Thanks Stan. I think this is a concise and reasonable critique- I mostly agree. A few thoughts.
My understanding is the CG recently did an internal calculation of SCC along the lines of the Rethink analysis, but one key difference is that they cut off damages around 2100 (not sure the exact year). This was just to have a like-for-like comparison with other interventions where they project benefits out for a fixed amount of time. They came to a conclusion of an SCC around 700. So if you take $1/ton as a reasonable benchmark for climate you get 700x, which doesn’t reach the CG bar but is in the ballpark. If I understand correctly, this CG SCC doesn’t include growth effects. So my point is that you don’t necessarily need long term and growth effects- with just one I think GDH gets competitive under reasonable assumptions, though perhaps not “winning”. (I haven’t actually seen this CG SCC analysis, but talked to people there about it. Hopefully it gets made public some day!)
For (b) I don’t think you need damages past 2200 to be competitive with GHD (especially if you do have endogenous growth effects), but the point is well-taken that you do need long-term effects, up to 2100 and beyond. My personal central estimate is that by 2100 we’ll have reached near net-zero, at warming of ~2.5 degrees C. But even though emission stops, the warming will persist for centuries, and therefore has the ability to continue to cause damages (compared to a no or lower-warming counterfactual). Of course adaptation is the wildcard. Will AI-powered growth radically increase resilience such that climate damages will drop to zero even in the face of persistent increased temperatures? I am not as optimistic as you are, but I see your perspective.
Hi Johannes, thank for all the detailed thoughts. I think we’re probably reaching diminishing marginal returns in debating point by point. I will say that we agree that the Rennert et al projections are too pessimisitic, and difficult far future projections can matter a lot with low discounting. Though I think we will need to agree to disagree on the magnitudes of these changes.
There is one technical point that I’d like to bring to your attention, though, that I think does critically matter. The Rethink application of a new discount rate is NOT a 0% discount rate. It is a 0% “rate of pure time preference”, which is one component of the discount rate. (I’m going to modify my original post to clarify.) Rethink’s calculation is built on calculations from the German Environmental Agency, so I will directly reproduce their explanations for how they did discounting:
“we use the social discount rate developed by Frank Ramsey (Ramsey 1928), which combines the two aspects above: expected consumption growth, weighted by its effect on the marginal utility of consumers, and the pure rate of time preference (PRTP).
In the GIVE model the consumption growth rate is a dependent variable. Therefore, it is not possible to specify the exact discount rate used for the climate costs”
I am not familiar enough with these models to get a good intuition for what discount rate would be, and therefore how much weight will be places on the far future. But as a reference, the Stern Review also used Ramsey discounting and a low (.1% PRTP), and resulted in an overall discount rate of 1.4%.
All of this to say, I think a simple mental model of “no discounting”, and therefore the vast majority of damages being in the far future, is not a good description of the Rethink analysis.