What was done
The authors studied the effects of minor-to-modest CO2 concentration increases of 14.5, 40 and 54.5 ppm on leaf area index, leaf area duration, shoot biomass and yield of spring wheat (Triticum aestivum cv. Dingxi 24) grown for two years in a semi-arid region of China under (1) natural conditions, (2) with irrigation and (3) with irrigation and added fertilizer.

What was learned
Among other things, Li et al. report that "elevated CO2 concentration owing to CO2 application leads to remarkable increase in leaf area index and shoot biomass, and also generates the higher value of leaf area duration that can benefit the photosynthesis in the growth stage and yield increase in crop." More specifically, they report that the 40-ppm increase in CO2 concentration increased grain yield by 4.2% (0.105%/ppm) under natural conditions, while with irrigation and fertilization (treatment 3) the 14.5-, 40- and 54.5-ppm CO2 concentration increases increased grain yield by 6.3% (0.434%/ppm), 13.1% (0.328%/ppm) and 19.8% (0.363%/ppm), respectively.

What it means
Even under natural (non-irrigated and unfertilized) conditions in a semi-arid region of China, the CO2-induced increase in wheat yield observed in this study (0.105%/ppm) was equivalent to the low-end response derived from the meta-analyses of Long et al. (2005, 2006). This finding is particularly gratifying, since the study of [in prior week's issue] Lobell and Field (2007) demonstrates that even this low-end response is sufficient to more than compensate for the negative effect of global warming experienced between 1981 and 2002 on global wheat yield. What is more, since this period of time is typically claimed by climate alarmists to have seen global temperatures rise at an unprecedented rate and to an unprecedented level, these findings suggest that even this supposedly catastrophic degree of global warming poses absolutely no threat to global wheat yields when it is accompanied by the current rate-of-rise in the air's CO2 content. And when water and fertilizers are applied to the soil, the aerial fertilization effect of atmospheric CO2 enrichment in the region of Li et al.'s study would appear to be even more effective, rising to a yield-response-level somewhere in the vicinity of 0.328-0.434%/ppm.

References
Lobell, D.B. and Field, C.B. 2007. Global scale climate-crop yield relationships and the impacts of recent warming. Environmental Research Letters 2: 1-7.

Long, S.P., Ainsworth, E.A., Leakey, A.D.B. and Morgan, P.B. 2005. Global food insecurity. Treatment of major food crops with elevated carbon dioxide or ozone under large-scale fully open-air conditions suggests recent models may have overestimated future yields. Philosophical Transactions of the Royal Society B 360: 2011-2020.

Long, S.P., Ainsworth, E.A., Leakey, A.D.B., Nosberger, J. and Ort, D.R. 2006. Food for thought: Lower-than-expected crop yield stimulation with rising CO2 concentrations. Science 312: 1918-1921.