Two decades of observations capture rapid warming of marginal permafrost across the 0 °C threshold

Abstract Marginal warm permafrost is nearing 0 °C, but its transition across this threshold remains poorly constrained by limited long-term observations. We analyzed up to two decades of ground-temperature records from 59 boreholes on the northeastern Qinghai–Xizang Plateau, including four sites undergoing advanced degradation near or across 0 °C. Bias-corrected ERA5-Land data showed rapid regional warming (median 0.56 °C decade −1 ), consistent warming across the 4100–4600 m elevation range containing most boreholes, and an upward shift in mean annual air temperature beginning in 2021. Several boreholes remained near 0 °C for prolonged periods despite atmospheric warming, indicating latent-heat buffering and delayed warming at depth. As buffering weakened, ground temperatures shifted upward and warmed more rapidly. At the most rapidly degrading site, a late-season subzero layer approximately 13 m thick disappeared between 2021 and 2023 after several years of near-zero temperatures. Profile changes were consistent with talik development and limited thermal recovery, although annual late-season measurements cannot confirm year-round talik conditions. Contrasting vegetation greenness coincided with differing ground-temperature responses, suggesting that local surface conditions influenced degradation timing. These observations show that marginal permafrost can undergo rapid, nonlinear degradation near 0 °C and that continued regional warming may promote similar transitions.