Various geoengineering schemes have been proposed to mitigate global warming. Some prominent schemes I don't like are accelerated weathering and stratospheric aerosol injection. I think marine cloud brightening is a better proposal than those.

accelerated weathering

To potentially absorb 1 ton of CO2, at least 2.3 tons of pure Mg silicate would be needed. Realistically speaking, "ore" won't be pure or react completely, so 3:1 is a more realistic ratio. Based on the cost of gravel and the availability of olivine deposits, digging up and crushing olivine to gravel would be $20-30/ton. Over a reasonable period of time, olivine only reacts with CO2 in a thin layer on the surface. To get good reaction, it must be ground very finely, which costs money. I expect that to cost >$30/ton for a 4:1 olivine:CO2 ratio. Some trucking and loading is inevitable, and olivine must be spread somewhere. I expect that to cost >$5/ton.

4*($25 + $30 + $5) = $240/ton CO2. That is much too expensive. If that cost was closer to viability I'd have spent more effort estimating it, but it's not worthwhile.

aerosol injection

Stratospheric aerosol injection proposals typically involve using special aircraft to spray SO2 at high altitudes. That oxidizes to sulfuric acid which forms small water droplets which reflect some light. Here are the reasons I don't like it very much:

  • At high altitude, SO2 and sulfate anions in droplets deplete the ozone layer.
  • Particle coalescence at relatively high concentrations is still unclear, and I believe it's greater than estimates used by proponents of stratospheric aerosol injection.
  • The requisite sulfur release that proponents estimate would be comparable to current human sulfur emissions, which causes some issues such as slight acidification.
  • The high-altitude particles would make the sky slightly white and hazy.
  • The effects on regional weather are unclear and potentially negative.
  • Unexpected types of negative effects are possible.
  • If negative effects are worse than expected, it can't be reversed.
  • Implementation would require development of a new type of aircraft, capable of efficiently carrying liquids to much higher altitudes than most aircraft fly at. At such high altitudes, air is much thinner, which affects lift and engine requirements proportionately. Development and tooling for even more-normal aircraft is very expensive; eg the Boeing 787 cost $32B to develop.

Sometimes I see people online saying "OBVIOUSLY WE SHOULD SPRAY SULFUR IN AIR RIGHT NOW!!!" I understand that culture is determined by an equilibrium between different views and people feel obligated to place their "vote" if they have a strong opinion, but these days, polls are common and easy. That being the case, someone making such comments because they read some magazine article, not being aware of the above issues or even trying to investigate details - I think that's a net negative contribution. As a more-general phenomenon, that makes discussion online harder and bothers me somewhat because I think humans can do better.

marine cloud brightening

Marine cloud brightening involves ships spraying salty water from towers such that small salt particles are formed and are lifted by rising air. Those salt crystals then reflect some sunlight. I like this proposal better than accelerated weathering and stratospheric aerosol injection.

Wood 2021 estimated the salt emission rate needed to approximately counteract current global warming at 50e9 ~ 70e9 kg/yr. I estimate costs at $80 ~ $600 / ton NaCl distributed, for $4e9 ~ $5e10 annual cost.

40~100nm salt particles are desirable for this. Producing such small salt particles is nontrivial, and economically feasible sprayer systems for this do not currently exist. Two proposed approaches are electrospraying with very small nozzles, and spraying supercritical water with larger nozzles. This paper tested spraying supercritical salt water. That's very corrosive, so they used gold-plated titanium, with ~50um nozzles. Energy requirements for making supercritical water are much larger, but without some water heating, evaporative cooling might prevent generated salt particles from rising effectively.

To any decent material scientist, (biphenyl dianhydride / p-phenylenediamine) polyimide film (aka UPILEX-S) is a potential option for small corrosion-resistant nozzles for 400+ C saltwater. I just thought I'd make this obvious note.

Spraying is expensive enough that increasing salt concentration could be worthwhile. Desalination is ~$0.50/m^3 purified water. Producing saturated saltwater from the concentrated stream could be done for approximately twice the net cost. However, increasing salt concentration tends to increase salt particle size. Also, with supercritical water, higher NaCl concentrations require higher pressures to prevent phase separation. Thus, I suspect increasing salt concentration is more suitable for electrospraying, which can produce ~150nm water droplets. A 25% salt concentration would be about optimal for that.

Salt-spraying ships could be steered around and turned off. Compared to stratospheric aerosol injection, the risks of marine cloud brightening are much less and the controllability is much better. With appropriate ship positioning, a limited amount of weather modification might also be possible. If such ships could prevent hurricanes from hitting cities, the economic benefits of that alone might be enough to pay for them.

By the way, I've sometimes seen concept art of such ships having Flettner rotors. That is quite dumb. Yes, such ships have been made, and they do technically work, but a Flettner rotor is strictly worse than a movable vertical sail of the same height - less effective, more expensive, and with extra energy consumption. Some people just like them because they assume something that looks different and sort of works must be more high-tech. (For the same reason, you often see vertical axis wind turbines in science fiction and games. No, they're not better.)

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Planes would not be required for stratospheric injection of SO2. It could in theory be done much more cheaply with balloons:

One interesting aspect of marine cloud brightening is that it can be used very locally.

Using it around the Antarctic to increase the amount of ice might be a good project. It will be a lot cheaper than changing global temperatures and at the same time increasing the amount of artic ice would be a clear and measurable success.

It seems that Big Oil companies generally care about being able to have policy proposals about how to address climate change that are not about limiting their ability to make profits. What would it take to make "marine cloud brightening in the Antarctic" part of their agenda?

The clouds would be localized, yes, but over longer time periods, heat is carried around by air and water currents.

One important distinction is between weather effects that are over time periods that are easy to predict and longer time periods where the cause and effect can't be predicted.

Redirecting hurricanes would be very valuable but if you look at the governance of the US, I think it's very hard to imagine that processes would be created that manage it well. 

You don't want the effects to be predictable enough that people can sue for weather that's inconvenient for their goals. At least in the beginning that might prevent technology deployment. 

What about lower altitude aerosol injection? It is already happening with our existing pollution anyway and we can say increase the altitude as we learn more. Specially I have seen it suggested over a regional area for a short amount of time, say 6 weeks over the Great Barrier Reef to avoid a particularly bad coral bleaching, or to avoid a major heatwave over population centers. It must be economic because it happens by accident with no effort.

Sulfur at low altitudes? You get the disadvantages of sulfur pollution with much less effectiveness per mass, and worse cost-effectiveness than using salt. There's a reason sulfur is removed from gasoline.

Do you have data on cost effectiveness for say salt then as a temporary measure. I expect people will want some short term experimentation with any chosen technique and be ok with low cost effectiveness to begin with

A previous discussion of marine cloud brightening.

I also think the marine cloud brightening is a good idea. I especially like how it can save stressed coral reefs in a pinch, since I aesthetically love coral reefs.

There are also other things worth considering which can be done also. For instance: whitening deserts, or carbon sequestration via converting biomass to inorganic carbon and using it as a fertilizer-modulating-sponge (improving fertilizer delivery efficiency) in agricultural topsoil (biochar). 

Normal biochar almost all biodegrades over several years. The cation exchange capacity of "terra preta" comes from accumulation of a relatively small % of polycarboxylate polycyclic aromatics.

But yes, burying biomass while somehow preventing decomposition has one of the lowest CO2 mitigation costs. My understanding is, the current best approach is drying it and adding some CaCl2 so it's too dry for stuff to grow.

With the right plant design, converting biomass to (levulinic acid + furfural + hydrochar) and burying the hydrochar is even better economically. However, land requirements are too large to use exclusively biomass for carbon sequestration or fuels.

Yeah, just another piece of the puzzle. In the short term, albedo changes are going to make a lot more difference.

Do you know if the biochar degradation would be slower if placed relatively deep (> 2ft) down into soil which won't be tilled, as could be done for an orchard?

Yes, biochar mostly won't biodegrade if it's buried in such a way that it doesn't get oxygen. However, making biochar costs money, so it's cheaper to dry biomass and add CaCl2.

If negative effects are worse than expected, it can't be reversed.

I agree that MCB can be reversed faster, but still being able to reverse in a few years is pretty responsive. There are strong interactions with other GCRs. For instance, here's a paper that argues that if we have a catastrophe like an extreme pandemic that disrupts our ability to do solar radiation management (SRM), then we could have a double catastrophe of rapid warming and the pandemic. So this would push towards more long-term SRM, such as space systems. However, there are also interactions with abrupt sunlight reduction scenarios such as nuclear winter. In this case, we would want to be able to turn off the cooling quickly. And having SRM that can be turned off quickly in the case of nuclear winter could make us more resilient to nuclear winter than just reducing CO2 emissions.

Somewhat relevant forecasting question for those who are interested (soon-to-open):

This question resolves positive if the International Maritime Organization (IMO) puts forth regulation that increases the limit of sulphur content in the fuel oil used on board ships to the pre-2020 limit of 3.5%, or greater, i.e. at least 3.5%. Otherwise this question resolves negative. Metaculus admins in conjunction with the community will be consulted should any disputes concerning this question's resolution arise over its lifetime.