Climate warming is intensifying terrestrial water scarcity and drought risks worldwide. Meanwhile, rising atmospheric CO₂ reduces plant stomatal conductance— the openness of leaf pores that governs both CO₂ intake and water loss — and improves water-use efficiency, which has been widely considered capable of alleviating land surface drying. However, a recent study reveals that this perceived benefit may have been optimistically overestimated.

On June 24, the team led by Prof. YUAN Xing published a paper in PNAS, uncovering a critical indirect impact that substantially limits the water-saving potential of vegetation under elevated CO₂, originating from atmospheric feedbacks.

Based on simulations from Earth system models participating in CMIP6-C4MIP (the carbon-cycle climate intercomparison project within the global CMIP6 modeling framework), the research team isolated the indirect impacts of vegetation changes on evapotranspiration through atmospheric feedbacks, and found that vegetation responses to rising CO₂ (including increased leaf area and reduced stomatal conductance) can alter surface energy balance, leading to warming and enhanced atmospheric evaporative demand, which ultimately drives greater surface water loss, undermining the water savings they were expected to deliver.

This previously overlooked indirect impact offsets 54% of the vegetation water-saving effect in northern mid-to-high latitudes under current climate conditions, and this ratio is projected to increase to 68% under a future 4×CO₂ scenario. Most concerning, at high latitudes, the net CO₂ physiological effect on evapotranspiration becomes statistically insignificant, meaning the anticipated water-saving benefit essentially disappears.

The indirect impacts of vegetation changes exhibit strong latitudinal differences (Fig. 1), with particularly pronounced effects in northern mid-to-high latitudes. In these regions, sustained water loss associated with atmospheric feedbacks can undermine the potential benefits of vegetation in mitigating land surface water scarcity, covering major agricultural regions such as the U.S. Corn Belt, European plains, and rice-growing regions of China.

Direct and indirect pathways through which CO₂ physiological effects (PHY) influence evapotranspiration. (Image by IAP)

As droughts become more frequent under climate change, vegetation’s role in regulating land surface water availability is becoming increasingly critical for agricultural production and water resource management. However, “the study warns against overreliance on CO₂ physiological effects as a natural solution to drought,” said YUAN.

Meanwhile, in low-latitude regions, the first author HAO Yi noted that “although CO₂ physiological effects may partially alleviate soil moisture drought, increasing compound heat and atmospheric drought stress are more likely to threaten ecosystem sustainability.”

The findings highlight that vegetation's CO₂ response alone cannot secure water resources or ecosystem stability under future climate change, underscoring the need for proactive adaptation strategies — including improved irrigation efficiency, drought-resilient crop breeding, and sustainable water resource planning.

Paper info:

Hao, Y., X. Yuan*, X. Xi, Z. Zeng, G. Forzieri, P. Wu, 2026: A potential overestimation of CO₂ physiological effects on evapotranspiration. Proceedings of the National Academy of Sciences of the United States of America, 123(26), e2534643123. https://doi.org/10.1073/pnas.2534643123