Short version: Probably aircraft, but anything that can get aloft and dump aerosols (e.g. SO2) into the upper atmosphere.
The scary thing is it's all quite achievable and isn't even that expensive or complex.
Regarding the issue of speed, the wikipedia article even notes one of the benefits as:
> Speed: A common argument is that stratospheric aerosol injection can take place quickly, and would be able to buy time for carbon sequestration projects such as carbon dioxide air capture to be implemented and start acting over decades and centuries.
And ironically, we don't even have to worry that much about side-effects since the effects of atmospheric aerosols are relatively temporary.
But, again: a) doesn't help with ocean acidification, which threatens entire foodchains, and b) gives an easy out to not do the hard and necessary work of cutting down on CO2 emissions.
That said, at this point, I would make a very sizable bet that a major country or countries will engage in some type of geoengineering within the next, oh... 10 years at the outside? The dangers of allowing global warming to continue to run away are just too high for some nation to not do it, prove it, and pave the way for others to follow.
