If a temporary SO2 sunscreen can reduce global temperature the way Pinataubo did, great.  And probably using coal-fired plants to do it, as Migeru suggests, would be to rely on technology already in place. But the expense of modifying scrubbers to permit SO2 release will not be undertaken voluntarily by utilities.

In any case, we also must mitigate CO2 on a large scale.  There is a limit to how much carbon dioxide forests, deserts, and soils and can take up before they start expelling CO2 into the atmosphere.  

Forgive me for repeating a comment I posted on Techno's Ocean Acidification diary:

The ocean is a tremendous carbon sink.

Oceanographers I've talked to are thinking that the changes in ocean temperature are too rapid for many species to adapt.  The acidification rate of the ocean may also be too rapid for adaptation of species.

At present carbon emissions from human activities continue to increase at a steep rate.  

Meanwhile, the increase in ocean temperature is speeding up the chemical processes of acidification.  This is ultimately bad news for critters with exoskeletons. And for the food chain.  One billion people rely on the ocean for food.

If the present acidification due to carbon absorption continues at the present rate, erosion from terrestrial rock will not supply enough buffering to counter the acidification.  At some point the ocean will fail as a carbon sink.  So the carbon will remain mostly in the atmosphere, creating a rather Venusian environment.

In this scenario, there's a tipping point that will be catastrophic unless drastic reduction of carbon emissions are undertaken.