Availability of risky geoengineering can make an ambitious climate mitigation agreement more likely

Some countries prefer high to low mitigation (H ≻ L). Some prefer low to high (L ≻ H). That fundamental disagreement is at the heart of the seeming intractability of negotiating a climate mitigation agreement. Modelling global climate negotiations as a weakest-link game brings this to the fore: Unless everyone prefers H to L, L wins. Enter geoengineering (G). Its risky and imperfect nature makes it arguably inferior to any country's preferred mitigation outcome. However, absent a global high-mitigation agreement, countries facing disastrous climate damages might indeed wish to undertake it, effectively ranking H ≻ G ≻ L. Meanwhile, those least affected by climate damages and, thus, least inclined to agree to an ambitious mitigation agreement, might be unwilling to engage in risky geoengineering, resulting in L ≻ H ≻ G. With these rankings, all players prefer H to G, and the mere availability of a credible geoengineering threat might help induce an ambitious climate mitigation agreement (H). The analysis here introduces the simplest possible model of global climate negotiations and derives the conditions for this outcome. These conditions may indeed be likely, as long as geoengineering is viewed as a credible albeit risky emergency response given the danger of low mitigation levels.