Drought risks based on changes in atmospheric evaporative demand due to plant response to CO2 levels

The temperature and CO₂ increase due to global warming are expected to exacerbate atmospheric water demand, worsening future drought conditions. Recent studies have revealed that evapotranspiration is regulated by stomatal response in response to CO₂ increase. However, understanding droughts defined based on evapotranspiration remains incomplete as it does not adequately integrate plant responses to anticipated drought conditions. In this study, we aimed to evaluate the frequency and extent of future drought events by comparing the Evaporative Stress Index (ESI) using two potential evapotranspiration (E_p) values capturing atmospheric evaporative demand. The first E_p utilized past data and predictions from the Coupled Model Intercomparison Project Phase 6, assuming a constant surface resistance (r_s) without considering plant responses. The second E_p accounted for the sensitivity of r_s to increased CO₂. Our findings indicate a significant increase in r_s due to elevated CO₂, leading to substantial changes in drought frequency and extent. While both non-vegetative response and plant response are expected to increase in future scenarios, an ESI that ignores plant responses tends to overestimate drought risk. Therefore, our study emphasizes the importance of integrating the sensitivity of r_s to evaporative demand and CO₂ level increases when assessing drought risk.