A decadal turning point in the CO ₂ sensitivity of the land carbon sink

Abstract The terrestrial carbon sink is a critical buffer in the climate system, yet its persistence under rising atmospheric CO 2 remains a major uncertainty in Earth system projections. Using an ensemble of 20 dynamic global vegetation models from the TRENDY v12 project, we decompose the CO 2 sensitivity of the land carbon sink, represented by net biome production (NBP), to diagnose long-term changes in intrinsic efficiency. We identify a robust, emergent turning point in the simulated intrinsic CO 2 sensitivity of NBP, shifting from a significant multi-decadal increase to a sustained decline around 1980. This reversal is reproduced in both carbon-only and coupled carbon–nitrogen model classes and is consistent with a progressive decoupling between CO 2 sensitivities of carbon inputs and losses (net primary production, NPP versus heterotrophic respiration, Rh). In contrast, the turning point is not readily detectable in the total apparent sensitivity, reflecting both the dominance of high-frequency climate and land-use variability and interacting environmental effects that may partially offset intrinsic CO 2 responses. Together, these results point to a weakening intrinsic efficiency of the terrestrial carbon sink, implying diminishing marginal land uptake per unit CO 2 increase and a potentially more constrained resilience of future land carbon sequestration than suggested by raw sink magnitudes alone.