Certainty Principle
"Personally I value the known price because I think we know what the social cost of carbon is better than we know what the perfect level of emissions is, but everyone's free to disagree on that point."
The social cost of carbon is the degree to which it increases heat retention. The degree to which it increases heat retention is directly related to the level of emissions above that which Earth's ecosystem can naturally absorb (i.e, the "perfect level of emissions"). If we assign the variables thus:
s = social cost
r = heat retention increase per unit of emission
e = amount of emissions the ecosystem can absorb (perfect level)
A = level of actual emissions (observed)
then the equation is
s = r(A-e)
This is a simple, direct relationship. So if you know the factor R and the observed variable A, you can determine factor e from s:
e = A - s/r
So, if you know r, and you know s, then you MUST be able to know e.
Given that, your claim only works if you don't know r. But the problem with that is that s is the derived variable, while A is the observed variable. So to find s, you must have known the factors r and e to apply them to A, and therefore you must know e. So either we don't really know the social cost of carbon emissions, or we do know the perfect level of emissions.
I'm certain of it.