We did try planet-scale geo engineering. It has worked so far, predictions were met. Heating through CO_2 and other heavy molecules works as predicted. Cooling through Sulfur compounds works even better than predicted, as evidenced by the sudden rise in temperatures after cessation of sulfur-rich fuels.

And in the end, all solutions we can attempt are technical and in some aspect untested. We just assume that the consequences of some of those solutions are small enough, but none have really been tested at scale.