Projected runoff declines from plant physiological effects on precipitation

The impact of plants on runoff under high atmospheric CO 2 is a major uncertainty for future water resources. Theory and Earth System Models (ESMs) suggest that stricter plant stomatal regulation under high CO 2 will reduce transpiration, potentially boosting runoff. Yet, across a 12-member ensemble of idealized ESM simulations that isolate plant responses to CO 2 , we show that lower transpiration robustly enhances runoff over only 5% of global land area. Precipitation changes are a significant signal of CO 2 physiological forcing over 31-57% of land areas across models, and are five times more important than transpiration changes in driving runoff responses. Crucially, ESMs largely disagree on where physiologically forced precipitation changes occur, but agree that plant responses in most locations are as likely to reduce runoff as increase it. These results imply that large model uncertainties in precipitation responses, rather than transpiration responses, explain why ESMs disagree on plant physiologically driven runoff changes.