Particulate matter in the air causes precipitation. Spraying large amounts of particulate matter into the sky is likely to drastically change global rainfall patterns - so you might get a ton of rain in the Sahara, where it’s useless, and none at all in the American Midwest. The result could be drastic food shortages and famines. Major volcanic eruptions have been associated with droughts and famines. I think geoengineering on this scale may be necessary, but it is a terrible option and an absolute last resort
When you say rain in the Sahara would be useless, are you saying it's already a carbon sink or something? After all it seems that rain in the Sahara would eventually cause forests to grow, given a few years.
Isn't the upside of this kind of geoengineering the reversibility? If we inject SO2, and we don't like the effect, we stop. IIRC it doesn't hang around that long.
There's likely a decade+ tail after the end of active particulate injection. If there is too much to begin with, you piss off all the electorates with years of chaotic weather, threatening food production, financial markets, and ecologies all over the world. Get it just right, you wreak less havoc, piss off fewer electorates, and lower global average temperatures. Get too small an effect, you risk overcorrection or the project failing before it has a chance to really work.
There's also the fact that you're altering a chaotic system, with fundamentally unpredictable side effects, some of which could last longer than a human lifespan. Wanna make California get monthly hurricanes? How about weekly tornados in Colorado? Bury New York city in ice?
It's not that any of those things are probable just that they're possible, and chaotic systems can drastically, catastrophically, and exponentially change configuration, and no matter how good your supercomputer simulations are, you fundamentally cannot predict the consequences.
All of human civilization teeters on the current climate configuration - geoengineering has terrifying potential. Humans have a horrible track record in attempts to predictably alter chaotic systems and ecologies. We damn well better know what we're doing before we use globally acting tools.
