Understanding and accurately predicting how global terrestrial primary production responds to rising atmospheric CO 2 concentrations is a prerequisite for reliably assessing the long-term climate impact of anthropogenic fossil CO 2 emissions. Here we demonstrate that current carbon cycle models underestimate the long-term responsiveness of global terrestrial productivity to CO 2 fertilization. This underestimation of CO 2 fertilization is caused by an inherent model structural deficiency related to lack of explicit representation of CO 2 diffusion inside leaves, which results in an overestimation of CO 2 available at the carboxylation site. The magnitude of CO 2 fertilization underestimation matches the long-term positive growth bias in the historical atmospheric CO 2 predicted by Earth system models. Our study will lead to improved understanding and modeling of carbon–climate feedbacks.

Abstract