Extensive studies have indicated significant effects of high vapor pressure deficit (VPD) and low soil moisture (SM) on global gross primary productivity (GPP), yet their relative importance and nonlinear effects remain uncertain. Here, leveraging CMIP6 outputs, we examine global GPP, SM, and VPD variations during a baseline period spanning from (1982 to 2014) and under four future scenarios, clarify the dominant influence on GPP by disentangling SM and VPD effects, explore critical thresholds and costs associated with the dominant factor affecting GPP, and propose an optimized global drought zoning plan. The results indicate a general increasing trend in GPP, with higher growth rates observed in the Northern Hemisphere. Except under SSP5-8.5, SM tends to increase, while atmospheric dryness, particularly in the Southern Hemisphere, remains evident. SM emerges as the primary driver of GPP variation, although its control diminishes under future scenarios compared to the baseline. An inverted U-shaped relationship exists between SM and GPP, with thresholds and costs of SM increasing alongside rising CO2 levels. Enhancing water use efficiency is crucial in mitigating high SM costs for vegetation growth in high CO2 emission scenarios. Only 13.4–16.5% of the global vegetated land maintains SM within its elastic range, while 41.4–59.6% of the global land exceeds this range, predominantly in northern South America and Southeast Asia. Regions falling below the elastic SM range expand significantly, particularly in the southern Sahara and Australia, increasing by 60% in high-emission scenarios. Future strategies should focus on targeted optimizations tailored to different SM conditions to mitigate constraints on vegetation growth and optimize carbon sequestration in global ecosystems.