The extensive survey of Kimball (1983) suggests that with an increase in [CO2] to 550 μmol mol−1 C3 yield should increase by 25% with a 99% lower confidence interval of 18%. Yet, not one of the three major C3 crops showed an actual yield increase that reached this lower confidence limit of 18% when tested under large scale replicated open-air elevation of [CO2] in FACE. For wheat actual values were a third of that observed in chambers, for rice two-thirds and soybean four-fifths. For the two major C4 crops, there was no significant increase in yield at all in FACE compared to the chamber-predicted increase of 7% and modelled increase of 10% (table 1). Evidence from chamber experiments has been contradictory regarding direct effects of elevated [CO2] on C4 photosynthesis