Atmospheric scientist here, currently non-practicing. You’re making the same mistake that climate denialists make, but in reverse. We actually know a great deal about climate feedback mechanisms now. We know the residence time of SO2 in the upper atmosphere. We know how to calculate the change in solar flux as a result. We can model what will happen — we know this because our past climate models did well with the uncontrolled experiment we're already running. Simply because the Earth system is complex doesn’t mean we don’t understand it enough to foresee catastrophic failure.

And if we overshoot cooling, we know how to fix it. Just pump methane into the atmosphere. Stuff lasts a few decades before breaking into ozone and CO2 (via NOx). We already know how that turns out because we’re doing it now.

Lack of perfect knowledge about the Earth-atmosphere system is not an excuse for inaction. The grand experiment was already begun in the 1800s, and it’s too late to call it off now. We must take both decarbonization and geoengineering strategies if we want to escape the worst of this.