The half-life of sulfur aerosols in the stratosphere is on the order of a few months. So if there turns out to be some overwhelmingly undesirable side effect,

The half-life of sulfur aerosols in the stratosphere is on the order of a few months. So if there turns out to be some overwhelmingly undesirable side effect, we can just stop putting the sulfur up there and the effects will dissipate relatively quickly. Nature does this experiment for us every so often: large volcanic eruptions, such as the 1991 Mount Pinatubo eruption, put massive amounts of sulfur into the stratosphere, much larger than the amount we'd need to use. So it's pretty likely we would have noticed the negative effects if there were any (beyond the fact that we got much more cooling than we wanted for a couple of years).