Enhancing evapotranspiration estimates under climate change: the role of CO ₂ physiological feedback and CMIP6 scenarios

Abstract. The future state of global evapotranspiration (ET) estimation under climate change remains uncertain. Current formulations primarily developed based on the high emission CMIP5 scenario, have been widely used to represent conditions under elevated greenhouse gas pathways. However, these formulations may not adequately capture the enhanced vegetation–climate interactions projected under the lower-emission scenarios of CMIP6. Without updates to account for evolving plant physiological responses to rising CO2, projections may overlook critical feedbacks between atmospheric CO2 concentrations, vegetation behavior, and hydrological processes. To address this, developing CMIP6-specific formulations is essential to leverage its improved datasets and reduce uncertainties in future ET simulations. In this study, we update the Penman-Monteith evapotranspiration (PM-ET) model by incorporating the CO2-vegetation coupling effect. This is achieved using outputs from four Coupled Model Intercomparison Project Phase 6 (CMIP6) global climate models (GCMs) under four Shared Socioeconomic Pathways (SSP1-2.6, SSP2-4.5, SSP3-7.0, SSP5-8.5). Results indicate a sustained historical increase in potential evapotranspiration (Ep). The inclusion of CO2 physiological effects reduces the deviation in projected ET trends by approximately 15 %–20 % compared to CMIP5-based frameworks, accounting for the increase in stomatal resistance driven by CO2 concentrations rising from ∼284 to ∼ 935 ppm. Furthermore, our model predicts an increasing dependence of ET projections on emission scenario, highlighting the growing influence of pathway-specific feedbacks. Overall, our approach demonstrates greater compatibility with CMIP6 simulations, allowing for more accurate representation of ET responses to future CO2 increases. These findings provide valuable insights for advancing the analysis of nonlinear vegetation-atmosphere interactions and hydrological uncertainty under climate and physiological forcings.