the changes of root zone SMR in two degrading permafrost sites. Results demonstrate that, as one important water source for vegetation green-up, the ratio of root zone SMR to the soil moisture storage after final thawed, Rsmr, could reach 22.8 % and 10.5 % at the two sites, respectively. In contrast to the negligible change at the high-latitude permafrost site, a rapid increase rate of 7.7 % per decade in Rsmr was found at the high-altitude permafrost site over the monitoring period of 2012–2018. Generally, vegetation green-up definitely benefited from the enrichment of soil moisture storage, but their sensitivities to climate warming and permafrost degradation were different at the two sites. The increase of Rsmr effectively buffered the shrinking trend of soil water availability of the root zone both over the pre-freezing and the green-up period at the high-altitude permafrost site. The changing role of root zone SMR might be threatened during the vegetation green-up in the longer future. Whereas, there would be no water shortage risk for the foreseeable future at the high-latitude permafrost site. Overall, this study emphasized the importance of SMR in enhancing soil water availability for vegetation green-up under the background of quick climate warming and permafrost degradation.

Accelerated organic matter decomposition in thermokarst lakes upon carbon and phosphorus inputs.

Mineralization of dissolved organic matter (DOM) in thermokarst lakes plays a non-negligible role in the permafrost carbon (C) cycle, but remains poorly understood due to its complex interactions with external C and nutrient inputs (i.e., aquatic priming and nutrient effects). Based on large-scale lake sampling and laboratory incubations, in combination with 13 C-stable-isotope labeling, optical spectroscopy, and high-throughput sequencing, we examined large-scale patterns and dominant drivers of priming and nutrient effects of DOM biodegradation across 30 thermokarst lakes along a 1100-km transect on the Tibetan Plateau. We observed that labile C and phosphorus (P) rather than nitrogen (N) inputs stimulated DOM biodegradation, with the priming and P effects being 172% and 451% over unamended control, respectively. We also detected significant interactive effects of labile C and nutrient supply on DOM biodegradation, with the combined labile C and nutrient additions inducing stronger microbial mineralization than C or nutrient treatment alone, illustrating that microbial activity in alpine thermokarst lakes is co-limited by both C and nutrients. We further found that the aquatic priming was mainly driven by DOM quality, with the priming intensity increasing with DOM recalcitrance, reflecting the limitation of external C as energy sources for microbial activity. Greater priming intensity was also associated with higher community-level ribosomal RNA gene operon (rrn) copy number and bacterial diversity as well as increased background soluble reactive P concentration. In contrast, the P effect decreased with DOM recalcitrance as well as with background soluble reactive P and ammonium concentrations, revealing the declining importance of P availability in mediating DOM biodegradation with enhanced C limitation but reduced nutrient limitation. Overall, the stimulation of external C and P inputs on DOM biodegradation in thermokarst lakes would amplify C-climate feedback in this alpine permafrost region.

Monitoring Thermokarst Lake Drainage Dynamics in Northeast Siberian Coastal Tundra

Thermokarst lakes in permafrost regions are highly dynamic due to drainage events triggered by climate warming. This study focused on mapping lake drainage events across the Northeast Siberian coastal tundra from 2000 to 2020 and identifying influential factors. An object-based lake analysis method was developed to detect 238 drained lakes using a well-established surface water dynamics product. The LandTrendr change detection algorithm, combined with continuous Landsat satellite imagery, precisely dated lake drainage years with 83.2% accuracy validated against manual interpretation. Spatial analysis revealed the clustering of drained lakes along rivers and in subsidence-prone Yedoma regions. The statistical analysis showed significant warming aligned with broader trends but no evident temporal pattern in lake drainage events. Our machine learning model identified lake area, soil temperature, summer evaporation, and summer precipitation as the top predictors of lake drainage. As these climatic parameters increase or surpass specific thresholds, the likelihood of lake drainage notably increases. Overall, this study enhanced the understanding of thermokarst lake drainage patterns and environmental controls in vulnerable permafrost regions. Spatial and temporal dynamics of lake drainage events were governed by complex climatic, topographic, and permafrost interactions. Integrating remote sensing with field studies and modeling will help project lake stability and greenhouse gas emissions under climate change.

Arctic Spring Systems Driven by Permafrost Aggradation

AbstractGroundwater springs in permafrost regions provide pathways for solutes and dissolved gases to escape from sub‐permafrost groundwater systems, which otherwise are completely isolated from the surface environment and atmosphere. Yet, fundamental questions as to the mechanisms driving groundwater flow to the surface remain unsolved. In this study, basal permafrost aggradation is explored as a mechanism for generating groundwater flow and driving groundwater spring systems. We employ process‐based numerical modeling to test the hypothesis of permafrost‐aggradation‐driven spring systems in a range of environmental settings. The model results show that permafrost aggradation can generate spring flow on a multi‐millennial timescale and with discharge rates up to a couple of liters per second. Permafrost aggradation deserves attention as a groundwater flow driving mechanism in areas of recent glacio‐isostatic uplift and glacial retreat.

Sediment Grain-Size Composition in the Permafrost Region of the Greater Khingan Range and Its Significance as a Material Source

Sediment grain-size distribution (GSD) provides rich information about sedimentary sources and can potentially do the same with regard to environmental and climatic changes. However, neither traditional descriptive statistics nor curving-fitting methods can fully address its complexity. We selected the Greater Khingan Range in northeastern China as the study area and used parameterized end-member analysis (EMA) of the GSD of four drilling cores to extract different end-member (EM) components. The results show that EM1 (mode particle size (Mo): 1.26–1.66 μm) originates from weathering and pedogenesis. The EMs with Mo values of 4.37–5.01 μm represent components transported by the upper westerly wind. EMs with Mo values of 7.58 μm and 11.48 μm represent wet dust deposition and dry dust deposition. The wind transport of particles in winter consists of low-level near-source transport and local-source transport (possibly from the flood plain of the Amur River). Due to the limitations of the EM model, the two sources have one or two EM components: AEM3 + AEM4, BEM3 + BEM4, CEM4, and DEM4. DEM5 is the only large particle-size component and may represent coarse-grained detritus generated via rock weathering. The components related to the winter monsoon and the 14C dating data suggest a weak-strong-weak-strong trend of the winter monsoon since the Marine Isotope Stage 3a (MIS 3a). Our results suggest that the strengthening of the winter monsoons in the previous few thousand years has caused the transportation of coarser grain sizes and further exacerbated permafrost degradation, providing a scientific reference for understanding climate change and the formation and evolution of permafrost in the Greater Khingan Mountains since the MIS 3a.

Characteristics of Microbial Abundance in Rhizosphere and Non-Rhizosphere Soils of Permafrost Peatland, Northeast China

The rhizosphere microenvironment is crucial to plant–soil physiological processes. The differences among microbial communities in the rhizosphere and non-rhizosphere peatland topsoil (0–15 cm) and subsoil (15–30 cm) in five plant communities dominated by Carex schmidtii, Chamaedaphne calyculata, Ledum palustre, Betula fruticosa, and Vaccinium uliginosum, as well as non-rhizosphere soil in discontinuous and continuous permafrost regions, were studied. We found that the bacteria and nifH gene abundances in the C. calyculata rhizosphere soil in the discontinuous permafrost region were higher than those in continuous permafrost region, while the nirK and nifH gene abundances in the non-rhizosphere soil of the discontinuous permafrost region were lower than those in the continuous permafrost region. The ratio of bacteria to fungi decreased and that of nirK to nirS increased significantly from the discontinuous to the continuous permafrost region, indicating that permafrost degradation can change soil microbial community composition. Fungal abundance was higher in the rhizosphere than the non-rhizosphere soils, suggesting that plant roots provide a more suitable environment for fungi. Moreover, the abundances of the topsoil bacteria; the fungi; and the nirK, nirS, and nifH genes were higher than those in the subsoil because of the organic matter from plant litter as a source of nutrients. The microbial abundance in the subsoil was also more affected by nutrient availability. To sum up, the microbial abundance varied among the different types of rhizosphere and non-rhizosphere soils, and the carbon and nitrogen cycling processes mediated by soil microorganisms may be greatly altered due to permafrost degradation under climate warming.

Thermokarst lake changes along the Qinghai-Tibet Highway during 1991–2020

Thermokarst lake development significantly affects hydrologic systems, infrastructure stability and biogeochemical processes, while the spatial and temporal changes in thermokarst lakes remain largely unknown. Here, we created a thermokarst lake dataset on the Qinghai-Tibet Plateau (QTP) using a threshold-based mapping method based on Google Earth Engine (GEE) data associated with visual inspection. The dataset includes a thermokarst lake inventory on the QTP at a 10 m resolution produced from Sentinel-2A images and a multitemporal inventory along the Qinghai-Tibet Highway (QTH) from Landsat and Sentinel-2A images. We analyzed the temporal and spatial changes in thermokarst lake area and their relationships to environmental factors. Our results showed that thermokarst lakes on the QTP permafrost region covered a total area of 1572 ± 184 km2, with most of the thermokarst lakes <10,000 m2 in area. The spatial distributions of thermokarst lakes are affected by the ground thermal stability, active layer thickness, vegetation type, and ground ice content. Over the past 30 years, the number and surface area of thermokarst lakes along the QTH have increased by 58.8 % and 83.1 %, respectively, with the increase in lakes likely caused by climate warming and precipitation increasing. This study provides deep insight in the long-term interannual variations and its driving factors in thermokarst lakes along the QTH.

Thermal characteristics and optimization of phase change energy storage subgrade in permafrost regions

In permafrost regions, the thawing of permafrost will cause serious damage to the road structure, and delaying the degradation of permafrost is the key to ensure the road performance. Based on the energy storage characteristics of phase change material (PCM) and the anti-seepage performance of geotextile, a phase change geotextile (PCG) with heat absorption and waterproof functions is prepared in this study. PCG is applied to the subgrade structure, and the phase change energy storage subgrade (PCESS) is proposed. This subgrade structure can effectively reduce the temperature and increases the artificial permafrost table in permafrost regions. In order to study the temperature regulation mechanism of PCESS, the effects of thermal properties of PCM and epoxy resin mixture (PCMERM) and structural parameters of PCG were analyzed by numerical models. The mean absolute errors (MAEs) were 0.88 and 1.08 with experiments conducted to validate numerical model. The analysis of influencing factors showed that the phase change temperature of PCMERM exhibits the best value to improve the regulation effect of PCESS under given conditions. Increasing the heat of fusion and dosage of PCMERM can enhance the regulation effect. When PCG was laid on the top of the subgrade, the regulation effect was the best. Compared with the traditional permafrost subgrade, PCESS showed long-term thermal stability. In addition, the prediction model for the artificial permafrost table change was established. The model predicted the artificial table in three cases, and the MAEs were 0.048, 0.019 and 0.039, respectively. These results suggest that the model can accurately predict the changes of the artificial permafrost table.

Effect of Frozen Ground on Seismic Response of Pile-Supported Bridge

With the rapid development of China’s economy, the linear traffic infrastructure in the permafrost region has also developed rapidly. Many engineering problems in pile-supported bridges are caused by the freeze–thaw cycles and seismic activity on the roadbed when linear traffic passes over the frozen ground. Therefore, this study focused on the seismic response of a pile-supported bridge in frozen ground. A three-dimensional (3D) nonlinear finite-element (FE) model was established based on a pile-supported bridge, and various modeling techniques and methods were presented in detail. Specifically, the p−y curves in the frozen or unfrozen soil were calculated based on the experimental results from modeling the soil–pile interactions. Based on the computed results, the seismic response of a pile-supported bridge structure in the frozen ground was comprehensively investigated and compared with that in the unfrozen ground. The effect of the frozen ground on the seismic response of a bridge structure was systematically explored. The modeling technique and the derived insights could be significant for pile-supported bridges in frozen ground.

Effects of freeze-thaw cycles on the size distribution and stability of soil aggregate in the permafrost regions of the Qinghai-Tibetan Plateau

As the basic units of soil structure, soil aggregate is essential for maintaining soil stability. Intensified freeze–thaw cycles have deeply affected the size distribution and stability of aggregate under global warming. To date, it is still lacking about the effects of freeze–thaw cycles on aggregate in the permafrost regions of the Qinghai-Tibetan Plateau (QTP). Therefore, we investigated the effects of diurnal and seasonal freeze–thaw processes on soil aggregate. Our results showed that the durations of thawing and freezing periods in the 0–10 cm layer were longer than in the 10–20 cm layer, while the opposite results were observed during completely thawed and frozen periods. Freeze–thaw strength was greater in the 0–10 cm layer than that in the 10–20 cm layer. The diurnal freeze–thaw cycles have no significant effect on the size distribution and stability of aggregate. However, < 0.25 mm fraction dominated wet sieving aggregate with the highest proportion during thawing period, while the < 1 mm fraction reached the highest during completely frozen period in the 10–20 cm layer (P < 0.05). Likewise, the mean weight diameter and water-stable aggregate were decreased during thawing period compared with the other periods, which were influenced by soil microbial biomass carbon and belowground biomass. Hence, the seasonal freeze–thaw processes destroyed macro-aggregate (> 0.25 mm) and reduced aggregate stability. Our study has scientific guidance for evaluating the effects of freeze–thaw cycles on soil steucture and provides a theoretical basis for further exploration on soil and water conservation in the permafrost regions of the QTP.

Correction: Thermal control mechanism of ventilated–closed block layer composite embankment of expressway in warm permafrost regions

Correction: Thermal control mechanism of ventilated–closed block layer composite embankment of expressway in warm permafrost regions

Towards the incorporation of hydrogeochemistry into the modelling of permafrost environments: a review of recent recommendations, considerations, and literature

This study is a meta-analysis of recent global research articles on hydrogeochemical modelling of permafrost regions to determine trends and consensus on research gaps and future research directions. The hydrogeochemical response of permafrost to climate change remains challenging to estimate and forecast despite evidence of large-scale impacts on freshwater and ecological cycles. We investigate the feasibility, need, and potential for hydrogeochemical modelling of permafrost landscapes by reviewing recommendations from previous modelling, review, and primer papers, including discussing ways to advance this type of modelling science. Key permafrost hydrogeochemical processes are discussed, including heat transfer and associated freeze–thaw regimes, biogeochemical processes and rates, and surface and subsurface flow. Modelling considerations (i.e., model dimension, scale, heterogeneity, and permafrost zonation) and model parameters are subsequently examined. Finally, limitations and additional considerations for advancing permafrost hydrogeochemical modelling efforts are reviewed. The findings of this review are summarized in recommendations, tables, and two schematics incorporating key considerations for future hydrogeochemical modelling initiatives in permafrost environments.

Greenhouse gas fluxes from different types of permafrost regions in the Daxing'an Mountains, Northeast China.

Global warming will increase the greenhouse gas (GHG) fluxes of permafrost regions. However, little is known about the difference in GHG fluxes among different types of permafrost regions. In this study, we used the static opaque chamber and gas chromatography techniques to determine the fluxes of carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) in predominantly continuous permafrost (PCP), predominantly continuous and island permafrost (PCIP), and sparsely island permafrost (SIP) regions during the growing season. The main factors causing differences in GHG fluxes among three types of permafrost regions were also analyzed. The results showed mean CO2 fluxes in SIP were significantly higher than that in PCP and PCIP, which were 342.10 ± 11.46, 105.50 ± 10.65, and 127.15 ± 14.27 mg m-2 h-1, respectively. This difference was determined by soil temperature, soil moisture, total organic carbon (TOC), nitrate nitrogen (NO3--N), and ammonium nitrogen (NH4+-N) content. Mean CH4 fluxes were -26.47 ± 48.83 (PCP), 118.35 ± 46.93 (PCIP), and 95.52 ± 32.86 μg m-2 h-1 (SIP). Soil temperature, soil moisture, and TOC content were the key factors to determine whether permafrost regions were CH4 sources or sinks. Similarly, PCP behaved as the sink of N2O, PCIP and SIP behaved as the source of N2O. Mean N2O fluxes were -3.90 ± 1.71, 0.78 ± 1.55, and 3.78 ± 1.59 μg m-2 h-1, respectively. Soil moisture and TOC content were the main factors influencing the differences in N2O fluxes among the three permafrost regions. This study clarified and explained the differences in GHG fluxes among three types of permafrost regions, providing a data basis for such studies.

The Minimum Temperature Outweighed the Maximum Temperature in Determining Plant Growth over the Tibetan Plateau from 1982 to 2017

The Tibetan Plateau (TP) plays a crucial role in the climate change of China as well as global climate change. It is therefore of great practical significance to study vegetation and its dynamic changes for regional ecological protection. The combination of a dry climate and notable temperature disparities can lead to intricate effects on the region’s vegetation. However, there are few studies exploring the complex effects of diurnal temperature variations on vegetation growth that differ from the effects of mean temperature on the TP, especially under different frozen ground types. Based on the long-time series maximum temperature (Tmax), minimum temperature (Tmin), and Normalized Difference Vegetation Index (NDVI) of the TP, we conducted a comparative study of the warming effects on plant growth under different frozen types. The results exhibit that it warms up faster at night (0.223 °C de−1; p &lt; 0.01) than during the day (0.06 °C de−1; p &lt; 0.01), resulting in a significant decrease in the temperature difference between day and night (−0.078 °C de−1; p &lt; 0.01) in the past few decades. The principal finding of this paper is that Tmin is the dominant temperature indicator for vegetation growth on the TP, which dominates 63.3% of the area for NDVI and 61.4% of the area for GPP, respectively. The results further identify a stronger correlation between air temperature and vegetation growth in seasonal frozen grounds (R = 0.68, p &lt; 0.01) and permafrost regions (R = 0.7, p &lt; 0.01) compared to unfrozen grounds (R = 0.58, p &lt; 0.01). Moreover, the physiological mechanism underlying the asymmetric influence of Tmin and Tmax on vegetation growth is further elucidated in this study. Given that future climate changes are expected to exacerbate these changes, it is imperative to explore additional avenues in pursuit of potential mechanisms that can offer adaptive strategies for safeguarding the ecology of the TP.

Depth-dependent driver of global soil carbon turnover times

Soil carbon fixation has the potential to offset anthropogenic carbon emissions and mitigate climate change. However, the carbon fixation capacity still remains uncertain at the global scale, and little is known about the patterns and controls of soil carbon turnover times. Here we synthesize 5188 radiocarbon measurements at the global scale, and random forest models are applied to assess the key drivers of soil carbon turnover times in different soil layers. We find that across the globe, the mean soil carbon turnover time (τ) is 4178 ± 106 years (mean ± standard error), but the turnover time varies significantly across different regions and land cover types, with the longest values of τ being observed in tundra and the shortest in temperate forests. Longer soil carbon turnover times are observed in the northern permafrost regions, where the mean τ value is nearly twice that of the non-permafrost regions. Furthermore, τ is generally longer in subsoil than that in topsoil across all ecosystems. Moreover, we find the key drivers of τ are depth-dependent. The most important factors affecting topsoil τ are microbes (bacteria, fungi), while soil mineral protection is the major contributor to subsoil τ. These results highlight the necessity to integrate depth-specific soil carbon turnover time and its associated drivers in carbon cycling models into future climate change scenarios.

Environmental spaces for palsas and peat plateaus are disappearing at a circumpolar scale

Abstract. Anthropogenic climate change threatens northern permafrost environments. This compromises the existence of permafrost landforms, such as palsas and peat plateaus, which have been assessed to be critically endangered habitats. In this study, we integrated geospatial datasets and statistical methods to model the suitable environments for palsas and peat plateaus across the Northern Hemisphere permafrost region. The models were calibrated using data from years 1950–2000. The effects of climate change on the suitable environments for the landforms were assessed by using low-, moderate-, and high-emissions scenarios (Representative Concentration Pathway climate scenarios: RCP2.6, RCP4.5, and RCP8.5, respectively) for two periods (2041–2060 and 2061–2080). Hotspots for palsa and peat plateau environments occurred in northern Europe, western Siberia, and subarctic Canada. Climate change was predicted to cause an almost complete loss (decrease of 98.2 %) of suitable environmental spaces under the high-emissions scenario by 2061–2080, while under low- and moderate-emissions scenarios the predicted loss was 76.3 % and 89.3 % respectively. Our modeling results are in line with previously published thermokarst data pointing out areas of recent degradation of palsa and peat plateau environments. Our results provide new insights into the distribution of the permafrost landforms in less studied areas such as central and eastern Siberia. In addition, the predictions provide new understanding of the changing geoecological conditions of the circumpolar region with important implications for greenhouse gas emissions.

Distress Characteristics in Embankment-Bridge Transition Section of the Qinghai-Tibet Railway in Permafrost Regions

The Qinghai-Tibet Railway has been operating safely for 16 years in the permafrost zone and the railroad subgrade is generally stable by adopting the cooling roadbed techniques. However, settlement caused by the degradation of subgrade permafrost in the embankment-bridge transition sections (EBTS) is one of the most representative and severe distresses. A field survey on 440 bridges (including 880 EBTSs) was carried out employing terrestrial laser scanning and ground-penetrating radar for comprehensively assessing all EBTSs in the permafrost zone. The results show that the types of distresses of EBTSs were differential settlement, upheaval mounds of the protection-cone slopes, subsidence of the protection-cone slopes, surface cracks of the protection cones and longitudinal and transverse dislocation of the wing walls. The occurrence rates of these distresses were 78.93, 3.47, 11.56, 3.36, 21.18 and 4.56%, respectively. The most serious problem was differential settlement, and the average differential settlement amount (ADSA) was 15.3 cm. Furthermore, the relationships between differential settlement and 11 influencing factors were examined. The results indicate that ADSA is greater on the northern side of a bridge than on the southern side and on the sunny slope than on the shady slope. It is also greater in the high-temperature permafrost region than in the low-temperature permafrost region and in the high-ice content area than in the low-ice content area. The EBTSs are more influenced by ice content than by ground temperature. The ADSA increases when the embankment height increases, the particle size of subgrade soil decreases and the surface vegetation cover decreases.