It’s worth spending some more time on the National Academy of Sciences reports on geoengineering prospects and concerns — the concerns mainly being about adding sun-blocking particles to the atmosphere to counteract global warming driven by the buildup of heat-trapping greenhouse gases.
Pierrehumbert’s prime concern (there are plenty more, all legitimate) is that any sun-blocking intervention done at climate scale would have to continue unabated for millenniums, or until CO2 removal was in high gear — or risk climatic whiplash if veils of reflective materials dissipated.
That should be enough to deter any countries from going global with such efforts.
But I’ve long seen plenty of other reasons why this is almost assuredly a nonstarter in any case. The main one is diplomatic, not technological. Who sets the thermostat?