Projected runoff declines from plant physiological effects on precipitation

The impact of plants on runoff under high atmospheric CO2 is a major uncertainty for the future of global water resources. An emerging consensus based on theory and Earth System Models (ESMs) suggests that stricter plant stomatal regulation under high CO2 will reduce transpiration, potentially boosting runoff. Yet, across a 12-member ensemble of idealized ESM simulations that isolate plant responses to CO2, we show that lower transpiration robustly enhances runoff over only 5% of global land area. Instead, we find that precipitation changes are five times more important than transpiration changes in driving runoff responses when only plants respond to CO2, and are a significant signal of CO2 physiological forcing over 31-57% of land areas across models. Crucially, the models 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, absent any effects from radiative warming. These results imply that large model uncertainties in precipitation responses, rather than transpiration responses, explain why ESMs disagree on plant physiologically driven runoff changes over most of the globe. Together, our findings implicate land-atmosphere rather than land-hydrologic responses as the key mechanistic source of uncertainty in runoff responses under CO2 physiological forcing. They further emphasize that any interpretation of plant-driven runoff responses must consider how precipitation itself will respond to CO2 physiological forcing.