Global models overestimate streamflow induced by rising CO2

The global streamflow plays a crucial role in the broader water cycle, intricately linked to human activities, ecology, and agriculture. The rise in atmospheric CO₂ has complex effects on global streamflow. In addition to feedbacks to climate change, CO₂ impacts on streamflow result from surface changes, including reduced streamflow induced by expanding vegetation and increased streamflow induced by reduced vegetation evapotranspiration due to stomatal closure. Global models, vital for policy planning, predict increased streamflow due to dominant positive impacts of elevated CO₂. More than 10 out of 14 global dynamic vegetation models concluded that increased CO₂ exacerbated runoff growth over the 1981-2020 period, especially in tropical and temperate regions. Yet, studying four decades of observed streamflow data, we find these models largely overestimate the increase in streamflow induced by elevated CO₂, particularly in tropical forest (tropical) and cold forest (cold), pointing to an unexpectedly drier world.