The Early faint Sun paradox is interesting. Having done a bit of
reading about it, it seems that one plausible explanation is that the
steady increase in the Sun's solar output may have been compensated by
a (less steady) decline in greenhouse gas concentrations, notably the
very powerful, but low concentration ones like methane.
Having said that, I still have trouble with the notion that forcings
always have the same level of feedbacks. I find it quite plausible on
the face of it that clouds, ice sheets, greenhouse gases etc.
sometimes could act as a near perfect thermostat, and sometimes they
might act to amplify even the tiniest of external perturbations. So,
in the ice ages, a solar related external perturbation of virtually
nothing got amplified to 6C, and over a longer time scale, a massive
solar perturbation didn't get amplified, but rather regulated away by
the Earth system response.
What do I conclude from that about how the system might respond to a
perturbation now? Well, I clearly don't buy Lindzen's line that it's a
near certain fact that a few W/m2 will now give virtually no response.
But I also fail to see, where the certainty comes from that as James
Annan put it, climate sensitivity is 3C. By analogy with past climate
alone it's certainly plausible to expect both the possibility of
massive amplification and a virtually perfect thermostat.
So, maybe in the ice ages, cloud feedbacks due to ice sheets were very
different to how cloud feedbacks might operate due to a forced
increase in CO2 concentrations together with massive aerosol
injections plus quite a few land use changes. And, while those
feedbacks might be thermostat like now, why shouldn't there be a
threshold, where clouds suddenly become strongly amplifying instead?
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