Geoengineering: playing God?

Problem: the Earth is getting warmer. Other problem: even if we stopped producing C02 completely tomorrow, the Earth would still continue to warm -- for years -- before it started to cool again. Arguably crazy solution: put up a solar shield and stop the warming ASAP. If this sounds a bit like that episode of The Simpsons where Mr. Burns blocked out the sun in Springfield to force people to use more of his nuclear power, you're not that far off -- except that this version of the sunblock scheme comes from scientists researching ways to slow climate change.

The idea, basically, is this: spray a lot of sulfur dioxide (or some other light-blocking, preferably non-carcinogenic agent) into the air until it deflects about 8% of the sun's rays. Wait until global temperatures reach pre-industrial levels. Party. This is to a lesser degree what happened when Mt. Pinatubo in the Philippines erupted in 1991: the particulate matter it rocketed into the upper atmosphere stayed there -- cooling the Earth a few tenths of a degree -- for nearly a decade. According to a new study, doing this artificially would only cost about $100 million, vs. the hundreds of billions it would cost to change the way most of the world produces energy. So what's the downside? That's kind of the scary part: so far, they're having a problem finding one, assuming the shield was placed and maintained properly. Maybe that's a problem in itself: this method is untested and its effects would be wide-ranging; asking "what's the worst that could happen?" sound suspiciously like famous last words.