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1. Solar geoengineering is not feasible for the next few decades.

a. Solar geoengineering poses major governance challenges.

b. These governance challenges are only likely to be overcome in at least 50 years’ time.

2. Solar geoengineering research is a moral hazard, and research might uncover dangerous weather manipulation methods.

3. Given this risk and given that we can delay research without obvious costs, there is a good case for delaying solar geoengineering research at least for a few decades.

Epistemic status: Seems correct to me, but some expert disagree (though I don’t think they have been exposed to these arguments).

Solar geoengineering is a form of climate intervention that reduces global temperature by reflecting sunlight back to space. The best studied form - stratospheric aerosol injection - involves the injection of aerosols, such as sulphur, into the stratosphere (the higher atmosphere). This mimics the effects of volcanoes, which can have globally significant effects via the same mechanism. For example, the Pinatubo eruption in 1991 cooled large parts of the Earth by about half a degree. Computer modelling studies have suggested that, if done in a certain way and in certain climatic conditions, solar geoengineering could eliminate many of the costs of global warming without having serious side-effects.[1] These models are of course limited and crude, but they do suggest that solar geoengineering could be useful tool, if it could be deployed and governed safely.

Consequently, interest in the technology is increasing, as discussed in this Economist article. The Open Philanthropy Project has in the past funded solar geoengineering governance research and computer modelling efforts.

Here, I will argue that we should delay solar geoengineering research for a few decades.

1. Solar geoengineering is not feasible for the next few decades

a. Solar geoengineering poses severe governance challenges

In my view, solar geoengineering is only likely to be used once warming is quite extreme, roughly exceeding around 4 degrees. The reason for this is that solar geoengineering would likely be extremely difficult to govern. I outline some of the governance challenges in section 3.4 of my paper on solar geoengineering.

Solar geoengineering, if done using the stratospheric aerosol injection method, would affect the weather in most or all regions.[2] Solar geoengineering would therefore politicise the weather in all regions, and would have diverse regional effects. Adverse weather events would likely be blamed on solar geoengineering by affected countries, even if they were not in fact caused by solar geoengineering. Public anger at such weather events would likely be severe if they thought a massive international weather alteration scheme were at fault. Computer models could at best offer highly imperfect attribution of weather events to climatic causes.

This suggests that for solar geoengineering to be feasible, all major global powers would have to agree on the weather, a highly chaotic system. Securing such an agreement would be extremely difficult in the first instance and also extremely difficult to sustain in the longer-term. States would also foresee the problems of sustained agreement, disincentivising successful agreement in the first place.

b. These governance challenges are only likely to be overcome in at least 50 years’ time.

In light of this, solar geoengineering is only likely to be used once climate change is very bad for all regions. Judging when this point will occur is difficult, but my best guess having looked at the climate impacts literature in some depth is that this would only likely happen after about 3-4 degrees of warming.

We have had about 1 degree of warming thus far and, according to an IMF report, a further 1 degree of warming would be economiclly positive for many regions, especially Canada, Russia and Eastern Europe, and even potentially China (IMF report page 15).

(Note that even this modest climate change is bad overall for the world.)

Russia is a crucial factor here: global warming seems likely to bring numerous economic benefits for Russia, freeing up the Russian Arctic for exploration and thawing potential farmland. It is very unlikely that they would agree to a global scheme that would likely damage their economic prospects. Without agreement from Russia, I find it difficult to see how solar geoengineering could ever be implemented.

Thus, it seems implausible that solar geoengineering would be practicable at 2 degrees of warming, and 4 degrees is a more plausible threshold, in my view.

However, 4 degrees of warming will take many decades to occur. On the highest emissions scenario considered by the IPCC, 4 degrees of warming would take at least 50 years to occur (IPCC synthesis, p59).

This means that solar geoengineering is only likely to get used by around 2070, giving us 50 years from now to find a solution.

One potential counter-argument would point to runaway feedback loops that cause rapid warming, such as release of massive amounts of methane from clathrates. I have looked at the evidence for this and the evidence overall seems slim and the median view in the literature is that this is a negligible risk for the next century at least. See section 4 of my write-up on climate and ex risk for more on feedback loops.

2. Solar geoengineering research is a moral hazard and research might uncover dangerous weather manipulation methods

Research into solar geoengineering itself carries two main risks.

A persistent worry about solar geoengineering research concerns moral hazard: the worry that attention to plan B will reduce commitment to plan A. Having solar geoengineering as a backup will decrease commitment to reducing carbon emissions, which almost all researchers agree to be the top priority. The best discussion of this is in Morrow’s paper,[3] and I discuss the considerations on moral hazard risk at length in sections 4-6 of my paper. Overall, I think this is a genuine risk with solar geoengineering research and a reason not to carry out research.

Another risk of solar geoengineering research is that it will uncover new technologies that could destabilise global civilisation. I discuss weaponisation risks in section 3.2 of my paper. For example, climate researcher David Keith has discussed the possibility that a certain type of nanoparticle could be much longer lasting than ordinary solar geoengineering and so could potentially precipitate an ice age if deployed for long enough. I don’t think this particular technology could actually be a feasible doomsday weapon, but there is a concern that further research could uncover dangerous unknown new geoengineering technologies.

In a nutshell, for those persuaded by the Vulnerable World Hypothesis, research into technologies that could dramatically alter the weather seems like the kind of thing we should avoid if we can.

3. We should delay solar geoengineering research

Solar geoengineering research has clear risks and, given that we cannot deploy it at least for the next 50 years, there is no need to incur these costs now. Instead, the more prudent course seems to be to wait and see how well standard mitigation efforts go and then, if these continue to fail, start researching solar geoengineering in earnest around the middle of the 21st century. This would give us at least 20 years to cover the technical details and a governance framework. This seems to me like enough time, given that:

  • Solar geoengineering is probably technically feasible with adaptations to various different current technologies.
  • Extant insights from the governance of other public goods, free rider problems, and free driver problems, could in large part be applied to solar geoengineering, adapted to account for the features of that technology.

I at least don’t think that we need 50 years of forward planning to figure this technology out if we need to use it. Committing research hours when we know it may actually be used makes more sense when research risks undermining fragile commitment to mitigation, and risks discovering dangerous new technologies.

Note that my view has changed on this and that in my paper on solar geoengineering, I made a tentative case for primarily governance-focused research.

[1] For layman’s discussion of a recent paper, see this Vox piece.

[2] The reason for this is that the particles would be distributed globally by stratospheric winds.

[3] David R. Morrow, “Ethical Aspects of the Mitigation Obstruction Argument against Climate Engineering Research,” Philosophical Transactions of the Royal Society of London A: Mathematical, Physical and Engineering Sciences 372, no. 2031 (December 28, 2014): 20140062, https://doi.org/10.1098/rsta.2014.0062.

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A persistent worry about solar geoengineering research concerns moral hazard: the worry that attention to plan B will reduce commitment to plan A. Having solar geoengineering as a backup will decrease commitment to reducing carbon emissions, which almost all researchers agree to be the top priority.

I'm not really sure why this would be a problem, though I read the sections in your paper - perhaps I just didn't understand properly. Moral hazard occurs when one group (Agent) pays another (Insurer) to cover the damages of some future event that Agent is partly responsible for. Because of this insurance, Agent has less incentive to avoid/mitigate the event. Insurer now has more incentive, but if it is cheaper for Agent to mitigate it than Insurer, total mitigation will go down (or total $ expenditure on mitigation will have to go up). This is inefficient, but due to imperfect contracting and monitoring hard to avoid.

But in the geoengineering case Agent and Insurer are the same - they're the researchers/governments. This doesn't seem so much like moral hazard as simply the substitution effect, in the same way that solar and geothermal energy are (imperfect) substitutes. Given the optionality inherent in research, it seems you need some strong irrationality story to say there will be a net-negative expected substitution effect.

I agree the weaponisation risks make sense as a reason not to do it, but they seem separate from the moral hazard idea.


I agree it's not technically the right name, but people generally know what it means which was important for a blogpost. In the paper I actually call it the mitigation obstruction argument. I explicitly discuss the irrationality assumption required for the mitigation obstruction argument in my paper. I think the question of how irrationally people/governments will respond to research is an open one.

Interesting ideas and post! First a critique. It didn't sound right that we should wait 50 years for improved governance. Governance isn't something that just improves on its own over time, it's not a force of nature. It manifests and changes in response to human needs. The aftermath of WW1 brought the League of Nations, WW2 brought on the UN and EU, the Cold War created NATO. If anything it seems that changes to governance happen AFTER a need arises, not preemptively. I know we all wish that weren't the case but it is what is.

The development of advanced geo engineering technology could serve as a catalyst for enhanced global governance. It might also lead to war but I think that's less likely than you do. My reasoning is first that it's likely that only great powers will have the ability to deploy such technology and second that the cost of war between great powers would far exceed the benefits of climate change for the aggressor. To confidently keep a country from being able to produce or buy the resources needed for climate change reversal would possibly require occupation. It's hard to imagine that being worth it or even possible.

That said I do buy into Bostrom's Vulnerable World framework, at least in the context of AI and perhaps biotech. It's totally plausible that geo engineering is another black or grey ball in the urn of innovation. The movie Snowpiercer and the weather control superweapon from Red Alert 2 (a video game) came to mind immediately while reading your post haha. I'm not convinced yet of the threat of geo engineering but I am totally in favor of improving global governance (get rid of permanent security council membership!) and applying more caution towards innovation.



I'm not completely sure I follow why your first paragraph is a critique. I don't expect governance to improve on its own. My claim is that we do not need 50 years of governance research to get governance to a sufficiently good level should we need to deploy solar geoengineering in the future. The hope is that we will be wise enough not to have to use it because we will start serious mitigation, and I'm worried that geoengineering research could be one of many factors that could derail those efforts.

It is true that developing geoengineering technology would create incentives to improve governance mechanisms for geoengineering. I'm not sure why that is a critique of my argument.

I agree that war is unlikely for the reasons you outline.

Hi John, thank you for this piece. I know it's been a long time since you posted this but I wanted to respond to some of your thoughts.

"In my view, solar geoengineering is only likely to be used once warming is quite extreme, roughly exceeding around 4 degrees" - +4C is already endgame and catastrophic in my opinion. Considering that most of the heat is being absorbed by oceans leading to acidification, we'll already be seeing significant sequestration losses as marine animals are unable to build calcium carbonate shells.

"This suggests that for solar geoengineering to be feasible, all major global powers would have to agree on the weather, a highly chaotic system." - individual actors may resort to solar geoengineering without worldwide consensus, especially if countries that aren't suffering from climate change are actively blocking mitigation attempts while still polluting. Understanding the possible ramifications before people begin to experiment through desperation is surely a good thing (e.g. India, China, Saudi Arabia, Brazil).

"We have had about 1 degree of warming thus far and, according to an IMF report, a further 1 degree of warming would be economiclly positive for many regions, especially Canada, Russia and Eastern Europe, and even potentially China (IMF report page 15)." - I think this is sketchy at best. The caveat footer 9 on page 14 should indicate how limited their conclusion is, not counting weather effects, migration, ecological effects, etc.

"Russia is a crucial factor here: global warming seems likely to bring numerous economic benefits for Russia, freeing up the Russian Arctic for exploration and thawing potential farmland." - the US and Canada have been far more disruptive for climate change global agreements, the Paris agreement was largely stymied by Republican Congress. Permafrost thawing doesn't free up usable farmland in significant amounts, these are still primarily extremely low viability / low human density forestland in Siberia. In fact, Russia is set to lose out significantly from permafrost thaw.

"Solar geoengineering research has clear risks and, given that we cannot deploy it at least for the next 50 years, there is no need to incur these costs now." - this argument doesn't hold weight for AGI research, and I don't think it should for solar geoengineering. SG is highly neglected and as a fraction of CC research is minimal. The research will take decades to filter through to policy and international agreements, so it is worth starting research (not implementing) well before we are forced to use it.

"This would give us at least 20 years to cover the technical details and a governance framework." - A lot of the warming is already locked into the ocean. Giving another 30 years before starting to research will likely be too late. I'm not in favour of implementing solar geoengineering now but researching the viability of these measures now seems to be promising, if not for application then for global security to dissuade rogue actors from implementing the measures with false / incomplete information / encourage preventative policy decisions. This requires fundamental technical research to assess the risks.

"This seems to me like enough time, given that: Solar geoengineering is probably technically feasible with adaptations to various different current technologies." - I'm not sure the current technologies that you are referring to that can be adapted, but the more promising interventions are all much larger scale such as sulfur aerosol injections and have almost no precedent (volcanic eruptions can only tell us so much).

Finally "Another risk of solar geoengineering research is that it will uncover new technologies that could destabilise global civilisation. I discuss weaponisation risks in section 3.2 of my paper. " - As your paper says, current information on SAI indicates that it will take a highly technically adept state actor decades of spending tens of billions of dollars and will still not be a permanent doomsday device (and will be obvious to other states and easily counteracted). All in all I find it difficult to imagine that SAI research will discover something that is easier and cheaper to generate a doomsday device than already exists in a conventional nuclear weapons stockpile. Additionally this doomsday device would also exterminate the user, whereas nuclear weapons can be directed at other states with no immediate, direct blowback (of course the political and social cost and likely retaliation from affiliated states are the reasons why we haven't seen this happen yet). So the implication is that the malicious actor would also have to be suicidal. This doomsday device would also take time to work, which would give time to find a counteraction, and if research inadvertently discovers this application the time to find a solution will be the time from that research until the time of implementation.

Adding to this, currently climate change is projected to be a major stressor on international politics which can exacerbate nuclear X-risk, as well as expanding vectors for natural pandemic risks, among others - so this should also be in consideration when considering if SAI may uncover new X-risks as the baseline p(X-risk) for the coming decades is likely to be a curve rather than flat.

In conclusion the injection of CO2 and methane into the atmosphere may already constitute a moral hazard and dangerous weather manipulation method, and I think that we should be researching (not implementing) potential technical geoengineering solutions in order to prevent the expected outcomes of climate change as well as many other potential (part) solutions (As mentioned by others SAI doesn't reduce CO2 levels and so does nothing for ocean acidification and other related issues). We should evaluate the risks and if (as I expect we will find) them to be too high due to uncertainty, we can use that information to construct international policy around this issue.


Hello thanks for these interesting comments

1. Do you think that 4 degrees is "endgame and catastrophic" in the sense of being a threat to the long-term flourishing of humanity, or something else?

I agree 4 degrees would be bad, but I don't see how that is relevant to my argument.

2. "individual actors may resort to solar geoengineering without worldwide consensus" I argue against it in my piece. If brazil starts doing stratospheric aerosol injection, this would affect weather in the US and other allies - it's not a plausible piece of statecraft in my opinion. You mention the risk of 'rogue actors' deploying it - I don't see an argument against what I said in my piece on this. You are stating one common view in the literature that is especially worried about unilateralism, but I find the other multilateralism take more persuasive.

I am happy to offer a bet on this - what do you think the odds are of a single state unilaterally deploying stratospheric aerosol injection for more than 6 months over the next 30 years? I'll offer £500 I win/ £500 you win.

Other things equal, understanding the ramifications of SG would be good, but there are costs to doing so, namely mitigation obstruction risk.

3. I agree it would've been better to look more in detail at the effects on Russia and that does update me towards it being bad for them.

4. Not sure I see why the connection to AGI research is relevant here. We should worry about neglect of a solution when that neglect is irrational. I think SG research is neglected for a reason - scientists and funders don't want to do it because they are worried about the moral hazard.

5. "A lot of the warming is already locked into the ocean. Giving another 30 years before starting to research will likely be too late." What do you mean by "a lot"? Emissions until 2080 will have a large effect on what level of warming there will be - it is still technically within our power to follow RCP2.6 or RCP8.5, which would have hugely different implications for the probability of 4 degrees. This is why a wait and see approach is valuable.

6. Current technologies - I was going off McLellan et al (2012) - "We conclude that (a) the basic technological capability to deliver material to the stratosphere at million tonne per year rates exists today". Smith and Wagner (2018) dissent from this but still say "However, we also conclude that developing a new, purpose-built high-altitude tanker with substantial payload capabilities would neither be technologically difficult nor prohibitively expensive".

You say "have almost no precedent (volcanic eruptions can only tell us so much)" - I don't see why this is relevant to the question at hand of the technical feasibility of getting aerosols into the air.

7. I agree that the weaponisation risk seems small. It's still hard to know what research will turn up in advance and if we can avoid this risk without much cost then we should do so. It is a downside of research it would be nice to avoid.

8. Climate is a stressor of international politics risks. I agree with that but don't see how it is inconsistent with my argument.

9. In what sense are CO2 emissions a moral hazard? It's usually classed as a free rider problem, not a moral hazard. If you mean that CO2 emissions are bad, I agree with you.

Yeah the thought that research will rule out SG is plausible and that would be a reason to research SG, especially with governance-focused research. I have some credence in that view and some in not researching it at all. The timelag feature that SG is unlikely to be deployed in the next 50 years pushes me towards delaying research being the way to go.

Hi John, thanks for the reply, I wasn't expecting one after this long but I am pleased about the thoroughness and thought you put into it.

  1. Yes, I think it's likely to be catastrophic (not extinction risk). If your assertion that it is only likely to be used when warming is 4C+, your whole argument is moot because that is already past the point of no return. From the base that 4C+ is an unacceptable situation, you would immediately be making the case that SG should be funded and accelerated by your own following argument - that currently policy and research are preventing it from being available until well after it is a viable temporary mitigation strategy (if it on balance positive).

  2. In the framing of the desperation-triggered, unilateral application of SG this would not be in the context of "statecraft". If millions are facing water shortages and famines in India they may have bigger concerns than their relations with allies, and equally could criticise them for not contributing to climate change solutions that are disproportionately affecting them. Scapegoating a unilateral SG actor for adverse weather is possible but they could also point to climate change for those effects and say they are trying to combat them, which no one else is doing. This very much gets into the mire of propaganda and spin and I don't think that a direct consequential chain of "public gets angry, state representing that public puts pressure on SG-using state, SG-using state acquiesces" is bulletproof as an argument. Saudi Arabia has such significant political and economic power that they can support terrorism with the West calling them allies. I think these things are far from obvious. I am not so concerned with unilateralism not for the state "peer pressure" reason you give but because it would be a significant financial burden for any one state. I also see it as a potential risk while you appear to discount it heavily, and use this to support 1b) which then kicks the ball down the road for 50 years.

This bet seems severely against your interests so I will treat it as rhetorical - your payout will only be accessible in 30 years and on those time scales inflation, life expectancy, technological change, X-risks and remembering the bet are all not in your favour. Whereas I have the potential to cash out at any time before that deadline. I don't particularly think that it's likely but I would still have taken that bet because of the conditions - 30 years is a long time.

  1. We are, depending on estimates, a decent way off having AGI, so by your argument there is no point doing AGI safety research now because we don't need it now, if there are any risks / costs associated. I think the same arguments in support of that work in this context, time required and complexity of the issue prompt early investment (and that early-stage research is neglected currently). This isn't about the neglect but the timeline - any research is going to incur costs now and pay off later. AGI safety has clear risks in potentially downplaying the risks of AGI and achieving a Dunning-Kruger effect for the entire human race, accelerating development of an inherently uncontrollable technology. It's a case of balancing the costs with the advantages of starting earlier.

  2. Over 90% of warming since the 1970s has been absorbed by the oceans, this conversation will only be of relevance if we are following higher RCP trajectories where it may be warranted to use controversial techniques, but there is a huge thermal sink that will work to bring the atmosphere back into equilibrium even using SG. This undermines the wait and see approach because it would be valuable to prepare in advance of the requirements.

  3. I didn't realise that this was referenced - if other people have investigated then that's fair enough. I didn't think there was any precedent for creating and dispersing particulate matter into the atmosphere and to keep them there, considering weather and localisation and other challenges. We are not able to replicate a volcanic eruption in terms of getting material airborne was my point, so the relevance of global cooling via volcanism isn't ironclad, direct evidence that SAI would work in the same way, as I expect it would depend on the distribution method and location (altitude etc.). If people who have looked into this see this as feasible with current tech then they will likely have more info than me.

  4. The counterfactual of not implementing SAI isn't a flat line for X-risk, in the absence of mitigating effects the risk of existential events increases over time as a result of the indirect effects of climate change unmitigated by SAI (other things being equal). This would somewhat discount the increasing X-risk of SAI.

  5. On rereading I may be misinterpreting, I thought that you were using moral hazard in the standard economic sense but you may be defining it just after as per previous papers as plan B undermining plan A. I agree in that case that my use of the term doesn't make sense, but I'm more familiar with it's use as increasing exposure to risk because the actor doesn't bear the full consequence of that risk, a sort of generalised externality. In that case CO2 emission is a moral hazard because the systemic risk isn't borne just by that actor, which incentivises overproduction.

Again, thanks for the response, I enjoyed the article and your reply has helped me understand the sources of disagreement a bit better. Some of them seem to be purely opinion-based or miscommunication. I also agreed with a lot of your points, although some not for the same reasons that you have given. I would still like to see small scale research on this topic being done and didn't see that much wrong with doing it. I will have to read more about the moral hazard argument sources you mentioned because they should be more convincing than those I have encountered previously.


Thanks for this - have some quick replies below

1. If solar geoengineering is not going to be used until we get to 4 degrees, then there is no point in researching it even if 4 degrees is catastrophic.

2. I agree that the constraints on state action are not perfect. As you say, the saudis fund terrorism and major powers flex their muscles at each other in more or less overt ways. The deployment of solar geoengineering would be on a different order - a huge and bold move. Do I think India would deploy solar geoengineering without the consent of China, risking the almost guaranteed ire of China? No.

The bet offer was not rhetorical and still stands if you would like it. We can pick an arbiter to make sure it happens. If you are worried about decaying attention, we could have a shorter timeframe? What do you think is the chance in the next 10 years that someone deploys it?

3. The debate about AI safety seems like a distraction to me - if you showed me that the case was analogous to solar geoengineering research, then I would argue that we should also delay AGI safety research for the same reasons. But it is disanalogous in numerous ways, so I don't see the point in exploring the analogy. Nevertheless... one rationale for AGI safety research is that some people think there is a non-negligible chance of AI in the next 20 years. Indeed, Toby Ord's median estimate is that we will get it in the next 20 years. If you believe that, then the case for AI safety research now is very clear. That is one disanalogy.

Secondly, the downsides of AGI research seem minimal. There is some dim possibility that AGI research could lead us to irrationally downplay the risks of AGI, but I have literally never seen this concern brought up before as a reason not to do AGI safety research. As far as I am aware, no-one is not doing AGI safety research because of that consideration. In contrast, in climate there is a pretty much cross-field taboo against against talking about solar geoengineering in a vaguely positive way. This is basically for the reasons I outline.

5. Our anthropogenic emissions between 2020 and 2080 have a huge effect on how hot it will get. e.g. We can still (technically) follow RCP2.6 and RCP8.5 On RCP2.6, median warming is less than 2 degrees, on RCP8.5, it is 4 degrees and beyond.

7. That seems right but the debate we're having is about whether to research it not deploy it.

Additional objections to stratospheric aerosol injections:

1. Stratospheric aerosol injections will stop global warming but not ocean acidification, which is caused directly by CO2 dissolving in seawater. This is a notable consequence if this "plan B" disrupts the "plan A" to reduce our carbon emissions to zero or below.

2. Once (conventional) aerosol injections start, they must not be stopped. Explanation:

I think it's fair to say that the main danger of global warming is its speed: ecosystems and human civilization would perhaps barely notice if the global mean land temperature were to rise 3°C gradually over the course of 10,000 years, but the same change in 100 years is difficult to bear (trivia: climate models and recent temperature records all agree that land temperatures will rise faster than sea temperatures; they disagree as to the extent of this phenomenon but, long story short, if global temperatures rise 2.2°C this corresponds to about 3°C warming on land, or more in the short term. I think that the goal to "keep global warming under 2°C" rather than "under 5°F on land" was a bit of a marketing mistake. Details in this paper.)

Stratospheric aerosols fall out of the stratosphere after a year or two, whereas much of our CO2 emissions will stay in the atmosphere for hundreds of years. Once stratospheric aerosol injections begin (assuming net CO2 emissions remain above zero), the quantity of aerosols must be continually increased to maintain a roughly constant temperature.

If the injections are ever suddenly stopped, most of the warming that would have occurred, over the decades or centuries that injections have been done, will occur immediately. This extremely rapid change is potentially very disruptive to humanity and global ecosystems, so an injection program should not begin without a very high confidence that we can ensure the injections will continue in perpetuity.

3. It may already be too late to avoid this problem, but in case of a global catastrophe, where modern society and most of its technology disappears for some reason, we'll want to rebuild society afterward. To this end, it may be significantly easier to rebuild if there is some oil left in the ground that is accessible to the reconstruction effort.


2. I don't think this is right, for reasons discussed in this Nature paper. Firstly, solar geoengineering could be used to slow the rate of warming even if it is deployed temporarily. You could deploy it over e.g. a fifty year period and thereby delay the point at which we reach peak warming, and then taper it out gradually. Secondly, as you say, an exception is if CO2 emissions stay above zero. Solar geoengineering could in principle buy us time to abate emissions and to take CO2 out of the atmosphere in which case it would not have to be deployed for the full lifetime of CO2 in the atmosphere. In this case, solar geo would slow the rate of warming and reduce peak warming.

Thirdly, I don't see why solar geoengineering would ever be stopped suddenly once we started. The reasons for this are discussed in the Parker and Irvine piece on solar geoengineering. All countries would have a reason to prevent it from stopping suddenly and would have the means to do so given how cheap it is. A catastrophe causing termination would have to be extraordinarily specific.

3. To clarify, is your point here that we should focus on mitigation because then we'll be left with some spare oil come a later catastrophe?

2. My assumptions were that geoengineering might reduce society's drive for mitigation (the switch to clean energy), and that it would be used to halt the temperature increase.

In the linked paper (Keith & Macmartin 2015) their proposal [actually they use the word "scenario" - I don't think they are going so far as to endorse it as a plan] is a bit different. They propose to use *half* as much aerosols as would be required to halt global warming (this is a bit tricky to get right, e.g. the radiative forcing of aerosols has much greater uncertainty than the forcing of CO2, so their proposal includes feedback to modify the injections as decades pass and observations are gathered about the effect of the aerosols). The paper says "We do not claim that this scenario is optimal. Rather we claim that good-quality policy-motivated scientific analysis requires an explicit scenario, and that this scenario is less obviously suboptimal than some scenarios employed in the literature." They point out that the harms of global warming increase superlinearly with temperature change, so I think they are saying that avoiding half the warming, or at least slowing global warming by half, is a reasonable compromise that avoids the worst harms without turning global warming into a total non-issue.

"Temporary deployment does not reduce long-term climate change. Warming in 2300, for example, is almost completely determined by cumulative carbon emissions and is unaffected by SRM that ends in 2200. Some commentators conclude that such temporary SRM offers no benefits, suggesting that it must be maintained forever." The paper counters that many climate change impacts depend on the rate of change - that if warming is slowed down, it is less harmful even if the total warming over 200 years is left the same. So I think the proposal here is to taper off the aerosol injections in such a manner that, in the worst case, we get the same warming over 200 years rather than 100.

They note that "It is clear that this scenario does not directly address thresholds that are a function only of the magnitude of the change rather than the rate, although it does delay reaching these thresholds, giving more time both to learn about the system and develop alternate strategies." The total amount of warming in their scenario *would* be decreased if we invent and deploy a technology that can remove CO2 from the air permanently (such technologies are very far from economical today). However, we can't guarantee we will invent an economical technology to do this. If we don't, Greenland may still melt under their scenario, but later than it would have otherwise ("in Fig. 1, the time to reach a temperature rise of 2 °C above pre-industrial increases from 2055 to 2068, while the time to reach a 2.5 °C rise increases by 32 years.").

Regarding stoppage of geoengineering due to catastrophe, they say, "While not discounting the possibility of social collapse, we note that humanity has operated technologies such as trans-oceanic communication links and electric power grids for more than a century in spite of horrific wars. Moreover, in considering the implications of a possible social collapse on the public policy of SRM [Solar Radiation Management], one must set the risks of termination against the (likely) greater human suffering that would arise directly from the collapse itself." So, if there's a global catastrophe, a sudden increase in global warming seems like a minor footnote in comparison.

I remain concerned that geoengineering is a distraction that could reduce the pressure to reduce CO2 emissions, but if geoengineering were to become a popular political position, I agree that Keith & Macmartin's proposal seems better than the "default" geoengineering proposal that people (including me) naively think of, i.e. to simply stop global warming regardless of CO2 emissions.

3. Yes.

I don't think the pernicious mitigation obstruction argument is sound. It would be equally plausible for just about any other method of addressing air pollution. For instance, if we develop better solar power, that will reduce the incentive for countries and other actors to work harder at implementing wind power, carbon capture, carbon taxes, tree planting, and geoengineering. All climate solutions substitute for each other to the extent that they are perceived as effective. But we can't reject all climate solutions for fear that they will discourage other climate solutions, that would be absurd. Clearly, this mitigation obstruction effect is generally smaller than the benefits of actually reducing emissions.

The pernicious mitigation obstruction argument could make more sense if countries only care about certain consequences of pollution. Specifically, if countries care about protecting the climate but don't care about protecting public health and crops from air pollution, then geoengineering would give them an option to mitigate one problem while comfortably doing nothing to stop the other, whereas if they have to properly decarbonize then they would end up fixing both problems. However, if anything the reverse is true. To the extent that the politics of climate change mitigation are hampered by the global coordination problem (which is dubious), and to the extent that the direct harms of air pollution are concentrated locally, countries will worry too little about the climate impacts while being more rational about direct pollution impacts. So geoengineering would mitigate the politically difficult problem (climate change) while still leaving countries with full incentives to fix the politically easy problem (direct harms of pollution), making it less of a mitigation obstruction risk than something like wind turbines.

Additionally, given the contentious side effects of geoengineering, the prospect of some actors doing it if climate change gets much worse may actually encourage other actors to do more to mitigate climate change using conventional methods. It's still the case that researching or deploying geoengineering would reduce the amount of other types of mitigation, but it would do so to a lesser degree than that caused by comparable amounts of traditional mitigation.

Another note: I think if we had a better understanding of the consequences of solar geoengineering, then the security consequences of unilateral deployment would be mitigated. Disputes become less likely when both sides can agree on the relevant facts.

This suggests that for solar geoengineering to be feasible, all major global powers would have to agree on the weather, a highly chaotic system.

Hm, I thought one of the main worries was that major global powers wouldn't have to agree, since any country would be able to launch a geoengineering program on their own, changing the climate for the whole planet.

Do you think that global governance is good enough to disincentivize lone states from launching a program, purely from fear of punishment? Or would it be possible to somehow reverse the effects?

Actually, would you even need to be a state to launch a program like this? I'm not sure how cheap it could become, or if it'd be possible to launch in secret.


Hello, good points! I discuss this supposed free driver/unilateralist's curse feature of solar geoengineering in section 3.2 of my paper. This is a recurrent worry in the literature, but I don't find it plausible. Quoting from ym paper:

"In my view, the risks of unilateralism are overstated. Firstly, as argued in Section 1, the cost estimates relied upon are likely to be a significant underestimate, plausibly by an order of magnitude. Secondly, as Parson (2014, 98) has argued “these scenarios overstate the distribution of capabilities and thus the risk of unilateral action, because they focus too narrowly on financial cost as the determinant of capability and neglect other, non-financial, requirements and constraints.” An SAI programme large enough to make a non-trivial sustained impact on the climate would be hard to conceal and vulnerable to military attack. “[U]nilaterally achieving a climate alteration that matters would require not just the money, technological capability, and delivery assets, but also the command of territory, global stature, and ability to deploy and project force necessary to protect a continuing operation against opposition from other states, including deterring their threats of stopping it through military action.” (Parson 2014, 99) This suggests that scenarios in which small states or rich individuals deploy SAI are vanishingly unlikely"

I then argue that multilateralism is actually the guiding logic of solar geoengineering:

" Indeed, Horton (2011) has persuasively argued that SAI is actually characterised by a logic of multilateralism. The success of an actor’s SAI programme would depend on whether other actors were also pursuing their own SAI programme and would be ineffective without coordination. Moreover: “States opposed to geoengineering have a number of tools at their disposal to counteract climate interventions. In the case of SAI, for example, fluorocarbon gases could be deployed to offset cooling effects. Alternatively, the strategic use of black carbon could neutralize artificial albedo enhancement.”(Horton, 2011, 62) In short, if powerful actors were opposed to an SAI programme by a state or a collective of states, they could effectively discourage it using ordinary military threats or by counteracting the effects of SAI. The foregoing suggests that the decision to deploy unilaterally would not be taken lightly, given the incentives created by conventional military threats and the ease with which SAI schemes can be disrupted. Even for a case in which a major power is facing very severe climate impacts, SAI without support from other major powers would likely either be counter-productive or ineffective. In my view, this suggests that unilateral deployment even by a powerful state or some coalitions of powerful states is not a serious danger, provided that there are some dissenting major powers (though it should be noted that many experts disagree)."

Imagine that India is deciding whether to launch a solar geoengineering programme that would dramatically affect the weather in China. I think it is clear that they would not proceed without Chinese agreement, given the enormous risk of war.

Hey John, thank you for the article! I feel that there is substantial confusion regarding whether to delay or even accelerate research into SRM and similar stuff. 

The argument against researching geoengineering methods seems to be that having SRM available in the short-term would do more harm than good.  However, this makes the fundamental assumption that research enables geoengineering in the first place. In my view, the "how to" deploy geoengineering is already public. Deployment (unilateral or not) could happen now.

If one assumes the „information hazard to be out of the box“, all that further geoengineering research does is reduce our uncertainty regarding the effects of environmental interventions. This seems to be a good thing. 

I also believe that (in the mid-term) geoengineering can be done for 1 billion $ or less as Marine Cloud Brightening seems to be potentially cheaper to implement and possibly harder to detect and trace. Furthermore, cost estimates do not account for accelerating technological progress that could cut costs relative to income even further in the next decades. 

Would love to hear your thoughts on this!


Hello! thanks for this

As I argue in the piece, I don't think deployment could happen now, at least for stratospheric aerosol injection. I don't think it will happen until there is significant within-country demand for SAI at least among all major powers. We are a long long way away from that. The governance challenges for things like marine cloud brightening are lower so I agree that could plausibly be used much sooner.  

The information/attention hazards depends not only on the idea of solar geoengineering but how much it is discussed. This is widely accepted in eg biorisk where many researchers will not mention published papers on gain of function research. It is clear that further scientific discussion and attention would increase the info/attention hazard. 

My main concern with SAI research is that it is a waste of money. The case is less clear for more regional solar geo

I largely agree! 

This post was awarded an EA Forum Prize; see the prize announcement for more details.

My notes on what I liked about the post, from the announcement:

While geoengineering isn’t one of the most popular EA cause areas, many people within EA think of it as a promising idea which merits further exploration. In 2016, Open Phil made a $2.5 million grant to support research on solar geoengineering technology, as well as the political and social implications of geoengineering.

In "The case for delaying solar geoengineering research", Halstead pushes back on the technical side of this research, arguing that:

  1. Due to geopolitical tension, we probably won’t want to actually use geoengineering in the next few decades.
  2. Research into weather alteration could lead us to discover dangerous technology, which means that we should avoid or delay the research if we can.
  3. And in fact, we can avoid or delay; we have a lot of time to observe the results of today’s climate change mitigation efforts before conducting high-risk geoengineering research.

His post adds considerable detail beyond what appears in the Open Phil writeup, and represents a change from his thoughts on the issue in 2018. The quality of Halstead’s analysis and his willingness to reconsider his past positions make this piece an excellent example of what we’d like to see on the Forum.

Countries are already willing to bear the local costs of reducing CO2 emissions in service of the global goal of reduced CO2 emissions. Russia already signed the Paris agreement. Poor countries who will lose statistical lives from energy regulations have signed the Paris agreement. So I think you are overstating the governance obstacles a little bit. Maybe this is addressed in those papers which discuss compensatory schemes, idk.

The geopolitical balance of power is not constant. In the next decades we will most likely see increasing geopolitical status for India, Brazil and possibly Africa. These countries have the strongest interests in preventing climate change and will make it easier to push a global movement for geoengineering.


Why not just pay Russia an (arguably fair) reparation?

There is a small probability that we are very wrong about climate sensivity and only in this case climate change is an existential risk. The reason for this is not in the climate science, but in the anthropic principle: if our climate is very fragile to the runaway global warming, we can't observe it, as we find ourselves only on planets where it didn't happen.

To fight runaway global warming we need different type of geo-engineering then for the ordinary climate management, as it should be able to provide quicker results for larger climate changes, and also require less research time and may be implemented unilaterally.

I call this type of geoengineering "plan C". it could be something like artificial effect of nuclear winter, may be started by nuclear explosions in dormant volcanos.

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