###### Abstract  
Carbon use efficiency (CUE) represents the proficiency of plants in transforming carbon dioxide (CO\({}_{2}\)) into carbon stock in terrestrial ecosystems. CUE extremes represent ecosystems' extreme proficiency in carbon transformation. Studying CUE extremes and their forming climate conditions is critical for enhancing ecosystem carbon storage. However, the study of CUE extremes and their forming climate conditions on the global scale is still lacking. In this study, we used the results from the daily Boreal Ecosystem Productivity Simulator (BEPS) model to detect the positive and negative CUE extremes and analyze their forming climatic conditions on a global scale. We found grasslands have the largest potential in changing global CUE, with the contribution being approximately 32.4% to positive extremes and 30.2% to negative extremes. Spring in the Northern Hemisphere (MAM) contributed the most (30.5%) to positive CUE extremes, and summer (JJA) contributed the most (29.7%) to negative CUE extremes. The probabilities of gross primary production (GPP) extremes resulted in CUE extremes (>25.0%) being larger than autotrophic respiration (Ra), indicating CUE extremes were mainly controlled by GPP rather than Ra extremes. Positive temperature anomalies (0-1.0 \({}^{\circ}\)C) often accompanied negative CUE extreme events, and positive CUE extreme events attended negative temperature anomalies (-1.0-0 \({}^{\circ}\)C). Moreover, positive (0-20.0 mm) and negative precipitation (-20.0-0 mm) anomalies often accompanied positive and negative CUE extremes, respectively. These results suggest that cooler and wetter climate conditions could be beneficial to enhance carbon absorptions of terrestrial ecosystems. The study provides new knowledge on proficiency in carbon transformation by terrestrial ecosystems. carbon use efficiency; extreme events; gross primary production; climate conditions; ecosystem model +
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growth and could deeply impact vegetation structure and functioning [5; 6]. CUE is a crucial eco-physiological parameter in shaping ecosystem carbon storage [7; 8; 9]. Thus, increasing CUE is critical for ecosystem management and mitigating global warming. Extreme events indicate the terrestrial ecosystem's unusual occurrences and episodes [10; 11]. CUE extremes represent unusual episodes of the proficiency of carbon transformation in the terrestrial ecosystem. Positive CUE extremes represent ecosystems' highest proficiency in carbon sequestrations. Conversely, negative CUE extremes represent more CO\({}_{2}\) being released into the atmosphere through Ra. Therefore, promoting positive and preventing negative CUE extremes is critical for enhancing carbon sequestrations in global terrestrial ecosystems. However, many previous studies focused on investigating the pattern and their climate drivers at global and regional scales for CUE. For example, [PERSON] et al. (2018) used process-based models to investigate the CUE pattern and its linkage with climate factors. [PERSON] et al. (2019) investigated the CUE variations and their climatic controls in China's terrestrial ecosystems. Additionally, for extreme productivity events, the studies always focus on studying GPP and NEP extremes at global and regional scales [10; 11], and there is still a lack of study on the global scale's positive and negative CUE extremes. CUE varies across space with plant function types (PFTs) and climate conditions [9; 12]. A previous study suggested that grasslands and croplands have higher annual CUE than forests, and the evergreen needle-leaf forest has a larger CUE than other forest types [4]. The spatial pattern of CUE is mainly regulated by temperature and precipitation. Temperature influences the variation of CUE by affecting both GPP and Ra. GPP increases to a temperature optimum and declines afterwards, while Ra increases exponentially with temperature. Positive temperature and solar radiation extremes could induce positive extremes of GPP in China's terrestrial ecosystems [10].