[Editors’ note: the author responses to the first round of peer review follow.]

Reviewer 2 claims that there is “no published data from FACE experiments in the tropics.” Her opinion is that “elevated CO 2 may have substantially less impact” on plant quality in the tropics and, “therefore, it is very uncertain what effect elevated CO2 will have on human nutrition there.”

This argument is flawed. There are published FACE, open-top chamber and greenhouse experiments carried out between the 35° N & S latitudes – the tropical and subtropical regions, where large parts of malnourished population reside (e.g., Ma et al. 2007; Jia et al. 2007, Pal et al. 2003, 2004; Singh et al. 2013; Khan et al. 2012; Azam et al. 2012), and they do show declines in the plant mineral content. Prompted by the eLife review, I made the regional analysis of all the CO 2 studies carried out between the 35° N & S parallels: it shows that the plant mineral content declines by 5% in the region. Furthermore, many countries in the tropics rely on imports of wheat, maize and soybeans, most of which are grown north of the N 35°parallel, where FACE and other experiments also reveal declines in the crop mineral content.

No reviewer found any logical flaws in my human-nutrition thought experiment, which relies on the rigor of mass balance laws. However, I understand that such “experiments” are not conventional even if their conclusions are valid. For this reason, I can tone down and shorten the health and obesity discussion. The revised paper will focus on firmly establishing a novel and important aspect of global change – the shift in the plant ionome induced by the rising CO 2 .

I emphasize that this is a novel result because the last definitive word on the issue was Duval et al. (2011) meta-analysis claiming the absence of any prevailing effect of elevated CO 2 on the plant minerals and, specifically, the lack of response of grain minerals to high-CO 2 – claims that are opposite to my results.

The power is in your hands to give my revised and stronger paper further consideration at eLife and to advance the progress on this important issue.

[Editors’ note: the author responses to the re-review follow.]

After discussion, a consensus agreement was reached that the manuscript could be accepted if it was substantially revised so that it was clear that impact of changes in nutrient content of the edible portion of food crops on human health has not been settled. We hope that in revising the manuscript, this uncertainty is explicitly addressed and that you could highlight the need for more research to address this very important but festering issue.

I revised the manuscript accordingly. Specifically:

I have added the following statement to the Discussion: “I emphasize that the impact of CO 2 -induced shifts in the quality of crops on human health is far from settled. The purpose of what follows is not to make definitive claims but to stimulate research into this important but festering issue.”

I have added a new subsection to the Discussion, titled “Data Scarcity,” noting that for many crops, the pertinent data are limited or non-existent.

The wording about the effects of the mineral decline on human nutrition was toned down from “will” to “can”/“might/potential”. (Furthermore, the abstract was changed to stress that the effects on human health are discussed (and, thus, are not parts of results.)

In addition, it was felt that the Introduction should be shortened, downplaying the thought experiment...

I deleted the passage referring to my 2002 ‘thought experiment’ from the Introduction and do not mention it anywhere else in the manuscript.

I deleted the reference to Loladze & Elser (2011) together with the sentence stating that the cellular stoichiometric homeostasis is sensitive to the environment.

I have shortened and simplified the passage about the dichotomy between CO 2 effects on N and minerals.

I have shortened and improved readability of the list of questions at the end of the Introduction. The new Introduction is shorter by ∼200 words.

...and significantly tempering the conclusions drawn in the Discussion.

Aside from changes indicated above, I have made the following changes to the Discussion:

1) Deleted Figure 9 showing the graphical output of Hall et al. (2011) dynamic model of weight gains in a female and a male.

2) Tempered and shortened conclusions about the impact on human health. Now the statements are reduced to: “The above ‘experiment’ suggests that a systemic and sustained 5% mineral depletion in plants can be nutritionally significant. While the rise in the atmospheric CO 2 concentration is expected to be nearly uniform around the globe, its impact on crop quality might unequally affect the human population: from no detrimental effects for the well-nourished people to potential weight gain for the calorie-sufficient but mineral-undernourished.”

In addition, to the above changes I revamped the Results by separating them into clear subsections. Furthermore, the readability throughout the paper was improved.