# marine cloud brightening

Various [geoengineering](https://en.wikipedia.org/wiki/Climate_engineering) 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](https://en.wikipedia.org/wiki/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](https://en.wikipedia.org/wiki/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](https://acp.copernicus.org/articles/21/14507/2021/) 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](https://ieeexplore.ieee.org/document/6713739) are electrospraying with very small nozzles, and spraying supercritical water with larger nozzles. [This paper](https://doi.org/10.1016/j.atmosres.2013.10.025) 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](https://en.wikipedia.org/wiki/Flettner_rotor). 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.)