Robin Hanson summarizes one case for it. I don’t know much about the technologies but my worries are mainly political. Won’t the Russians benefit greatly from a warming world, both because they are a bit cold and because they will access a warmer Arctic? Would the UN Security Council approve climate engineering? Probably not. If the United States did it on its own, could that be perceived as an act of war? If geo-engineering is cheap (which is part of its very promise), and unilateral action is acceptable, don’t other countries also get to take their shot at influencing the environment and moving us toward an optimal climate? What does the resulting equilibrium look like? Who moves last? How would we feel if someone changed the U.S. climate to make many parts of the country colder?