Declining Contribution of Plant Physiological Effects to Global Drought Characteristics With Rising CO2 Using State-of-the-Art Earth System Models

Vegetation physiology responses to rising atmospheric CO₂ can alter the global hydrological cycle, thereby influencing drought occurrence. It has long been controversial and poorly understood how vegetation physiological effects influence meteorological drought characteristics with increasing CO₂. To investigate that, we employ multiple CO₂ sensitivity experiments of the state-of-the-art Earth System Models (ESMs) in the Coupled Model Intercomparison Project Phase 6 (CMIP6). We quantify drought characteristics in response to rising CO₂ using two drought indices: the Standardized Precipitation Index (SPI) and the Standardized Precipitation Evapotranspiration Index (SPEI), with SPEI calculated using both the Penman-Monteith method (SPEI_PM) and energy-only method (SPEI_Rn). Our findings reveal that plant physiological effects can robustly induce more intense, frequent, and prolonged droughts under elevated CO₂ levels. Spatially, drought intensity as measured by SPI, SPEI_PM, and SPEI_Rn, resulting from CO₂ physiological forcing, is projected to increase over 61%, 69%, and 78% of global terrestrial areas, respectively. Notably, we found that the contribution of plant physiological effects (${\beta }_{\text{PHY}}$) to drought characteristics, including intensity, frequency, and duration, exhibits a significant and spatially extensive declining trend with rising CO₂ across most land areas. This declining trend is robustly depicted in both the multi-model mean and individual models. Vegetation coverage plays an important role in the spatial pattern of ${\beta }_{\text{PHY}}$. CO₂ physiological forcing therefore exerts greater impacts in the tropics, particularly over tropical forests. Our results demonstrate that drought characteristics are expected to become less dependent on plant physiological effects with increasing CO₂, a consideration essential for accurate drought projections.