Plant responses to climate change are central to projected drought and aridity changes. Some theories and analyses of earlier generations of Earth System Models (ESM) suggest that transpiration will decline with higher atmospheric carbon dioxide concentrations [CO2], enhancing runoff and soil moisture, and countering continental drying predicted by warming alone. Here we use the latest-generation of ESMs to show that the opposite effect prevails: plants themselves contribute to projected soil drying, with smaller effects on runoff, and this picture emerges by considering nonlinear interactions between warming and the physiological effects of high [CO2] on plant-based evapotranspiration (ET, transpiration plus leaf evaporation). Across the ensemble, the strengths of these nonlinearities in plant-based ET scale with a weakened sensitivity of terrestrial carbon loss from warming—a key feedback in the carbon cycle. Our results suggest that the linearity assumption underpinning analyses of carbon feedbacks used over the last twenty years are not appropriate for the latest generation of ESMs, with implications for the accurate interpretation of projected changes in the carbon cycle and its consequences for future climate, drought, and water availability.