I doubt they can predict all of the second order effects of doing this all over the earth; it could easily lead to an ecological, climate, or humanitarian disaster. This is a terrible idea. A unilateral action is completely uncalled for.
Nobody wants to block the sun. But ecological, climate, and humanitarian disaster are inexorably headed our way already. If positive feedbacks seriously kick in, this has a chance of buying us some time until we can reduce carbon levels.
Besides, we've already seen this employed, by volcanoes, with global effects lasting one to two years. Disaster did not ensue.
Kind of like research on deflecting asteroids.
"the U.S. Government and Accountability Office (GAO)"..."estimated that about $2 million were directed to “albedo modification and less conventional CDR approaches,” so less than 0.1% of the U.S. climate change budget focused on the strategies discussed in our report." (page 185 of the report)