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Comment by estearum | Hacker News Reader

Unknown second order effects

Nearest real world success is continuous low volume maritime dispersal which has completely different dispersal dynamics than high-altitude bursts, and the continuous low volume maritime dispersal is non-viable

No way to undo it once done

If humans can’t perpetually release aerosols — and I mean perpetually, for the next millions of years — then the global climate “snaps back” violently within weeks and almost certainly eradicates all known life in the entire universe.

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The climate shock from stopping aerosols would be a crisis for the planet, but we would have more than weeks to stop it. First it would take months for the aerosols to leave the upper atmosphere, and then years for the earth to heat up to its new equilibrium temperature - catastrophe, but not likely the end of all life.

https://www.carbonbrief.org/solar-geoengineering-risk-termin...

My understanding is that also volcano eruptions have temporarily cooled down global climate. So, such an abrupt, high-volume dispersal seems to work too, although probably not what we would want. If both sudden volcano eruptions and maritime emissions cool down the climate, I can't see why spraying stuff from airplanes wouldn't work too.

Of course there are going to be unknown side-effects, and suddenly stopping it would be bad. But it might still be better than doing nothing at all. It's a shitty band-aid fix, but I would still take it over "hothouse earth" type scenarios.

I'm no expert in this subject and I don't have any strong opinions on it. The point of this comment isn't to debate one side or the other.

That said, your comment stands out to me to be self-contradictory and unscientific (by way of being alarmist and not backing up an extraordinary claim ).

> Unknown second order effects

This sounds right.

> Nearest real world success is continuous low volume maritime dispersal which has completely different dispersal dynamics than high-altitude bursts, and the continuous low volume maritime dispersal is non-viable

Since I don't know a lot about this topic I'll take your word for it.

> No way to undo it once done

This doesn't sound quite right, my intuition says more likely "no known way to undo it once done".

> If humans can’t perpetually release aerosols — and I mean perpetually, for the next millions of years — then the global climate “snaps back” violently within weeks

Wait... So, to undo it all we have to do is stop doing it? Doesn't this contradict the statement right before it?

> almost certainly eradicates all known life in the entire universe

This statement makes me suspicious of the credibility of the rest. This is an extraordinary claim and I think deserves way more explanation if you want to convince anyone who doesn't already agree with you. It would be a lot easier to accept "decimates human civilization" than "eradicate all life on earth". Life is extremely resilient.

How exactly would it eradicate all life on earth?

Read on maritime SO2, it was stopped because it's second order effects were too negative (mostly acid rain if i recall right)

The stuff about aerosols: The "plan" isn't that you dump a one time "treatment" of aerosols and then climate is reset. It's a continuous aerosol injection advitam aeternam to offset the warming - constant upkeep.

About comments on destruction of all life: the biosphere is so conplex it's hard to even grasp the gist of it. Global temp affects every level of it. Were this "treatment" be a little too strong/maladjusted, it could very well cause runaway mass death.

And we only have proof of life on earth - if we kill life on it, as far as we know, it's over for life itself. Can't be careful enough, and aerosol dispersal isn't that.

there is absolutly no possibility of injecting enough of anything into the atmosphere in such a way as the CO² cost of doing so dosn't cause more heating than the "airosol" reflects into space, which in itself is purely conjectural, there bieng, absolutly zero engineering paractice or experience to go from,.... well, not counting an unlimited supply of hubris

It's the aerosol levels that "snap back" (although that takes months to a few years, not weeks).

The effect that has on the climate depends on how much CO2 is in the atmosphere at the time.

If during the time we were doing the aerosol program we've also continued increasing CO2, using increasing aerosol releases to prevent that from causing warming, then when we stop with the aerosol we get that deferred warming over a few years or less.

We end up at the roughly the same place we would have been if we had not done the aerosols at all, except instead of getting from where we are now to them over several decades we get their over a few years.

Let's say that is 3℃ above current average temperature. A 3℃ rise over 2 years is a lot harder to handle than a 3℃ rise over 30 years.

Worse, if we started aerosols a lot of countries might decide that it is OK to keep increasing emission rates, so if we ever stop the aerosols were are looking at a rise to much higher than we would have had if we had not used aerosols.

Wow gee wiz, it’s almost like my original comment was: go do some reading.

Regarding the “no way to undo” and the “violent snap back”, we know the desire albedo effects dissipate and therefore require continuous maintenance.

However, these aerosols also cause hard-to-reverse reactions to other things like damaging the ozone layer and causing rain pattern shifts.

Yeah, most climate catastrophe scenarios are of the severity you describe. This one is not.

The entire point of SAI is to “save up” damage to the environment. So over 100 years of SAI and then stopping, you will incur 100 years of atmospheric and temperature changes within a few months.

And that’s only over a hundred years. If we depend on this and do it for a thousand years, now it’s a thousand years of damage applied in months.

This is far, far, far faster than any biological system evolves. Sure maybe some microbes that can survive in a gigantic range of environments could survive, but no, probably no complex life forms would.

> Wow gee wiz, it’s almost like my original comment was: go do some reading.

My understanding is that the whole purpose of HN is to discuss interesting topics with intellectual curiosity. "Go do some reading" type statements aren't really conducive. What would be more appropriate is recommending specific sources, or just taking a moment to elaborate since the whole point is discussion.

I appreciate your elaborations in this last comment. I don't appreciate the dismissive tone of your first line or your earlier comment.

> Regarding the “no way to undo” and the “violent snap back”, we know the desire albedo effects dissipate and therefore require continuous maintenance.

> However, these aerosols also cause hard-to-reverse reactions to other things like damaging the ozone layer and causing rain pattern shifts.

This makes sense. I guess where the logic breaks down for me is the conflation between the time it would take us to recognize the second order effects and stop the process, the amount of violent snap back that would occur, and the time to reverse the second order effects.

To be clear, I understand the risk you are pointing at and it is a significant risk, it still seems like you are exaggerating it.

It's either we do this for thousands of years (in which case the second order effects must be minor to make it sustainable for that long), or we do it for a short time because second order effects aren't sustainable.

It's the logical relationship between the reversibility, second order effects, and magnitude of snap back risks that isn't adding up for me.

All this said, as I've engaged in this topic and thought more about it, my current stance is that we shouldn't be introducing new things into the climate to address the consequences of other changes we have made. A safer approach seems like economically sustainable ways to undo the root-cause damage we have done. (E.g. CO2 capture sounds better than novel aerosol injection).

So I think we probably agree in principle, I just still find the comment I responded to originally alarmist and not very convincing.

> It's either we do this for thousands of years (in which case the second order effects must be minor to make it sustainable for that long), or we do it for a short time because second order effects aren't sustainable.

Just like building petrochemical-dependent societies!

Err, actually, there’s a third option: we put ourselves into a pickle.

Pretty much no hard problem would exist if the dynamic you’re describing were necessarily valid in general, and you’ve done nothing to demonstrate it’s valid in this particular case.

It is absolutely possible for the side effects to be hard to detect, widespread, hard to mitigate once detected, and for us nevertheless to be otherwise dependent upon continuing to produce those effects. See: fossil fuels.

But fossil fuels do not have the same snapback risk. This actually does.

> It is absolutely possible for the side effects to be hard to detect, widespread, hard to mitigate once detected, and for us nevertheless to be otherwise dependent upon continuing to produce those effects.

Oh I completely agree that it's possible, but there are some very material differences between those examples.

The purpose of fossil fuels is increasing access to energy and The downside is climate change. The purpose of the aerosol injection would be climate management and the downside would be unintended climate change. If it's not working as intended we are far more likely to stop doing it because of the direct relationship between the purpose and the issues.

Also, we got hooked on fossil fuels before we had the science to understand the long term consequences to the climate.

So to summarize there are at least two very material differences:

- fossil fuels were essential to reducing energy costs whereas I don't see a direct economic benefit to aerosol injection, just the purpose of managing climate damage. Am I missing something?

- our scientific understanding at the beginning of adoption will be materially different and we are a lot more likely to detect issues earlier on. Not certainly, but materially more likely.

To me the combination of these two things makes it a fundamentally different dynamic.

I am sure it sounds like I'm advocating for aerosol injection, but I'm actually just playing devil's advocate and trying to strengthen my understanding by pointing out the holes I'm seeing in your explanations.

If there's ever a specific source that you think would help fill a gap in my understanding I am receptive to checking it out.

Stopping spreading and undo the damage from spreading are not contradictable.

Eradicating all life is not unscientific, you might be better at throwing real arguments of why you don't like this idea instead of boasting.

> Wait... So, to undo it all we have to do is stop doing it? Doesn't this contradict the statement right before it?

It's not quite that simple.

The intuition that you're subtly relying on is the idea that the response or effect of one of these geoengineering treatments is linear. But unfortunately, that's not something you can assume about a dynamical system. In reality, the climate system can undergo certain types of hysteresis where "undoing" the forcing doesn't revert the initial perturbation, because you're suddenly on a different response curve. Probably the most famous example of this in climate dynamics is the way that the ice-albedo effect sets up a hysteresis in the trajectory towards a "snowball Earth" scenario. Apologies for the lack of links/references; Wiki has decent write-ups on this, and it's typically covered in the first chapter of a climate dynamics textbook.

The potential response to suddenly stopping a climate change mitigation strategy has a very well-popularized name: a "termination shock." In fact, Neal Stephenson used exactly this concept in his titular novel on the topic in 2021.

As a climate scientist, my mental model to better understand the risk of termination shocks and unintended consequences boils down to how fast the response of the climate system is. Marine brightening is "less risky" because the meteorological response to these interventions is extremely fast; a cloud-precipitation system will respond on the order of minutes to hours, and unless the intervention continues unabated, it will clean the air quickly, limiting the repsonse. Stratospheric aerosol injection is more complicated, but we have a very good analogue - very large scale volcanic eruptions like Mt Pinatubo. The response to these sorts of events is measured more on the timescale of 2-5 years, although knock-on effects (such as a shift towards more diffuse solar radiation reaching the surface, which has significant effects on terrestrial and oceanic biogeochemistry) could very much persist longer than that - and don't "snap back" nearly as quickly. A continuous, Pinatubo-like intervention would compound and introduce coupled atmosphere/ocean responses that could decade years or longer to fully play out. And that's _in addition_ to the near immediate (1-2 year) response in global average temperature, which would bounce back to most of the pre-intervention level very quickly.

These things are complex. There's a lot we don't know. But, there's also a lot we _do_ know. I would encourage anyone who does not have significant experience in climate dynamics to remain curious and avoid jumping to conclusions based on simple intuition; they're probably wrong.

Thank you for this response. Of those that replied to me, yours seems the most balanced and scientific, and I learned the most from. I wish more often people engaged on HN like you have here.

Given your expertise in this, I'm curious what your take is on CO2 capture, not in terms of economic viability, but in terms of climate risk...

For example, if we were to discover a process that removed CO2 from atmosphere and converted it into a product profitably such that there was an economic incentive/positive feedback loop to remove CO2.

My intuition is that if we removed the CO2 too quickly or too much of it we may have unwanted consequences, but if the rate was managed and we slowed down and stopped at a certain equilibrium, would this be a theoretically ideal way to address the problem?

Some things to consider.

First, what is "too quickly" with reference to CO2 removal from the atmosphere? At present, human civilization emits over 40 gigatons - or 40 trillion kilograms - of CO2 per year. And that increases the atmospheric burden by about 2.5 parts per million per year. So today, before you even start _reducing_ atmospheric CO2, you need to be sucking down at least 40 trillion kilograms of CO2. I struggle to imagine a scenario outside of total science fiction where that would be remotely possible.

Second, the equilibrium climate response to changes in greenhouse gas forcing take on the order of decades or centuries to realize. This is because the dynamics of the climate system are heavily buffered. For example, the ocean acts as a giant heat capacitor that slowly interchanges with the atmosphere. Were you to instantaneously halve the CO2 in the atmosphere, you'd likely see a pretty classic exponential decay in global average temperature (and other more nuanced responses); in the present climate, it's not clear we have already passed specific "tipping points" that would induce that hysteresis I described in the previous comment (in fact - one could read "climate tipping point" as a synonym for dynamical system hysteresis). Theoretically, one could try to "dial in" some particular equilibrium climate state, but it's not obvious over what timescale you'd have to intervene and what the cost of such an intervention would be.

The cool thing is none of this needs to be purely "theoretical." You could simulate all of this _today_ if you had a setup to run a global climate model. A "4X CO2" experiment where you branch from an equilibrium spin-up climate state and immediately apply a global quadrupling of CO2 has been a completely standard experiment as part of CMIP for over two decades. The opposite experiment is an established protocol for both the Carbon Dioxide Removal Intercomparison Project [1], which features an annual ramp down of CO2 at a 1% per year rate, and the Cloud Feedback Model Intercomparison Project [2], which features a more direct counterpart, with an abrupt decrease of atmospheric CO2 by 50%. There is a large body of literature discussing the results of these classes of experiments, but this is outside of my primary research focus and I can't turn you to particularly good papers off-hand. But they're easy enough to find.

[1]: https://www.geomar.de/en/cdrmip [2]: https://www.cfmip.org/experiments/cfmip3cmip6

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