Hydrologic implications of vegetation response to elevated CO₂ in climate projections

Climate model projections using offline aridity and/or drought indices predict substantial terrestrial drying over the twenty-first century^1–11. However, these same models also predict an increased runoff^12–15. This contradiction has been linked to an absence of vegetation responses to an elevated atmospheric CO₂ concentration [CO₂] in offline impact models^12,14,16,17. Here we report a close and consistent relationship between changes in surface resistance (r_s) and [CO₂] across 16 CMIP5 models. Attributing evapotranspiration changes under non-water-limited conditions shows that an increase in evapotranspiration caused by a warming-induced vapour pressure deficit increase¹⁸ is almost entirely offset by a decrease in evapotranspiration caused by increased r_s driven by rising [CO₂]. This indicates that climate models do not actually project increased vegetation water use under an elevated [CO₂], which counters the perception that ‘warming leads to drying’ in many previous studies^1–11. Moreover, we show that the hydrologic information in CMIP5 models can be satisfactorily recovered using an offline hydrologic model that incorporates the [CO₂] effect on r_s in calculating potential evapotranspiration (E_P). This offers an effective, physically-based yet relatively simple way to account for the vegetation response to elevated [CO₂] in offline impact models.