=global warming =geoengineering =industrial design
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.)