SignificanceCold regions contain vast stores of permafrost carbon. Rapid warming will cause permafrost to thaw and plant respiration to accelerate, with a resultant loss of CO2, but could also increase the fixation of CO2by plants. A network of 32 eddy covariance sites on the Tibetan Plateau, which has the largest store of alpine permafrost carbon on Earth, shows that this region functions as a net CO2sink. Our sensitivity analyses, experiments, and model simulations consistently showed that the fixation of CO2by plants outpaces the loss of CO2from permafrost and accelerates plant respiration. This indicates a plant-dominated CO2balance on the Tibetan Plateau, which could provide a negative feedback to climate warming.