...and therefore we shouldn't be doing anything? That's not a rational decision.
Assuming there are only two options (do nothing or put up an SO2 sunscreen), and both options are poorly understood, then it is still the rational thing to go through with the option where the better outcome is predicted. No matter how large the uncertainties are.
In the present case, however, there's no conflict between expectation and risk. As best I understand it, the negative effects of unchecked warming also produce what seem to be far greater risks -- setting positive feedback loops in motion, changing weather patterns, possibly switching off the Gulf Stream... Words and ideas I offer here may be used freely and without attribution.
As it happens, we already know the consequences (smog, acid rain, global temperature drop if scale is large enough) of a massive SO2 release, as you have pointed out. So the modeling should not be too fraught with uncertainties.
Carbon sequestration, which is on everyone's lips these days, is a most likely a problem with more uncertainties--how carbon dioxide will affect subterranean rocks and soils; question of leakage and release after a few hundred years, etc.
