[OldNavyVet]

According to Steve Jones (Darwin’s Chost, page 231): “five hundree million years ago the air had twenty times as much carbon dioxide as it contains now.”

According to http://chemistry.about.com/od/chemistryfaqs/f/aircomposition.htm

This is composition of air in percent by volume, at sea level at 15°C and 101325 Pa.

Nitrogen — N2 — 78.084%

Oxygen — O2 — 20.9476%

Argon — Ar — 0.934%

Carbon Dioxide — CO2 — 0.0314%

That means that air of 500 million years ago contained around 0.628% of CO2.

[Jim Robinson]

Man, if we had that much CO2 today, we'd all blimp up like dinosaurs algore.