This conclusion suggests a larger difference between FACE and chamber studies, than noted in the earlier reviews by Amthor (2001) and Kimball et al. (2002). What might explain this difference? Amthor (2001) reviewed all previously published studies of wheat grown throughout its life cycle in elevated [CO2]. In common with this study, the yield increase in FACE was less than in chamber studies. It indicates an overestimation in chamber studies relative to FACE, although the difference is considerably less than indicated by table 1. However, most chamber studies used [CO2] of ca 680–700 ppm and Amthor (2001) assumed a linear increase in yield with increase in [CO2] from 350 to 700 ppm. This would underestimate the yield increase that would be observed in chambers at 550 ppm if the response is curvilinear.