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Feedbacks from the carbon cycle in boreal and arctic ecosystems can significantly affect climate change, but the effects of climate change on the high-latitude carbon cycle during the dormant period remain uncertain. By analyzing the long-term atmospheric CO2 concentration record from Point Barrow in Alaska, we show that warming significantly boosts net CO2 release in autumn over the period 1974-2014. But this warming-stimulated effect has been attenuated since 1997. This deceleration of net CO2 release with warming is ascribed to the attenuation in respiration response to temperature rather than changing relationship between temperature and productivity or changes in atmospheric transport, fossil fuel emissions, or air-sea CO2 exchanges. The attenuated respiration response is likely due to decoupling between temperature and plant-derived carbon inputs to soil for decomposition. Contrary to previous suggestions, warming no longer results in a higher autumn net CO2 release.

Plain Language Summary Boreal and arctic ecosystems are highly sensitive to climate change. Most of previous studies focus on terrestrial carbon cycle responding to warming during carbon uptake period, while much less on the dormant season that characteristic with net carbon release. We provide the first evidence that autumn warming no longer accelerates net carbon losses in boreal and arctic ecosystem as previously suggested. This deceleration of net CO2 release with autumn warming is attributed to the attenuation in respiration response to temperature, which interestingly, is most likely due to the recently reported weakening relationship between temperature and productivity. Our finding counteracts recently reported warming-induced loss of net CO2 uptake during carbon uptake period, which provides a negative feedback to climate warming.