Technical Abstract: This letter to New Phytologist defends Long et al. (2006), in which we reported that stimulation of seed yield in response to elevated [CO2] is lower in FACE experiments than in enclosure studies of the world¿s four most important food crops. We suggested that the implications of this finding were that (1) modeling studies using CO2 fertilization factors derived from enclosure experiments may have over-estimated future food supply; and (2) additional field experiments are needed to better understand the mechanism of response and drive research and development efforts to improve crop yields under future climatic conditions. Tubiello et al. (2007a) have vigorously challenged these findings and subsequently dismissed our results in Tubiello et al. (2007b) and in the latest Intergovernmental Panel on Climate Change (IPCC) report (Easterling et al., 2007). In this letter, we use a different statistical model to again show that FACE experiments are significantly different from chamber studies, and we highlight important differences between modeling results and experimental results. While enclosure studies have been critical in discerning mechanisms of response, they are not the best experimental facilities for estimating CO2 response ratios of yield. We reiterate the concern that more FACE experiments are needed in addition to the chamber studies in order to generate the best possible understanding of crop responses to elevated [CO2] and to improve the performance of crops under future conditions.