Extensive studies have underscored alterations in the carbon use efficiency (CUE), alongside the diverse factors that shape its dynamics over the preceding three decades. Nonetheless, the predictive trajectory of the global CUE and the pivotal factors that will govern its evolution in the forthcoming era remain veiled in uncertainty. In this study, leveraging CMIP6 model outputs, we project CUE trends for baseline (1982-2014) and future (2015-2100) periods, identify key drivers using boosted regression trees, assess the sensitivity of CUE-controlling factors (Svalue)—indicating higher/lower factor influence on CUE—and their variability across four representative scenarios, and explore the relationship between Svalue and socio-economic indicators. Our findings indicate a decline in CUE during the twenty-first century, particularly pronounced in the SSP5-8.5 scenario, with a decreased rate (1.2 × 10−2 ± 5.2 × 10−4/decade) significantly higher than in SSP1-2.6. Geospatially, reductions in CUE are projected across 56.9%, 74.5%, 83.1%, and 88.6% of global vegetated land under four future scenarios, respectively, primarily anticipated within 30-60° N latitudes of the Northern part of the global. Temperature emerges as the primary driver of CUE variability, except in Africa. Svalue decreases over time, indicating intensified temperature impacts on CUE. The timing of Svalue changes advances with increased CO2 emissions, prolonging warming pressure on ecosystems under high CO2 emission pathways. Notably, a threshold effect exists in the precipitation and Svalue changes, characterized by a notably higher threshold under scenarios of elevated CO2 emissions. Economic development and population control mitigate the negative temperature-CUE relationship, yet this effect diminishes with rising CO2 levels. To bolster the global ecosystem, C sink capabilities should focus on promoting renewable energy innovations and implementing effective population control strategies.