* The warming after the ice ages was not set off by the forcing effect of CO2 increases
* As the planet warmed up through other factors (orbit variations, tilt of axis etc) CO2 was released from the seas and acted as a forcing agent further increasing global temperature (by how much we cannot say for sure).
This does lead to the obvious next question posed and answered in New Scientist –
Higher temperatures lead to more CO2 and more CO2 leads to higher temperatures, why doesn’t this positive feedback lead to a runaway greenhouse effect?
There are various limiting factors that kick in, the most important being that infrared radiation emitted by Earth increases exponentially with temperature, so as long as some infrared can escape from the atmosphere, at some point heat loss catches up with heat retention.
What do we conclude from this? That the planet may recover from CO2 increase but most of it’s inhabitants won’t survive?
Is there a saturation point for the amplification effect of CO2?
We can be pretty confident that there is no runaway positive feedback, because of evidence much further back than the ice cores of carbon dioxide levels of order 1500 ppm in the Mesozoic.
There is a saturation effect in that the bands where water and carbon dioxide are already not that far off from being saturated, leaving big windows in the IR where water and carbon dioxide do not absorb. These windows will continue to do the whole Stefan-Boltzmann thing as the marginal increase in absorption from the bands gets smaller and smaller.