Stomatal density is one of the plant traits influencing leaf gas exchange and is known to be affected by the plant’s environment. Understanding its degree of plasticity to various abiotic factors is therefore important. We conducted a meta-analysis of a wide range of experiments in which plants were grown under different levels of CO₂, light, temperature, and water availability, and derived generalized dose-response curves. Although both stomatal density and stomatal index showed a significant negative correlation with CO₂ levels, these relationships were weak and only marginally consistent across the analyzed experiments. In contrast, the effect of growth light intensity was positive, highly consistent, and substantially stronger than the impact of atmospheric CO₂. Temperature also positively influenced stomatal density, while water availability showed no consistent effects. Based on these dose-response curves, we highlight several caveats when using stomatal density or stomatal index for paleo-CO₂ reconstruction. The weak CO₂ response, coupled with the strong confounding impact of light intensity, poses significant limitations to the accuracy of such estimates.