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Elevated carbon dioxide (eCO(2)) is well known to stimulate plant photosynthesis and growth. Elevated carbon dioxide's effects on crop yields are of particular interest due to concerns for future food security. We compiled experimental data where field-grown wheat (Triticum aestivum Linnaeus) was exposed to different CO2 concentrations. Yield and yield components were analyzed by meta-analysis to estimate average effects, and response functions derived to assess effect size in relation to CO2 concentration. Grain yield increased by 26% under eCO(2) (average ambient concentration of 372 ppm and elevated 605 ppm), mainly due to the increase in grain number. The response function for grain yield with CO2 concentration strongly suggests a non-linear response, where yield stimulation levels off at 600 ppm. This was supported by the meta-analysis, which did not indicate any significant difference in yield stimulation in wheat grown at 456-600 ppm compared to 601-750 ppm. Yield response to eCO(2) was independent of fumigation technique and rooting environment, but clearly related to site productivity, where relative CO2 yield stimulation was stronger in low productive systems. The non-linear yield response, saturating at a relatively modest elevation of CO2, was of large importance for crop modelling and assessments of future food production under rising CO2.