A global-scale drought assessment considering vegetation response to atmospheric CO₂ changes

Climate change is increasing atmospheric evaporative demand (E_p), a key driver of future drought. However, conventional E_p estimation methods neglect the physiological responses of vegetation to rising atmospheric carbon dioxide concentration and vapor pressure deficit. To assess this impact, we projected future agricultural drought risk by applying two E_p models to outputs from 10 Coupled Model Intercomparison Project Phase 6 Earth System Models: a conventional formula (E_p-RC) and a two-source model (E_p-Veg) that dynamically considers changes in surface and aerodynamic resistances. We then used these E_p estimates to drive a precipitation-based (Standardized Precipitation-Evapotranspiration Index, SPEI) and an evaporation-based (Evaporative Stress Index, ESI) drought index. The resulting projections of drought duration and area through 2100 under different Shared Socioeconomic Pathways were compared against a benchmark soil moisture-based index (SSI). Our results show that E_p-RC consistently overestimates future drought severity and area compared to E_p-Veg. The projections using E_p-Veg were more consistent with the SSI benchmark, confirming that accounting for the water-saving effects of vegetation provides a more realistic risk assessment. However, a significant and growing discrepancy between the ESI and SSI projections remains, even when using the refined E_p-Veg model. This suggests that while modeling vegetation physiology is crucial, inherent structural limitations within the E_p framework, driven by land-atmosphere interactions, are also major uncertainty sources in future drought assessments.