Quote:
Originally Posted by Retrograde
The other thing about higher concentrations of CO2 in the distant past is that when you go back hundreds of millions of years the difference in solar intensity between then and now is significant.
300 million years ago during the Corboniferous era TSI was roughly 2.5% less than today due to the sun's long-term life cycle (which partly explains why the abundance in atmospheric CO2 then wasn't a huge threat to life).
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Indeed, there are a number of 'boundary conditions' that affect the climate system but only change on geological timescales. Solar energy output is one. Another is the distribution of land masses and their effect on ocean currents. We know, for instance, that the opening of the Southern Ocean and the establishment of the circum-Antarctic current both froze Antarctica and lowered world temperatures. Prior to that warm equatorial water reached to pole and kept Antarctica warm and vegetated. Another possibility is the strength of the magnetic field. If it varies (does it?) then it should affect the solar energy reaching the ground.