Gernot Wagner has a scary op-ed in Business Week with this conclusion:

Solar geoengineering is potentially so powerful that one actor might be able to lower temperatures for the globe. It’s only a matter of time before pressure will increase to do just that, regardless of how fast the world slashes CO₂ emissions. With more frequent extreme heat and weather, it’s not hard to foresee conditions miserable enough to make an attempt at a little relief seem worth the risk to some. Limited research is already making one thing clear: 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.”