Crop response to elevated CO₂ and world food supply. A comment on "Food for Thought..." by Long et al., Science 312:1918-1921, 2006

Recent conclusions that new free-air carbon dioxide enrichment (FACE) data show a much lower crop yield response to elevated CO₂ than thought previously - casting serious doubts on estimates of world food supply in the 21st century - are found to be incorrect, being based in part on technical inconsistencies and lacking statistical significance. First, we show that the magnitude of crop response to elevated CO₂ is rather similar across FACE and non-FACE data-sets, as already indicated by several previous comprehensive experimental and modeling analyses, with some differences related to which "ambient" CO₂ concentration is used for comparisons. Second, we find that results from most crop model simulations are consistent with the values from FACE experiments. Third, we argue that lower crop responses to elevated CO₂ of the magnitudes in question would not significantly alter projections of world food supply. We conclude by highlighting the importance of a better understanding of crop response to elevated CO₂ under a variety of experimental and modeling settings, and suggest steps necessary to avoid confusion in future meta-analyses and comparisons of experimental and model data.