Fran Manns wrote:
Present day CO2 level were greatly exceeded by Carboniferous CO2 levels.
http://www.geocraft.com/WVFossils/Ca...s_climate.html


For the figure see the link.

Global Temperature and Atmospheric CO2 over Geologic Time
Similarly, atmospheric concentrations of carbon dioxide (CO2) in the Early
Carboniferous Period were approximately 1500 ppm (parts per million), but by
the Middle Carboniferous had declined to about 350 ppm -- comparable to
average CO2 concentrations today!

Earth's atmosphere today contains about 370 ppm CO2 (0.037%). Compared to
former geologic times, our present atmosphere, like the Late Carboniferous
atmosphere, is CO2- impoverished! In the last 600 million years of Earth's
history only the Carboniferous Period and our present age, the Quaternary
Period, have witnessed CO2 levels less than 400 ppm.

Global Temperature and Atmospheric CO2 over Geologic Time

Late Carboniferous to Early Permian time (315 mya -- 270 mya) is the only
time period in the last 600 million years when both atmospheric CO2 and
temperatures were as low as they are today (Quaternary Period ).

(...)

One thing to keep in mind - if my understanding of stellar
physics is correct, the sun was also significantly less bright
than it is now. Solar luminosity is thought to be increasing as
the sun ages. Perhaps within a billion years it may be too
high to support life (photodissociation of water molecules, but
I digress).

So perhaps the Earth could support higher CO2 levels in the
distant past and still be suitable for life than it can now?

-Eric B