Gross primary productivity (GPP; ecosystem-level photosynthesis) represents the largest terrestrial carbon flux and is highly sensitive to temperature. Despite global warming, the trends and controlling factors of optimum temperature ((Formula presented) ) and maximum rates of GPP ((Formula presented) ) remain uncertain. We investigated the drivers of (Formula presented) and (Formula presented) trends during 2000–2019 using global observations of ground-based eddy covariance and satellite-based sun-induced chlorophyll fluorescence. Although (Formula presented) increased worldwide, (Formula presented) increased only in tropical and temperate regions but remained unchanged globally and in arid and cold regions. Thermal acclimation via shifting (Formula presented) was constrained by atmospheric and soil dryness, explaining less than 20% of the global (Formula presented) rise. In contrast, (Formula presented) trends were more strongly driven by stomatal regulation improving water-use efficiency and by enhanced canopy development under dryness constraints. These findings challenge the expectation that thermal acclimation is central to projecting GPP under warming and highlight dynamic physiological-structural shifts that sustain terrestrial carbon uptake.