*I* don’t know. It depends on what you mean. Or perhaps as RP Jr said, “It is a little like saying, would you prefer a poverty rate of 10% or 8%? Well, lower is better, the question is how do you get there? Not by arguing about ideal poverty rates I’d say”. Which is a nice way of phrasing it, although there are problems with the analogy.
But the main reason for this post was to pull out of the comments over at inkstain this between Eli and RP:
ER: “Do you believe that any level of GHG CO2 equivalent mixing ratios would be so dangerous/costly as to be avoided through serious mitigation. If so where (My answer is 550-600 ppm, although to avoid that action will have to be taken almost immediately. In this I differ significantly with Tol and Nordhaus on when we have to start although not the end point.)”
RP: “450, though likely not in the cards. 550-600 also unlikely. As you know I don’t think that this is the best way to frame the problem or think about action”
I find this a bit curious… 450 is a very low value to consider dangerous, and as CO2 equivalent we’re practically there already (where are we at CO2e, anyway? Anyone know?).
[Update: I have two answers for this (which I belatedly realise you can get from the SPM fig 2: 430 (add all the +ve's) or 380 (include the negatives too, since net forcing is just about equal to CO2 forcing). The latter answer is probably better -W]