Observed talik development triggers a tipping point in marginal permafrost of the Qinghai-Xizang Plateau

Permafrost, a critical component of Earth’s climate system, is increasingly subject to abrupt thaw events, which jeopardize infrastructure, reshape landforms, alter hydrological regimes, and disrupt ecosystems, thereby posing substantial threats to global sustainability. However, the underlying mechanisms that trigger these abrupt transitions remain incompletely understood. Here, we present decade-long in-situ observations from HRQ1, a marginal permafrost site in the Headwater Area of the Yellow River, northeastern Qinghai-Xizang Plateau. These data reveal the formation and growth of a talik, indicative of a permafrost tipping point. Absent before 2017, the talik subsequently formed and progressively deepened, extending to the maximum observation depth of 300 cm by 2024. The transition from perennially frozen to thawed conditions was accompanied by a substantial increase in mean annual soil temperature (MAST) throughout the entire soil profile. From 2015 to 2023, MAST in the upper 200 cm rose from sub-zero (-0.30 to -0.49 °C) to consistently above 0 °C (0.07 to 1.08 °C). Concurrently, maximum daily soil temperatures in deeper layers (200–300 cm) became positive, indicating thaw propagation into the relict permafrost. This warming coincided with a marked increase in unfrozen soil moisture, particularly within the expanding talik. The rapid, non-linear deepening of the talik, far exceeding rates attributable to conductive heat transfer alone, was driven by a strong convective mechanism (Rayleigh-Darcy instability). This advective process was triggered when the soil profile became fully saturated, a condition resulting from the convergence of intensified rainfall and enhanced water retention linked to decadal vegetation greening. Intriguingly, despite the accelerated subsurface warming, the annual amplitude of ground surface temperature decreased from 29.0 ± 2.8 °C to 24.5 ± 3.6 °C following talik formation, likely due to the buffering effect of increased vegetation cover, which modified the surface energy balance. Our results demonstrate that climatic warming and wetting can initiate a cascade of internal feedbacks, propelling marginal permafrost beyond an abrupt tipping point. These findings emphasize the acute vulnerability of marginal permafrost and highlight the urgent necessity for sustained monitoring to assess ecosystem stability and quantify associated greenhouse gas emissions.