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Cold region ecosystems store vast amounts of soil organic carbon (C), which upon warming and decomposition can affect the C balance and potentially change these ecosystems from C sinks to carbon dioxide (CO2) sources. We quantified the decadal year‐round CO2 flux from an alpine steppe‐ecosystem on the Tibetan Plateau using eddy covariance and automatic chamber approaches during a period of significant warming (0.13°C per 10 years; and 0.18°C in the non‐growing season alone: 1st October to next 30th April). The results showed that ongoing climate change, mainly warming within the topsoil layers, is the main reason for the site's change from a sink for to a source of CO2 in the atmosphere. Non‐growing‐season ecosystem respiration accounted for 51% of the annual ecosystem respiration and has increased significantly. The growing seasons (1st May to 30th September) were consistent CO2 sink periods without significant changes over the study period. Observations revealed high‐emission events from the end of the non‐growing season to early in the growing season (1st March to fifteenth May), which significantly (p < 0.01) increased at a rate of 22.6 g C m−2 decade−1, ranging from 14.6 ± 10.7 g C m−2 yr−1 in 2012 to 35.3 ± 12.1 g C m−2 yr−1 in 2017. Structural equation modeling suggested that active layer warming was the key factor in explaining changes in ecosystem respiration, leading to significant changes in net ecosystem exchange over the period 2011–2020 and indicated that these changes have already transformed the ecosystem from a CO2 sink into a source. These results can be used to improve our understanding of the sensitivity of ecosystem respiration to increased warming during the non‐growing period. Plain Language Summary: Cold region ecosystems store vast amounts of soil organic carbon (SOC), which upon warming and decomposition can affect the net carbon balance and potentially change these ecosystems to become a source of carbon dioxide (CO2) to the atmosphere. We have measured year‐round CO2 fluxes over 10 years from an alpine steppe‐ecosystem on the Tibetan Plateau. The results show that the region has experienced pronounced warming during the study period and that the resulting near‐surface soil warming is a key parameter for explain why the ecosystem over 10 years have changed from being a net sink to become a net source of CO2 to the atmosphere. Measurements year‐round demonstrate that the shift in the CO2 balance is mainly due to a marked increase in decomposition of SOC during the non‐growing‐season. Furthermore, observations reveal several high‐emission events at the end of the non‐growing season and early in the growing season, which have increased in importance during the study period. The results are important to improve our understanding of the sensitivity of cold ecosystem respiration to warming and to highlight the importance of winter processes and emissions events on the annual ecosystem carbon budget. Key Points: Warming of the Tibetan Plateau has consequences for the net carbon balanceSignificant warming has resulted in a net carbon loss from soil respirationThe steppe‐ecosystem has changed from being a sink to a source of CO2 to the atmosphere [ABSTRACT FROM AUTHOR]

The methane (CH4) cycle on the Qinghai-Tibet Plateau (QTP), the world's largest high-elevation permafrost region, is sensitive to climate change and subsequent freezing and thawing dynamics. Yet, its magnitudes, patterns, and environmental controls are still poorly understood. Here, we report results from five continuous year-round CH4 observations from a typical alpine steppe ecosystem in the QTP permafrost region. Our results suggest that the QTP permafrost region was a CH4 sink of -0.86±0.23 g CH4- Cm-2 yr-1 over 2012-2016, a rate higher than that of many other permafrost areas, such as the Arctic tundra in northern Greenland, Alaska, and western Siberia. Soil temperature and soil water content were dominant factors controlling CH4 fluxes; however, their correlations changed with soil depths due to freezing and thawing dynamics. This region was a net CH4 sink in autumn, but a net source in spring, despite both seasons experiencing similar top soil thawing and freezing dynamics. The opposite CH4 source-sink function in spring versus in autumn was likely caused by the respective seasons' specialized freezing and thawing processes, which modified the vertical distribution of soil layers that are highly mixed in autumn, but not in spring. Furthermore, the traditional definition of four seasons failed to capture the pattern of the annual CH4 cycle. We developed a new seasonal division method based on soil temperature, bacterial activity, and permafrost active layer thickness, which significantly improved the modeling of the annual CH4 cycle. Collectively, our findings highlight the critical role of fine-scale climate freezing and thawing dynamics in driving permafrost CH4 dynamics, which needs to be better monitored and modeled in Earth system models. [ABSTRACT FROM AUTHOR]