Limited contribution of recent elevated CO2 to global streamflow changes

Global streamflow, a critical resource for ecosystems, agriculture, and human activities, is influenced by various factors, including rising atmospheric CO₂ (eCO₂). Through direct regulation of vegetation physiology and structure, eCO₂ can either increase or decrease streamflow. However, despite a 21% rise in CO₂ over the past 40 years, its impact on streamflow remains unclear and subject to ongoing debate. This study evaluates the effects of eCO₂ on global streamflow from 1981 to 2020, focusing on its direct regulation of vegetation. Using a dataset of 1,116 unimpacted catchments, we find that precipitation is the dominant driver of streamflow changes, accounting for over 70% of observed variability. In contrast, eCO₂ exhibits a negligible influence, with its median contribution approaching zero across catchments. At the global scale, attribution analyses conducted via the regularized optimal fingerprinting method for 14 global ecological model simulations confirm that climate change predominantly explains streamflow trends. No significant evidence supports attributing these changes to eCO₂ or land-use change. Observation-constrained models further enhance the robustness of these findings by reducing uncertainties inherent in global ecological models. These results highlight the limited role of vegetation responses to eCO₂ in driving global streamflow changes, underscoring the primacy of climate variability. This improved understanding of hydrological responses to rising CO₂ is vital for refining future water resource management and adaptation strategies under changing climatic conditions.