Since aerosols last much longer in the stratosphere than they do in the rainy troposphere, the amount of aerosol - forming substance that would need to be injected into the stratosphere annually is far less than what would be needed to give a similar cooling effect in the troposphere, though so far as
the stratospheric aerosol burden goes, it would still be a bit like making the Earth a permanently volcanic planet (think of a Pinatubo or two a year, forever).
b) Deployment of a fast - working high - leverage technology (such as — possibly —
stratospheric aerosols) may not have the same
burden - sharing problems, but would surely require a global agreement for other reasons.