Permafrost, ground remaining below 0°C for at least two years, is crucial for hydrological, ecological, and environmental stability. While long-term climate warming's effects on permafrost are well-studied, the significant impact of short-term extreme events, particularly increasing high temperatures and heatwaves, is gaining attention. Existing research focuses on the event-scale impacts on permafrost changes, disaster mechanisms (like rockfalls and landslides triggered by ground ice melt), and ecosystem processes (reduced vegetation growth and altered soil fauna). However, understanding the processes by which extreme events affect permafrost at seasonal and interannual scales remains uncertain. The unprecedented summer heatwave of 2022, with temperatures exceeding 40°C in many Northern Hemisphere regions, provides a valuable case study. This research uses long-term monitoring data from six active layer sites and three permafrost boreholes in the central Qinghai-Tibet Plateau (QTP), combined with field surveys and reanalysis data, to investigate: 1) whether the QTP experienced the 2022 heatwave; 2) if so, the resulting changes in active layer and permafrost thermal conditions, including ALT and MAGT; and 3) the heatwave's contribution to seasonal thaw depth. The findings improve our understanding of permafrost changes and refine permafrost modeling.

Numerous studies have investigated the impacts of long-term climate warming on permafrost, using observations and simulations. However, a growing body of research highlights the significant role of short-term extreme events. Studies have shown that extreme high temperatures can increase ground temperature, melt ground ice, and trigger secondary events like snow droughts or wildfires, altering the buffer layer's influence on permafrost thermal states. In ice-rich regions, these events can destabilize slopes, causing rockfalls and landslides. Furthermore, extreme winter warming can disrupt ecosystem processes, reducing summer vegetation growth and impacting soil fauna. While these studies primarily examine event-scale impacts, the mechanisms by which extreme events influence active layer and permafrost thermal states at seasonal and interannual scales require further investigation. This study addresses this gap by focusing on the 2022 Northern Hemisphere heatwave, examining its effects on a specific permafrost region.

This study utilizes long-term monitoring data from six active layer sites (China01, China04, China06, QT01, QT06, QT09) and three permafrost borehole sites (QTB01, QTB06, QTB09) in the central QTP. Data included summer air temperature (SAT), degree days thawing (DDT), active layer thickness (ALT), and mean annual ground temperature (MAGT). SAT was calculated from June to August daily air temperatures (1961-2022). DDT, representing cumulative energy transfer, was calculated by summing positive daily air temperatures during thawing periods. ALT was determined from soil temperature profiles using the 0°C isotherm. MAGT was the ground temperature at the depth of zero annual amplitude (DZAA). The Stefan model was used to estimate ALT in years with missing data, assuming a homogeneous active layer. This model relates ALT to DDT and an edaphic factor (E) encompassing thermal conductivity, soil moisture, latent heat of fusion, and soil density. The model's accuracy was validated against observed ALT data (R²=0.9, RMSE=12.96 cm, MAE=10.31 cm). The contribution of the heatwave to seasonal thaw depth was quantified using a modified Stefan model, separating normal thaw from heatwave-induced thaw. A contribution fraction was calculated as the ratio of heatwave-induced thaw depth to total ALT. Multiple linear regression analyses were performed to assess the influence of DDT and degree days freezing (DDF) on MAGT, using various time lags. Pearson correlation, least squares regression, and Mann-Kendall tests were used for statistical analyses. ERA5-Land reanalysis data was used to supplement in-situ air temperature data, after validation against in-situ measurements. Snow depth data, vegetation coverage, biomass, and topographic factors (altitude, slope aspect, slope gradient) were also incorporated using field surveys and SRTM DEM data.

The study area experienced the 2022 summer heatwave, with mean SAT in 2022 significantly exceeding the 1961–2021 average (mean increase: 3.2°C). DDT in 2022 was also exceptionally high (1.6 times the 1961–2021 mean). Four active layer sites (China06, QT01, QT06, QT09) exhibited record ALT in 2022, with an average increase of 29.4 cm compared to the 2000–2021 mean. These sites also showed the highest summer mean ground temperatures in 2022 compared to previous years. MAGT at three borehole sites reached their highest values in 2022, exceeding previous averages. The Stefan model revealed that the heatwave's contribution to seasonal thaw depth varied across sites (6.6%–13.6%), with the highest contribution in 2022. Correlation analyses showed significant positive correlations between SAT, DDT, and ALT at several sites. However, responses to the 2022 heatwave varied widely among sites. China04 showed minimal response despite high SAT and DDT, attributed to high vegetation cover, high soil organic matter, and abundant ground ice near the permafrost table. QT06's response was primarily climate-driven, with lesser influence from vegetation and soil factors. Some sites showed insensitivity to climate warming, possibly due to substantial subsurface ice content. Multiple linear regression analysis indicated that the MAGT at the three borehole sites had significant increasing trends, which were likely dominated by winter climate change rather than summer due to the higher winter warming rate and the limited buffering effect of the snow cover. The average contribution fraction at all six sites was 21.8% in 2022, 2.3 times higher than the average during 2000–2021.

The findings demonstrate that the 2022 heatwave significantly impacted permafrost in the central QTP, leading to increased ALT and MAGT. However, the response varied considerably among sites due to the interplay of multiple factors: climate (SAT, DDT), vegetation (coverage, biomass), soil (organic matter, moisture, texture), and topography (altitude, slope aspect, gradient). High vegetation cover and soil organic matter content buffered the effects of the heatwave in some areas. The inconsistent response highlights the complexity of permafrost dynamics and the need to consider multiple factors when modeling these changes. The study also underscores the importance of understanding the interaction between long-term climate warming and short-term extreme events. While long-term trends are essential, short-term extremes can trigger significant and rapid changes in permafrost conditions.

This study reveals the substantial impact of the 2022 summer heatwave on permafrost in the central QTP, evidenced by increased ALT and MAGT. The variable response across sites highlights the complexity of permafrost dynamics, influenced by climate, vegetation, soil, and topography. Future monitoring should integrate ALT and ground subsidence measurements to better capture permafrost change. Further research should focus on the differential responses of various permafrost types to both long-term climate warming and short-term extreme events, ultimately improving permafrost models incorporating extreme events.

The study's reliance on a simplified Stefan model for ALT estimation in years with missing data may introduce some uncertainty. The limited availability of continuous data for factors such as vegetation, snow cover, and soil texture might constrain the analysis. The spatial extent of the study is relatively limited, and the findings may not be fully generalizable to other permafrost regions. The impact of ground subsidence was not directly measured and only inferred; more direct measurements would strengthen the conclusions.