About the CO2 saturation question. Angstrom concluded in the early 1900 that blocking of IR by CO2 would saturate at low concentrations based on laboratory experiments and that these lines overlapped water absorption anyway. This put the idea of human caused climate change to rest for several decades. An engineer named Callendar argued in 1938 that you could saturate a laboratory container but that the atmosphere would saturate in layers as C02 moved higher into the atmosphere. So each layer would act as a barrier for IR from the layer below, producing a cumulative effect, like gradually piling on more blankets. Upper layers have less water and cooler temperatures making the absorption more dependent on CO2. You can do a couple of layers by hand calculation but you need a computer to do multiple layers. Callendar did not initially convince anyone but eventually in the 1950s, better resolution of spectral lines and computer models calculating the effect of saturation in different layers showed that CO2 could have a bigger effect than initially thought.
I haven't heard anything approaching a compelling response either.
But there have been some fascinating incorrect and partially correct
responses, and these can at least help us better understand the physics.
The CO2 spike is in a way quite remarkable. If you had told me that
humans are adding so many gazillion tons of CO2 to the atmosphere
every year and asked me to guess what fraction of that added CO2 stays
in the atmosphere (rather than being absorbed by plants or water), I
would have guessed that the fraction is either close to zero or close
to 100%. That is, either the absorption mechanisms can easily keep up
with the emissions, or they can't keep up at all. But in fact, the
fraction is close to 50%. I find this a remarkable coincidence and
although I'm sure the experts understand the reasons in detail, I'm
not aware of any simple explanation.
There actually are some climate skeptics who deny the basic physics of
the greenhouse effect. Sadly, some of them are professional
physicists, and have even gotten a paper published in a peer-reviewed
journal. Just Google "Gerlich and Tscheuschner" if you're not already
aware of this embarrassing incident. Be sure to read Arthur Smith's
devastating rebuttal.
I think you need more than "common sense" to translate the known CO2
increase into a quantitative predicted temperature increase. An
extremely naive argument would say that if CO2 is responsible for a
third of the current 33C greenhouse effect, then doubling CO2 would
increase the average temperature by another 11C. A more sophisticated
argument would say that doubling CO2 won't have any effect at all,
because the CO2 absorption band is already "saturated" by existing CO2
concentrations and so adding more CO2 doesn't cause any additional
absorption at these wavelengths. The correct analysis acknowledges
that the middle of the absorption band is saturated but then goes on
to consider the "shoulders" of the absorption band as well as energy
transport processes in the atmosphere. The resulting prediction,
without feedbacks, is a temperature increase on the order of only
about 1C. That's for a doubling of pre-industrial CO2 levels, so I
don't think it's correct that "simple" models predict a "few-degree"
increase at current CO2 levels.
The reason why the skeptics can get away with their nonsense is
precisely because the effects are small and the correct explanations
are complicated. As a physicist, I find this situation extremely
frustrating.
Dan
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"A society which reverences the attainment
of riches as the supreme felicity will
naturally be disposed to regard the poor
as damned in the next world, if only to
justify making their life a hell in this."
R. D. Tawney