Direct vegetation response to recent CO rise shows limited effect on global streamflow.

Global streamflow, crucial for ecology, agriculture, and human activities, can be influenced by elevated atmospheric CO (eCO) though direct regulation of vegetation physiology and structure, which can either decrease or increase streamflow. Despite a 21.8% rise in CO over 40 years, its impact on streamflow is not obvious and remains highly debated. Using a full differential approach at the catchment scale and an optimum finger approach globally, both constrained by observed streamflow, here, we find that vegetation responses to eCO in 1981-2020 has limited impact on streamflow via direct regulation. The median eCO contribution approaches zero across 1116 unimpacted catchments, and global streamflow changes cannot be solely attributed to eCO. These results offer key insights into the intricate dynamics of CO and other factors shaping streamflow changes over the past four decades. Such understanding is vital for attributing current streamflow changes under eCO conditions.