Alpine treelines ecotones are critical ecological transition zones and are highly sensitive to global warming. However, the impact of climate on the distribution of treeline trees is not yet fully understood as this distribution may also be affected by other factors. Here, we used high-resolution satellite images with climatic and topographic variables to study changes in treeline tree distribution in the alpine treeline ecotone of the Changbai Mountain for the years 2002, 2010, 2017, and 2021. This study employed the Geodetector method to analyze how interactions between climatic and topographic factors influence the expansion of Betula ermanii on different aspect slopes. Over the past 20 years, B. ermanii, the only tree species in the Changbai Mountain tundra zone, had its highest expansion rate from 2017 to 2021 across all the years studied, approaching 2.38% per year. In 2021, B. ermanii reached its uppermost elevations of 2224 m on the western aspects and 2223 m on the northern aspects, which are the predominant aspects it occupies. We also observed a notable increase in the distribution of B. ermanii on steeper slopes (> 15 degrees) between 2002 and 2021. Moreover, we found that interactions between climate and topographic factors played a more significant role in B. ermanii's expansion than any single dominant factor. Our results suggest that the interaction between topographic wetness index and the coldest month precipitation (Pre(1)), contributing 91% of the observed variability, primarily drove the expansion on the southern aspect by maintaining soil moisture, providing snowpack thermal insulation which enhanced soil temperatures, decomposition, and nutrient release in harsh conditions. On the northern aspect, the interaction between elevation and mean temperature of the warmest month explained 80% of the expansion. Meanwhile, the interaction between Pre(1) and mean temperature of the growing season explained 73% of the expansion on the western aspect. This study revealed that dominant factors driving treeline upward movement vary across different mountain aspects. Climate and topography play significant roles in determining tree distribution in the alpine treeline ecotone. This knowledge helps better understand and forecast treeline dynamics in response to global climate change.

The global cryosphere is retreating under ongoing climate change. The Third Pole (TP) of the Earth, which serves as a critical water source for two billion people, is also experiencing this decline. However, the interplay between rising temperatures and increasing precipitation in the TP results in complex cryospheric responses, introducing uncertainties in the future budget of TP cryospheric water (including glacier and snow water equivalents and frozen soil moisture). Using a calibrated model that integrated multiple cryospheric-hydrological components and processes, we projected the TP cryospheric water budgets under both low and high climatic forcing scenarios for the period 2021-2100 and assessed the relative impact of temperature and precipitation. Results showed (1) that despite both scenarios involving simultaneous warming and wetting, under low climatic forcing, the total cryospheric budget exhibited positive dynamics (0.017 mm yr-1 with an average of 1.77 mm), primarily driven by increased precipitation. Glacier mass loss gradually declined with the rate of retreat slowing, accompanied by negligible declines in the budget of snow water equivalent and frozen soil moisture. (2) By contrast, high climatic forcing led to negative dynamics in the total cryospheric budget (-0.056 mm yr-1 with an average of -1.08 mm) dominated by warming, with accelerated decreases in the budget of all cryospheric components. These variations were most pronounced in higher-altitude regions, indicating elevation-dependent cryospheric budget dynamics. Overall, our findings present alternative futures for the TP cryosphere, and highlight novel evidence that optimistic cryospheric outcomes may be possible under specific climate scenarios.

Permafrost underpins engineering in cold regions but is highly sensitive to climate change. The mechanisms linking climate warming, precipitation changes, and permafrost degradation to infrastructure stability remain poorly understood on the Qinghai-Xizang Plateau (QXP). Here, we present a multi-factor framework to quantify climate impacts on permafrost engineering stability. Our findings reveal a 26.7% decline in permafrost engineering stability from 2015 to 2100, with areas of extremely poor stability expanding by 0.3 x 104 km2 per decade (SSP2-4.5) and 0.6 x 104 km2 per decade (SSP5-8.5). Meanwhile, regions with relatively better stability shrink by 2.0 x 104 km2 and 2.9 x 104 km2 per decade, respectively. These changes driven primarily by a warming and wetting climate pattern. Moreover, engineering stability is maintained in northwestern and interior regions, whereas warmer, ice-saturated areas in the central plateau and southern Qilian Mountains degrade rapidly. Notably, cold permafrost is warming faster than warm permafrost, increasing its vulnerability. These insights provide a critical basis for guiding the future design, construction, and maintenance of permafrost infrastructure, enabling the development of adaptive engineering strategies that account for projected climate change impacts.

The Tibetan Plateau (TP) has experienced accelerated warming in recent decades, especially in winter. However, a comprehensive quantitative study of its long-term warming processes during daytime and nighttime is lacking. This study quantifies the different processes driving the acceleration of winter daytime and nighttime warming over the TP during 1961-2022 using surface energy budget analysis. The results show that the surface warming over the TP is mainly controlled by two processes: (a) a decrease in snow cover leading to a decrease in albedo and an increase in net downward shortwave radiation (snow-albedo feedback), and (b) a warming in tropospheric temperature (850 - 200 hPa) leading to an increase in downward longwave radiation (air warming-longwave radiation effect). The latter has a greater impact on the spatial distribution of warming than the former, and both factors jointly influence the elevation dependent warming pattern. Snow-albedo feedback is the primary factor in daytime warming over the monsoon region, contributing to about 59% of the simulated warming trend. In contrast, nighttime warming over the monsoon region and daytime/nighttime warming in the westerly region are primarily caused by the air warming-longwave radiation effect, contributing up to 67% of the simulated warming trend. The trend in the near-surface temperature mirrors that of the surface temperature, and the same process can explain changes in both. However, there are some differences: an increase in sensible heat flux is driven by a rise in the ground-atmosphere temperature difference. The increase in latent heat flux is associated with enhanced evaporation due to increased soil temperature and is also controlled by soil moisture. Both of these processes regulate the temperature difference between ground and near-surface atmosphere.

Permafrost in marine sediments exhibits a lower freezing point and significant unfrozen water content. This paper investigates the role of the soil freezing characteristic curve (SFCC) in permafrost degradation. Three SFCCs, representing thawing-freezing characteristics of soils with varying clay content and salinity, were established based on experiments and existing data. These SFCCs were then applied in numerical analyses to simulate permafrost thawing under various warming scenarios, using measured ground temperatures and permafrost profiles for a site at Longyearbyen in Svalbard (Norway). It is shown that the ground temperature in non-saline permafrost soil increases more rapidly than saline permafrost, due to a greater downward net heat flux to the permafrost in the former case. Conversely, the thawing rate is more pronounced for saline permafrost soil, attributed to its lower freezing point and latent heat consumption. A more nonlinear ice-melting process is observed for permafrost soil with a lower salinity. The temperature rise follows three stages: a constant-rising, a damp-rising, and an accelerated-rising rates. The duration of the damp-rising rate becomes shorter for saline permafrost under a great warming condition. The study underscores the high significance of the soil-freezing characteristic curve for accurate estimations of permafrost degradation.

This study analyzes the forest flammability hazard in the south of Tyumen Oblast (Western Siberia, Russia) and identifies variation patterns in fire areas depending on weather and climate characteristics in 2008-2023. Using correlation analysis, we proved that the area of forest fires is primarily affected by maximum temperature, relative air humidity, and the amount of precipitation, as well as by global climate change associated with an increase in carbon dioxide in the atmosphere and the maximum height of snow cover. As a rule, a year before the period of severe forest fires in the south of Tyumen Oblast, the height of snow cover is insignificant, which leads to insufficient soil moisture in the following spring, less or no time for the vegetation to enter the vegetative phase, and the forest leaf floor remaining dry and easily flammable, which contributes to an increase in the fire area. According to the estimates of the CMIP6 project climate models under the SSP2-4.5 scenario, by the end of the 21st century, a gradual increase in the number of summer temperatures above 35 degrees C is expected, whereas the extreme SSP5-8.5 scenario forecasts the tripling in the number of such hot days. The forecast shows an increase of fire hazardous conditions in the south of Tyumen Oblast by the late 21st century, which should be taken into account in the territory's economic development.

Thermokarst landslide (TL) activity in the Qinghai-Tibet Plateau (QTP) is intensifying due to climate warminginduced permafrost degradation. However, the mechanisms driving landslide formation and evolution remain poorly understood. This study investigates the spatial distribution, annual frequency, and monthly dynamics of TLs along the Qinghai-Tibet engineering corridor (QTEC), in conjunction with in-situ temperature and rainfall observations, to elucidate the interplay between warming, permafrost degradation, and landslide activity. Through the analysis of high-resolution satellite imagery and field surveys, we identified 1298 landslides along the QTEC between 2016 and 2022, with an additional 386 landslides recorded in a typical landslide-prone subarea. In 2016, 621 new active-layer detachments (ALDs) were identified, 1.3 times the total historical record. This surge aligned with unprecedented mean annual and August temperatures. The ALDs emerged primarily between late August and early September, coinciding with maximum thaw depth. From 2016 to 2022, 97.8 % of these ALDs evolved into retrogressive thaw slumps (RTSs), identified as active landslides. Landslides typically occur in alpine meadows at moderate altitudes and on gentle northward slopes. The thick ice layer near the permafrost table serves as the material basis for ALD occurrence. Abnormally high temperature significantly increased the active layer thickness (ALT), resulting in melting of the ice layer and formation of a thawed interlayer, which was the direct causing factor for ALD. By altering the local material, micro-topography, and thermal conditions, ALD activity significantly increases RTS susceptibility. Understanding the mechanisms of ALD formation and evolution into RTS provides a theoretical foundation for infrastructure development and disaster mitigation in extreme environments.

Seasonally frozen ground (SFG) is a significant component of the cryosphere, and its extent is gradually increasing due to climate change. The hydrological influence of SFG is complex and varies under different climatic and physiographic conditions. The summer rainfall dominant climate pattern in Qinghai Lake Basin (QLB) leads to a significantly different seasonal freeze-thaw process and groundwater flow compared to regions with winter snowfall dominated precipitation. The seasonal hydrological processes in QLB are not fully understood due to the lack of soil temperature and groundwater observation data. A coupled surface and subsurface thermal hydrology model was applied to simulate the freeze-thaw process of SFG and groundwater flow in the QLB. The results indicate that SFG begins to freeze in early November, reaches a maximum freezing depth of approximately 2 meters in late March, and thaws completely by June. This freeze-thaw process is primarily governed by the daily air temperature variations. During the early rainy season from April to June, the remaining SFG in deep soil hinders the majority of rainwater infiltration, resulting in a two-month delay in the peak of groundwater discharge compared to scenario with no SFG present. Colder conditions intensify this effect, delaying peak discharge by 3 months, whereas warmer conditions reduce the lag to 1 month. The ice saturation distribution along the hillslope is affected by topography, with a 10 cm deeper ice saturation distribution and 3 days delay of groundwater discharge in the steep case compared to the flat case. These findings highlight the importance of the freeze-thaw process of SFG on hydrological processes in regions dominated by summer rainfall, providing valuable insights into the hydro-ecological response. Enhanced understanding of these dynamics may improve water resource management strategies and support future research into climate-hydrology interactions in SFG-dominated landscapes.

The degradation of permafrost in the Northern Hemisphere is expected to persist and potentially worsen as the climate continues to warm. Thawing permafrost results in the decomposition of organic matter frozen in the ground, which stores large amounts of soil organic carbon (SOC), leading to carbon being emitted into the atmosphere in the form of carbon dioxide and methane. This process could potentially contribute to positive feedback between global climate change and permafrost carbon emissions. Accurate projections of permafrost thawing are key to improving our estimates of the global carbon budget and future climate change. Using data from the latest generation of climate models (CMIP6), this paper explores the challenges involved in assessing the annual active layer thickness (ALT), defined as the maximum annual thaw depth of permafrost, and estimated carbon released under various Shared Socioeconomic Pathway (SSP) scenarios (SSP1-2.6, SSP2-4.5, SSP3-7.0 and SSP5-8.5). We find that the ALT estimates derived from CMIP6 model soil temperatures show significant deviations from the observed ALT values. This could lead to inconsistent estimates of carbon release under climate change. We propose a simplified approach to improve the estimate of the changes in ALT under future climate projections. These predicted ALT changes, combined with present-day observations, are used to estimate vulnerable carbon under future climate projections. CMIP6 models project ALT changes of 0.1-0.3 m per degree rise in local temperature, resulting in an average deepening of approx. 1.2-2.1 m in the northern high latitudes under different scenarios. With increasing temperatures, permafrost thawing starts in Southern Siberia, Northern Canada, and Alaska, progressively extending towards the North Pole by the end of the century under high emissions scenarios (SSP5-8.5). Using projections of ALT changes and vertically resolved SOC data, we estimate the ensemble mean of decomposable carbon stocks in thawed permafrost to be approximately 115 GtC (gigatons of carbon in the form of CO2 and CH4) under SSP1-2.6, 180 GtC under SSP2-4.5, 260 GtC under SSP3-7.0, and 300 GtC under SSP5-8.5 by the end of the century.

Arctic fjords are hotspots of marine carbon burial, with diatoms playing an essential role in the biological carbon pump. Under the background of global warming, the proportion of diatoms in total phytoplankton communities has been declining in many high-latitude fjords due to increased turbidity and oligotrophication resulting from glacier melting. However, due to the habitat heterogeneity among Svalbard fjords, diatom responses to glacier melting are also expected to be complex, which will further lead to changes in the biological carbon pumping and carbon sequestration. To address the complexity, three short sediment cores were collected from three contrasting fjords in Svalbard (Krossfjorden, Kongsfjorden, Gronfjorden), recording the history of fjord changes in recent decades during significant glacier melting. The amino acid molecular indicators in cores K4 and KF1 suggested similar organic matter degradation states between these two sites. In contrast to the turbid Kongsfjorden and Gronfjorden, preserved fucoxanthin in Krossfjorden indicated a continuous increase in diatoms since the mid-1980s, corresponding to a 59 % increase in biological carbon pumping, as quantified by the delta C-13 of sedimentary organic carbon. The increasing biological carbon pumping in Krossfjorden is further attributed to its hard rock types in the glacier basin, compared to Kongsfjorden and Gronfjorden, which are instead covered by soft rocks, as confirmed by a one-dimensional model. Given the distribution of rock types among basins in Svalbard, we extrapolate our findings and propose that approximately one-fifth of Svalbard's fjords, especially those with hard rock basins and persistent marine-terminated glaciers, still have the potential for an increase in diatom fractions and efficient biological carbon pumping. Our findings reveal the complexity of fjord phytoplankton responses and biological carbon pumping to increasing glacier melting, and underscore the necessity of modifying Arctic marine carbon feedback to climate change based on results from fjords underlain by hard rocks.