Would unilateral solar geoengineering be in the interest of any nation? I'll argue that for Stratospheric aerosol geoengineering (SA geo) pure unilateralism wouldn't be but that for marine cloud brightening (MCB) and cirrus cloud thinning (CCT) it could be. 1/

As MCB and CCT could be applied in specific geographic regions, with climate effects primarily localized in those regions (although not exclusively), unilateral deployment within / around a nation would primarily affect that nation. 2/

A nation could argue plausibly that their local, unilateral MCB / CCT deployment was a national concern. Such an intervention would have remote effects but these would be very challenging to detect and attribute. 3/

For SA geo the picture is very different. Any SA geo deployment would be global in its effects as it is impossible to contain the released aerosols to within a nation's borders. A unilateral deployment of SA geo could not plausibly be argued to be a solely national concern. 4/

Why would a country wish to deploy SA geo? And would these reasons argue in favour of unilateral deployment or something more inclusive? Let's consider a few reasons: 1) reducing national climate risks, 2) relative advantage over competitors, 3) prestige / national pride. 5/

1st, reducing national climate risks. As it is impossible to restrict the effects of SA geo to one nation, a unilateral deployment would produce similarly global effects to a fully-multilateral one. Hence any unilateral deployment would need to consider these global impacts. 6/

Now, the views of each country about any proposed or realized SA geo deployment will be a product both of their perceptions of the physical climate impacts of such a deployment as well as other motivations, including the desire to be involved in decisions. 7/

If the motivation for SA geo deployment, unilateral or otherwise, is national climate risk reduction, a benefit, then the pushback from others, in the form of sanctions, reputational damage, etc., will be a cost that must be weighed against this benefit. 8/

Our potential unilateral deployer motivated by climate risks could compromise a little and potentially reduce push-back by pursuing a more globally-minded risk reduction strategy, that has large national benefits, and invite others to join a mini-lateral, hegemonic coalition. 9/

Here, there is a spectrum of possibilities from highly-exclusionary (OPEC) or unpopular (coalition of the willing) coalitions to large coalitions that are still dominated by one hegemonic power (NATO). Such a move could be worth it for a nation motivated only by climate risks 10/

Given that many nations are facing severe climate risks, and assuming climate model projections of the potential benefits of SA geo are accurate, I believe many nations which would oppose unilateral deployment could be persuaded to join such a mini-lateral coalition 11/

If our nation, motivated by national climate risks, can be confident of finding similarly motivated allies (which I believe they could be) then there seems to be no reason to prefer unilateralism over narrow multi-lateralism at minimum as this would reduce push-back. 12/

What if our country has the second motivation, relative advantage? Our unilateral deployer could seek to optimize its national climate outcomes without consideration of others, or perhaps seek to deliberately harm others. 13/

However, again, these climate benefits (or harms to competitors) would come at the expense of international push-back. This push-back would be all the more intense if these others realized that our deployer was pursuing such narrowly self-interested and destructive goals. 14/

There is a precedent for this, as the US pursued weather modifcation for military purposes in the Vietnam war generating considerable opposition and leading other nations to sign the ENMOD treaty which banned such behaviour. 15/

Based on my understanding of the science, any SA geo intervention would be so unspecific in its impacts that any attempt to harm some other nation, e.g. disrupting precipitation patterns by shifting the tropical rain bands, would harm nations the world over. 16/

Selfishly pursuing relative advantage through means of climate modification would likely generate such opposition that any advantage gained would surely be outweighed by the international opposition it would generate. 17/

A more subtle means of gaining advantage would be to pursue prestige or national pride through means of deploying solar geoengineering, our third motivation. However, as noted above, selfish unilateral deployments are likely to garner for more opposition than prestige. 18/

If motivated by prestige, then rather than setting itself up as a unilateral pariah, it could instead position itself as a savior, researching and developing SA geoengineering as a self-less contribution to tackling global climate risks. 19/

Again there are precedents for this kind of thing. After early soviet success in space, the US seriously considered pursuing a global desalination effort before settling on the race to the moon as a means of competing for global prestige (read in The Moon, @Eaterofsun ). 20/

To conclude, a pure unilateral deployment of SA geoengineering seems unlikely given the push back it would generate (unilateral, regional MCB and CCT are much more likely). There seems little to be lost from broadening out to form some form of coalition and much to be gained. FIN