The optimum air temperature for photosynthetic ecosystem productivity (T_opt) determines the annual maximum of terrestrial carbon uptake. Previous studies have quantified T_opt and explored its acclimation in response to climatic warming. However, there remains uncertainty regarding the extent to which T_opt has changed globally in recent decades and how it might evolve in the future. Our analysis, using both satellite- and ground-based datasets, reveals a slight increase in T_opt over the past two decades (+0.14°C, 2000-2019), contrasting with a significant 0.46°C rise in maximum global growing-season temperature (T_max). The lack of change in T_opt was attributed to an insignificant trend of T_max in cold areas and to drought inhibiting thermal acclimation of photosynthesis in temperate and arid regions. If global surface temperatures exceeded pre-industrial levels by 2°C, the mean T_max over the globe was predicted to increase slightly more than T_opt (1.5 vs. 1.2°C), with the gap widening under a 4°C temperature increase (4.1 vs. 3.4°C). In the +4°C scenario, soil drought dominated the widespread decline in photosynthetic acclimation across tropical and temperate vegetation, slowing the rate of T_opt increase at the end of the century (+0.04°C, 2080-2099). Conversely, increasing atmospheric CO₂ concentrations failed to have significant effects on T_opt and its acclimation. These findings imply that the absorption of atmospheric CO₂ by terrestrial vegetation, and subsequent carbon sequestration, may be further hampered by the limited acclimation capacity of T_opt to rising global temperatures.