> I would appreciate comments regarding how
> trustworthy this result is.
You wouldn't have a link to a free version of the paper?
I do notice that in the abstract they only mention one proxy line of
evidence. They do not mention stomata:
http://gsabulletin.gsapubs.org/content/121/9-10/1441.abstract
"Evidence for past CO2 spikes comes from expanded and refined stomatal
index data of fossil Ginkgo and related leaves. ...
The magnitude of the coming anthropogenic greenhouse pales in
comparison with past greenhouse spikes at times of global mass
extinctions."
My opinion is that the proxy evidence for CO2 concentrations is too
poor to distinguish 1000 and 7000 ppm reliably hundreds of millions of
years ago, and that we also need a better picture of other forcings in
the far past to conclude much.
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