Permafrost tipping point triggered by warming-driven loss of old carbon

Permafrost carbon vulnerability, particularly concerning temperature thresholds and old carbon mobilization, remains a critical uncertainty in climate projections. Through a five-year, multi-level warming experiment on the Tibetan Plateau, we investigate these dynamics using >40,000 hourly flux measurements combined with vertical CO₂ concentration and δ¹³C-CO₂ profiling. Here we demonstrate under low-to-moderate warming (<2 °C), respiratory carbon loss (R_eco) increments exceed photosynthetic carbon uptake (GPP) gains by 1-16 fold, driving a quantitative shift toward ecosystem carbon source. Extreme warming (2-4 °C) triggers a surge in growing-season deep carbon loss to 59% R_eco, while GPP declines precipitously. The decoupling between R_eco and GPP drives a qualitative transition to strong carbon source, implying the existence of a tipping point within 2-4 °C. Projected to end-of-century warming levels (2.69 °C) across Tibetan permafrost regions, this could release 24-47 g CO₂ m^-2 yr^-1 old carbon. These findings establish quantitative thresholds for permafrost carbon vulnerability and inform carbon-climate feedback projections in global cold regions.