Most of the models and studies that have fed into recent assessments, such as the UN–IPCC third assessment report of future staple crop production, derive their ‘CO2-fertilization’ effect from Kimball (1983) (Gitay et al. 2001). The models used to predict future crop yields mathematically approximate carbon gain from either radiation use efficiency or net photosynthesis (e.g. CERES-wheat, Ritchie & Otter 1985; Godwin et al. 1989; CERES-rice, Singh et al. 1989; CERES-maize, Jones & Kiniry 1986; Ritchie et al. 1989; SOYGRO, Jones et al. 1989; EPIC, Stockle et al. 1992)