Black carbon (BC) aerosols are short-lived climate pollutants with important, but uncertain, climate impacts. In this Review, we synthesize observations of atmospheric BC concentrations, sources, optical properties, lifetimes and climate effects, drawing comparisons with atmospheric model simulations. Isotopic fingerprinting reveals regional differences in BC sources, with biomass burning contributing 93 +/- 3% in sub-Saharan Africa, 56 +/- 7% in South Asia and 28 +/- 5% in East Asia. Atmospheric BC loadings have declined in South America, East Asia, Europe and North America, and stabilized in Africa and South Asia owing to clean air policies and advances in technology and practices. The optical properties of BC influence its climate effects. The global-mean mass absorption coefficient (MAC550) of atmospheric BC is 12.3 +/- 5.8 m2 g-1, being highest in Africa, Europe and South Asia. MAC550 is enhanced near universally by 1.6 +/- 0.4 owing to ageing during long-range transport. In major emission regions, the aerosol absorption optical depth and the direct aerosol radiative forcing ratio between the bottom and the top of the atmosphere are lower in model simulations than in observations by factors of 2 and 1.5, respectively. Relative to long-term observations, model simulations estimate higher BC deposition fluxes but lower concentrations and sunlight absorption. These discrepancies have implications for the accuracy of model representations of humidity, clouds, precipitation and climate forcing. Future research should prioritize comparisons of emission inventory and model estimates with observations to enhance model accuracy and guide mitigation efforts.

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.

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.

The response of Arctic vegetation to climate warming exhibits pronounced spatial heterogeneity, driven partly by widespread permafrost degradation. However, the role of thermokarst lake development in mediating vegetation-climate interactions remains poorly understood, particularly across heterogeneous landscapes of northeastern Siberia. This study integrated multi-source remote sensing data (2001-2021) with trend analysis, partial correlation, and a Shapley Additive Explanation (SHAP)-interpreted random forest model to examine the drivers of normalized difference vegetation index (NDVI) variability across five levels of thermokarst lake coverage (none, low, moderate, high, very high) and two vegetation types (forest, tundra). The results show that although greening dominates the region, browning is disproportionately observed in areas with high thermokarst lake coverage (>30%), highlighting the localized reversal of regional greening trends under intensified thermokarst activity. Air temperature was identified as the dominant driver of NDVI change, whereas soil temperature and soil moisture exerted secondary but critical influences, especially in tundra ecosystems with extensive thermokarst lake development. The relative importance of these factors shifted across thermokarst lake coverage gradients, underscoring the modulatory effect of thermokarst processes on vegetation-climate feedbacks. These findings emphasize the necessity of incorporating thermokarst dynamics and landscape heterogeneity into predictive models of Arctic vegetation change, with important implications for understanding cryospheric hydrology and ecosystem responses to ongoing climate warming.

Ground ice, cryostratigraphical and sediment analyses have been done on samples from 16 boreholes covering the different landforms in the lower part of the valley Longyeardalen, where the largest settlement in Svalbard, Longyearbyen, is located. This allows the production of the first ever top 1 m permafrost ice content map showing the spatial distribution of ground ice (excess ice content) for the Longyearbyen area based on the collected ground ice data and the quaternary geology map of the valley. The valley was infilled since deglaciation with up to 45 m of mainly alluvial sediment and marine mud, whereas colluvial and till deposits with thicknesses from less than 1 m to more than 7 m are dominating the hillsides surrounding the valley. Rock glaciers and ice cored moraines are the landforms with the highest ice content, with assumed over 20% excess ice in the top metre of permafrost. Till and solifluction material has a medium ice content with 10%-20% excess ice content, whereas colluvial deposits have a low ice content with 5%-10% excess ice content. These landforms all have an active layer thickness between 1.6 and 2.2 m. Alluvial deposits in the valley floor has the lowest ice content with 0%-2% excess ice content. Pore ice, suspended ice and reticulate cryostructures dominates the ground ice types, with layered, lenticular and porphyritic cryostructures also present. Marine sediments are widespread and only found in the lower parts of the valley beneath the marine limit. These findings are important to understand and to be prepared for increased landslide risk that is expected due climate warming thawing the top of permafrost and bringing more rainfall in the near future.

Ecosystem carbon use efficiency (CUE) is a key indicator of an ecosystem's capacity to function as a carbon sink. While previous studies have predominantly focused on how climate and resource availability affect CUE through physiological processes during the growing season, the role of canopy structure in regulating carbon and energy exchange, especially its interactions with winter climate processes and nitrogen use efficiency (NUE) in shaping ecosystem CUE in semi-arid grasslands, remains insufficiently understood. Here, we conducted a 5-year snow manipulation experiment in a temperate grassland to investigate the effects of deepened snow on ecosystem CUE. We measured ecosystem carbon fluxes, soil nitrogen concentration, species biomass, plants' nitrogen concentration, canopy height and cover and species composition. We found that deepened snow increased soil nitrogen availability, while the concurrent rise in soil moisture facilitated nutrient acquisition and utilization. Together, these changes supported greater biomass accumulation per unit of nitrogen uptake, thereby enhancing NUE. In addition, deepened snow favoured the dominance of C3 grasses, which generally exhibit higher NUE and greater height than C3 forbs, providing a second pathway that further elevated community-level NUE. The enhanced NUE, through both physiological efficiency and compositional shifts, promoted biomass production and facilitated the development of larger canopy volumes. Larger canopy volumes under deepened snow increased gross primary production through improved light interception, while the associated increase in autotrophic maintenance respiration was moderated by higher NUE. Besides, denser canopies reduced understorey temperatures throughout the day, particularly at night, thereby suppressing heterotrophic respiration. Ultimately, deepened snow increased ecosystem CUE by enhancing carbon uptake while limiting respiratory carbon losses. Synthesis. These findings demonstrated the crucial role of biophysical processes associated with canopy structure and NUE in regulating ecosystem CUE, which has been largely overlooked in previous studies. We also highlight the importance of winter processes in shaping carbon sequestration dynamics and their potential to modulate future grassland responses to climate change.

The Three-Rivers Headwater Region (TRHR) is located on the Tibetan Plateau, within a transitional zone between seasonally frozen ground and continuous permafrost. Over 70 % of the region is predominantly covered by alpine grasslands, a vulnerable ecosystem increasingly threatened by ongoing permafrost degradation. This study utilized satellite data to analyze permafrost degradation by measuring active layer thickness (ALT) and the soil non-frozen period (NFP), and to investigate their impacts on alpine grassland growth. Results showed significant permafrost degradation from 2000 to 2020, with ALT thickening at a rate of 7.79 cm/decade (p < 0.05) and NFP lengthening by 1.1 days/yr (p < 0.05). Simultaneously, grassland vegetation exhibited a significant greening trend (0.0014 yr(-1), p < 0.01). Using the partial least squares (PLS) regression method, the study evaluated the relationships between grassland dynamics and permafrost degradation, while jointly accounting for climate variables (temperature, precipitation, and sunshine duration). ALT thickening was the dominant explanatory variable for grassland growth in 11.09 % of the region, and it was positively correlated in relatively cold western and alpine areas, but negatively correlated in the relatively warm eastern and central regions. NFP extension was the dominant explanatory variable for grassland growth in 10.38 % of the region, although its positive correlation weakened as climate conditions transitioned from relatively cold-dry to relatively warm-wet. Although permafrost degradation was positively correlated with grassland greening in relatively cold regions, the diminishing benefit of NFP extension and the adverse effects of ALT thickening may increasingly undermine grassland stability in relatively warm regions under further climate warming.

Thawing permafrost, driven by climate change and anthropogenic activities, presents escalating challenges for infrastructure in northern regions. Traditional design strategies focused on maintaining frozen ground are increasingly unsustainable due to accelerated warming and extreme weather events. This paper introduces a Thaw Settlement Evaluation Framework for assessing thaw settlement at regional and site-specific scales. The framework comprises (1) regional thaw settlement hazard assessments to identify zones prone to thaw settlement and (2) site-specific thaw strain estimations for quantitative settlement predictions. By consolidating established methodologies, such as thaw hazard indices and empirical thaw strain estimation tools, the framework provides adaptable workflows tailored to the specific needs and data availability of the project. Key outputs include thaw settlement hazard levels, thaw strain estimation, and stress-strain characterization, which together offer actionable insights to support infrastructure planning, design, and maintenance in permafrost regions. By consolidating resources and streamlining decision-making, the framework equips practitioners, planners, and decision-makers with practical guidance to address challenges of thawing permafrost.

The reasonable value of good gradation characteristic parameters is key in designing and optimising soil-rock mixed high fill embankment materials. Firstly, the DJSZ-150 dynamic-static large-scale triaxial testing instrument was used for triaxial compression shear tests on compacted skeleton structure soil-rock mixture standard specimens. The changes in strength and deformation indicators under different gradation parameters and confining pressure were analysed. Then, based on the Janbu empirical formula, relationships between parameters K, n, and (sigma 1-sigma 3)ult and the coefficient of uniformity Cu and coefficient of curvature Cc were explored. Empirical fitting formulas for Duncan-Chang model constants a and b were proposed, establishing an improved Duncan-Chang model for soil-rock mixtures considering gradation characteristics and stress states. Finally, based on significant differences in particle spatial distribution caused by gradation changes, three generalised models of matrix-block stone motion from different particle aggregation forms were proposed. Results indicate the standard specimen's strength and deformation indicators exhibit significant gradation effects and stress-state correlations. The improved Duncan-Chang model effectively simulates the stress-strain relationship curve under different gradations and confining pressure, with its characteristics explainable based on the matrix block stone motion generalised model.

Most gravel roads leading to rural areas in Ghana have soft spot sections as a result of weak lateritic subgrade layers. This study presents a laboratory investigation on a typical weak lateritic subgrade soil reinforced with non-woven fibers. The objective was to investigate the strength characteristic of the soil reinforced with non-woven fibers. The California Bearing Ratio and Unconfined Compressive Strength tests were conducted by placing the fibers in single layer and also in multiple layers. The results showed an improved strength of the soil from a CBR value of 7%. The CBR recorded maximum values of 30% and 21% for coconut and palm fibers inclusion at a placement depth of H/5 from the compacted surface. Multiple fiber layer application at depths of H/5 & 2 h/5 yielded CBR values of 38% and 31% for coconut and palm fibers respectively. The Giroud and Noiray design method and the Indian Road Congress design method recorded reduction in the thickness of pavement of 56% to 63% for coconut fiber inclusion and 45% to 55% for palm fiber inclusion. Two-way statistical analysis of variance (ANOVA) showed significant effect of depth of fiber placement and fiber type on the geotechnical characteristics considered. (sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic),(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic). (sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic). (sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic). (sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic). (sic)(sic)(sic)(sic),CBR(sic)(sic)7%(sic),(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic). (sic)(sic)(sic)(sic)(sic)(sic)(sic)H/5(sic)(sic)(sic)(sic)(sic)(sic),CBR(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)30%(sic)21%. (sic)H/5(sic)2H/5(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic),(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)CBR(sic)(sic)(sic)(sic)38%(sic)31%. Giroud&Noiray(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)56%(sic)63%,(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)45%(sic)55%. (sic)(sic)(sic)(sic)(sic)(sic)(ANOVA)(sic)(sic),(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic)(sic).