High-latitude and high-altitude regions contain vast stores of permafrost carbon. Climate warming may result in the release of CO2 from both the thawing of permafrost and accelerated autotrophic respiration, but it may also increase the fixation of CO2 by plants, which could relieve or even offset the CO2 losses. The Tibetan Plateau contains the largest area of alpine permafrost on Earth. However, the current status of the net CO2 balance and feedbacks to warming remain unclear, given that the region has recently experienced an atmospheric warming rate of over 0.3 °C decade−1. We examined 32 eddy covariance sites and found an unexpected net CO2 sink during 2002 to 2020 (26 of the sites yielded a net CO2 sink) that was four times the amount previously estimated. The CO2 sink peaked at an altitude of roughly 4,000 m, with the sink at lower and higher altitudes limited by a low carbon use efficiency and a cold, dry climate, respectively. The fixation of CO2 in summer is more dependent on temperature than the loss of CO2 than it is in the winter months, especially at higher altitudes. Consistently, 16 manipulative experiments and 18 model simulations showed that the fixation of CO2 by plants will outpace the loss of CO2 under a wetting–warming climate until the 2090s (178 to 318 Tg C y−1). We therefore suggest that there is a plant-dominated negative feedback to climate warming on the Tibetan Plateau.

No abstract

Terrestrial ecosystems absorb atmospheric CO 2 through photosynthesis and convert it into sugars and starches (Keenan & Williams, 2018), though soil carbon (C) loss returns them to the atmosphere (Giardina et al., 2014;Keidel et al., 2015;Yvon-Durocher et al., 2012). Among various regions, the cold high-latitude and high-altitude regions are unique from temperate and boreal ecosystems owing to their vast permafrost soil C storage, rather than in plants. However, these cold ecosystems suffer from much stronger climate warming than the global average; for instance, warming at a rate of 0.3°C per decade in the Arctic, compared to a rate of 0.12°C per decade for the global land surface (IPCC, 2013). Under such conditions, the fate of the permafrost to climate warming is ultimately governed by the balance between photosynthetic assimilation and ecosystem respiration (Ding et al., 2017;McGuire et al., 2016).Averaging over 4,000 m in altitude, the Tibetan Plateau (TP) is home to the world's largest alpine permafrost, which contributes 15.3 Pg of soil C storage (1 Pg = 10 15 g) (Ding et al., 2016). Roughly 46% of the TP is underlain by permafrost, making it the highest and most extensive alpine permafrost on Earth (Ran et al., 2020). Temperatures in the TP region have risen by 0.26°C per decade since the 1960s (Kuang & Jiao, 2016), which is comparable to the Arctic and could potentially cause permafrost thaw and release of C. Although, two opposing pieces of evidence have been reported regarding the effects of warming on the C balance of the TP. On the one hand, permafrost degradation has been reported and projected across the TP (Zou et al., 2017). The large C pool size together with significant permafrost thawing suggests a risk of C emissions and positive climate feedback across the permafrost region on the TP (Chen et al., 2016). On the other hand, several studies, using both satellite observations and model simulations (Piao 2011;Zhang et al., 2013;Zhuang et al., 2010), have found that the TP is greener under the current warming and wetting climate, as well accumulating soil C content (Ding et al., 2017), which together suggests a stronger net C gain of alpine ecosystems, rather than permafrost C loss. Therefore, these contrasting views means it remains unclear as to whether the C balance has been altered by the changing climate on the TP.

No abstract