The Tibetan Plateau (TP), often referred to as the 'Asian Water Tower', plays a critical role in regulating the hydrological cycle and influencing global climate patterns. Its unique topography and climatic conditions result in pronounced seasonal freeze-thaw (FT) dynamics of the land surface, which are critical for understanding permafrost ecosystem responses to climate change. However, existing studies on FT dynamics over the TP are limited by either short observational periods or deficiency in accuracy, failing to capture the long-term FT processes comprehensively. This study presents a novel satellite-based approach for monitoring the FT dynamics over the TP from 1979 to 2022, utilizing passive microwave observations. We developed a new algorithm that integrates discriminant function algorithm (DFA) with a seasonal threshold algorithm (STA), employing the freeze-thaw index (FTI) as the classification variable to determine optimal FT thresholds. The strong performance of the algorithm was confirmed by in-situ validation, with an overall accuracy of 91.46%, a Kappa coefficient of 0.83, and an F1-score of 0.92, outperforming other remote sensing-derived FT products such as SMAP (OA = 89.44%, Kappa = 0.79, F1 = 0.89). Results reveal significant changes in surface freeze-thaw dynamics over the past four decades. Between 1988-2022, frozen days exhibited a significant decreasing trend of -0.19 daysyear(-)(1), primarily attributed to the delayed freeze onset (0.19 daysyear(-)(1)), while thaw onset showed no significant trend. Spatially, permafrost regions experienced a more pronounced decrease in frozen days and earlier thaw onset compared to seasonally frozen regions. Moreover, marked interannual trend differences in FT processes were observed across elevation gradients, with higher elevations showing more negative trends in frozen days and thaw onset. This study provides a reliable and up-to-date analysis of surface FT process changes over the TP, informed by long-term satellite-based observational perspectives. These analyses revealed marked spatial heterogeneity in surface FT dynamics across the TP region, underscoring the impacts of climate change on the cryosphere and hydrology.

Rapid climate change in the Northern Hemisphere cryosphere threatens ancient permafrost carbon. Once thawed, permafrost carbon may migrate to surface waters. However, the magnitude of permafrost carbon processed by northern freshwater remains uncertain. Here, we compiled '1800 radiocarbon data of aquatic dissolved organic carbon (DOC) and particulate organic carbon (POC) in the Arctic and Qinghai -Tibet Plateau (QTP) to explore the fate of permafrost carbon under climate warming over the past 30 years. We showed that the contribution of aged carbon has significantly increased since 2015. Approximately 70 % of DOC and POC was derived from aged carbon for QTP rivers. In Arctic waters, an average of '67 % of POC was derived from aged carbon, however, '75 % of DOC was derived from modern carbon, mainly due to low temperatures and protection by vegetation limiting the export of aged DOC. For both regions, DOC 14 C age was positively correlated with the active layer thickness, whereas the POC 14 C age was positively correlated with the mean annual ground temperature, suggesting that gradual thaw accelerated the mobilization of aged DOC while abrupt thaw facilitated the export of aged POC. Furthermore, POC 14 C age was positively correlated with the soil organic carbon density, which was attributed to well-developed pore networks facilitated aged carbon output. This study suggests that permafrost carbon release is affected by both permafrost thermal properties and soil organic carbon density, which should be considered in evaluation of permafrost carbon -climate feedback.

Permafrost soils contain approximately twice the amount of carbon as the atmosphere and this carbon could be released with Arctic warming, further impacting climate. Mosses are major component of Arctic tundra ecosystems, but the environmental drivers controlling heat penetration though the moss layer and into the soil and permafrost are still debated, especially at fine spatial scales where microtopography impacts both vegetation and soil moisture. This study measured soil temperature profiles (1-15 cm), summer thaw depth, water table depth, soil moisture, and moss thickness at a fine spatial scale (2 m) together with meteorological variables to identify the most important controls on the development of the thaw depth during two Arctic summers. We found a negative relationship between the green moss thickness (up to 3 cm) and the soil temperatures at 15 cm, suggesting that mosses insulated the soil even at high volumetric water contents (>70%) in the top 5 cm. A drier top (2-3 cm) green moss layer better insulated deep (15 cm) soil layers by reducing soil thermal conductivity, even if the moss layers immediately below the top layer were saturated. The thickness of the top green moss layer had the strongest relationships with deeper soil temperatures, suggesting that the top layer had the most relevant role in regulating heat transfer into deeper soils. Further drying of the top green moss layer could better insulate the soil and prevent permafrost thawing, representing a negative feedback on climate warming, but damage or loss of the moss layer due to drought or fire could reduce its insulating effects and release carbon stored in the permafrost, representing a positive feedback to climate warming.

Soil organic matter (SOM) stability in Arctic soils is a key factor influencing carbon sequestration and greenhouse gas emissions, particularly in the context of climate change. Despite numerous studies on carbon stocks in the Arctic, a significant knowledge gap remains regarding the mechanisms of SOM stabilization and their impact on the quantity and quality of SOM across different tundra vegetation types. The main aim of this study was to determine SOM characteristics in surface horizons of permafrost-affected soils covered with different tundra vegetation types (pioneer tundra, arctic meadow, moss tundra, and heath tundra) in the central part of Spitsbergen (Svalbard). Physical fractionation was used to separate SOM into POM (particulate organic matter) and MAOM (mineral-associated organic matter) fractions, while particle-size fractionation was applied to evaluate SOM distribution and composition in sand, silt, and clay fractions. The results indicate that in topsoils under heath tundra POM fractions dominate the carbon and nitrogen pools, whereas in pioneer tundra topsoils, the majority of the carbon and nitrogen are stored in MAOM fractions. Moreover, a substantial proportion of SOM is occluded within macro-and microaggregates. Furthermore, the results obtained from FTIR analysis revealed substantial differences in the chemical properties of individual soil fractions, both concerning the degree of occlusion in aggregates and across particle size fractions. This study provides clear evidence that tundra vegetation types significantly influence both the spatial distribution and chemical composition of SOM in the topsoils of central Spitsbergen.

Assessing long-term changes in Aerosol Optical Depth (AOD) together with Aerosol Radiative Forcing Efficiency (ARFE, defined as radiative forcing per unit visible AOD) provides critical insight into the evolving role of different aerosol species in regional climate forcing. In this study, we analyse two decades of AOD trends (2001-2020) across eight climatically diverse regions using a multivariate regression framework, and quantify species-specific radiative effects with the Santa Barbara DISORT Atmospheric Radiative Transfer (SBDART) model. The regions were chosen to represent contrasting trends in total AOD. Our results show that sulfate aerosols, which account for the largest share of AOD over India (similar to 36-45 %), are the primary driver of increasing AOD and associated atmospheric warming. Black carbon (BC), although contributing only a minor fraction to total AOD (2-10 %), emerges as the dominant warming agent across most regions, with particularly strong forcing signals over the Middle East. In contrast, sea-salt (SS) aerosols exert the largest cooling influence, most prominently over the Southern African (SAF) region, partially offsetting warming from absorbing species. Europe, despite an overall decline in AOD, exhibits a slight increase in SS that sustains a regional cooling effect. These findings demonstrate that species composition, vertical distribution, and optical properties govern ARFE more strongly than the total AOD magnitude alone. By linking AOD trends with species-resolved radiative forcing efficiency across multiple regions, this study advances the interpretability of ARFE as a climate indicator and highlights its potential to inform policy-relevant assessment of aerosol-driven warming and cooling.

Recent climate warming has accelerated permafrost thaw and dynamics of thermokarst lakes (TLs) on the Tibetan Plateau (TP). Yet, owing to the lack of long-term monitoring of TLs, our understanding of lake evolution processes and their driving factors remains uncertain. Here, using the global surface water product and timeseries Landsat imagery, we identified 58,538 TLs (0.01-3 km2) and determined the primary occurrence year of lake changes from 1990 to 2022. Our results indicated that TLs on the TP are primarily located in the central inland region, over 82 % of lakes experienced area expansion, and only 15 % in the northwest show decrease in area. Annual number of lake expansion peaked in 2016, whereas lake shrinkage was most common in 2019. The calculated lake area errors, field investigations, and validation of lake disturbance time demonstrated high accuracy and consistency. We applied the optimal machine learning regression model to distinguish the different drivers for lake expansion and shrinkage. The topographic and climatic factors are primary drivers for lake expansion, while differences in evaporation trend and soil temperature trend might contribute to lake shrinkage. This study highlights the vulnerability of permafrost on the TP to climate change, which can contribute to carbon sequestration estimation and infrastructure maintenance.

The thermal stability of permafrost, a foundation for engineering infrastructure in cold regions, is increasingly threatened by the dual stressors of climate change and anthropogenic disturbance. This study investigates the dynamics of the crushed rock revetted embankment at the Kunlun Mountain Section of the Qinghai-Tibet Railway, systematically investigating the coupled impacts of climate warming and engineering activities on permafrost thermal stability using borehole temperature monitoring data (2008-2024) and climatic parameter analysis. Results show that under climate-driven effects, the study area experienced an air temperature increase of 0.2 degrees C per decade over the 2015-2024. Concurrently, the mean annual air thawing degree-days (TDD) rose by 13.8 degrees C center dot d/a, leading to active-layer thickening at a rate of 3.8 cm center dot a- 1at natural ground sites. From 2008 to 2024, the active layer had thickened by 0.7-0.8 m. At the embankment toe (BH 5), the active-layer thickening rate (3.3 cm center dot a- 1) was 25 % lower than that at the natural ground borehole (3.8 cm center dot a- 1); correspondingly, the underlying permafrost temperature increase rate at the toe (0.3 degrees C per decade) was lower than that at the natural borehole (0.5-0.6 degrees C per decade). Permafrost warming rates decreased with depth. Shallow layers (above -2 m) were significantly influenced by climate, with warming rates of 0.3-0.6 degrees C per decade. In contrast, deep layers (below -10 m) showed warming rates converging with the background atmospheric temperature trend (0.2 degrees C per decade). Thermal regime disturbance was most pronounced at horizontal distances of 3.0-5.0 m from the embankment. Nevertheless, the crushed-rock revetment maintained a permafrost table 0.6 m shallower than that of natural ground, confirming its thermal diode effect (facilitating convective cooling in winter), which partially offset climate warming impacts. This study provides critical empirical data and validates the cooling mechanism of crushed-rock revetment, which is essential for predicting the long-term thermal stability and informing adaptive maintenance strategies for railway infrastructure in warming permafrost regions.

Near-surface temperature and moisture are key boundary conditions for simulating permafrost distribution, projecting its response to climate change, and evaluating the surface energy balance in alpine regions. However, in desertified permafrost zones of the Qinghai-Tibet Plateau (QTP), the observations remain sparse, and reported trends vary considerably among sites. This lack of consistent evidence limits the ability to represent microenvironmental processes in models and to predict their influence on permafrost stability. From September 2021 to August 2024, we conducted continuous observations at a desertified permafrost site on the central QTP, covering the vertical range from 150 cm above to 100 cm below the ground surface (boundary layer). Measurements included air and ground temperature, air humidity, soil moisture, wind speed, and net radiation. Results showed that the mean annual air temperature increased with decreasing height at a gradient of approximately 0.42 degrees C/m, while mean annual air humidity remained nearly constant at 56.8 +/- 1.1 % (150-0 cm). In the near-surface soil layer (0 similar to -10 cm), temperature rose by 3.6 +/- 0.1 degrees C and moisture decreased by 34.0 +/- 2.7 %. The mean annual ground temperature increased with depth at a rate of about 0.55 degrees C/m, whereas soil moisture decreased between -20 and -60 cm (52.86 %/m) and increased between -60 and -100 cm (56.30 %/m). Seasonal patterns showed marked difference: in the freezing season, the calculated total temperature increment within the boundary layer (1.91 degrees C) was 61 % lower than the observed value (4.88 degrees C), while in the thawing season, it was 58 % higher (4.38 degrees C > 2.77 degrees C). These results reveal strong vertical gradients and seasonal contrasts in thermal and moisture regimes, emphasizing the need to integrate coupled temperature-moisture processes into boundary layer parameterizations for cold-region environments. Improved representations can enhance permafrost modeling and inform infrastructure design in regions experiencing both warming and desertification.

Accurate soil thermal conductivity (STC) data and their spatiotemporal variability are critical for the accurate simulation of future changes in Arctic permafrost. However, in-situ measured STC data remain scarce in the Arctic permafrost region, and the STC parameterization schemes commonly used in current land surface process models (LSMs) fail to meet the actual needs of accurate simulation of hydrothermal processes in permafrost, leading to considerable errors in the simulation results of Arctic permafrost. This study used the XGBoost method to simulate the spatial-temporal variability of the STC in the upper 5 cm active layer of Arctic permafrost during thawing and freezing periods from 1980 to 2020. The findings indicated STC variations between the thawing and freezing periods across different years, with values ranging from-0.4 to 0.28 W & sdot;m-1 & sdot;K-1. The mean STC during the freezing period was higher than that during the thawing period. Tundra, forest, and barren land exhibited the greatest sensitivity of STC to freeze-thaw transitions. This is the first study to explore the long-term spatiotemporal variations of STC in Arctic permafrost, and these findings and datasets can provide useful support for future research on Arctic permafrost evolution simulations.

Current research predominantly emphasizes elemental carbon (EC) light absorption, while ignoring its relationship with aerosol hygroscopic scattering. In this study, concentrations and optical properties of aerosol components were measured during a full-year monitoring campaign at an urban site in Suzhou. Results from a multiple linear regression model suggested that secondary organic carbon was a primary contributor to high mass absorption efficiency of EC in summer. Through the estimation of aerosol scattering coefficients under both dry and ambient atmospheric conditions, it was found that hygroscopic growth accounted for more than 35.0 % of the total aerosol scattering coefficient. Hygroscopic growth of nitrate and sulfate enhanced their annual mean scattering contributions by 42.1 % and 45.2 %, respectively. A negative correlation between EC concentration and the hygroscopic growth factor (f(RH)) was observed under varying relative humidity (RH) conditions. Associated with the decrease in f(RH), reductions in PM2.5 scattering coefficients (14.0 f 2.2, 29.4 f 5.2, and 24.5 f 8.2 Mm-1) were linked to EC concentration increases of 0.37 f 0.1, 0.40 f 0.1, and 0.21 f 0.1 mu g/m3 under low, medium, and high RH conditions, respectively. An increase in EC concentration by 0.19-0.37 mu g/m3 elevated the PM2.5 absorption coefficient by 2.66-5.41 Mm-1, and reduced the scattering coefficient by 10.53-17.91 Mm-1. Collectively, increased EC concentrations reduced aerosol single scattering albedo (SSA), particularly under high RH conditions. This study reveals that EC not only reduces aerosol extinction coefficients but also shifts aerosol radiative forcing in a positive direction by suppressing hygroscopic scattering.