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Mine some ice from an asteroid and drop it into the ocean, with larger blocks every few years as pollution increases :-)

Much of the ice would vaporise on entry. Water vapor is a greenhouse gas.

Yes. But more importantly, that was a Futurama reference: https://www.youtube.com/watch?v=6cjx4gJFME0

It can either both prevent sunlight from entering or leaving depending on the altitude.