Is there any known or predicted capture technology that does this more efficiently than by growing plants or algae and then burying them underground and/or using them as biofuels?

Inducement of massive algea blooms is extremely cost effective, can store massive amounts of carbon very quickly, and horrendously damaging to large lake, and ocean ecosystems, where you can use them. There are two problems, the first is that its only effective as storage if you make it damaging enough to ensure mostly anoxic decomposition, so the damage is required. Its reusable though, so if you waste a huge lake, there is no reason not to do this over and over again. The second problem is that if you do it in oceans, then ensuring it sinks to the bottom, becomes key, though this is straightforward if you do it on a scale to solve global warming in a single sweep. Though you would sacrifice an ocean and everyone living nearby for decades. The one time someone tried it, it was a far more massive success than anticipated, but also killed the fishing and tourism industry on several nearby, and not so nearby islands, for several years.

A slower variant is to build artificial floating reefs, increasing the amount of marine snow. That improves the ecosystem, but requires significant investment.

According to this guy [1] in less than a decade it will be cheaper to produce hydrocarbons out of air and water + surplus renewable energy than by pumping them out of ground. Which is in turn cheaper than biofuels.

[1] https://caseyhandmer.wordpress.com/2021/11/01/scaling-carbon...

Yes. CO2 can be frozen out of the air as snow at sufficiently low temperatures.

This could be done at scale in central Antarctica using enormous refrigerating complexes[1], but it would be the most ambitious engineering project ever undertaken by humanity.

Generally this is not seriously discussed by experts as a solution to global warming not because it is not feasible, but because it would diminish the sense of urgency and discourage the much more prudent and affordable approach of simply reducing emissions.

[1] https://journals.ametsoc.org/view/journals/apme/52/2/jamc-d-...

> it would diminish the sense of urgency and discourage the much more prudent and affordable approach of simply reducing emissions.

If air capture costs more, then simple economics will keep us more focused on reducing emissions. If it somehow costs less, then doing the air capture is a win.

Most likely, air capture will cost more in some cases (power plant emissions), and less in other cases (long-haul jets). Ideally we'd set a price on carbon, award credits for verifiable air capture/sequestration, and let the market sort all this out.

> Generally this is not seriously discussed by experts as a solution to global warming not because it is not feasible, but because it would diminish the sense of urgency

Let us say it is not being discussed because it is not feasible. Unless of course we build massive nuclear power plants in Antarctica with all what that entails. We are not any time soon in a position where we can produce any nontrivial amount of solar or wind energy in the hostile environment of that continent, plus it's dark night down there for half a year each year. Meaning the only remaining option would be to ship coal or oil down there to burn it so we can cool air to –140°C, obviously a non-starter if there ever was one.

> and discourage the much more prudent and affordable approach of simply reducing emissions.

This. The entire plan is madness: you'd burn two tons of oil and coal to get rid of part of what burning one ton of oil and coal leave behind in the atmosphere. It is not clear to me at this point if it is at all feasible to use fossil fuel to get more CO2 out of the atmosphere than burning it puts into the atmosphere in the first place. Because in this household we obey the laws of thermodynamic. And if it's possible at all it's not easy to see why continuing to burn oil and coal and capturing the CO2 at other sites should be better than not burning part of those fuels and capturing the CO2 right at their point of emission should be the better option. It is a hare-brained plan.

> Let us say it is not being discussed because it is not feasible. Unless of course we build massive nuclear power plants in Antarctica with all what that entails.

Maybe it really is because it's not feasible, I concede that's not something I can really know. But nuclear power plants are not necessary. As the paper sets out, there is abundant wind energy in Antarctica. Setting up a medium-size (1200 MW) wind farm on the Antarctic coast is actually not a crazy proposal, since the construction can be undertaken by ship alone.

Moreover it's very clear that the energy required to freeze one ton of CO2 is substantially less than the useful energy obtained by its combustion (this is intuitive from the magnitude difference between heat of combustion and enthalpy of sublimation: combusting one mol of pure coal to CO2 liberates 393 kJ, freezing one mol of CO2 out of the air consumes 26 kJ). In no way does that violate thermodynamics; the CO2 still exists, it just isn't doing any harm.

This project would not just ameliorate global warming, it could allow useful exploitation of all the remaining global fossil fuels.

Maybe perpetually shaded area would work even without active refigeration? Sun never rises overhead in Antarctica. But yes it's still about building a megastructure in very harsh environment.

I'm not sure if it's more efficient than the methods you mentioned, but Project Vesta makes some pretty bold claims using the carbonate-silicate cycle. More details:

https://news.ycombinator.com/item?id=20403570

https://www.vesta.earth/

> surplus renewable energy

is where you lost me. Hell coal is projected to get burned by the megaton for another half century or so because China and India have those resources and that demand. Mankind does not currently have surplus renewable energy.

There isn't a single global account of electricity generation and expenditures. If your windmills are humming in Montana, and demand for electricity relatively nearby right now is already met, you have surplus energy to either use or store; China doesn't factor into it.

Maybe if you were allocating resources across the world's economies, it would be better to invest in solar panels in Shanghai than capture carbon in Billings, but that's not the actual situation.

> Mankind does not currently have surplus renewable energy.

On the contrary - we have a lot of surplus renewable energy, and it's a problem [1].

Energy isn't fungible. 1 MWh in Texas at 12:00 on 4th July is not the same as 1MWh in London at 23:30 on 25th December.

Energy consumption vary a lot through the day and year. Energy production of solar and wind vary a lot as well, and these variances aren't correlated with each other.

Most big scale energy grids are created with the assumption that every millisecond energy produced == energy consumed. When this isn't true - frequency in the grid rises or drops. If it drops too much you just have blackouts, if it rises too much - devices blow up AND you have blackouts until you replace the blown up devices.

Even if a grid as a whole has energy deficit - it's often true that one part of the network is producing too much but the power lines between them might not be "thick" enough to transfer all that energy to the part of the grid that has deficit at the moment.

If we moved completely away from fossil fuels towards renewables - to serve energy needs of customers we would need a lot of overcapacity (because you can't count on sun and wind producing at 100% power all the time). Usually the overcapacity for wind is 2x and for solar is 10x compared to traditional sources. Better batteries might change that, but it still won't be 1x. So when there's a very good weather grids with a lot of renewables will by definition produce too much energy.

There's a lot of factors, and in practice even in countries with less than 50% renewable power installed - often they have too much renewable energy. This will only get worse as we replace more energy production with renewables.

Big batteries like the one in Australia are very profitable and solve short-term (sub-hour) balancing, but aren't big enough to shift the solar production peak in the noon to the consumption peak late evening.

So indeed we have a lot of surplus renewable power already that gets wasted every day and causes problems, and it will only get worse.

[1] https://www.sierraclub.org/sierra/what-do-we-do-too-much-ren...