[astazangasta]

You're right, I didn't read enough or do enough back-of-the-napkin math. I read more; this seems plausible in life cycle terms, but now I have doubts about the economics. At $30/ton, 30Gtons becomes $900 billion a year. For comparison US discretionary spending is $1.4 trillion. It's not clear to me where this level of spending is going to come from (jewelry sales?), for an activity that has no useful economic output other than sequestering carbon. In addition I'm not sure that $30/ton is an accurate estimate of all of the capital outlays and so on required - entire industries have to be created to facilitate this process at scale. Much of this activity has to be done in decentralized fashion, i.e., it is not a single polity or entity that must partake in olivine mining to make this viable, which adds cost and political overhead.

I'm also not clear on what putting 30 gigatons of rock onto coastal sea shelves each year is going to do in terms of ecosystem impact, and how tolerant local polities will be of this. For example, Costa Rica gets something like 6% of its GDP from tourism, a lot of which is ecotourism; it seems unlikely they will be happy about a significant mining and rock-dumping operation taking over its beaches.

EDIT: Also, on the life cycle question, I'm unclear on how real the "1 ton for 1.25 tons of CO2" claim is - would a real beach weathering actually produce this much absorption per ton? Would it happen on a 1-year time scale so that we could actually offset this much every year? Uncertain; if you have citations demonstrating this please post them, if you're not above arguing on the Internet.

[groby_b]

Olivine is mined at $25/ton right now, which presumably takes capital requirements into account, so that's less of a concern from my POV.

The need for $900 billion to make this work (or even $300bln if the olivine price really drops as they predict) is the hard part. But then, the alternative is pretty much climate collapse, so it's still a bargain. Question is if people will realize that soon enough. But yeah, definitely not paid through jewelry. I'd imagine an actual emissions tax with teeth might help.

As for ecosystem impact, let's for now just look at a per-country amount. CR is 5M people, ~1.6tons of CO2/yr, so 8 megatons. With 800 miles of coastline, that's a lot of rocks - if they were only deposited on the beach. But it can be dumped in the water as well, if the currents are right. See e.g. https://projectvesta.org/science/#dflip-df_90/26/ (Not CR, but Europe, and basically says "pour it in the Channel")

I don't have the weathering rate, and the claim in their booklet (p. 32) actually doesn't make sense to me, so that's the big question for me as well. AIUI, this is the big unknown - they claim the old numbers don't apply, but they need to run a large-scale test to actually tell what the numbers would be.

I'm not, to be clear, saying "don't be skeptical". I'm merely saying as far as CO2 absorption ideas at scale go, this is one of the saner ones. It's not immediately unfeasible, and we should probably test it.

(As you can tell, I'm a fan of arguing on the Internet ;)