Table 1.

Comparison of chamber and FACE findings on percentage increase in yield with elevation of [CO₂] to 550–575 μmol mol⁻¹.

crop	lineara	hyperbolicb	FACE	some recent model projectionsc
wheat	22	25	8d	19e, 23f, 33g
rice	16	18	10h	26i
maize	6	7	n.s.j	10f
sorghum	23	34	n.s.k
soybean	17	19	15l	16f
mean (C3)	23	25	11	19m−27n

n.s., not significantly different from zero.

^aMean projected increase in yield reported linearly extrapolated to 550–575 μmol mol⁻¹ from the averages provided by Kimball (1983) for 660 μmol mol⁻¹ and for wheat from the average observed in open-top chambers across western Europe in the ‘ESPACE-wheat’ project (Bender et al. 1999).

^bMean projected increase assuming a hyperbolic response of yield to increasing [CO₂], saturating at 2000 μmol mol⁻¹, e.g. Amthor (2001).

^cEffect of elevation to 550 μmol mol⁻¹ extracted by comparison of projections with and without a direct effect of [CO₂].

^dIrrigated spring wheat, Arizona (Kimball et al. 1995).

^eEurope (Ewert et al. 2005).

^fUSA (Thomson et al. 2005).

^gUSA & Canada (Rosenzweig & Iglesias 1998).

^hRice, Honshu Island, average of 3 years and three nitrogen treatments, reported in fig. 4 of Kim et al. (2003).

ⁱORYZA1 model projection (Matthews et al. 1997).

^jRainfed maize, central Illinois (A. D. B. Leakey, M. Uribelarrea, E. A. Ainsworth, S. L. Naidu, A. Rogers, D. R. Ort & S. P. Long, unpublished data).

^kIrrigated sorghum, Arizona (Ottman et al. 2001).

^lRainfed soybean, central Illinois, average of 3 years (Morgan et al. 2005).

^mMean of all major crops in Europe, including C₄.

ⁿUSA (Darwin & Kennedy 2000).