<p>Rising atmospheric CO₂ concentrations, temperature and vapour pressure deficit substantially influence plant photosynthesis and terrestrial carbon uptake, yet how these drivers interact to alter photosynthesis across different climate regimes remains unclear. Here, using globally distributed FLUXNET measurements and satellite-derived machine learning estimates of gross primary production (GPP) for 1982–2022, we reveal an asymmetric shift in vegetation productivity between drylands and humid regions. This shift is led by a substantial slowdown in the rate of increase in dryland GPP since 2001, primarily due to water constraints associated with the rising vapour pressure deficit. By contrast, humid regions exhibit a sustained increase in GPP in response to rising temperatures and atmospheric CO₂. Notably, dynamic global vegetation models and Earth system models fail to capture this divergence in both historical simulations and future projections. Given increasing atmospheric aridity and the continued expansion of drylands, we anticipate a broad water constraint on global photosynthetic capacity that may limit the land carbon sink. Consequently, we advocate prioritizing adaptive strategies in drylands and nature-based solutions in humid regions to enhance global climate action.</p>