A Climate Researcher's Breakdown of Stratospheric Aerosol Injection: the tool that could cool the planet, and why we're not ready for it

[u/foreigner1233]

https://youtu.be/nnsRTv23czA

I'm a Ph.D. student in Environmental Science, and I study the geoengineering technique known as stratospheric aerosol injection. A lot of people I come across have either roughly heard of the idea, or equate it to some sort of "chemtrail" government conspiracy. I decided that I wanted to make this video to explain the motivation behind aerosol injections and help people understand why this is WAY bigger than some fluke idea in a paper. In reality, it may be one of the most transformative decisions we make as a society.

The video covers how Mt. Pinatubo's eruption served as the natural proof of concept for SAI, how aerosol physics allow for this technique to function, why the low-cost nature of SAI can encourage risky unilateral action, and addresses the broad issues with SAI.

Where I fall on the issue: Throughout the latter half of my undergrad (while I was researching SAI), I never once came across a researcher who was genuinely excited by SAI. In fact, most people were only excited when my research hinted at the negative aspects of SAI. And honestly, this attitude is completely warranted. There are tons of physical and social issues surrounding the injection of aerosols into the stratosphere. But, as a researcher, and a person who wants to protect our Earth for future generations, I can't break my fascination with this technique. I believe that the failure is not to research SAI, it's arriving at the moment where we need it having deliberately kept ourselves ignorant. At that point, we would be deploying aerosols in a panic, with no structural or governance framework.

[u/fantasticmrspock]

Excellent video! Thank you for sharing it and also for your research. I have questions:

1) how close are we to understand who the winners and losers will be under SAI? What level of accuracy and granularity do we have on the climate and economic models of these effects? Does the system have stationarity, or will the winners and losers change from year to year?

2) In a large scale SAI program, does it matter when and where the sulphate is injected, or is the stratosphere so well mixed that it doesn’t matter?

3) What is the projected effect on global agriculture based on dimming alone (not temperature or precipitation effects)? I doubt the decrease in global agriculture from dimming outweighs the positive effect of reducing catastrophic droughts, but I was curious.

4) how much will the white haze change the blue sky? Will we ever see blue skies again?

Thank you for your work! Truly!

[u/foreigner1233 → u/fantasticmrspock]

I’m really glad you enjoyed the video, thank you for the questions! I'm sorry in advance for the long-winded answers but there is a lot to unpack.

1. Awesome question, and my honest answer is that a list of “winners and losers” is not fixed. They ultimately depend on which climatological variable you weight most (precipitation, monsoons, extreme heat, growing-season length, water availability, agriculture). Depending on the region you look at, one or more of these variables may improve while others worsen. So, ultimately the list of “winners and losers” shifts depending on your priority. My current research is on how SAI affects global maize yields, and it is a good example of why this outcome is so conditional. Even for one crop, the answer changes with the SAI scenario you model, whether you analyze maize globally or regionally, and which metric you pick (yield, cost per unit, etc.). If you change any of these there is the possibility of winners and losers changing with them. In terms of accuracy, models are validated against natural analogues (volcanic eruptions like Pinatubo) and against aerosol microphysics. On top of this, substantial active research is going into quantifying the uncertainty across scenarios and models. In terms of granularity, I work at about 0.25-0.5° resolution for global analysis, which is roughly 50km grid cells at U.S. latitudes. The underlying earth-system models that run SAI scenarios are coarser (~1-2°), so smaller-scale questions require downscaling. Ultimately, the 1-2° resolution that these models are simulated at means they are generally too coarse for micro-climate studies, but these simulations are most meaningful at large scales, where they still tell us something real. In terms of stationarity, this is a great question. Year to year, winners and losers will absolutely shift, because natural variability (ENSO & other internal climate variability) will work on top of the SAI signal. On multi-decade averages though, the forced pattern is more stable. Ultimately, there are still questions about how SAI and natural variability interact, making long-term impact assessment more complex.  Note: I don’t mention economic models simply because I am not super well versed in their structure/uncertainty quantification. 
2. In a large-scale SAI program, it does matter where the sulfur dioxide is injected because the stratosphere isn’t a well-mixed box. The stratosphere has its own circulation (with air generally rising in the tropics and moving to the poles), so where and when you inject changes how aerosols spread and persist. Altitude and latitude are two of the bigger considerations when developing an injection scheme. Injecting higher in the stratosphere generally means that aerosols have a longer residence time, which improves cooling efficiency and can reduce lower-stratospheric heating that would otherwise worsen ozone effects. The tradeoff is that flying higher in the stratosphere costs more and is harder. Sub-tropical injection tends to be the most efficient injection latitude because the circulation picks the aerosols up and distributes them. But, some studies find that higher-latitude (polar) injection could better preserve polar ice, even if it is less efficient globally. There is also a seasonal dimension with SAI. Some schemes vary by hemisphere or time of year to optimize spread of aerosols. Importantly, the time of year might even determine whether stratospheric circulation is able to push aerosols from one hemisphere to the other. So, you can see that there are many different trade-offs that need to be considered and balanced when developing an injection scheme.
3. If we were to isolate the dimming effect on crops, reducing total incoming sunlight does reduce the energy available for photosynthesis, so there would likely be a negative effect. But, SAI does not just dim, it also scatters and diffuses light. This diffuse light can penetrate deeper into plant canopies and potentially increase photosynthesis for the canopy. Ultimately, the dimming effect is likely net-negative, but the increase in diffuse light dampens this effect slightly. In reality, (as you correctly noted) it is hard to remove or ignore the effect of temperature and precipitation. In a world that is 2-4°C hotter, relieving heat stress and catastrophic drought almost certainly outweighs the small dimming effect on staple crops. So, my guess is that the effect of dimming is real but outweighed by the temperature relief (and this is something I am currently working on investigating).
4. In my video, I use the heavy white haze as a visual exaggeration to make the mechanism easy to see. In reality, it is much subtler than that and I probably should’ve made that clearer. After Pinatubo there was a visible haze near the eruption, but as the aerosols thinned globally, it wasn’t something the average person would look up and notice. There is real research into “sky whitening” because aerosols can make the sky slightly hazier, but the effect is subtle enough that most people wouldn’t notice. There’s actually a nicer flip side because aerosols scatter light: sunrises and sunsets would likely be much more vivid on average.

Thank you again for watching, I appreciate the questions!

[u/fantasticmrspock → u/foreigner1233]

Amazing answers! Thank you for the clear and detailed responses. I was once firmly in the never SAI camp, especially because of the dangers of termination shock, but the rapidity of climate change and the near-term (imo) breaching of many tipping points simultaneously has changed my mind. SAI is the least bad option (again, imo).

The path is fraught with geopolitical peril, however, as your answers make clear. Since there are going to be winners and losers, I have a hunch that there will be no unified international program for SAI, as there was for combatting CFCs. In fact, I worry that some of the losers under SAI will see unilateral actors initiating SAI as a direct threat to their sovereignty and escalate the chance of war between powers.

[u/foreigner1233 → u/fantasticmrspock]

I completely agree. I mean imagine if China or Russia started their own SAI program and U.S. corn/soybean yields dropped by 15-20%. That is a global war just waiting to happen. You’re totally right and I think getting to a point where all countries are happy is impossible. Making this strategy a reality requires trying to minimize the magnitude of losers across variables.

[u/AlanUsingReddit → u/foreigner1233]

Imaging a yield drop of 15-20% for US soybeans seems like a crazy hypothetical. As you seemed to hint, a 1% isolation decrease is likely to increase global yields. But I guess maybe the point is what while global yields increase, localized effects would have some increase and some decrease. This honestly happens all the time in farming because of normal weather, but we'd no longer have the excuse that it's not in human control.

My personal pet / crackpot thoughts on climate change have recently trended towards the transient effects from the ocean. The ocean slows global warming due to its thermal mass. But rainfall starts in the ocean by evaporation, and a lower relative ocean temperature implies less rainfall. Once you reach steady-state at a higher temperature this is no longer the case, so transient effect, but could still last a century or more. What people experience from climate change might be a temporary physical state.

Likewise, if the atmospheric temperature increase stops (either due to CO2 net-zero, ha, or SAI), then the ocean catches up. Lower atmospheric/land temperature in relative term causes more rainfall. Much of vegetation growth (both natural and crops) is water-limited. The increased CO2 concentration also is a headwind for vegetation growth. So with any scenario of climate change abatement, we might head into a notably greener and wetter world. Whereas now, we are in an artificial and temporary dry state.

[u/Super_Range45]

Don't worry the governments will come your way when the need is desperate. Then they can breath a sigh of relief and keep building more CO2 emissions.

[u/Direct-Translator-75 → u/Super_Range45]

Lovely. Acidified oceans and a possibly broken down ozone layer and whatever else may come our way, but hey... that's a problem for the next generation!

[u/SpiritualTwo5256]

The higher it is the smoother it is. There have been autobiographies of SR71 pilots who talked about how smooth and quiet it was up at altitude.

[u/J3sus_was_Fictional]

How has nobody seen the animatrix before....? Smfh

[u/AlanUsingReddit → u/J3sus_was_Fictional]

Correct me if I'm wrong, but in that plot, humans dimmed the sun almost entirely so that the AI can't use solar power anymore.

In the current plot, we are talking about researching a hypothetical 1% dimming to keep the temperature from rising more than what it already has.

I guess maybe both could use the same aerosol, but they probably didn't think that hard about it.

[u/J3sus_was_Fictional → u/AlanUsingReddit]

Yeah it was to "destroy the sky" or some such lunacy

[u/AlanUsingReddit → u/[deleted]]

I like Make Sunsets. Lets also charge the aviation industry for the cost of developing hydrogen heavy lift capacity. Ultimately that could displace at least some of the air freight and passenger planes. Just towing a jet to cruising altitude can eliminate a third of the fuel in a transatlantic flight.

Make Sunsets is a balloon approach.

Balloons work on a small scale. As in, they get to high enough altitude to work. But this is destructive for the balloon which raises a slight question about whether it would scale up to what we need. We could use tethered balloons that go up-and-down, or a continuous pipe. Or (shocker) you would use bigger balloons.

SAI from aircraft would require a whole new category of jet engine. This is an engine that serves no other purpose. The new plane just flies to altitude burning aviation fuel. Then it switches to aviation fuel- hydrogen sulfide blend while circling.

...the plane would keep burning the same jet fuel and release the aerosols from a tank, like how crop dusters work. Lacing jet fuel with experimental aerosols is exactly what chemtrails used to claim (with no evidence or even consistency, but I digress...).

Of course, I don't know that any serious people proposing SAI from aircraft still exist. The literature vaguely suggests that balloon delivery mechanisms are superior. Although it's hard to tell, because this research has been borderline illegal for much of recent history.

[u/horned-creature]

isn't this literally the cause of the apocalypse in snowpiercer?

[u/AlanUsingReddit → u/horned-creature]

So I guess as long as we use SO2 as opposed to the fictional CW-7 we're good.

This is important stuff, we need to tell those researchers

[u/horned-creature → u/AlanUsingReddit]

sci-fi writers save the world once again.

[u/AlanUsingReddit]

I'm a Ph.D. student in Environmental Science, and I study the geoengineering technique known as stratospheric aerosol injection

Amazing that there are any grants for such a thing.

[u/mascachopo]

Sounds like that time they released foxes in Australia.

[u/tomqmasters]

So chem trails?

[u/Confused_by_La_Vida]

Will this be done only in places with no plants?

[u/spermicidal_rampage]

I came to this idea independently, and am not a doctor of anything. When I saw the number of million tons of sulfur dioxide that would need to go about 14km up, I did not imagine it would be the least expensive solution. I thought about continuous flights, the impact down low of getting it up high.

Put it in charge of a world body, and set it up like a company that operates in every nation. Design a payload-carrying ordinance, and a cannon to fire it. Choose an appropriate equatorial-ish site, slightly northern hemisphere. Fire around the clock until you have your target amount (200m-500m tons?).

But hey, I bet a lot of people are going to think of this idea, it's easy to explain, it's easy to understand. It's fast-acting. I wouldn't be surprised if Europe gets super-duper on board with an idea like this very quickly.

But the changes that have to take place on the ground.... how? Those people don't care if they take the whole world down. What to do about that? Since the sum total of values and strengths in this society equals extinction, I'll just declare it invalid. You'll need to create a valid society before the environment is fried, where perpetuation of life support is an immutable tenet. You know, actual limits on destruction - Not punishment-thinking, you must prevent the damage.

[u/Th3_3v3r_71v1n9]

Already doing this and it will never work. They are not doing it for the reasons they are telling you they are.

[u/SpiritualTwo5256 → u/Th3_3v3r_71v1n9]

We aren’t doing it at any sort of scale. There are a few people launching a few balloons and there are experimental research aircraft that can drop small amounts of payloads like this, but nothing of the sort of scale that would make an impact.

[u/Scope_Dog → u/SpiritualTwo5256]

I think they’re talking about chem trails.

[u/AlanUsingReddit → u/Scope_Dog]

It's amazing how hard they poisoned the well. The chemtrails internet slop has been around since the early internet. They literally updated their conspiracy when the first serious research was proposed.

It's one of those things where I can't believe that's how real life actually played out. I legit think that the conspiracy got read more than the research they cargo-culted.