Warming climate amplifies vapor pressure deficit limits on gross primary productivity

Ongoing climate warming may profoundly impact terrestrial gross primary productivity (GPP), a key component of the global carbon cycle. However, uncertainty in the relative roles of atmospheric water demand (vapor pressure deficit, VPD) and root-zone soil moisture (SM) limits predictions of drought impacts on GPP. Here, we show that growing-season GPP was more strongly constrained by VPD than SM globally, based on observation-constrained model estimates, satellite retrievals and Dynamic Global Vegetation Model simulations. The importance of VPD increased with higher temperatures and more severe and prolonged droughts. This pattern reflects VPD's critical role in regulating stomatal conductance and plant hydraulic function, both of which are essential for preventing xylem embolism. We further found that P₅₀, the water potential at 50% loss of hydraulic conductivity, was the strongest predictor of their relative influence. Collectively, rising VPD may accelerate declines in terrestrial carbon storage under future warming.