I think this is a very important point to understand: regardless of if you believe particular geoengineering approaches are too dangerous or too little understood to apply, at some point when the threat & the damage from global warming is no longer possible to ignore, it will not be surprising if a single actor begins unilaterally executing a relatively cheap & feasible geoengineering project:

> Solar geoengineering isn’t only technically feasible, it’s a bargain. Next to the trillions in costs from unmitigated climate change, and even the expense of cutting CO₂, solar geoengineering costs practically nothing. If anything, it’s too cheap. A program that releases SO₂ to decrease average temperatures by about 0.1C would cost less than $5 billion per year. This should prompt the world to prepare for its inevitability. Dozens of countries have both the capacity and possible motivation. The operative word is “when,” not “if.”

Ignoring the unwanted environmental side effects of pumping SO2 into the atmosphere, one thing this article does not mention is that the results of geo engineering will be non uniform. Just like global warming will causes non uniform temperature & climate change over the globe, similarly geoengineering approaches will have non uniform results -- some regions will be relative winners and some regions will be relative losers -- getting a larger share of the unwanted consequences of geo engineering with few or no corresponding benefits.