The best-studied proposal [for climate geoengineering], to pump sulfate aerosols into the upper atmosphere to block sunlight, would cause its own troubles. The sulfates would slow or reverse the recovery of the ozone layer; they might also reduce global rainfall, and the rain that did fall would be more acidic. And those are just the foreseeable effects. Aerosols are the least understood aspect of the climate system.
The possibility that international collective action might not be entirely reliable brings up the fourth and perhaps most intractable barrier to geoengineering: the geopolitics. Imagine if, say, Chinese-produced clouds of sulfuric acid blew across the Pacific or if American efforts to reduce flooding on our shores triggered drought in Central Asia. How would nations respond to such provocations as anything but an act of war?
High cost, unintended consequences, uncertainty, short attention spans, international bickering: if these problems sound familiar, it is because climate skeptics have made the very same criticisms of plans to cut emissions, such as the Kyoto Protocol. The difference is that geoengineering is even worse. Emissions cuts may be challenging, but the science is well established, most of the technology already exists, the costs can be spread over the natural capital-replacement cycle, public awareness is high, and international institutions such as carbon markets are taking root. The time to act is now.
The problem here is that nobody knows if this stuff indeed saves crops. It might as well just save one or two precious crops and then cause some other, major fuckup nobody took the time to calculate beforehand.
I distrust anything large scale at this point - we are simply not knowledgeable and disciplined as a species to pull this stuff off, and maybe just should slow down for now, not add more problems to the already existing ones.
In the general sense with messing with things, I agree. I'd rather go for massive industry regulation (that actually works) and CO2 and other green house gas sequestration.
We need to both cut back on output 'and' take some back out to do the least harm possible. Since we're nowhere near neutral, thirty years ago could've been a good starting time...
Sure, there are ups and downs of various methods, but my point is, we're already waaaay past safety margins. Whining about possible catastrophy is like complaining about spilling your drink while the car is grinding up on a guard rail. (except there are no guard rails for the entire planet, so it's straight off the cliff for us, baby!)