Leaf temperature regulates plant physiology, affecting photosynthesis, water use and heat stress. Under elevated [CO2], increases in leaf temperature are commonly attributed to reduced stomatal conductance, yet direct field evidence remains scarce. Using a multi-season free-air CO2 enrichment experiment, we show that a +150 μmol mol⁻1 increase in [CO2] increases the leaf–air temperature difference (ΔT) by ~1.2–1.7 °C in wheat, independently of stomatal regulation. Parallel ΔT–stomatal conductance and ΔT–transpiration relationships indicate that stomatal processes do not explain the warming. Sequential modeling analyses reveal that canopy structural and soil variables account for only a small fraction of the ΔT offset. These findings suggest that CO2-induced warming reflects canopy energy-exchange processes not captured by current models. This highlights a critical gap in Earth system and crop models, which may underestimate future heat-stress risks by relying primarily on stomatal-driven warming pathways.