Mike Konczal points out a major problem with geo-engineering by pumping sulfur into the atmosphere:
[L]et’s then assume that the sulfur is causing too many side effects, and we want to shut it down. Then what happens? The sulfur rains out over the course of a short time period, say a year, and then the Earth heats up 5 degrees very, very quickly. No gradual increase over this time period; we have the same carbon amount as we had before. We haven’t lost any weight, we were just wearing bigger pants. That would be a nightmare situation, and as such even if the side-effects were terrible it would be difficult to “turn off” such a plan.