How To Lie While Telling the Truth: Part II
It has long been known that increasing the concentration of CO2 increases the yield of C3 plants, including most crop species. The usual estimate is that doubling CO2 concentration, roughly what is projected for the end of this century, increases yield by 30% or more. This is an inconvenient fact for people who want to argue that AGW will reduce food supplies. Clearly what they need is a scientific article to cite, proving that CO2 is actually a bad thing for crops.
A quick google found the following table of minerals for wheat:
It's worse than that. The figures, at least according to the news story and the abstract of the article, are for concentration. Increasing CO2 increases the yield of C3 crops. If the concentration of zinc goes down by nine percent but the total crop yield goes up by more than nine percent, total yield of zinc goes up, not down.
Further, the abstract makes it clear that the effect varies from one cultivar to another—so if there really is a problem with the concentration of zinc and iron, farmers will have an incentive to switch to cultivars for which there isn't.
If, with a doubling of CO2 (the abstract does not say what concentration was used in the experiments), crop yield goes up by 30% but the concentration of two minerals goes slightly down and the yield of protein only increases by 24%, that is a "threat to human nutrition."
A striking example of propaganda disguised as science.
[I have sent a message to the lead author of the article inviting him to respond here]
Another paper, not behind a pay wall, by many of the same authors. It makes it explicit that "we believe the simplest approach is to model diets that are unchanged with respect to calories and composition."