Effects of Increasing [CO2] on the Dominant Drivers of Terrestrial Evapotranspiration

Evapotranspiration (ET) is a critical component of the Earth’s system that plays a central role in the water, energy, and carbon cycles. The ET dynamics are potentially altered by rising atmospheric carbon dioxide concentration ([CO2]), which could drive both warming and changes in vegetation physiology. However, the interaction between ET and other drivers, such as climate and vegetation, under rising [CO2] remains unclear. In this study, we determine and compare the dominant drivers of ET from Earth system model simulations and observation-based datasets with different [CO2] concentrations at annual and seasonal scales. Models largely agree on radiative effects on atmospheric conditions but diverge in simulating the physiological effects of [CO2], such as changes in leaf area index (LAI). In terms of the dominant drivers of ET, models agree at the monthly time scale but less at the annual time scale, likely due to limited large-scale model constraints at longer timescales. Under increasing [CO2], we find no widespread shift in dominant ET drivers at either time scale. However, one model suggests that 6% of land areas may transition from vegetation to water control of ET, although the accuracy of this shift is uncertain. The lack of large-scale observations of ET and differences in model representations of ET dynamics hinder our understanding of the land-atmosphere feedback under future global change. Our research highlights the need for improved model representation of key hydraulic dynamics drivers (e.g., LAI, soil moisture, and stomatal conductance) for more accurate climate and ecosystem predictions.