The idea of a future American administration unilaterally deciding to tinker with geoengineering and somehow making things 100 times worse, is one of my nightmare scenarios.

Back of the napkin, thinking out loud, pumping a bunch of sulfur into the stratosphere might mitigate for a short-term the worst effects of climate change. Same with a few other compounds. Like OP replied, it would also mean that for the next several centuries we're going to have to continue to pump those compounds into the stratosphere, and it also assumes there won't be knock-on effects that we are simply ignorant of at this time.

The problem with geoengineering is we only have one biosphere. You can't really do experiments, you can't know for sure what affects whatever action you take will have around the globe. This, in addition to it being relatively cheap and easy, being the kind of short-term solution that American politics loves, leaves me certain that it will be an American president who finally darkens the skies.

https://youtu.be/AEagAcFP1II