By analyzing the last 50-60 years of climate changes in Arctic and Subarctic Yakutia, we have identified three distinct periods of climate development. The cold (1965-1987), pre-warming (1988-2004), and modern warming (2005-2023) periods are clearly identifiable. Yakutia's Arctic and Subarctic regions have experienced mean annual air temperature increases of 2.5 degrees C and 2.2 degrees C, respectively, compared to the cold period. The thawing index rose by an average of 171-214 degrees C-days, while the freezing index dropped by an average of 564-702 degrees C-days. During the pre-warming period, all three characteristics show a minor increase in warmth. Global warming intensified between 2005 and 2023, resulting in elevated permafrost temperatures and a deeper active layer. Monitoring data from the Tiksi site show that warming has been increasing at different depths since the mid-2000s. As a result, the permafrost temperature increased by 1.7 degrees C at a depth of 10 m and by 1.1 degrees C at a depth of 30 m. Soil temperature measurements at meteorological stations and observations at CALM sites both confirm the warming of the permafrost. A permafrost-climatic zoning study was conducted in Arctic and Subarctic Yakutia. Analysis identified seven regions characterized by similar responses to modern global warming. These study results form the foundation for future research on global warming's effects on permafrost and on how northern Yakutia's environment and economy adapt to the changing climate.

The freezing index (FI) is one of the most important indicators that shows the variation of permafrost. However, the relationship between climate change and the thermal conditions of permafrost is not understood well. This study analyzed the variation of FI based on 5-cm soil temperature derived from 74 meteorological stations from 1977 to 2016 on the Qinghai-Tibet Plateau (QTP). Furthermore, the factors affecting the FI variation and its relationship with permafrost degradation were also discussed. The results showed that FI was much smaller in the interior than other areas of the QTP, and it increased at a rate of 53.0 degrees C d/10a during the 40 years. FI in the main body of the QTP was relatively stable than surrounding areas; it was more stable in the northern part than in the southern part. On average, the FI variation coefficient was larger than 10%, indicating the large fluctuation of FI during the 40 years. FI decreased with the increasing altitude; it was more sensitive to the altitude in the south of 33 degrees N than in the north. The variation of FI was closely related to the maximum freezing depth (MFD) and the active layer thickness (ALT). It was observed that MFD decreased and ALT increased by approximately 1.4 cm and 1.6 cm, respectively, with each 10.0 degrees C d increase in FI. The results exhibited the thermal condition variation of the permafrost in QTP and revealed a degrading trend of the permafrost.

The freezing index (FI) is an important index used in investigations of climate change, frozen ground degradation and frost heave resistance engineering design. In view of the fact that the deterministic effects of latitude and elevation are not considered in the frequency calculation of FI, we proposed an index-freezing method that considers the certainty effects of both elevation and latitude by referring to the index-flood method in this paper. The correlations between the FI and certainty factors (elevation and latitude) were obtained by multiple regression analysis. The effects of latitude and elevation were then removed by nondimensionalisation, and dimensionless FI sequences were subsequently obtained. Finally, the index-freezing method was verified by regional probability analysis. Using the daily average temperature data recorded at 10 major meteorological stations over the 1960-2020 period in Ningxia, the calculation process of the FI and its frequency distribution were provided. The results showed that the proposed FI method can not only remove the certainty effects of elevation and latitude but can also consider the uncertainty associated with interannual FI variations, thus providing more scientific, reasonable and accurate results. The generalised extreme value (GEV) distribution is the optimal frequency distribution of the nondimensional regional FI. The estimation errors of the missing data tests were mostly within 10%, and the residual sum of squares (RSS) and root-mean-square error (RMSE) values were also lower than those obtained through spatial interpolation, thus indicating that the interpolation preci-sion of the proposed FI method was optimal.

Due to the complex topography and localized climate, active layer thickening and permafrost warming varied distinctly in different regions on the Qinghai-Tibet Plateau (QTP). Based on the borehole-temperature data at 93 sites from 2012 to 2018, we analyzed the temporal and spatial characteristics of active layer thickness, permafrost temperature, and relevant climatic factors in 8 typical geomorphological units on the QTP. The active layer thickened at 86 sites and thinned at 7 sites. The permafrost warmed at 89 sites and cooled at 4 sites. The median values of the annual increase rate of active layer thickness were from 0.04 to 0.13 m/a for the monitored regions. The highest rate reached 0.46 m/a, indicating severe permafrost degradation in local areas. The mean annual soil temperatures at a 6-m depth generally increased faster for cold permafrost, and the active layer thickened more significantly in warm permafrost sites. Among these regions, Kekexili Mountains showed a lower increase rate of active layer thickness, and the temperature rise of permafrost in the Fenghuoshan Mountains was more significant. The temporal change of snow cover duration was closely related to the active layer thickness variation in the northern permafrost regions on the QTP (Kunlunshan Mountains and Chumaerhe High Plain). In contrast, the temporal variation of freezing index was the dominant factor in the southern regions (Wuli Basin, Tongtianhe Basin, and Tanggula Mountains). No linear correlation between the temporal variations of climatical factors and active layer thickness variation was found for the regions in the middle of QTP (Kekexili Mountains, Beiluhe Basin, and Fenghuoshan Mountains ). The comprehensive effects of freezing index and snow cover duration result in the different relationships between air temperature variation and permafrost change in different regions on the QTP. These findings are beneficial for understanding the relationship between climate change and permafrost evolution.

Mongolia is one of the most sensitive regions to climate change, located in the transition of several natural and permafrost zones. Long-term trends in air freezing and thawing indices can therefore enhance our understanding of climate change. This study focuses on changes of the spatiotemporal patterns in air freezing and thawing indices over Mongolia from 1960 to 2020, using observations at 30 meteorological stations. Our results shows that the freezing index ranges from -945.5 to -4,793.6 degrees C day, while the thawing index ranges from 1,164.4 to 4,021.3 degrees C day over Mongolia, and their spatial patterns clearly link to the latitude and altitude. During the study period, the trend in the thawing index (14.4 degrees C-day per year) was larger than the trend in the freezing index (up to -10.1 degrees C-day per year), which results in the net increase of air temperature by 2.4 degrees C across Mongolia. Overall, the increase in the thawing index was larger in the low latitudes and altitudes (e.g., the Gobi-desert, steppes, the Great lake depression and major river valleys) than in high latitudes and altitudes (mountain regions), while it was the opposite for the freezing index. The highest values for both thawing index and freezing index (i.e. the least negative values) have occurred during the last 2 decades. As the trends in the freezing and thawing indices and mean annual air temperature confirm intensive climate warming, increased permafrost degradation and shallower seasonally frozen ground are expected throughout Mongolia.

Freeze-thaw cycles (FTC) are known to have an effect on railway track stability, safety, and performance. FTC are expected to become more frequent in the future due to climate change. This paper presents the results of a field investigation in which the mechanism of FTC development within the track embankment and its effect on the performance of railway tracks including track surface deformation and track geometry degradation are studied. Field observations suggested that the frost depth within the track embankment is influenced by the freezing index and winter snow cover. They also showed that a warmer and drier winter leads to more intermittent FTC and even though the average frost heave is lower than for a colder winter, the frost heaves occurring at culvert locations creates a larger differential deformation and thus may lead to a worse operating condition. The comparison of geometry measurements before freezing and after thawing indicated that the track geometry is in poorer and rougher condition during springs that were preceded by increased FTC. It was also concluded that track in proximity to culverts suffered the highest geometry degradation. Overall, the limited field observations of this study suggest that future winters, mild with less precipitation and higher occurrence of FTC, may increase the rate of track deterioration and more maintenance will be required to keep track within safe limits.

Changes in soil thermal regimes in cold climates have widespread impacts on hydrology, ecology, and the carbon cycle. The annual freezing and thawing index, which is generally calculated using daily temperature, has been widely used to estimate the freezing depth, active layer thickness, and the distribution of permafrost. However, continuous and reliable daily temperature data are scarce in cold climates, while monthly and annual temperature data are more readily available. If daily temperature data are unavailable, these indices can be estimated based on monthly or annual temperature data. In this study, we developed a resampling method for estimating the annual freezing and thawing index and compared the results with those produced by the existing methods. Daily temperature data with a 0.5 degrees resolution over the Northern Hemisphere during 1901-2012 were used to calculate the freezing/thawing index, and then the monthly and annual temperature were calculated and three different approaches were used to estimate the daily temperature and the freezing/thawing index. When the monthly data were used, the resampling method produced the smallest relative error (RE) and mean bias error (MBE), and the largest correlation in estimating the two indices, compared to the two other methods. Although the annual temperature data usually underestimate the freezing/thawing index, the RE is still <5% over most of the high-latitude regions. The results suggest that if the daily temperature can be reliably estimated using the resampling method, the thermal regimes of permafrost can be reliably estimated using modelled monthly temperature and/or reconstructed past monthly/annual temperature. These estimations can also be used to validate modelled paleo-permafrost and its variations. Additionally, our results indicate that after the 1970s the annual freezing index (DDF) increased substantially, while the frost index (FI) decreased substantially.

The freezing index (FI) is one of the most important indicators that shows the variation of permafrost. However, the relationship between climate change and the thermal conditions of permafrost is not understood well. This study analyzed the variation of FI based on 5-cm soil temperature derived from 74 meteorological stations from 1977 to 2016 on the Qinghai-Tibet Plateau (QTP). Furthermore, the factors affecting the FI variation and its relationship with permafrost degradation were also discussed. The results showed that FI was much smaller in the interior than other areas of the QTP, and it increased at a rate of 53.0 degrees C d/10a during the 40 years. FI in the main body of the QTP was relatively stable than surrounding areas; it was more stable in the northern part than in the southern part. On average, the FI variation coefficient was larger than 10%, indicating the large fluctuation of FI during the 40 years. FI decreased with the increasing altitude; it was more sensitive to the altitude in the south of 33 degrees N than in the north. The variation of FI was closely related to the maximum freezing depth (MFD) and the active layer thickness (ALT). It was observed that MFD decreased and ALT increased by approximately 1.4 cm and 1.6 cm, respectively, with each 10.0 degrees C d increase in FI. The results exhibited the thermal condition variation of the permafrost in QTP and revealed a degrading trend of the permafrost.