Photosynthetic optimum temperature plays a minor role in the increase of terrestrial carbon uptake from 2000 to 2019

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 ( T opt ) and maximum rates of GPP ( GPP max ) remain uncertain. We investigated the drivers of T opt and GPP max trends during 2000–2019 using global observations of ground-based eddy covariance and satellite-based sun-induced chlorophyll fluorescence. Although GPP max increased worldwide, T opt increased only in tropical and temperate regions but remained unchanged globally and in arid and cold regions. Thermal acclimation via shifting T opt was constrained by atmospheric and soil dryness, explaining less than 20% of the global GPP max rise. In contrast, GPP max 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.