The study examines the impact of autumn warming on the carbon balance of northern ecosystems. Over the past two decades, spring and autumn temperatures in northern latitudes have increased by about 1.1°C and 0.8°C, respectively, leading to a simultaneous greening trend characterized by an extended growing season and enhanced photosynthetic activity. The authors analyze atmospheric CO₂ concentration data and ecosystem CO₂ fluxes to find that atmospheric records show a trend towards an earlier autumn-to-winter carbon dioxide build-up, suggesting a shorter net carbon uptake period. This trend is not solely explained by changes in atmospheric transport and is further supported by ecosystem flux data, indicating increasing carbon losses in autumn. Using a process-based terrestrial biosphere model (ORCHIDEE) and satellite vegetation greenness index observations, the study finds that both photosynthesis and respiration increase during autumn warming, but the increase in respiration is greater. In contrast, warming increases photosynthesis more than respiration in spring. The simulations and observations suggest that northern terrestrial ecosystems may currently lose carbon dioxide in response to autumn warming, with a sensitivity of about 0.2 PgC °C⁻¹, offsetting 90% of the increased carbon dioxide uptake during spring. If future autumn warming occurs at a faster rate than in spring, the ability of northern ecosystems to sequester carbon may be diminished earlier than previously thought.The study examines the impact of autumn warming on the carbon balance of northern ecosystems. Over the past two decades, spring and autumn temperatures in northern latitudes have increased by about 1.1°C and 0.8°C, respectively, leading to a simultaneous greening trend characterized by an extended growing season and enhanced photosynthetic activity. The authors analyze atmospheric CO₂ concentration data and ecosystem CO₂ fluxes to find that atmospheric records show a trend towards an earlier autumn-to-winter carbon dioxide build-up, suggesting a shorter net carbon uptake period. This trend is not solely explained by changes in atmospheric transport and is further supported by ecosystem flux data, indicating increasing carbon losses in autumn. Using a process-based terrestrial biosphere model (ORCHIDEE) and satellite vegetation greenness index observations, the study finds that both photosynthesis and respiration increase during autumn warming, but the increase in respiration is greater. In contrast, warming increases photosynthesis more than respiration in spring. The simulations and observations suggest that northern terrestrial ecosystems may currently lose carbon dioxide in response to autumn warming, with a sensitivity of about 0.2 PgC °C⁻¹, offsetting 90% of the increased carbon dioxide uptake during spring. If future autumn warming occurs at a faster rate than in spring, the ability of northern ecosystems to sequester carbon may be diminished earlier than previously thought.