Plant physiological adaptation to the global rise in atmospheric CO 2 concentration (CO 2 ) is identified as a crucial climatic forcing. To optimize functioning under rising CO 2 , plants reduce the diffusive stomatal conductance of their leaves (g s ) dynamically by closing stomata and structurally by growing leaves with altered stomatal densities and pore sizes. The structural adaptations reduce maximal stomatal conductance (g smax ) and constrain the dynamic responses of g s . Here, we develop and validate models that simulate structural stomatal adaptations based on diffusion of CO 2 and water vapor through stomata, photosynthesis, and optimization of carbon gain under the constraint of a plant physiological cost of water loss. We propose that the ongoing optimization of g smax is eventually limited by species-specific limits to phenotypic plasticity. Our model reproduces observed structural stomatal adaptations and predicts that adaptation will continue beyond double CO 2 . Owing to their distinct stomatal dimensions, angiosperms reach their phenotypic response limits on average at 740 ppm and conifers on average at 1,250 ppm CO 2 . Further, our simulations predict that doubling today's CO 2 will decrease the annual transpiration flux of subtropical vegetation in Florida by ≈60 W·m −2 . We conclude that plant adaptation to rising CO 2 is altering the freshwater cycle and climate and will continue to do so throughout this century.climate change | physiological forcing | plant evolution P lants respond to the complex of environmental signals they perceive by plastic changes in their phenotype to increase individual fitness (1). The most apparent environmental change that induces phenotypic adaptations in plants is the global increase in atmospheric CO 2 concentration (CO 2 ) (2). In response to this rise in CO 2 , plants reduce the diffusive stomatal conductance of their leaves [g s (mol·m −2 ·s −1 )] to increase drought resistance (3) and reduce physiological costs associated with water transport (4). Plants can reduce g s by dynamically closing their stomata within minutes (5, 6), and structurally within the lifespan of an individual by growing leaves with altered stomatal density [D (number of stomata·m −2 )] and pore size at maximal stomatal opening [a max (m 2 )] (7, 8). Structural adaptations thereby reduce maximal stomatal conductance [g smax (mol·m −2 · s −1 )], which critically reduces actual g s , especially when stomata are fully open during times with ample light and water (9).Reduction of g s via structural adaptation of g smax has the potential to reduce transpiration fluxes and, thus, cause land surface warming in addition to changes in the global hydrological cycle with rising CO 2 (10). This climatic effect is termed the physiological forcing of CO 2 , which acts beside and independent of its radiative forcing. Despite advances to quantify this physiological forcing with global climate models (11, 12), these models rely on semiempirical relations to simulate g s from environmental variables (...