Basic reporting

The article is clearly written, interesting, and well prepared. The figures support the reported conclusions.

A few points of clarification are needed:

1) Introduction, line 1: According to the UN Population Division, the human population exceeded 7 billion in October 2012 (on Halloween, notably, although this is obviously just an approximation).

2) Introduction, on the assumption of the fungibility of crops: Obviously, this is quite a large assumption in the context of the present analysis. One could imagine lots of reasons for farmers electing to have multiple crops, ranging from balancing their dietary requirements to bet-hedging against crop-specific pathogens, weather, and crop-price fluctuations. Some brief discussion of this later in the paper would be warranted.

3) Results section: One point on which I was not clear was crop transport. Some crops might be produced near to where their demand is concentrated, even if that locale is suboptimal. Is this factored into the analysis? I presume not. Again, this might be mentioned briefly in the Discussion.

Experimental design

The design of the analysis is effective and well considered, and falls within the scope of the journal. The paper contains a great deal of interesting analysis and interpretation.

Validity of the findings

My sense is that the analyses are reasonably robust and effectively interpreted, using the best available information and data sets at hand. The conclusions seem broadly justified by the analyses, notwithstanding the need for some minor points of clarification as indicated above.

Comments for the author

I found much of interest in this paper. It is appropriately framed as a sort of thought experiment, and addresses some very big and important questions.