The potential geopolitical consequences are huge. The United States could unilaterally initiate such a project, but developing countries such as China would be resolutely opposed. By dimming the planet, the U.S. would impose its own will on the rest of the world. The rest of the world, which includes nuclear powers hostile to the US, might be very, very angry. So that's something we would need to prepare for. Indeed, food-stressed countries like China and North Korea might view geoengineering as an existential threat to their societies.