Solar radiation management:
That is, spraying millions of tons of sulfur aerosols into the lower stratopshere, either from aircraft or gigantic hoses, in order to block the amount of sun that reaches the earth. The goal of this scheme is to mitigate global warming, but the dangers are many: Widespread drought, worse than what climate changes would cause by itself; wild fluctuations in global temperatures that would eventually trigger feedback mechanisms and make global warming even more catastrophic than it would have ordinarily been.