jats:titleAbstract</jats:title>jats:secjats:titleAim</jats:title>jats:pWithin Cjats:sub3</jats:sub> plants, photosynthesis is a balance between COjats:sub2</jats:sub> supply from the atmosphere via stomata and demand by enzymes within chloroplasts. This process is dynamic and a complex but crucial aspect of photosynthesis. We sought to understand the spatial pattern in COjats:sub2</jats:sub> supply–demand balance on a global scale, via analysis of stable isotopes of carbon within leaves (Δjats:sup13</jats:sup>C), which provide an integrative record of COjats:sub2</jats:sub> drawdown during photosynthesis.</jats:p>jats:secjats:titleLocation</jats:title>jats:pGlobal</jats:p></jats:sec>jats:secjats:titleTime period</jats:title>jats:p1951–2011.</jats:p></jats:sec>jats:secjats:titleMajor taxa studied</jats:title>jats:pVascular plants.</jats:p></jats:sec></jats:sec>jats:secjats:titleMethods</jats:title>jats:pWe assembled a database of leaf carbon isotope ratios containing 3,979 species–site combinations from across the globe, including 3,645 for Cjats:sub3</jats:sub> species. We examined a wide array of potential climate and soil drivers of variation in Δjats:sup13</jats:sup>C.</jats:p></jats:sec>jats:secjats:titleResults</jats:title>jats:pThe strongest drivers of carbon isotope discrimination at the global scale included atmospheric pressure, potential evapotranspiration and soil pH, which explained 44% of the variation in Δjats:sup13</jats:sup>C. Addition of eight more climate and soil variables (each explaining small but highly significant amounts of variation) increased the explained variation to 60%. On top of this, the largest plant trait effect was leaf nitrogen per area, which explained 11% of Δjats:sup13</jats:sup>C variation.</jats:p>jats:pjats:boldMain conclusions:</jats:bold> By considering variation in Δjats:sup13</jats:sup>C at a considerably larger scale than previously, we were able to identify and quantify key drivers in COjats:sub2</jats:sub> supply–demand balance previously unacknowledged. Of special note is the key role of soil properties, with greater discrimination on low‐pH and high‐silt soils. Unlike other plant traits, which show typically wide variation within sets of coexisting species, the global pattern in carbon stable isotope ratios is much more conservative; there is relatively narrow variation in time‐integrated COjats:sub2</jats:sub> concentrations at the site of carboxylation among plants in a given soil and climate.</jats:p></jats:sec>