[groby_b]

It seems like you didn't read the paper, or really anything on this.

But, to make it easy for you:

* 1 ton of Olivine will absorb 1.25 tons of CO2[1]

* Transportation cost is 161.8 grams of CO2 per ton mile[2]

* They are looking at mines within a 300km[1] (or ~190 miles) radius. That's a maximum transportation cost of ~31kg CO2. That's a rounding error.

* Mining cost for a ton of Olivine is ~$30/ton right now. Let's say it's all for hydrocarbon fuels, and they're cheap, so 30 gallons of fuel. Let's make it 50 gallons, because I'm lazy. A gallon of fuel produces 20 pounds of CO2[3], so 1000 pound of CO2. That's half a ton.

* Let's assume for reasons beyond our ken we'll spend another quarter ton of CO2 on this. Likely due to people arguing on the Internet.

That still leaves us with half a ton of CO2/ton of Olivine.

Maybe do at least back-of-napkin calculations before claiming things will certainly fail.

[1] https://projectvesta.org/ [2] http://business.edf.org/blog/2015/03/24/green-freight-math-h... [3] https://www.fueleconomy.gov/feg/contentIncludes/co2_inc.htm

[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 ;)

[Accujack]

Mines or potential mining sites for extracting Olivine on this scale don't exist within short distances of these tropical beaches. Many of the beaches are on islands which are essentially coral heads above the water. Digging down to any deposits of other rock are going to require either significant effort in terms of dewatering, caissons, and equipment or else mining underwater.

This is a good idea that's probably not practical. It sounds ok if you accept that that much rock can be mined, processed, and moved within the constraints given, but they're probably not even close with these estimates.

On top of that, you're talking about completely changing the composition of the beaches in much of the tropics, displacing the existing beach materials and probably causing mass die-offs and disruption in the ecosystems attached to them.

Plus, you'll have to keep hauling sand. Beaches in active areas like the tropics aren't static. The sand migrates down them and out into the ocean. Sometimes islands like the Bahamas even have to dredge sand back out of the water to rebuild beaches.

Underwater sand isn't going to absorb much atmospheric CO2.

[dentalnanobot]

Actually I think underwater sand can absorb dissolved CO2 in water - that’s how it’s alkalising effect on oceans works. I think I remember Vesta’s docs saying there are places in the ocean (between the UK and Europe IIRC) with enough current flow that you could just put the Olivine directly on the sea-bed.

[tom_mellior]

> Many of the beaches are on islands which are essentially coral heads above the water.

Afaik most Caribbean islands are of volcanic origin. Barbados is a notable exception, it's a coral island.

[Accujack]

Actually, the caribbean is the area I'm most familiar with. Lots of small islands in the Bahamas chain with sand and coral beaches.

Perhaps islands in deeper water are volcanic, but I still doubt that mining that much olivine anywhere near the place it would be used is possible.