Why is the response to a forcing exactly, or at least to a pretty good
approximation, linear (ie if 3.6 W/m2 cause 3 C, then it follows that
1.8 W/m2 will cause 1.5 C and 7.2 W/m2 will cause 6C)? Is there some
easy to grasp explanation for that? And how does that square with so
called tipping points?
And how can a forcing be measured?
It can't be measured by just measuring incoming radiation, I presume,
because water vapour is assumed to be a feedback (so 3.6 W/2 say from
CO2 will cause another 7 W/m2 from water vapour or something like
that). But how does that work for things like the indirect cloud
effect of aerosols? How does that get translated into a forcing in W/
m2? I mean how do you distinguish between forcing and feedback
there?
I get this basic idea that we try to look at the atmosphere as if only
one factor had changed (say CO2) with everything else (eg water
vapour) held constant, but how on Earth does that work in practise?
( because well we can't actually hold everything else constant)
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