Thank you for addressing the reviewer comments. Both reviewers were positive about the paper and I also think it is an important contribution to the paleo-CO2 literature. The revisions proposed address most of their concerns. I note a few issues below that I would like to see addressed before final acceptance and ask that you submit a revised version of the paper that includes the revisions you proposed and addresses these further comments:

1) Line 98-101. Can you clarify this discussion of uncertainty? The issue is whether all studies using only stomatal density ingore the two sources of error you mention, or just some do. The wording here is not clear on that point.

Line 102. Smaller than what? With fossil taxa? Please clarify.

Line 116-118. This sentence strikes me as awkward it refers to "elements" but then phrases them as questions. Can you reword?

Line 121. Can you provide a more informative title for the table?

Line 170-171. How are the ambient CO2 values known?

Line 239. Regarding Milligan et al., please just provide the details of the method. Even if the paper is in press the reader would benefit.

Line 320-321. I think it would help here to explain that you do not have measurements of d13C-CO2.

Line 342-346. I understand why you use the 2/3 range, but you should explain here why. Also, what does it mean to say that 28% overlap the target at 95% confidence if they are really noisy (that is, is this a useful statement). And, how is the target value established? Assumed 400 ppm?

Line 526. Can you replace "tough" with a more specific term?

With best wishes, Ed Brook

Thanks very much for taking care of the last set of corrections so quickly!