Theory predicts that rising CO₂ increases global photosynthesis, a process known as CO₂ fertilization, and that this is responsible for much of the current terrestrial carbon sink. The estimated magnitude of the historic CO₂ fertilization, however, differs by an order of magnitude between long-term proxies, remote sensing-based estimates and terrestrial biosphere models. Here we constrain the likely historic effect of CO₂ on global photosynthesis by combining terrestrial biosphere models, ecological optimality theory, remote sensing approaches and an emergent constraint based on global carbon budget estimates. Our analysis suggests that CO₂ fertilization increased global annual terrestrial photosynthesis by 13.5 ± 3.5% or 15.9 ± 2.9 PgC (mean ± s.d.) between 1981 and 2020. Our results help resolve conflicting estimates of the historic sensitivity of global terrestrial photosynthesis to CO₂ and highlight the large impact anthropogenic emissions have had on ecosystems worldwide.