Regarding rightists: It would seem easier to go from "there is no problem" to "the problem can be cheaply contained", particularly without a strict requirement

Regarding rightists: It would seem easier to go from "there is no problem" to "the problem can be cheaply contained", particularly without a strict requirement for mass lifestyle, infrastructure, or industrial changes. Similarly, it's hard to imagine opportunists not using any leftist refusal to take concrete action as ammunition. This will motivate them to at least give token support. If the problem gets worse, geoengineering will naturally be turned to, not because it's some best solution that would make everyone maximally happy, but because it's the easiest solution and requires the least coordination. Lastly, note that you mention international consensus, but it's easy to imagine mere regional/factional consensus being enough. Nations already often engage in pollution or depletion of natural resources that crosses borders. Other players not cooperating only means scaling up independent operations to compensate.