Significance: Cold 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.

The Tibetan Plateau (TP) is home to the most extensive alpine permafrost, where there are over 30% of China's marshlands. Though climate warming of 0.26°C per decade may have altered the carbon (C) balance by promoting permafrost C release. To date, most studies have been conducted in the eastern TP region (<3,500 m in altitude), even though 51% of the TP's marshlands are located at altitudes above 4,500 m, where climate warming is even faster than at lower altitudes. Thus, we set up an eddy covariance system at a very high marshland (4,760 m in altitude) in the hinterland of the TP and recorded observations for three consecutive years. The results showed that the marshland functioned as a net C sink (187 ± 29 g C m⁻² yr⁻¹, Mean ± SD), which is even stronger than at lower altitude marshlands, possibly because of cold‐constraints on respiration. Temperature and vapor pressure deficit together dominated the variation in C exchanges. Moreover, the ecosystem functioned as a net C sink when the average daily temperature exceeded 8°C. Photosynthesis was consistently more sensitive to temperature variation, compared with C release, at the annual, seasonal, and daily scales, which was then validated across other marshlands on the TP. All these pieces of evidence suggest that high‐altitude marshlands on the TP are still functioning as C sinks, and photosynthesis is more sensitive to temperature than C release implies that a warmer climate will benefit the C accumulation, before there is too much soil C loss.