How plants regulate their exchange of CO₂ and water with the atmosphere under global warming is critical for their carbon uptake and their cooling influence. We analyze the isotope ratio of atmospheric CO₂ and detect no significant decadal trends in the seasonal cycle amplitude. The data are consistent with the regulation towards leaf CO₂ and intrinsic water use efficiency growing proportionally to atmospheric CO₂, in contrast to recent suggestions of downregulation of CO₂ and water fluxes.