Thawing Index Research Articles

Spatiotemporal variations of permafrost extent in Mongolia during 1950–2022

Permafrost in Mongolia is located in the transition zone between high-latitude and high-altitude permafrost regions of the Northern Hemisphere, with large temperature differences and complex subsurface characteristics. In this study, the reliability of the skin temperature data from the ERA5-Land product covering Mongolia is assessed via site observations. The ERA5-Land skin temperature dataset shows a cold bias, which is more pronounced in the cold season. Following calibration based on elevation differences, significant improvements are observed at both the annual scale (92 % improvement in RMSE (root mean square error) and 98 % improvement in MBE (mean bias error)) and the seasonal scale (78 % improvement in RMSE and 82 % improvement in MBE). Additionally, the spatial variations in the surface freezing index (SFI) and surface thawing index (STI) are most pronounced in the central and northeastern Mongolia. The SFI exhibits a significant decreasing trend of 7.16 °C·d/year, while the STI shows a significant increasing trend of 4.49 °C·d/year. Furthermore, the permafrost extent in Mongolia is simulated from 1950 to 2022 using the frost number (Fn) model and the temperature on top of permafrost (TTOP) model. The validated results indicate that the accuracy of the Fn model is relatively high, with an overall accuracy of 0.9 and a Kappa coefficient of 0.47. The permafrost extent in Mongolia has declined from 734.7 × 103 km2 in the 1950 s to 480.1 × 103 km2 in the 2010 s, with a prominent decrease of 3.2 × 103 km2/decade after 1994. According to the variations in permafrost extent during past 72 years, the Hovsgol and Khentii Mountain ranges have experienced significant permafrost degradation.

Freezing and thawing characteristics of seasonally frozen ground across China

Observations from 1,047 meteorological stations from September 1, 2006 to August 31, 2015 revealed regional differences in the freezing and thawing processes of seasonally frozen ground (SFG) across China. SFG generally undergoes a one-way freezing process (i.e., top-down), and the stations with a large freeze depth generally experienced long freeze durations. During the thawing process, soil is generally characterized by two-way thawing (i.e., top-down and bottom-up) in the region north of 35′N, especially north of 30′N (except in northeastern China). The onset of thawing from the bottom occurs earlier than that from the top at most stations in the two-way thawing region. The stations exhibiting one-way thawing (i.e., bottom-up) were mainly located on the southern edge of eastern China (east of 110°E) and in southern part of Xinjiang and southeast part of the Qinghai–Tibet Plateau. The freezing process lasts several days to more than four months longer than the soil thawing process, and this difference tends to be larger in high-latitude and high-altitude regions. All of the sites experienced a discontinuous freeze–thaw process, the station-average duration of which was less than a quarter of that of the continuous freeze–thaw process. Strong associations of soil freeze depth with air temperature (as characterized by the air freezing index and air thawing index) implied a dominant influence of air temperature on the soil freeze–thaw process. During the freezing process, this relationship was partially modulated by snow cover in snowy regions, such as northeast China, northwest China, and the eastern Tibetan Plateau. This paper provides the first overview of regional differences in the freezing and thawing processes of SFG over China, and the findings improve our understanding of the soil freeze–thaw process and provide important information to support research into regional landscapes, ecosystems, and hydrological processes.

Air temperature changes in SW Greenland in the second half of the 18th century

Abstract. The thermal conditions of south-western Greenland in the second half of the 18th century were estimated using two unique series of meteorological observations. The first series (Neu-Herrnhut, 1 September 1767 to 22 July 1768, hereinafter 1767–1768) is the oldest long-term series of instrumental measurements of air temperature available for Greenland. The second (Godthaab, September 1784 to June 1792) contains the most significant and reliable data for Greenland for the study period. The quality-controlled and corrected data were used to calculate daily, monthly, seasonal and yearly means. The daily means were further used to calculate day-to-day temperature variability (DDTV), thermal seasons, growing degree days (GDDs), the air thawing index (ATI), positive degree days (PDDs) and air freezing index (AFI) degree days. Air temperature in Godthaab (now Nuuk) was, on average, warmer than the present day (1991–2020) in 1767–1768 and colder in 1784–1792. Compared to the early 20th-century Arctic warming (ETCAW) period, the data for the two sub-periods show that the late 18th century was as warm or even warmer. Except winter 1767/1768, winters and springs in the study period were longer, while summers and autumns were shorter than at present. The analysed climate indices usually did not exceed the maximum and minimum values from 1991–2020. Mean daily air temperature in the studied historical periods rarely exceed ±2 SD (standard deviation) of the long-term mean calculated for the contemporary period. Air temperature distribution was usually close to normal, in both historical and contemporary periods.

Application of the Machine Learning Method to Determine Spring Load Limits and Winter Weight Premium

Freight transportation plays a crucial role in sustaining the Canadian economy. However, heavy truck transportation also puts enormous pressure on roadway networks. Spring Load Restrictions (SLR) are implemented to minimize road damage caused by heavy traffic during the thaw-weakening season, and Winter Weight Premium (WWP) is used to reduce the impact of SLR on trucking operations by allowing higher axle loads in winter. However, existing policies apply fixed dates each year for these restrictions, regardless of the actual structural capacity of the pavement. Different methods have been proposed to improve the application of SLR and WWP; however, they rely mainly on indirect indices, such as the cumulative thawing index and cumulative freezing index, which pose challenges in their calculation. This study explores the practical implementation of machine learning models for accurately determining the start and end dates of SLR and WWP. In a novel approach, machine learning models directly derive the start and end dates of SLR and WWP from frost and thaw depths in the pavement structure which are determined by pavement temperature and moisture content. In contrast to previous studies that neglected the influence of soil moisture content on determining the start and end dates of SLR and WWP, this study examines the variation in soil moisture content to evaluate the validity of existing theories. The findings reveal a high level of agreement between the machine learning model’s estimations of frost and thaw depths and the measured values, with R2 values exceeding 0.91.

Effects of vegetation extreme degradation on soil hydrothermal processes in alpine wet meadow on the central Qinghai–Tibet Plateau

Ongoing climate warming and humidification have triggered a series of environmental responses, including vegetation succession, significant permafrost degradation, hydrological shifts, alterations in water resources, and increased frequency of freeze-thaw events. Notably, vegetation modulates the water cycle, regulates soil temperatures, and sustains permafrost stability. However, the extent to which the degradation of alpine vegetation impacts soil hydrothermal processes in permafrost regions is unclear. Therefore, we measured the soil moisture and temperature of the alpine wet meadow (AWM) and extremely degraded alpine wet meadow (EDAWM) ecosystems within the permafrost regions of the Qinghai–Tibet Plateau in situ. The objectives of this study were to explore the freeze-thaw cycles and hydrothermal dynamics within the active layer and to understand the mechanisms behind the effects of extreme alpine vegetation degradation. The results revealed that the AWM ecosystem had a longer soil freezing duration and a higher soil freezing rate than those of the EDAWM ecosystem. Additionally, the freezing index was higher in EDAWM than that in AWM, while differences in the thawing index were insignificant. The variance in the thaw-freeze ratios between the two ecosystems indicated that extreme vegetation degradation in AWM altered soil heat absorption and dissipation in the plant root zone and the deeper active layer. Moreover, EDAWM exhibited a decrease in soil bidirectional freezing processes, particularly from the permafrost table upwards. The extreme degradation in AWM changed soil physical properties and organic matter content, reducing ground temperatures in the active and permafrost layers of EDAWM, particularly during winter. The reduced heat transfer in EDAWM resulted in an active layer depth 9 cm shallower than that in AWM. Without vegetation cover, soil moisture in EDAWM was more prone to evaporation or deeper infiltration, leading to lower soil moisture content than that in AWM. Furthermore, an increase in soil moisture content decreased temperature in shallow soils in AWM but increased it in shallow soils in EDAWM. In summary, extreme vegetation degradation impaired air-heat exchange in AWM soil. These insights provide a scientific and theoretical basis for predicting permafrost evolution in the Qinghai–Tibet Plateau, highlighting the complex interactions among vegetation degradation, soil hydrothermal processes, and climatic factors.

Risk Assessment of Freezing–Thawing Hazards in the Daxing’anling Forest Region

The Daxing’anling forest region represents a crucial forestry hub in China and confronts some of the nation’s most severe freezing–thawing hazards. This study delved into the temporal trends and spatial distributions of various parameters related to freezing and thawing, including air temperature, ground surface temperature, freezing index, thawing index, and freezing–thawing frequency. Furthermore, this study assessed and delineated freezing–thawing hazards within the research area. The findings revealed a rapid increase in air temperature and ground surface temperature within the Daxing’anling forest region yet a lower rate of increase in ground surface temperature compared to Northeast China. Latitude had the strongest influence on mean annual air temperature, mean annual ground surface temperature, air freezing index, air thawing index, ground surface freezing index, ground surface thawing index, air freezing–thawing frequency, and ground surface freezing–thawing frequency, followed by longitude and elevation. Overall, freezing index, and air freezing–thawing frequency increased from south to north, whereas mean annual air temperature, mean annual ground surface temperature, air thawing index, ground surface thawing index, and ground surface freezing–thawing frequency decreased from south to north. The assessment outcomes underscore the importance of closely monitoring freezing–thawing hazards in regions north of the 50th parallel.

Dynamics of Freezing/Thawing Indices and Frozen Ground from 1961 to 2010 on the Qinghai-Tibet Plateau

Freezing/thawing indices are important indicators of the dynamics of frozen ground on the Qinghai-Tibet Plateau (QTP), especially in areas with limited observations. Based on the numerical outputs of Community Land Surface Model version 4.5 (CLM4.5) from 1961 to 2010, this study compared the spatial and temporal variations between air freezing/thawing indices (2 m above the ground) and ground surface freezing/thawing indices in permafrost and seasonally frozen ground (SFG) across the QTP after presenting changes in frozen ground distribution in each decade in the context of warming and wetting. The results indicate that an area of 0.60 × 106 km2 of permafrost in the QTP degraded to SFG in the 1960s–2000s, and the primary shrinkage period occurred in the 2000s. The air freezing index (AFI) and ground freezing index (GFI) decreased dramatically at rates of 71.00 °C·d/decade and 34.33 °C·d/decade from 1961 to 2010, respectively. In contrast, the air thawing index (ATI) and ground thawing index (GTI) increased strikingly, with values of 48.13 °C·d/decade and 40.37 °C·d/decade in the past five decades, respectively. Permafrost showed more pronounced changes in freezing/thawing indices since the 1990s compared to SFG. The changes in thermal regimes in frozen ground showed close relations to air warming until the late 1990s, especially in 1998, when the QTP underwent the most progressive warming. However, a sharp increase in the annual precipitation from 1998 began to play a more controlling role in thermal degradation in frozen ground than the air warming in the 2000s. Meanwhile, the following vegetation expansion hiatus further promotes the thermal instability of frozen ground in this highly wet period.

Permafrost Thermal Responses to Asymmetrical Climate Changes: An Integrated Perspective

AbstractAn integrated perspective of permafrost dynamics is a key bridge between permafrost and global socioeconomic assessments. This study investigated the air temperature changes (1976–2020) among permafrost zones in the Northern Hemisphere and their potential impacts on permafrost. We found that continuous permafrost zones experienced faster warming than other regions. The freezing index declined 724°C‐day while the thawing index increased only 166°C‐day over continuous permafrost zones. This may explain why the temperature of cold permafrost increased rapidly but the active layer thickness changed only slightly. Assuming permafrost carbon emissions arise only from thaw processes may miss a significant source of the emissions. An often‐neglected factor is that cold‐season snow amplifies permafrost warming caused by summertime air temperature changes. Due to seasonal effects, using mean‐annual air temperature to depict permafrost evolution under integrated assessment frameworks may lead to significant errors.

Study on Temporal and Spatial Variation in Soil Temperature in Artificial Ground Freezing of Subway Cross Passage

Temperature is the fundamental variable used in artificial ground freezing (AGF) design to assess the frozen curtain’s state and carry out an analysis of its mechanical behavior. Therefore, it is important to appropriately understand the temperature variation in the soil during freezing and thawing throughout the construction process of AGF. In this paper, a soil physical state analysis model was established to obtain the one-dimensional distribution curve of the soil temperature field and the temperature variation curve of the soil with temporal, which can be used to explore the soil temperature characteristics during the construction of AGF, so as to scientifically evaluate the physical state of frozen soil and reduce the construction risk. The soil can be divided into three zones during the construction process of AGF, namely the frozen zone, the frozen fringe, and the unfrozen zone. Additionally, Hangzhou muddy silty clay was selected for the indoor model test to verify the correctness of the analytical model. The results show the following: (1) Due to the influence of the latent heat of the phase change, the physical state and temperature of the soil on both sides of the frozen fringe are not the same. It is not appropriate to use the same temperature index to judge whether the soil is frozen or thawed in the project. (2) For Hangzhou muddy silty clay, the freezing index is −1 °C, and the thawing index is 0.9 °C. The research results can provide some guidance for the judgment of the soil state during the AGF of subway cross passages.

Improved permafrost stability by revegetation in extremely degraded grassland of the Qinghai-Tibetan Plateau

Revegetation is an effective approach for restoring extremely degraded grassland (DG) in the Qinghai-Tibetan Plateau (QTP). However, little is known about its effects on permafrost stability. Our study investigated changes in the characteristics of DG and revegetated grassland (RG) in alpine permafrost regions of the QTP by means of in situ monitoring and sampling. Compared with DG, soil temperature was lower in warm months and slightly higher in cool months both at 2 and 10 cm depths after revegetation, while soil moisture generally decreased. Revegetation advanced the onset and increased the duration of completely frozen stage. The number of freeze–thaw days decreased at 2 cm but increased at 10 cm depth. The freeze–thaw strength weakened at 2 cm depth in spring and autumn, and at 10 cm depth in autumn, but increased at 10 cm depth in spring. The thawing index at the two depths and active layer thickness in RG were also significantly lower than those in DG. Revegetation significantly affected the particle size distribution and stability of soil aggregates by increasing the proportion of large macroaggregates. Thus, revegetation can effectively improve the permafrost stability of degraded grassland in the QTP and enhance the service functions of alpine grassland ecosystems.

Ground Surface Freezing and Thawing Index Distribution in the Qinghai-Tibet Engineering Corridor and Factors Analysis Based on GeoDetector Technique

The land surface temperature obtained from remote sensing was widely used in the simulation of permafrost mapping instead of air temperature with the rapid development of remote sensing technology. The land surface freezing and thawing index (LFI and LTI), which is commonly regarded as the ground surface freezing and thawing index (GFI and GTI), can produce certain errors in the simulation of permafrost distribution on the Qinghai–Tibet Plateau. This paper improved the accuracy of the thermal condition of the surface soil in the Qinghai–Tibet Engineering Corridor (QTEC) by calculating the LFI (or LTI) and N-factors. The environmental factors affecting the spatial distribution of the GFI and GTI were detected by the GeoDetector model. Finally, the multiple linear relationships between the GFI (or GTI) and the environmental factors were established. The results from 25 monitoring sites in the QTEC show that the Nf (ratio of GFI to LFI) is 1.088, and the Nt (ratio of GTI to LTI) is 0.554. The explanatory power of the interaction between elevation and latitude for the GFI and GTI is 79.3% and 85.6%, respectively. The multiple linear regression model with six explanatory variables established by GFI (or GTI) has good accuracy. This study can provide relatively accurate upper boundary conditions for the simulation of permafrost distribution in the QTEC region.

Spatial–Temporal Characteristics of Freezing/Thawing Index and Permafrost Distribution in Heilongjiang Province, China

Under the trend of climate warming, the high-latitude permafrost in Heilongjiang Province is becoming seriously degraded. The question of how to quantitatively analyze the spatial and temporal trends of multi-year permafrost has become fundamental for current permafrost research. In this study, the temporal and spatial variations of annual mean air temperature (MAAT), annual mean ground temperature (MAGST) and freezing/thawing index based on air and surface temperature data from 34 meteorological stations in Heilongjiang Province from 1971–2019, as well as the variation characteristics of permafrost distribution, were analyzed based on the freezing index model. The results showed that both MAAT and MAGST in Heilongjiang Province tended to decrease with the increase of altitude and latitude. For interannual variation, the MAAT and MAGST warming rates tended to be consistent across Heilongjiang Province, with multi-year variation from −8.64 to 5.60 °C and from −6.52 to 7.58 °C, respectively. From 1971–2019, the mean annual air freezing index (AFI) and ground surface freezing index (GFI) declined at −5.07 °C·d·a−1 and −5.04 °C·d·a−1, respectively, whereas the mean annual air thawing index (ATI) and ground surface thawing index (GTI) were elevated at 7.63 °C·d·a−1 and 11.89 °C·d·a−1, respectively. The spatial distribution of the multiyear mean AFI, ATI, GFI and GTI exhibited a latitudinal trend, whereas the effect of altitude in the northern mountainous areas was greater than that of latitude. Permafrost was primarily discovered in the Daxing’an and Xiaoxing’an Mountains in the north, and sporadically in the central mountainous regions. The southern boundary of permafrost shifted nearly 2° to the north from 1970 to 2010s, while the southern boundary of permafrost in Heilongjiang Province was stable at nearly 51° N. The total area of permafrost narrowed from 1.11 × 105 km2 in the 1970s to 6.53 × 104 km2 in the 2010s. The results of this study take on a critical significance for the analysis of the trend of perennial permafrost degradation at high latitudes in Heilongjiang Province and the whole northeastern China, as well as for mapping the distribution of large areas of permafrost using the freezing index model. This study provides a reference for natural cold resource development, ecological protection, climate change and engineering construction and maintenance in permafrost areas.

Response of Freezing/Thawing Indexes to the Wetting Trend under Warming Climate Conditions over the Qinghai -Tibetan Plateau during 1961–2010: A Numerical Simulation

AbstractSince the 1990s, the Qinghai-Tibetan Plateau (QTP) has experienced a strikingly warming and wetter climate that alters the thermal and hydrological properties of frozen ground. A positive correlation between the warming and thermal degradation in permafrost or seasonally frozen ground (SFG) has long been recognized. Still, a predictive relationship between historical wetting under warming climate conditions and frozen ground has not yet been well demonstrated, despite the expectation that it will become even more important because precipitation over the QTP has been projected to increase continuously in the near future. This study investigates the response of the thermal regime to historical wetting in both permafrost and SFG areas and examines their relationships separately using the Community Land Surface Model version 4.5. Results show that wetting before the 1990s across the QTP mainly cooled the permafrost body in the arid and semiarid zones, with significant correlation coefficients of 0.60 and 0.48, respectively. Precipitation increased continually at the rate of 6.16 mm decade−1 in the arid zone after the 1990s but had a contrasting warming effect on permafrost through a significant shortening of the thawing duration within the active layer. However, diminished rainfall in the humid zone after the 1990s also significantly extended the thawing duration of SFG. The relationship between the ground thawing index and precipitation was significantly negatively correlated (−0.75). The dual effects of wetting on the thermal dynamics of the QTP are becoming critical because of the projected increases in future precipitation.

Spatial Variability of Active Layer Thickness along the Qinghai–Tibet Engineering Corridor Resolved Using Ground-Penetrating Radar

Active layer thickness (ALT) is a sensitive indicator of response to climate change. ALT has important influence on various aspects of the regional environment such as hydrological processes and vegetation. In this study, 57 ground-penetrating radar (GPR) sections were surveyed along the Qinghai–Tibet Engineering Corridor (QTEC) during 2018–2021, covering a total length of 58.5 km. The suitability of GPR-derived ALT was evaluated using in situ measurements and reference datasets, for which the bias and root mean square error were approximately −0.16 and 0.43 m, respectively. The GPR results show that the QTEC ALT was in the range of 1.25–6.70 m (mean: 2.49 ± 0.57 m). Observed ALT demonstrated pronounced spatial variability at both regional and fine scales. We developed a statistical estimation model that explicitly considers the soil thermal regime (i.e., ground thawing index, TIg), soil properties, and vegetation. This model was found suitable for simulating ALT over the QTEC, and it could explain 52% (R2 = 0.52) of ALT variability. The statistical model shows that a difference of 10 °C.d in TIg is equivalent to a change of 0.67 m in ALT, and an increase of 0.1 in the normalized difference vegetation index (NDVI) is equivalent to a decrease of 0.23 m in ALT. The fine-scale (<1 km) variation in ALT could account for 77.6% of the regional-scale (approximately 550 km) variation. These results provide a timely ALT benchmark along the QTEC, which can inform the construction and maintenance of engineering facilities along the QTEC.

Variation of Ground Surface Freezing/Thawing Index in China under the CMIP6 Warming Scenarios

As an important parameter in permafrost research, the annual ground surface freezing/thawing index is widely used in the variation of permafrost. In addition, it is also an important indicator in climatology, providing a large amount of theoretical basis for the assessment of climate change. Based on the ground surface temperature data recorded at 707 meteorological stations from 1960 to 2020, the ground surface freezing/thawing index in China were calculated. The results showed that over the past six decades, the thawing index has shown an upward trend, whereas the freezing index has shown a downward trend, and the trend is stronger around 2000. The results of the R/S-based analysis indicate that the freezing/thawing index will remain on a decreasing/increasing trend for some time to come. Based on the five warming scenarios published by Coupled Model Intercomparison Project Phase 6 (CMIP6), the spatial–temporal variation characteristics of the ground surface freezing/thawing index in China during 2020–2100 was simulated. It was found that under SSP3-7.0 and SSP5-8.5 scenarios, the freezing/thawing index may be 0 °C-days in 2080 and 2070, respectively, which may imply that the ground surface freezing process in some regions of China may disappear.

Spatial and temporal characteristics of the site-specific N-factor over the Qinghai-Tibet Plateau

The N-factor is a vital intermediate parameter in simulating the distribution and dynamics of permafrost. To successfully model or map the permafrost distribution, it is necessary to fully understand the spatial and temporal characteristics of the N-factor and its correlation with local factors. In this study, we preliminarily analyzed the N-factor on the Qinghai-Tibet Plateau (QTP) after calculating the freezing and thawing indices with the long-term observed near-surface air temperature (Ta) and ground surface temperature (GST) from 66 national meteorological stations. Results demonstrated that the freezing N-factor (Nf) ranges from 0 (including 11 stations with daily GST always above 0 °C) in the southeast to 1.29 (at Nuomuhong) in the northwest. The thawing N-factor (Nt) at most of the stations ranges from 1.20 to 1.70, which is lowest at Batang (1.14) and highest at Wudaoliang (2.59), respectively. Mean Nf decreased from 0.85 to 0.58 at a rate of −0.03/a in 1998–2007, then fluctuated around 0.64 in 2008–2020; while mean Nt did not change so abruptly as that of mean Nf, which varied around 1.45 and 1.65 for the respective periods. The variability of Nf in the east of the QTP is larger than that in the middle part and the changing rate of Nf is negative at most stations. The changing rate of Nt ranges from −0.10/10a at Wudaoliang to 0.03/10a at Zedang in 1980–2020. The stepwise multi-factor linear regression revealed that latitude is a vital factor influencing Nf (R = 0.16, p < 0.01), while Nt is negatively correlated with mean annual air temperature (MAAT) (R = −0.18, p < 0.01), longitude (R = −0.07, p < 0.01) and altitude (R = −0.03, p < 0.01). In the context of climate warming, the decrease in the ground surface freezing index (GFI) and air freezing index (AFI) and the increase in the ground surface thawing index (GTI) and air thawing index (ATI) with the same amounts are likely to cause the decrease of both Nf and Nt. As the start and end of the freezing periods may vary greatly, the change of Nf is larger than that of Nt. We conclude that more synergistic observations of near-surface air, land surface, and ground surface temperatures and their interactions with local factors are required to clarify the spatial heterogeneity and the parameterization of N-factors on the QTP.

Historical and long-term climate trends in warm permafrost regions: A case study of Bethel, AK

Permafrost is very sensitive to climate factors, with warm permafrost particularly vulnerable to climate change. This paper aims to provide a detailed case study of climate trends of a high-latitude Alaskan site underlain by warm permafrost based on historical data and predictions from the Coupled Model Intercomparison Project Phase 5 (CMIP5). The climate data, including air temperature, precipitation, wind speed, and their annual and seasonal trends from 1950 to 2099, are presented for Bethel, Alaska. The downscaled data are compared with the historical data in the overlap period. Both air and design freezing and thawing indices are calculated using various methods. In comparison, the downscaled model predictions align well with air temperature and wind speed records but not for precipitation, indicating the need for further downscaling at a finer resolution. The historical annual temperature records show a warming rate of 0.40 °C per decade, which is predicted to almost double to 0.78 °C per decade in the 21st century, leading to shorter winters and longer summers. The historical annual precipitation increased by a rate of 14.8 mm per decade from 1950 to 2020, and this trend is predicted to continue but at a slightly lower rate. The wind speed decreased consistently from 1984 to 2020 by 0.9 km/h per decade. However, the CMIP5 predicts that such a trend will diminish, with the mean annual wind speed remaining almost unchanged in this century. The air freezing index exhibits a consistent downtrend from 1950 to 2020, whereas the air thawing index remained flat until 1980 and rose afterward; the CMIP5 predicts that both trends will continue in the 21st century. The design air freezing and thawing indices are sensitive to the assessment periods for both methods. An assessment period shorter than 30 years, e.g., ten years, is recommended to find design indices. It is conservative to use the monthly mean air temperature method for evaluating the design air freezing index and apply the extreme event method for the design thawing index.

Spatiotemporal Variations in the Air Freezing and Thawing Index Over the Mongolian Plateau From 1901 to 2019

The Mongolian Plateau is located in the permafrost transitional zone between high-altitudinal and high-latitudinal permafrost regions in the Northern Hemisphere. Current knowledge of the thermal state and changes in the permafrost on the Mongolian Plateau is limited. This study adopted an improved calculation method of the Mongolian Plateau air freezing and thawing index using the monthly air temperature reanalysis dataset from the Climate Research Unit (CRU). The spatial and temporal variation characteristics from 1901 to 2019 were further assessed by the Mann–Kendall (M–K) test and spatial interpolation methods. The results indicate that the spatial distributions of the freezing and thawing index show clear latitudinal zonality. Over the study period, the air freezing index decreased by 4.1°C·d/yr, and the air thawing index increased by 2.3°C·d/yr. The change point in the air thawing index appeared in 1995 (p &amp;lt; 0.05) based on the M–K method, in contrast to the so-called hiatus in global warming. Our results reveal rapid warming on the Mongolian Plateau, especially in the permafrost region, and are useful for studying permafrost changes on the Mongolian Plateau.

Climate warming in the Qinghai-Tibet Plateau and Mongolia as indicated by air freezing and thawing indices

The majority of the Qinghai-Tibet Plateau (QTP) and Mongolia are underlain by permafrost. We have examined trends in air temperature and associated freezing/thawing index by using a non-parametric statistical method for the QTP and Mongolia from 1961 to 2011. The annual air temperature and associated freezing/thawing index exhibit similar patterns, suggesting similar warming trend in the two regions. The annual warming trends of air temperature are 0.33 °C/decade and 0.37 °C/decade in the QTP and Mongolia, respectively. The freezing index show significantly decreasing trends with −56.7 °C·days/decade and −57.5 °C·days/decade, while the thawing index present obvious increasing trends of 68.2 °C·days/decade and 68.3 °C·days/decade in the QTP and Mongolia, respectively. We find that the variations of air temperature and freezing/thawing index exhibit prominent spatial heterogeneity, and the warming trends is attributed to different seasonal warming. The warming trends in the QTP are dominated by winter warming, it is coincide with previous studies. Contrary to the QTP, autumn warming mainly accounts for the warming trends in the Mongolia. In addition, a winter cooling trend is observed in the Mongolia during the last two decades. These findings will be helpful to better understand the spatial heterogeneity of permafrost changes.

Phenological Changes in Alpine Grasslands and Their Influencing Factors in Seasonally Frozen Ground Regions Across the Three Parallel Rivers Region, Qinghai-Tibet Plateau

Alpine ecosystems are sensitive and vulnerable to climate change. In this study, we extracted three phonological parameters, viz. the start of the growing season (SOS), the end of the growing season (EOS), and the length of the growing season (LOS) from the moderate resolution imaging spectroradiometer (MODIS) normalized difference vegetation index (NDVI) dataset during 2000–2019 for alpine grasslands across the three parallel rivers region (TPRR), and also examined the spatiotemporal heterogeneity of the three phenological parameters in seasonally frozen ground regions (hereafter referred to as SFGR) in response to climate change. The results showed that the multiyear mean SOS gradually delayed from 100 to 160 days along higher elevation, EOS advanced as a whole (from 320 to 280 days), and LOS shortened (from 210 to 130 days). The multiyear mean spatial distribution of all the three parameters showed significant north-to-south differences in the TPRR. In general, the variation trends in all the phenological parameters were not significant (p &amp;gt; 0.05) in the past 20 years, where SOS was advanced by 0.16 days year−l, EOS delayed by 0.08 days year−l, and consequently LOS extended at a rate of 0.07 days year−l, likely due to the warming and drying climate during the study period. In addition, annual mean temperature (AMT) was negatively correlated with SOS (50.26%) and positively correlated with EOS and LOS (72.25 and 60%, respectively). As for the annual mean total precipitation (AMP), it was positively correlated with SOS and EOS (50.77 and 52.97%, respectively) and negatively correlated with LOS (52.07%). Furthermore, a higher freezing index led to a delayed SOS and an advanced EOS and a shortened LOS at the regional scale. Similar to AMT, a higher thawing index advanced the SOS, delayed the EOS, and extended the LOS. Our results confirmed the importance of climate and permafrost status on vegetation phenological processes and also contributed toward understanding the response of cold region ecosystems to global climate change.