Global photosynthesis is increasing with elevated atmospheric CO₂ concentrations, a response known as the CO₂ fertilization effect (CFE), but the magnitude of CFE remains uncertain. We quantify CFE by combining observations from a globally distributed eddy covariance measurement network with a novel mechanistic attribution framework. The framework is based on three eco-evolutionary optimization theories that analytically couples the carbon and water exchange processes. We report the first direct observational evidence for the dominant effect of CO₂ on gross primary production (GPP) enhancement since the 2000s, with additional other contributions primarily from warming. Soil moisture and specific humidity are the two largest contributors to GPP interannual variation through their influences on plant hydraulics. We also apply this process-based, theory-driven framework to scale up CFE globally using remotely sensed green leaf area and reanalysis data, suggesting a higher-than-expected CFE in tropical forests, which is unobserved in many remote sensing GPP products that omit the direct CFE on gas exchange. These results highlight the critical role that CFE has had on the global carbon cycle in recent decades.