(I manage the FP Climate Fund, so my incentives run counter to my take). I hadn’t had the time to examine this paper in detail, but I am quite skeptical of this Rethink Priorities paper. I think it is quite easy to string together assumptions that yield a high social cost of carbon, but I wouldn’t treat this as an unbiased estimate.
For example, if I understand this correctly based on your description, they use the Rennert et al (2022) paper to derive the SCC from which they make adjustments.
https://www.nature.com/articles/s41586-022-05224-9/figures/1
The assumptions of that paper are clearly extremely pessimistic, probably by 2022 standards, but definitely by what would now be the consensus view.
For example, they assume close to 20/Gt annual emissions in 2100 as their median scenario and high emissions continue well into the 23rd century. In other words, we are more than a 100 years late in achieving net-zero in their median scenario despite all technological trendlines rendering this quite implausible.
Combining this with a low discount rate will give a high SCC, but I don’t think this is close to a reasonable baseline for what a median expectation should be. (Obviously good to have a low discount rate from an EA perspective, but this requires that the modeling of the future is a bit more careful). Essentially, this means that most marginal carbon reduction modeled for the SCC will happen in worlds where this is implausibly valuable thereby inflating the SCC value.
This alone probably leads to an overestimate of the SCC of a factor of 5x or more and this came up from looking at the paper for 5min.
Thanks, Dan, for your reply. I think I still disagree pretty strongly and I’ll briefly outline how and why: (1) I don’t think we should give credence on studies based on government usage, (2) 22 century emissions are clearly very implausible, (3) there are reasons for this to really matter and (4) being able to find this in 5min should make us deeply skeptical of the wider paper. (5) As you, I deeply care about effective climate action but I don’t think we should propagate bad evidence to inflate the importance vis-a-vis other causes.
Should we give special credence to something because the Biden admin and the German EPA used it?
I think the answer is pretty clearly “no”.
As you also mention, SCCs are inherently very uncertain estimates, but they also are clearly political projects; they are often produced or at least amplified to serve political purposes such as justifying more or less ambitious policies.
Given the biases in the climate science community, I think it is upon us to evaluate studies on their merits and to not assume that studies that are well-cited or used by climate hawkish administrations are therefore inherently more credible.
In case that sounds conspiratorial, it’s worth remembering that RCP 8.5 (the most extreme emissions scenario by the IPCC) was essentially abused prominently for 5-10 years as a baseline after it was obvious to all serious observers it was not a baseline but worse than a conceivable worst case scenario.
Given this record of the emissions scenario modeling community, we should always check what they assume.
Why it’s obvious that 22nd century emissions shouldn’t have that high a median
This is obvious to Dan, but for the wider group. The reason the 22nd emissions are so implausible is because they essentially assume that nothing much changes by the 22nd century, 3-5 investment cycles from now. Yes, decarbonizing the world by 2050 might be hard, but decarbonizing the world by 2100 or 2150 is not hard. It’s simply not plausible to believe in a 22nd century that is both richer than today, a hundred years in the future, exposed to a lot of climate damage, and not able to decarbonize.
This is much more than a 15% difference!
This is not about a 15% difference and a 5x over-estimate from this alone is quite plausible. The calculation you run is – as you note – not informative because it ignores the discounting and the implausibility of the Rennert et al paper lies primarily in the 22nd century+ and those will get outsized importance if changing the discounting assumptions.
Yes, by 2100 Rennert looks mildly pessimistic but not widely so, but that’s not the point – once discounting is very low the 22nd century matters and drives most of the result.
The 22nd century is discounted by more than 75% on the RFF explorers lowest discount rate (2%) so this is not a useful tool to explore whether implausible 22nd century assumptions massively inflate the SCC estimate.
And, indeed, we should expect the 22nd century to drive a massive difference when discounting is low. By Rennert et al’s assumption temperature keeps increasing well into the 22nd century, climate damage is non-linear, so on low discounting this will drive a massive amount of the damage and lead to strongly changed estimates.
I am not an economist, but when I Claude the emissions assumptions of the Rennert et al paper and stipulate to make conventional assumptions about climate damage at 0 discount rate, 96% of the damage is after 2100.
https://www.nature.com/articles/s41586-022-05224-9/figures/1
And we can see here why this is. In the Rennert world temperature keeps rising till after Captain Kirk with 25% probability mass above 4.5C. This is very different from what I think we both believe about the future.
This goes beyond this one example I picked this example because it was quick for me to find, having seen this pattern (making deeply implausible assumptions in papers that get a lot of policy traction) so often. But I am sure one could find other instances. The point that it is so easy to find a highly biased assumption cited as mainstream evidence is that one should distrust the broader paper more.
Why I care As you and many readers here, I deeply care about effective climate action and we are together on this project of making the climate response more effective.
But I do think we should not propagate bad evidence to inflate the importance of climate vis-a-vis other causes in the same way that we would also – and correctly – criticize a motivation of AI risk that only relied on Yudkowsky’s p(doom) estimate.
Implausible assumptions rarely come alone and when a paper makes such implausible assumptions (that are not simpler than more reasonable assumptions, i.e. this is not driven by parsimony) then I think we should be quite skeptical of the conclusions of that paper.