Fractionation of carbon isotopes by plants during CO ₂ uptake and fixation (Δ _leaf ) varies with environmental conditions, but quantitative patterns of Δ _leaf across environmental gradients at the global scale are lacking. This impedes interpretation of variability in ancient terrestrial organic matter, which encodes climatic and ecological signals. To address this problem, we converted 3,310 published leaf δ ¹³ C values into mean Δ _leaf values for 334 woody plant species at 105 locations (yielding 570 species-site combinations) representing a wide range of environmental conditions. Our analyses reveal a strong positive correlation between Δ _leaf and mean annual precipitation (MAP; R ² = 0.55), mirroring global trends in gross primary production and indicating stomatal constraints on leaf gas-exchange, mediated by water supply, are the dominant control of Δ _leaf at large spatial scales. Independent of MAP, we show a lesser, negative effect of altitude on Δ _leaf and minor effects of temperature and latitude. After accounting for these factors, mean Δ _leaf of evergreen gymnosperms is lower (by 1–2.7‰) than for other woody plant functional types (PFT), likely due to greater leaf-level water-use efficiency. Together, environmental and PFT effects contribute to differences in mean Δ _leaf of up to 6‰ between biomes. Coupling geologic indicators of ancient precipitation and PFT (or biome) with modern Δ _leaf patterns has potential to yield more robust reconstructions of atmospheric δ ¹³ C values, leading to better constraints on past greenhouse-gas perturbations. Accordingly, we estimate a 4.6‰ decline in the δ ¹³ C of atmospheric CO ₂ at the onset of the Paleocene-Eocene Thermal Maximum, an abrupt global warming event ∼55.8 Ma.