Rapid degradation of frozen soil environments in thermokarst-affected alpine grasslands on the Qinghai-Tibet Plateau under climate change

Abstract

Soil freeze–thaw state influences multiple terrestrial ecosystem processes, such as soil hydrology and carbon cycling. However, knowledge of historical long-term changes in the timing, duration, and temperature of freeze–thaw processes remains insufficient, and studies exploring the combined or individual contributions of climatic factors—such as air temperature, precipitation, snow depth, and wind speed—are rare, particularly in current thermokarst landscapes induced by abrupt permafrost thawing. Based on ERA5-Land reanalysis, MODIS observations, and integrated thermokarst landform maps, we found that: 1) Hourly soil temperature from the reanalysis effectively captured the temporal variations of in-situ observations, with Pearson’ r of 0.66–0.91. 2) Despite an insignificant decrease in daily freeze–thaw cycles in 1981–2022, other indicators in the Qinghai-Tibet Plateau (QTP) changed significantly, including delayed freezing onset (0.113 d yr⁻¹), advanced thawing onset (−0.22 d yr⁻¹), reduced frozen days (−0.365 d yr⁻¹), increased frozen temperature (0.014 °C yr⁻¹), and decreased daily freeze–thaw temperature range (−0.015 °C yr⁻¹). 3) Total contributions indicated air temperature was the dominant climatic driver of these changes, while indicators characterizing daily freeze–thaw cycles were influenced mainly by the combined effects of increased precipitation and air temperature, with remarkable spatial heterogeneity. 4) When regionally averaged, completely thawed days increased faster in the thermokarst-affected areas than in their primarily distributed grasslands—alpine steppe (47.69%) and alpine meadow (22.64%)—likely because of their stronger “warming effect” of precipitation. Locally, paired comparison within 3 × 3 pixel windows from MODIS data revealed consistent results, which were pronounced when the thermokarst-affected area exceeded about 38% per 1 km². Conclusively, the “warming and wetting” climate has significantly altered soil freeze–thaw processes on the QTP, with the frozen soil environment in thermokarst-affected areas, dominated by thermokarst lakes, undergoing more rapid degradation. These insights are crucial for predicting freeze–thaw dynamics and assessing their ecological impacts on alpine grasslands.