This thread's take is pretty naive. The topic of geoengineering predates the 80's if not longer, and is a typical case of Tragedy of the Commons.

Let's suppose, for the sake of the argument, that the technical side of the issue (how, how much, to what extend) is solved. Now we have our best scientists planning accurately what to spread where and in which amount to make the planet cool to its pre-industrial levels. This is a gigantic effort, and mobilizes all countries on Earth. The perfect ground for some countries to think "well, if I spray X% less, my climate will be milder, I will outgrow the neighbours' country crops and provide a better future for my people". Sabotages are discovered, pledges are broken, wars are fought.

Anticipating on that, undisputed World Leader Country X plans for everyone everywhere all at the same time for what it consider is only fair: maintain the status quo. And proceeds unilaterally with the plan. Sabotages are made, wars are fought.

Back to today's world. We currently can't collectively solve climate change by reducing emissions, although the course of action is dead-simple, because millennia of geopolitics and self-serving policies get in the way. The technical ability to geo-engineer the problem away could be nothing more than more complex way to shout ourselves in the foot, faster and louder.