Since 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.

Changes in soil temperature (ST) and soil moisture (SM) are essential for climate change and ecosystem assessments. Previous investigations on the ST and SM on the Tibetan Plateau (TP) are mainly based on the situ observation and the satellite products. In this study, the improved Community Land Model version 4.5 (CLM4.5), with proper parameter optimization and surface datasets update, is used to estimate the response of ST and SM in the TP to climate change in the long‐term time series from 1961 to 2010. After validating the reasonability of the simulated results using the observations, the spatial distribution of changes in ST and SM in annual and seasonal time series since 1960s, 1980s, 1990s, are investigated respectively and the changes of precipitation (Pr) and surface evaporation (Ev) are also analysed to understand the cause of changes objectively. As a whole, the soil was warming and wetting at the maximum value of 0.31°C/decade and 0.77%/decade since the 1960s. However, the warming process in soil mainly occurred in the 1980s while the wetting tendency is detected since the 1990s extensively. Except for the influence of air warming, the enhanced Pr and Ev might also be indispensable factors that caused the intensive wetting process but damped warming process in soil. Summer is the favourable season for the thermal and hydraulic variation since the 1980s. There exists the striking warmer and drier trend in the eastern TP since 1980s while the colder and wetter condition in the western TP since the 1990s. The magnitude of variation in soil is magnified from 1990s under the continuing impact of climate change.

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.

Freezing/thawing indices are important indicators of dynamics of frozen ground on the Qinghai-Tibet Plateau (QTP), especially for 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 of frozen ground distribution in each decade in the context of warming and wetting. The results indicate that an area of 0.60×106 km2 permafrost of QTP degraded to SFG in 1960s-2000s, and the primary shrinkage period occurred in 2000s. The air freezing index (AFI) and ground freezing index (GFI) decreased dramatically at the rates of 71.00℃·d/decade and 34.33℃·d/decade from 1961 to 2010, respectively. In contrast, the air thawing index (ATI) and ground thawing index (GTI) increased strikingly with the values of 48.13℃·d/decade and 40.37℃·d/decade in the past five decades. Permafrost showed more pronounced changes in freezing/thawing indices since 1990s compared to SFG. The changes of thermal regimes in frozen ground shows closely relations to air warming until the late 1990s, especially in 1998 when the QTP has undergone the most progressively warming. However, a sharp increase in the annual precipitation from 1998 seems to take a stronger control role on thermal degradation in frozen ground than the air warming in 2000s. Meanwhile, the prominent vegetation expansion further promotes thermally unstable of frozen ground in this highly wetting period.