I'll give an answer that considers the details of the Copenhagen Consensus Center (CCC) and geoengineering, rather than being primarily a priori. I've spent a day and a half digging around and have zero prior knowledge. I spent too much time reading Lomborg and CCC in retrospect, so I mention him disproportionately relative to other sources.
Cross-posted to my blog.
Here's what I notice:
1. Lomborg and his CCC seem very cost-benefit focused in their analysis. A few others are too, but see point 4. Basically, it's easy to compare climate interventions to other interventions, but hard to figure out how much damage warming and other climate change will cause, so you can't really figure out the benefit part of the cost-benefit analysis.
2. Lomborg and his CCC has recieved a ton of criticism for systematically making errors that underestimate the effect of climate change, and never making errors that overestimate the effect of it. One detailed account of him making such an error that could not have been made in good faith, is given here. He also literally lies in his cost-benefit analysis (by more than 10x).
There's a lot of articles about Lomborg e.g. taking a 700k salary and getting donations from Koch and Exxon, which showed up before I found the above examples of him lying about data. I reacted to the info on his salary/funding by saying, "this is indicative of him being sketchy, but instead of just changing my estimate of how likely he is to be sketch by however much ("updating") and calling it a day, I'm going to take this as a cue to dig into things until I have a firm understanding of whether this guy is systematically lying or not". Turns out he's a liar (see previous paragraph).
3. Page 33 of this CCC paper notes that,
To place SRM 1 [a plan which, by definition, reduces the amount of heat from the sun that stays in the earth's atmosphere by 1 watt per square meter] in perspective, 1 W m-2 is about 0.3% of the [average over the earth] incoming solar radiation of 341 W m-2 (Kiehl and Trenberth, 1997; Trenberth et al., 2009).
Which corresponds to a 0.6 C average temperature change. This should put in perspective what a huge effect a 0.3% change in how much heat gets stays in earth's atmosphere has on global temperatures. If all the glaciers melted, the earth's temperature would rise 10 C, because glaciers are good at reflecting heat back away from the earth (back of envelope, me); if the earth became totally frozen, the temperature would drop by 55 C (see this page), which is why the thing this post is about, MCB, can realistically change global temperatures by a couple degrees C. Note that 1/2 of the global temperature change that already happened was because some glaciers and snow already melted and stopped reflecting heat away from the earth. Basically, the science behind MCB is pretty solid and I'd expect it to basically work.
AFAICT the IPCC estimates of how much warming there will be in the future seem to take into account the fact that the melting of the glaciers will further speed warming in itself, in addition to the warming you get from rising CO2 levels. Except they don't ever explicitly say whether that was considered as a factor, so I can't be totally sure they took that into account, even though it sounds like the most obvious thing to consider. (I skimmed this whole damn thing, and it wasn't said either way!) I guess my next course of action could be to annoy an author about it, though I think I'll be lazy and not.
4. We have little enough data on "how much economic damage has global warming done so far" that we can't make decent extrapolations to "how much economic damage will global warming do later". Like, you have papers saying that the economic damage from 3 C of warming could be 1%, 5-20%, 23%, or 35% of the GDP. When you have zip for data, you fall back on your politics.
5. The obvious game theory consideration of, "it's better if someone other than you spends money on global warming". The normal lefty position of, "our institutions aren't set up to coordinate well on this sort of problem, and every action against climate change, until we change, will predictably be a stopgap measure". The unusual conservative position of, "just do the cost-benefit analysis for MCB". How much damn energy I've spent filtering out the selectivity in what scraps of data scientists and economists want to show me. /rant
Here's what I'm taking away from all that:
CCC isn't reliable in general, but others have made estimates of the cost of worldwide MCB. I'm inclined to believe CCC about MCB in particular, as their numbers match up with others'. MCB is the most cost-effective climate intervention by a ~50x margin, and the estimated cost of worldwide MCB is 750M-1.5B USD annually. The exact technology needed to do MCB hasn't been fleshed out yet, but could be engineered in a straightforward way.
By CCC's own analysis, deploying worldwide MCB is >10x more cost-effective than standard global poverty interventions, and the fact that OPP and Givewell have far more funding than they know what to do with (even though they're lying and saying they don't), makes MCB even more attractive than this in practice.
Personally, I suspect that fleshing out the details of how MCB could be done in practice, would be more cost-effective than instituting a full-blown implementation of MCB, as having a well-defined way to implement it would reduce the friction for other(s) to implement it. Once I have hella money, it's something I'd fund (the research on how to do it, but certainly not the actual MCB). Like, to get things to the point of having a written plan, "hey government, here's exactly how you can do MCB if you want, now you can execute this plan as written if/when you choose". I expect other interventions (re: factory farming) to be more effective than the actual MCB at preventing suffering.
Thanks for reading, and thanks for bringing MCB to my attention. Stay awesome.