That's $100 billion one-time cost to reduce global temperature by 3 degrees, which is *100 years* worth of global warming.
Now, obviously, there are some caveats -- like, no one's *really* sure if it would work, and sulfur dioxide causes acid rain if it gets into the wrong part of the atmosphere, and if it gets into the *really* wrong part of the atmosphere (the part we breathe) it kills people (although I don't think it's as deadly as ozone).
That wasn't the big objection at the international workshop addressing the matter. No. The big objection was that if we had a way to counteract carbon emissions and save the planet without having to force cutbacks in world industry, it might be a moral hazard that would keep us from concentrating on our efforts to cut back on world industry.
I've seen this before.
"We have a drug that tricks the body into thinking that you just exercised, giving most of the same health benefits. But if we allow it to be distributed, who would actually bother to exercise? We must ban this drug before we even find out if it works or is safe due to the moral hazard it presents!"
...er, and I know there've been others, not all of them technological. The basic format is 'We've been trying to solve problem X by method A, but not everyone has been able to succeed because method A is hard. Now we have method B which is easy -- we must ban it, or the rates of success using method A will plummet!'
It's called a 'moral hazard' because the process of succeeding using method A has been talked up as a good thing in its own right, since the high rate of failure makes it hard to sell based on its results. Method A has become the morally right thing to do, and people forget *why* we were bothering to do it in the first place.
Usually, people come to their senses -- the moral objections that actually get traction are a little more sensible. It still really makes my skin crawl whenever I see it, though.
(...also, I think it would be really cool to terraform Earth. We should do it just for the lulz!)