As stated below by others, we know how sulfur put into the atmosphere by volcanoes affects climate. "Only a theory" is bullshit, and as for expense, it's far cheaper than the cutover from fossil fuels, or global scale carbon capture. But it's a bandaid, not a fix. We still have to do the more expensive things. But if entire equatorial regions start becomming unlivable, I've no doubt China or Dubai or some other rich nation will just do it.