Accelerated silicate weathering seems like the best bet. Naturally occurring silicate compounds of magnesium and calcium react with ambient CO2 in the presence of water to form stable carbonates and silicon dioxide:
CaSiO3 + CO2 -> CaCaO3 + SiO2
The thermodynamics are much more favorable than "combustion-in-reverse" (trying to reform hydrocarbons from CO2). The products are stable and the process can deal with point CO2 sources, distributed sources, atmospheric CO2, and ocean acidification -- the whole shebang.
But the natural kinetics of silicate weathering are very slow. Kinetics can be improved by orders of magnitude if you grind the silicate minerals to dust; it's a surface area limited reaction. Kinetics get better yet if you apply the mafic rock dust to e.g. acid sulfate soils used for agriculture in the tropics. Those soils already need a pH boost to avoid crop stunting by soluble aluminum. Doing the adjustment with silicates instead of limestone means a two-for-one benefit (crop productivity plus drawing down CO2). But I think that to see widespread adoption there will have to be some targeted donor aid to poorer countries; ground limestone is cheaper than ground silicates if you don't care about CO2 abatement, and the rich world's farmers don't have a lot of acid sulfate soils in need of treatment within their home countries.
0) Donor government identifies a country where acid soils are already cultivated for agriculture. The recipient country should also be reasonably stable so you don't have to plan around armed conflict. Palm oil production in Malaysia looks like a good candidate. It's important to get buy-in from the (potential) recipient country's government too.
1) Within the chosen country, identify a region that has a lot of acid soil cultivation and that has easily accessible deposits of ultramafic rocks nearby. (Shouldn't be too hard, as such rocks are very common.)
2) Provide electrically powered crushers and/or grinders for size reduction, plus power sources if remote from the grid (ideally solar, maybe diesel generators as are common for mining sites; even if it's fossil powered the net CO2 is deeply negative). There may be some necessary R&D optimizing said crushers for low-maintenance conditions, since ultramafic rocks are among the hardest and toughest.
3) Pay locals to run the crushers and transport discounted-or-free rock dust to farmers in place of limestone. There might be some gaming of the system if e.g. drivers are really just dumping rock dust on the nearest empty land instead of finding farmers to take it, but that's tolerable. The project is already 75% done if you just get the rock crushed to dust in the first place. So key metric is ensuring that the crushers are still operable and are actually being used to crush rocks as intended.
Another idea, after I've written this out: there is less but still some acid soil under cultivation in the richer nations of the world, e.g. parts of Australia. Might want to try it first in the developed world where monitoring, language barriers, and infrastructure are all less challenging.
EDIT: a somewhat less effective but still potentially interesting idea is to put the ground rock in near-shore ocean environments where wave action will help abrade/weather the particles. Probably not as good as acidic warm soil in the tropics, but still a speedup over the natural way.
I'd love to hear people's proposals for this. There's of course natural methods such as reforestation and soil building, but I have a feeling that we'll want to augment that with more synthetic means.
Carbon chains, or in the simplest case, methane, are expensive to produce, as one might expect. By far it's better to never have burned that fossil fuel than to try to reclaim it from the atmosphere or ocean. Setting a $80/ton CO2 carbon tax (Exxon's internal estimate for new project planning) or even a $40/ton CO2 carbon tax (Shell's internal estimate for new project planning) is an obvious step. When even the fossil fuel companies are using this tax level in their internal estimates, it's just stupid that our government and political leaders aren't pushing for that or double or triple it.
My personal hope is that by 2040 we will have lots of excess electricity from solar and wind because we overprovision them. And we'll overprovision them because they will be the cheapest electricity, and we need enough of them such that at their low point of output they still meet our needs. In that case it seems that we may have cheap intermittent electricity. At that point we can start making hydrogen, then combining it with CO2, to start getting rid of greenhouse gases.
Some people think that we'll be able to take methane (natural gas) and make hydrogen and precipitate out the carbon as graphite. that's probably the easiest form of carbon to dispose of, whereas a liquid like methanol or even diesel is going to pose lots of problems.
It seems that CO2 in the oceans is going to be a bit more easy to remove than atmospheric CO2. So it seems that there are all sorts of chemistries that would be useful for that.
But still, we need a carbon tax to fund such carbon sequestration activities. Without a market mechanism, there's not going to be much investment. We need to both do research, then develop them into large industries really quickly.
Carbon tax, carbon tax, carbon tax. Don't vote for anybody or any party that doesn't advocate for it. This is the biggest single issue for our children and their economy.
If we are already past the point where conservation will help, a carbon tax seems like closing the barn doors after all the cows escaped. the market is already going down the alternate fuel route.
What I want to know is there any concrete plan to build something that will remove excess CO2 from the air and ocean that can be built by the government. Build this is going to go over a whole lot better than tax this.
Of course, they are heavier into natural gas than their fossil fuel competitors, so a carbon tax impacts them the least, therefore giving them a competitive advantage.
The only reason it's not politically viable is because a minority of Americans have forced one political party to say so. That can change quickly. They think that they're protecting "their side" but even "their side" disagrees with them.
There's no excuse, absolutely no excuse, to be against a carbon tax. It's the most libertarian, market friendly way to deal with carbon emissions.
CaSiO3 + CO2 -> CaCaO3 + SiO2
The thermodynamics are much more favorable than "combustion-in-reverse" (trying to reform hydrocarbons from CO2). The products are stable and the process can deal with point CO2 sources, distributed sources, atmospheric CO2, and ocean acidification -- the whole shebang.
But the natural kinetics of silicate weathering are very slow. Kinetics can be improved by orders of magnitude if you grind the silicate minerals to dust; it's a surface area limited reaction. Kinetics get better yet if you apply the mafic rock dust to e.g. acid sulfate soils used for agriculture in the tropics. Those soils already need a pH boost to avoid crop stunting by soluble aluminum. Doing the adjustment with silicates instead of limestone means a two-for-one benefit (crop productivity plus drawing down CO2). But I think that to see widespread adoption there will have to be some targeted donor aid to poorer countries; ground limestone is cheaper than ground silicates if you don't care about CO2 abatement, and the rich world's farmers don't have a lot of acid sulfate soils in need of treatment within their home countries.