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Naïve question: What's the deal with the cheapest CO2 offset prices?

It seems, though, that the current price of credible offsets is much lower than the social cost of carbon, and possibly so low that just buying offsets starts to look competitive with GiveWell top charities.

I'm not an expert on this. (I run an offsetting program for a small organization, but that takes about 4h/year. Otherwise I don't think about this much.) I'm also not anywhere near advocating that we should sink tons of money into offsets. But this observation strikes me as unintuitive enough that I suspect I'm missing something.

Cost of offsetting:
I've generally seen the UN FCCC's carbon offset market presented as credible, if not the biggest or most scalable. They make it pretty easy direct money to specific projects, and most of them pass the smell test as being both verifiable and additive. One project that popped up last year involved converting the operations of a platinum mining company in Bihar from burning coal to another burning another fossil fuel in a slightly-lower-emissions way. That's easy to verify, and there was a clear argument for why it wouldn't make economic sense for them to transition without the offset money.

Current prices on that market are around $1.75/tonne, and at other times of year recently, I've seen numbers dip as low as ~$0.33/tonne.

Value of offsetting:
My impression is that the social cost of carbon is somewhere in the $30–300/tonne range, suggesting that cheap credible offsets are plausibly pretty high-leverage, but I don't have good framework for thinking about this kind of impact.

I found this easier to visualize in light of this line from a recent Future Perfect piece:

[...] Bressler found that adding 4,434 metric tons of carbon dioxide into the atmosphere would result in one heat-related death this century.

Combining that with the numbers above yields a price-per-life-saved in the $1k-10k range, which is within the same order of magnitude as charities like AMF, IIRC.

So?
What am I missing? Is there something fishy with that use of of dollars per life saved? Are these cheap UN-monitored offsets actually bullshit? Or is it actually just very impactful to buy this kind of offset while they're still cheap (...even if it's still not the most effective way to spend money, or even the most effective way to spend money on climate impacts)?

One project that popped up last year involved converting the operations of a platinum mining company in Bihar from burning coal to another burning another fossil fuel in a slightly-lower-emissions way. That's easy to verify, and there was a clear argument for why it wouldn't make economic sense for them to transition without the offset money

I am also confused about the general question, but I found this intervention interesting to think about. It seems like the legitimacy of this comes down to the elasticity of demand for coal in India (basically, if someone buys 1 ton less of coal, will someone else buy that same ton for a lower price, or will coal producers make one ton less?). I couldn't find any data on elasticity of demand for coal in India, but this paper estimates it for China as 0.3 to 0.7, which is maybe an OK proxy? And I don't know if it's reasonable to model the elasticity of demand of coal and of the other fossil fuel as the same (eg, it would be terrible if not buying 1 ton of coal reduces the total coal by 0.2 tons produced, but buying 1 ton of oil increases total oil produced by 0.8 tons). 

Overall it feels non-obvious to me whether it's legit, though I lean towards "probably, but about half as effective as a naive calculation suggests"